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INJECTION MOULD DESIGN BY R W G Pye
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A textbook for the novice and a design manual for the thermoplastics industry
R. G. W. Pye
Fourth Edition
I~WPI Affiliated East-West Press Pvt Ltd New Delhi
EDUTECH NTTF INDIA PVT. LTD. BANGALORE-560 058.
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Affiliated East-West Press Pvt Ltd New Delhi Copyright © Ronald G W Pye and The Plastics and Rubber Institute 1989
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the Publishers.
Printed at Rekha Printers Private Limited, New Delhi II 0 020 Published by Affiliated East-West Press Private Limited 105 Nirmal Tower, 26 Barakhamba Road, New Delhi 110 001
ISBN
81-7671-010-5
Licensed for sale in India only.
To Trina For her encouragement and indulgence
Contents
xi
PREFACE ACKNOWLEDGEMENTS
PART ONE
l
Mould making
1.1 1.2 1.3 1.4 1.5 1.6 1. 7 1.8 2
3
ELEMENTARY MOULD DESIGN
General Machine tools Castings Electrodeposition Cold bobbing Pressure casting Spark machining Bench fitting
xii 1 3
3 4 18 20 21 23 25 27
General mould construction
31
2.1 2.2 2.3 2.4 2.5
31 34 45 50 71
Basic terminology Mould cavities and cores Bolsters Ancillary items Attachment of mould to platen
Ejection
3.1 3.2 3.3 3.4 3.5 3.6
General Ejector grid Ejector plate assembly Ejection techniques Ejection from fixed half Sprue pullers
74
74 74 80 92 129 131
vii
CONTENTS
4
5
6
7
Feed system
135
4.1 General 4.2 Runner 4.3 Gates
135 135 146
Parting surface
168
5.1 5.2 5.3 5.4 5.5
168 168 170 177 179
General Flat. parting surface Non-flat parting surface Relief of parting surfaces Venting
Mould cooling
180
6.1 6.2 6.3 6.4 6.5
180 181 193 216 218
General Cooling integer-type mould plates Cooling insert-bolster assembly Cooling other mould parts Water connections and seals
Standard mould systems
225
7.1 7.2 7.3 7.4
225 230. 247 254
General considerations Standard two-part mould systems Deviations from the standard mould Comparative terminology
PART TWO
INTERMEDIATE MOULD DESIGN
255
8 Splits
257
8.1 8.2 8.3 8.4 8.5
257 260 288 295 296
General Sliding splits Angled-lift splits Summary Standard parts for the splits type mould
9 Side cores and side cavities
9.1 9.2 9.3 9.4
General Design features Types of side core and side cavity Standard mould parts
10 Moulding internal undercuts 10.1 General 10.2 Form pin 10.3 Split cores viii
301 301 309 325 336 340
340 341 346
CONTENTS
ll
12
13
10.4 Side cores 10.5 Stripping internal undercuts 10.6 Standard mould parts
350 350 353
Mould for threaded components
354
11.1 11.2 11.3 11.4 11.5
354 357 403 408
General Moulds for internally threaded components Moulds for externally· threaded components Mould construction Standard unscrewing type mould systems
Multi-daylight moulds
414
12.1 12.2 12.3 12.4
414 416 433 437
General Underfeed moulds Triple-daylight moulds Standard parts for underfeed moulds
Runnerless moulds
444
13.1 13.2 13.3 13.4 13.5 13.6
444
General Nozzle types Hot-runner unit moulds Insulated runner moulds Hot-runner plate moulds Standard parts for the hot-runner type mould
PART THREE
14
410
ASPECfS OF PRACfiCAL MOULD DESIGN
447 454 501
504 507 511
Procedure for designing an injection mould
513
14.1 General
513 515 517 519 522 524 525
14.2 Stage A: Primary positioning of inserts 14.3 Stage B: Ejector system 14.4 Stage C: Ejector grid 14.5 StageD: Complete the top half of the drawing 14.6 Stage E: Complete the plan view 14.7 Stage F: Complete the cross-section 14.8 Stage G: Complete the drawing
15 Checking mould drawings 15.1 General 15.2 Pin ejection mould 15.3 Sleeve ejection mould 15.4 Stripper plate mould 15.5 Splits type mould 15.6 Side Core type mould 15.7 Underfeed type mould
530 533
533 534 536 537 539 541 544 ix
CONTENTS
16
15.8 Hot-Runner type mould 15.9 Stepped parting surface mould design
546 549
Worked examples of simple injection moulds
553
16.1 Gendal 16.2 Mould design draughting practice 16.3 Stepped cutting planes 16.4 Preparatory study 16.5 Example 1 (Pin ejection type mould I) 16.6 Example 2 (Sleeve ejection) 16.7 Example 3 (Pin ejection II) 16.8 Example 4 (Pin ejection III) 16.9 Example 5 ('D' pin ejection) 16.10 Example 6 (Compressed air ejection) 16.11 Example 7 (Stripper plate ejection I) 16.12 Example 8 (Stripper plate ejection II) 16.13 Example 9 (Stripper bar ejection) 16.14 Example 10 (Valve ejection and pin gate) 16.15 Example 11 (Splits type mould I) 16.16 Example 12 (Splits type mould II) 16.17 Example 13 (Side-core type mould I) 16.18 Example 14 (Side-core type mould II) 16.19 Example 15 (Split core design) 16.20 Example 16 (Unscrewing type mould) 16.21 Example 17 (Underfeed type mould I) 16.22 Example 18 (Underfeed type mould II) 16.23 Example 19 (Hot runner unit type mould) 16.24 Example 20 (Two-stage ejection)
553 554 563
GLOSSARY OF TERMS USED IN INJECTION MOULD DESIGN APPENDIX INDEX
X
573 579 581 585
589 597
603 608 614 621 628
638 652
658 669 677
682 689
698 711 720 729
743 746
Preface
The primary object of the first edition of this book was to provide a handbook on design for mould draughtsmen and designers in industry. In addition to fulfilling this function, the book has been used increasingly by the novice as an introductory guide, because it progresses in simple stages from the consideration of basic principles and components to more detailed explanation of the more complex types of special purpose mould. When the publishers asked me to consider another edition of this work, I had no hesitation in recommending that the new extension be used primarily to help the beginner. In this respect the chapter on worked examples has been enlarged from three to twenty examples in order to cover all of the basic designs discussed in the preceding chapters. Secondly, as there has been a rapid increase in the use of standard parts in the mould making industry since the publication of the previous edition, the chapter on this subject has been completely rewritten. In addition, the use of standard parts has been included in various chapters which relate to specific designs. The revised edition has been divided into three sections: elementary mould design, intermediate mould design, and aspects of practical mould design. Part One covers mould-making methods including standard mould parts. Other primary considerations such as feed systems, parting surfaces and mould cooling are also covered. Part Two progresses to specific designs such as splits, side-core, unscrewing, underfeed, and hotrunner types. Part Three is included primarily for the novice, and includes chapters on topics such as procedure for designing an injection mould, checking mould drawings and worked examples etc. In this edition, for size limitation reasons, no attempt has been made to include the more theoretical aspects of mould design. Topics such as computer aided design, aspects of rheology, heat transfer, fluid flow, etc., have been excluded. It is the author's intention to include these important' theoretical subjects in a companion volume which is now in preparation. R.G.W. Pye Barnet London 1989 xi
Acknowledgements
The author wishes to place on record his sincere thanks to a number of people: to Mr R.M. Ogorkiewicz for his advice and his constructive criticism of the original manuscript; to Mr R. Baker, Mr P. Bullivant, Mr J. Collins, Miss B. Humphries, Dr R. Phillips and Mr J. Robinson for their valuable assistance in the preparation of the first edition; to Mr A. Byford, Mr L. Davenport, Mr J. Harris and Mr J. Haylar, who between them taught the author the fundamentals of mould design; to Mr J. Robinson for his advice and comments on all editions; to his wife for her help and forbearance during the writing of this work. Thanks are also due to the following companies for permission to reproduce photographs: Bakelite Xylonite Limited DME Europe E. Elliot Limited Fox and Offord Limited Alfred Herbert Limited H. B. Sale Limited J. E. Snow (Plastics) Limited Tooling Products (Langrish) Limited Wherever possible, the individual designer and/or company has been credited with a specific design or device mentioned in the book. This leaves the author to acknowledge the contribution of the many thousands of anonymous designers who between them, over several decades, have formed the basis of modern injection mould design. It should be pointed out that many of the designs and devices described and illustrated in this book are the subjects of valid patents.
xii
PART ONE
Elementary mould design
1
Mould making
1.1 GENERAL A competent mould designer must have a thorough knowledge of the principles of mould making as the design of the various parts of the mould depends on the technique adopted for its manufacture. This chapter is included primarily for the beginner who does not have a background knowledge of the various machining and other mould making techniques. To cover the topic of mould making thoroughly would require a companion work equal in size to this monograph and therefore this introduction to the subject must, of necessity, be superficial. However, we hope that the very fact that it is included in a monograph on design will emphasise the importance of mould making as a subject and will also encourage the beginner to a further and more complete study in this field. The majority of moulds are manufactured by the use of conventional machine tools found in most modern toolrooms. From the manufacturing viewpoint we classify the mould into two parts (i) the cavity and core, and (ii) the remainder of the of the mould. The latter parts is commonly referred to as bolster work. The work on the cavity and core is by far the most important as it is from these members that the plastics moulding takes its form (see Chapter 2 for definitions). The work on the cavity and core can further be classified depending upon whether the form is of a simple or a complex nature. For example, the cavity and core for a circular or rectangular box-type moulding is far simpler to make than a cavity and core to produce, say, a telephone handset moulding. The mould parts for the simple form are produced on such machine tools as the lathe and the milling machine, whereas the more complex form requires the use of some kind of copying machine. The bolster work is not as critical as the manufacture of the cavity and core forms but, nevertheless, accuracy in the manufacture of the various parts is necessary to ensure that the mould can be assembled by the fitter without an undue amount of bench-work. Now, while the bolster work is always produced on conventional machine tools, the cavity and core, particularly the former, can be produced by one of a number of other- techniques. These include investment casting, electro-deposition, cold bobbing, pressure casting and spark machining.
3
MOULD MAKING
1.2 MACHINE TOOLS The purpose of any machine tool is to remove metal. Each machine tool removes metal in a different way. For example, in one type (the lathe) metal is removed by a single point tool as the work is rotated, whereas in another type (the milling machine) a cutter is rotated and metal is removed as the work is progressed beneath it. Which machine tool is to be used for a particular job depends to a large extent upon the type of machining required. There is, however, a ·certain amount of overlapping and some machine tools can be utilised for several different operations. In the illustrations which follow, typical machining operations are illustrated but it must not be assumed that the particular machine tool is restricted to the operation shown. The machine tools which will be found in the modern toolroom are as follows: (i) Lathes for turning, boring and screwcutting, etc. (ii) Cylindrical grinding machines for the production of precision cylindrical surfaces. (iii) Shaping and planing machines for the reduction of steel blocks and plates to the required thickness and for 'squaring up' these plates. (iv) Surface grinding machines for the production of precision flat surfaces. (v) Milling machines for the rapid removal of metal, for machining slots, recessi's, boring holes, machining splines, etc. (vi) Tracer-conirclled milling machines for the accurate reproduction of complex cavity and core forms. In addition to the above list of major machine tools there is, of course, ancillary equipment without which no toolroom would be complete. This includes power saws, drilling machines, toolpost grinders, hardening and polishing facilities, etc. 1.2.1 Lathe
The primary purpose of the lathe is to machine cylindrical forms. The contour is generated by rotating the work with respect to a single-point cutting tool. For machining the outside surface, the cutter is moved parallel to the axis of rotation. This operation is called turning. Alternatively, metal may be removed from the inside of the work in which case the operation is called boring. When the tool is moved across the face of the work it is called facing. The principal parts of the lathe are illustrated in Figure 1.1. The workpiece is secured at one end in a chuck and supported at the other end by a centre, fitted in the tailstock. The chuck is mounted on the headstock spindle and driven by an electric motor via a gearbox and transmission system (the last two items are not shown). The headstock and tailstock are both attached to the machine bed, and the position of the tailstock is adjustable to accommodate various lengths of workpiece. 4
MACHINE TOOLS HEADSTOCK SPIIiOLE
TAILSTOCK
CROSS -SLIDE
Figure 1.1- Lathe
The cutting action is by means of a single-point cutting tool mounted in a toolholder. The cutting tool is positioned, prior to the commencement of the cutting operation, so that the cutting point is in line with the axis of the work. The tool can be moved, primarily, in two directions. For normal turning and boring, a longitudinal movement is required and this is achieved by moving the carriage along the slideways of the bed. For facing the end of the work-piece, a transverse movement is required and this is achieved by moving a cross-slide along the slideways of the carriage. Note that these slideways are at right angles to the bed slideways. Both the longitudinal and transverse movements can be power operated. The speed of the carriage (or cross-slide) relative to the rotational speed of the work is adjustable and this, together with the depth of cut chosen, determines the finish obtained on the work. For rough machining a relatively deep cut with a fast feed is used, but for finishing a shallow cut with a fine feed is required. When a large amount of metal has to be removed, a number of successive roughing cuts are made until the required diameter is approached. The part is then finished to size with one or two finishing cuts. The lathe is extremely versatile and is used for making a large variety of mould parts. For example, guide pillars, guide bushes, circular .support blocks, ejector rods, ejector rod bushes, push-back pins, etc., are all manufactured on the lathe. In addition to this bolster work the cavity and core are also produced on a lathe if the moulding form is cylindrical.
5
MOULD MAKING
Turning is a relatively fast machining operation and for this reason moulds for circular components are cheaper to produce than corresponding moulds for components of any other form. Internal and external thread forms are also easily generated, when required, as for example on the end of an ejector rod (Figure 3.7). A slight complication arises if the thread is required on the core (see Figure 11.18) or in the cavity (Figure 11.50) to produce a complementary moulded thread as shown. In these cases it is necessary to make some allowance for the plastics material shrinkage on the mould thread pitch (i.e. the mould thread pitch must be machined slightly larger than required to allow for the material shrinkage on cooling). To describe the manufacture of a typical mould part, we take a guide pillar (Figure 1.2) to illustrate the sequence of operations. The first step is to cut off a suitable length of steel bar and mount this in the lathe chuck. The end of the bar is faced and subsequently
,~,.,
D
(b)
I
(c)
L__J~------------~
(d)
~(f) Figure 1.2-Stages in manufacture of guide pillar
6
MACHINE TOOLS
cei"ttre-drilled (Figure 1.2a). The tailstock centre is then positioned in this cei"ttre-drilling as illustrated in Figure 1.1. 11le bar is now progressively reduced in diameter by a turning operation in a number of stages (Figure 1.2b-e). Finally, the guide pillar is parted off (Figure 1.2f) and the scrap piece of steel removed from the chuck. To permit the head of the guide pillar to .be faced and reduced to the required thickness, the workpiece is again mounted in the chuck but this time it is reversed and held on the fitting diameter. 'This completes the lathe work. The guide pillar is then hardened and the important fitting diameters are subsequently ground to size on a cylindrical grinding machine. 1.2.2 Cylindrical grinding machine
This machine tool is used for precision grinding cylindrical mould parts. Metal is removed by the action of a rotating abrasive grinding wheel which is brought into contact with a contra-rotating workpiece. The axes of both the grinding wheel and the workpiece are parallel for normal operation. An important feature of the grinding machine is that it can cut hardened metal. This characteristic, together with the close tolerances and the high surface finish obtainable, makes this machine tool an essential piece of toolroom equipment. A simplified drawing of a cylindrical grinding machine is shown in Figure 1.3. The workpiece is mounted at one end in a chuck and is supported at the other end by a centre fitted in the tailstock. The chuck is HEADSTOCK SPINDLE
CRIHDIHG WHEEL
TABLE
MACHINE BED
TAIL STOCK
Figure I .3- Cylindrical grinding machine
7
MOULD MAKING
mounted on the headstock spindle and driven by an electric motor, attached to the headstock. Both the headstock and the tailstock are mounted on a table; the tailstock is adjustable to accommodate various lengths of work. The table, fitted on slideways of the machine bed, is a driven reciprocating member. The length of stroke is adjustable. The workpiece, therefore, has a rotary motion and a longitudinal motion with respect to the grinding wheel. The grinding wheel spindle (not visible) is driven via a belt transmission by an electric motor mounted on top of the wheel-slide. The latter member is fitted on the slideways of the machine bed. These slideways are at right angles to the axis of the workpiece, and therefore movement of the wheel-slide moves the grinding wheel towards or away from the workpiece. In normal grinding of cylindrical parts, the wheel-slide is adjusted forward until the rotating grinding wheel just contacts the contra-rotating workpiece. Further forward movement of the wheel-slide sets the depth of cut. The table is then caused to reciprocate, thereby grinding the outside surface of the workpiece over a preset distance. This operation is repeated and the depth of cut progressively increased until the required diameter on the workpiece is obtained. 1.2.3 Shaping and planing machines
A mould normally includes a number of steel plates suitably secured together. Each of these plates must have parallel faces and, ideally, the four sides should be square. Now, as the primary purpose of a shaping machine is to produce flat surfaces, this machine tool is used in the initial preparation of mould blocks. The principle of the shaping machine is illustrated in Figure 1.4. The workpiece is mounted on a table and a reciprocating single-point tool removes metal in a series of straight cuts. After each forward stroke, the table is traversed a preset increment in preparation for the next cut. 1 he ram is a driven reciprocating member which is guided in slideways at the top of a vertical column. The length of stroke of the ram is adjustable. The tool is attached to the ram via a tool-holder and head. The depth of cut by the tool is preset by vertical adjustment of the head. The table, to which the work is securely attached, is mounted on slideways on the cross-rail. Movement of the table across the face of the cross-rail is effected by the rotation of a lead screw (not shown), actuated by the ram via a simple mechanism. The cross-rail can be adjusted vertically on slideways at the front of the column. Adjustment of this member is carried out only during the setting-up operation. Once the workpiece is close to the tool the cross-rail is clamped in the desired position before the cutting operation commences. Because of the single-stroke cutting action of this machine, the surface finish obtained on the work is always in the form of a series of fine grooves. The depth and width of the groove depend on the depth of cut and the traverse increment chosen. While this surface finish is suitable for
8
MACHINE TOOLS RAM
(a)
TABLE
(b)
Figure 1.4-Shaping machine: (a) squaring up mould plate; (b) reducing thickness of mould plate
certain mould parts (for example, the sides of mould plates) it is normal practice to follow the shaping operation with a surface grinding operation to produce a finer surface finish. In Figure 1.4 the sides of a mould block are shown being squared up (a), and the block is shown in the process of being reduced in thickness (b). For the squaring up and surface facing of large blocks of steel an alternative machine tool, called a planing machine is often used. This machine is similar to the shaping machine in that it employs a reciprocating action and produces a flat surface by a series of straight cuts using a single-point tool. However, with the planing machine it is the work which is reciprocated, the tool being fixed in a head above the workpiece. Figure 1.5 illustrates a simplified version of a planing machine. The work is rigidly attached to the table which is reciprocated on the slideways of the bed. The single-point cutting tool is mounted in the cutter head and this member is adjustable in the vertical direction on the slideways of the cross-slide. This latter member, in turn, is mounted on the slideways of the cross-rail, permitting the tool to be traversed across the face of the workpiece. The cross-rail is supported above the table upon the side columns. In operation the depth of cut is preset by suitable adjustment of the cutterhead. The table is caused to reciprocate and the single-point tool slices a shaving of steel from the workpiece. At the end of the cutting stroke the head is automatically traversed by a preset increment in preparation for the next cutting stroke. This continues until the complete surface has been planed. If necessary the depth of cut is increased and the above procedure repeated, until the required thickness of plate is obtained. 9
MOULD MAKING CROS~
-RAIL
CROSS- SLIDE
CUTTER HEAD
\\ \
SIDE COLUMNS
lEO
Figure 1. 5- Planing machine
1.2.4 Surface grinding machine
We have previously discussed the cylindrical grinding machine for the grinding of cylindrical surfaces (Section 1.2.2). Now the surface grinding machine performs a similar function for flat surfaces, and grinding normally follows the shaping or planing operation. An excellent surface finish combined with accuracy can be achieved on hard or soft steel with this machine tool. There are several basic designs of surface grinder and the principle of one type is shown in Figure 1.6. The workpiece is mounted on a table which is reciprocated beneath a rotating abrasive grinding wheel. Metal is removed in a series of straight cuts, the table being traversed a preset increment after each cutting stroke until finally the entire surface has been ground. The grinding wheel is mounted on a shaft which is parallel to the surface of the workpiece. The shaft is mounted in bearings within the wheel head and the depth of cut is preset by vertical movement of this member. The length of stroke of the table is adjusted to suit the length of work before the cutting operation commences. A simple mechanism (not shown), operated by the table movement, actuates the cross-slide in the transverse direction. The cross-slide is mounted in slideways on the machine bed. 1.2.5 Milling machine
Milling is an operation in which metal is removed from a workpiece by a rotating milling cutter. The workpiece can be moved in three directions at
10
MACHINE TOOLS
GRINDING
WHEEL
Figure /.6-Surface grinding machine
right angles to each other, with respect to the cutter. The three directions are longitudinal, transverse and vertical, respectively. There are two basic types of milling machine. In one the axis of the cutter is perpendicular to the surface of the workpiece (Figure 1. 7a) and this is called a vertical milling machine. In the other, the axis of the milling cutter is parallel to the surface of the workpiece and this is called a horizontal milling machine (Figure 1.7b). Both types are used extensively in the manufacture of various parts of the mould. The table assembly is identical for both machines and we will therefore discuss this feature first. The table is mounted on the slideways of the saddle, which allows the table to be moved longitudinally. The saddle is mounted on slideways of the knee and these slideways are at right angles to the saddle slideways, which permits the workpiece to be traversed. Finally, the knee, which is the main supporting member, is mounted on the vertical slideways of the column. This allows for the workpiece to be adjusted vertically. In the horizontal machine the depth of cut is preset by this vertical movement. However, with the vertical machine the vertical movement of the knee is used primarily to bring the workpiece close to the milling cutter, the actual depth of cut being preset by vertical movement of the spindle on which the cutter is mounted. The vertical milling machine (Figure 1. 7a) incorporates a vertical milling head mounted on a horizontal extension of the column. The head comprises a spindle suitably driven by an electric motor via a belt transmission system. The spindle, which is rotated at high speed, can be fed in the vertical direction either manually, or automatically via the drive system. In Figure 1. 7a an endmill cutter is fitted into the spindle. This type of cutter incorporates cutting edges both on its periphery and on its 11
MOULD MAKING ELECTRIC MOTOR
ARBOR
SPINDLE
HILLING CUTTER OVERARM ASSEH&LV
--
COLUMN
VERTICAL --HILLING HEAD TABLE
KNEE
(a)
(b)
Figure 1.7-Milling machine: (a) vertical milling machine; (b) horizontal milling machine
underside. Thus the cutter can be sunk into the workpiece which can then be moved either longitudinally or transversely depending upon the form required. Figure 2.19 illustrates a pocket being machined in a mould plate by an endmill cutter. The horizontal milling machine (Figure 1.7b) incorporates a driven horizontal arbor upon which is mounted the milling cutter. Deflection of the arbor during the cutting stroke is prevented by the overarm assembly. The most common type of milling cutter used on this machine is the side and face cutter which is essentially a disk of tool steel which has cutting teeth on the periphery (i.e. the face) and on the sides. This type of cutter is used both for cutting slots and for surface machining. By using milling cutters with a contoured face various profiles can be machined on the workpiece. The operation of both types of milling machine is similar. The movement of the work in all three directions can be manually or automatically operated. It is normal practice to operate the machine in only one direction at a time. For example, consider the machining of a large slot in a bolster. The sequence of the operations is shown in Figure 1.8. The operator presets the depth of cut and uses a longitudinal cutting stroke to machine 12
MACHINE TOOLS
(a)
(b)
(c)
(d)
(e)
Figure 1.8-Sequence of operations in machining deep slot on horizontal milling machine
the first slot (a). At the end of this cutting stroke, the work is traversed a preset amount and another longitudinal cut made, and so on, until the required width of slot is obtained (b). The depth of the cut is then increased and again a longitudinal cut made (c). At the end of this and of each subsequent stroke the work is traversed in preparation for the next cut. The procedure is repeated-( d) shows an intermediate position-until 13
MOULD MAKING
the required depth of slot is obtained. The depth of cut, the traverse increment adopted and the cutting rate depend upon several factors, which include the type of steel being machined, the surface finish required and the type of cutter used. While, as we have stated, these machines are used extensively in the manufacture of various parts of a mould, they cannot easily be used for the manufacture of three-dimensional form, which is often required for the cavity and core. For these complex shapes some form of copy milling machine is needed. 1.2.6 Manufacture of simple mould plate
Now we have considered the basic machine tools, we will give an example of the procedure for the manufacture of a simple mould part. Let us assume that we want to make a cavity plate for a box-type component. We would proceed as follows: (i) Cut off a suitable length of steel from a bar on the power saw. (ii) Square up the block and machine both sides on the shaping machine (Figure 1.4a and b). (iii) Grind both surfaces to the specified size on the surface grinding machine (Figure 1.6). (iv) End-mill the cavity form on the vertical milling machine (Figure 1.7a). (v) Mill a slot in the two sides of the mould plate (for clamping purposes) on the horizontal milling machine (Figure 1.7b). This completes the first stage of the machining operation. The mould plate now passes to the bench fitter who does a certain amount of hand finishing (Section 1.8.1) and marking out for subsequent ancillary machining operations which in this example would include the following: (vi) Bore and counterbore a hole in the centre of the plate for the sprue bush on the vertical milling machine or on the lathe. (vii) Bore and counterbore the holes to accommodate the guide bushes on the vertical milling machine or on a jig boring machine (a special type of vertical milling machine). (viii) Bore the water cooling channels on a radial drilling machine. From this point the mould plate passes back to the bench fitter whose responsibility it is to hand finish the mould plate and subsequently to assemble the mould. 1.2. 7 Tracer-controlled milling
The principle of this type of machine tool is similar to that of the vertical milling machine in that an end mill cutter is used to remove metal in a series of cuts. With tracer-controlled milling, however, the required form is generated by causing a tracer, directly coupled to a cutting head, to follow a template or a model. The master form is accurately reproduced. One machine of this type is the Pratt, Whitney and Herbert 'Keller', and a simplified drawing of the machine is shown in Figure 1.9. The machine consists essentially of an angle plate, mounted on a stationary work table. 14
MACHINE TOOLS TRACER
I
TRACER HEAD SLIDE
COLUMN
8£0
END MILL CUTTER
Figure I. 9- Tracer-controlled milling machine
The template, or model, and the work are mounted one above the other on the angle plate as shown. An end mill cutter is mounted in a spindle which is individually driven by a motor housed in the spindle head. The traverse (in-and-out) motion of the cutter is obtained by having the spindle mounted on the horizontal slideways of the slide. Similarly, the vertical (up-and-down) motion of the cutter is obtained by having the slide mounted on the vertical slideways of the column. Finally, the longitudinal (forward-and-back) motion of the cutter is obtained by having the column mounted on slideways of the bed. Each of these three motions is produced by an independent drive and the cutter can therefore be moved in three directions simultaneously. The tracer is mounted in a tracer head vertically above and in line with the cutter. Before the cutting operation commences the tracer head slide is adjusted on the vertical slideways of the spindle head so that the respective positions of the tracer and master, and the cutter the work, are identical. The transverse position of the tracer with respect to the cutter is also preset by adjustment of the tracer head which is mounted in slideways on the tracer head slide. There are two types of control on the basic machine, namely profile tracing and three-dimensional tracing. In either case a small deflection of the tracer point will make and break electrical contacts to alter automatically the cutting direction via magnetic clutches and lead screws.
15
MOULD MAKING
This is a technique by which the tracer controls only the horizontal and vertical movements of the spindle head. The required cutting depth is preset before the operation starts. The tracer is caused to follow a master, normally a thin metal template, which guides the cutter in two dimensions to reproduce the master shape exactly. The set-up of the machine for the profiling operation is shown in Figure 1.9. In operation the tracer and cutter assembly is caused to move, say, vertically upwards. Immediately the tracer strikes the template form, the direction of movements is automatically reversed and the assembly moves vertically downwards. At the end of each stroke the assembly is moved longitudinally a preset increment in preparation for the next cutting stroke. This procedure continues until the entire form of the template has been swept. The cutter can only remove a certain amount of steel during each cutting stroke so when the template form has been swept once at a particular cutting depth, the depth of cut is increased and the procedure repeated. By adopting this technique a metal template can be used to produce a relatively complex two-dimensional form. Figure 1.10 shows the sequence of operations in the machining of a deep elliptical cavity. Note that while the template is made from relatively thin sheet metal (Figure 1. 9) any depth of cavity can be milled out that is within the capacity of the machine. PROFILE TRACING.
Figure 1.10-Stages in
16
manuj&~cture
of deep elliptical cavity
MACHINE TOOLS
For complex cavity and core forms which cannot be machined using the profiling technique, the profile tracer head is replaced by a three-dimensional tracing head. In operation the tracer is caused to travel over the surface of the model in a series of parallel horizontal sweeps. At the end of each sweep an automatic vertical step feed occurs. The operator can alternatively select vertical sweeps combined with a horizontal step feed. The tracer head is designed so that the cutter, the movement of which it controls, is moving at all times. When the tracer is not in contact with the model, the tracer and cutter automatically move inwards. Immediately contact with the model is made the tracer is deflected slightly which, via electrical contacts, energises the horizontal (or vertical) travel circuit or the traverse (in-and-out) circuit, or both circuits simultaneously. The precise direction of the motion depends on the direction in which the tracer is deflected. It is not practicable for the cutter to reproduce the contours of a deep master form in one sweep of the model. Instead this must be done in stages and the maximum depth of cut is preset before the operation commences. Figure 1.11 illustrates the sequence of machining operations
THREE-DIMENSIONAL TRACING.
(b)
Figure 1.11-Sequence of machining operations of three-dimensional core
17
MOULD MAKING
of a core for a toy boat hull. On the first sweep (a,b) only the top portion of the form is reproduced in the steel. Successive sweeps at progressively greater cutter depths (c,d,e) see the emergence of the required form, until finally an accurate reproduction of the model is produced in steel (f). Plate 1 shows a cavity form in the process of being copied.
1.3 CASTINGS The manufacture of cavities and cores in steel by the conventional casting method using sand moulds is not satisfactory owing to the poor finish obtained and to the porosity which occurs on, or just below, the surface of the casting. The expenditure involved in plugging, machining and finishing these conventional castings makes this method of mould making uneconomic. The Shaw investment casting process does not, however, share the disadvantages associated with sand casting and is therefore applicable to the manufacture of cavities and cores. The process is carried out by specialists and the mouldmaker supplies the company with a pattern of the required mould part. As the final casting will be an accurate reproduction of the pattern supplied, this must be manufactured to close tolerances and have a good surface finish. To allow for the contraction of the steel on cooling the pattern is made approximately 0.020 mm/mm (in/in) oversize. The procedure adopted in the manufacture of a mould part by this process is illustrated diagrammatically in Figure 1.12. (a) The prepared pattern (in this case a core) is mounted in a pattern box. The internal dimensions of the pattern box correspond to the required outside dimensions of the mould plate plus the shrinkage allowance. (b) A slurry consisting of highly refractory particles suspended in a bonding material (ethyl silicate) is poured into the pattern box. The slurry progressively hardens and, at a suitable point, the pattern and pattern box are removed. (c) The refractory mould is immediately ignited and this is followed by a firing in a furnace at approximately 900°C, which eliminates alcohol and water evolved during the process. (d) When the refractory mould is removed from the furnace its structure consists of refractory particles bound together with residual silica. The elimination of the alcohol leaves behind a network of very fine cracks. (e) Molten steel at 1600°C is now poured into the refractory mould. (f) When the steel solidifies the cast mould plate is removed from the refractory mould and the mould plate is returned to the mouldmaker for the final finishing and fitting operations. The reasons why the refractory mould is suitable for the manufacture of mould parts, whereas the sand mould is not, can be summarised as follows: (i) The structure of the refractory mould allows for expansion as the steel is poured without the cracking. 18
CASTINGS (a) PATTERN MOUNTED IN PATTERN BOX
(b) SLURRY POURED INTO PATTERN BOX
(c) 'REFRACTORY' HOULD IGNITED AND FIRED AT
9oo•c
(d) FINAL 'REFRACTORY' MOULD
~--.
(e) HOLTEN STEEL POURED INTO REFRACTORY MOULD
(f) WHEN SOLIDIFIED THE 'CORE' IS REMOVED FROM REFRACTORY MOULD
Figure 1.12-Stages in manufacture of core by Shaw investment casting process
(ii) Entrapped gas within the molten steel is allowed to escape through the fine hairline cracks; this results in a non-porous surface. (iii) As the refractory mould is an identical replica of the pattern, the surface of the pattern is faithfully reproduced on the casting. (iv) The low value of thermal conductivity of the refractory mould allows the steel to cool at a very slow rate. The major limitation associated with the process is that it is not possible to guarantee an overan tolerance better than 0.005 mm/mm (in/in) which means that for large mould castings there is the possibility of considerable error. For example, the discrepancy on a component 500 mm (20 in) 19
MOULD MAKING
long may be as much as 2.5 mm (0.1 in) which would certainly necessitate a subsequent machining operation. However, in many cases, as for example in the toy industry, extreme accuracy in the dimension of the final product is not as important as the form, in which case the process often shows considerable saving over conventional machining methods. 1.4 ELECfRO-DEPOSITION Electro-deposition is an electrochemical process used to reproduce accurately a cavity or core form from a given pattern. The pattern can be made in an easily worked material and is the reverse form to that required. That is, a male pattern is required for a cavity and a female pattern for a core. Normally it is much easier to machine a male pattern than the reverse cavity form and it is for this reason that most applications for this technique are for intricate cavity work. The principle of electro-deposition is illustrated in Figure 1.13. (a) A pattern of the required form is made, normally, from either brass. an acrylic or an epoxy resin. The process requires that the pattern conducts electricity and the surfaces of the non-conducting materials must therefore be made conductive. One method, is to coat the pattern with a thin film of silver (1.5 x 10-4mm thickness). (b) Nickel-cobalt is deposited on to the electrically conductive pattern in a plating vat until a shell of about 4 mm (5/32 in) thickness is obtained. (c) Hard copper is deposited on to the nickel shell in a second plating vat. As copper can be deposited at a much faster rate than nickel, a considerable thickness of this backing material is built up relatively quickly. THIN LAYER OF SILVER
(b) MOULD INSERT
HOULO BOLSTEI'.
tc)
Figure /.13-Stages in manufacture of cavity by electro-deposition technique; (a) pattern is made conductive with thin coating of silver; (b) nickelcobalt deposited on to pattern; (c) hard copper deposited on to nickel shell; (d) 'electroformed' insert machined, and fitted to bolster
20
COLD HOBBING
(d) After the pattern is removed, the outside of the electroformed insert is machined to a shape and size that can be accommodated in a bolster. The insert is held in position either by drilling and tapping it and holding back with screws through the bolster, or by machining a flange on the bottom of it and using a frame-type bolster (Section 2.3.3). This process has advantages over some other mould making techniques; these advantages may be summarised as follows: (i) A male pattern in a soft material is simpler to manufacture than a die-sunk cavity in nickel-chrome steel. (ii) Complex parting surfaces may be achieved relatively easily with this technique. (iii) Nickel-cobalt is non-corrosive and it is therefore suitable for all plastics materials. (iv) Provided a highly polished master is used the finishing costs on the electroformed insert are nominal. (v) The electroformed cavity or core insert is in one piece which offers considerable advantages in production with free-flowing plastics materials (e.g. nylon). These materials to flow very readily into minute cracks and crevices of a built-up cavity or core assembly. The main limitations of the process are in size and strength. The maximum size of electroformed insert which can be produced is iimited to the size of vat available. Moreover, while nickel-cobalt is relatively hard (44-55 Rockwell C), the copper backing is less hard and this might allow a depression to form on the impression face due to a localised high force.
1.5 COLD HOBBING Cold hobbing is a process in which a hardened steel master hob is forced into a soft steel blank under considerable pressure. Hobbing is used for the production of cavities which by virtue of their shape would be difficult to die-sink on conventional machine tools. The basic principle of bobbing is illustrated in Figure 1.14. (a) The master hob is mounted above a hobbing blank in a hobbing press (the platens only of which are illustrated). The soft steel hobbing blank is fitted in a substantial chase. (b) and (c) The top platen of the hobbing press is caused to move downwards and the hardened steel hob is progressively forced into the soft steel blank. The pressure exerted by the hob causes the soft steel to flow in the cold state. (d) When the required depth is reached, the top platen is raised, leaving the hob behind in the hobbing blank. (e) The hobbing blank must now be removed from the chase and one method of doing this is illustrated. The chase is placed upside-down on two parallels and a knock-out bar is positioned above the hobbing blank. The top platen of the press is then lowered to release the tightly fitting blank from the bolster.
21
MOULD MAKING
PLATEN
!
MASTER HOB
CHASE PLATEN
(b)
(a)
~
It
! (d)
(c)
!
EXTRACT Oil
r.
KNOCK-OUT BAR
(f)
I I
I I
I
I
(9)
(~)
Figure 1.14-Stages in manufacture of cavity insert by cold hobbing technique
(f) The next operation is to remove the hob from the hobbing blank. Here again there are several methods, one of which is illustrated. A screw extractor is used to apply the necessary force to withdraw the hob. (g) All that remains now is to machine the hobbing blank to a suitable shape, to case-harden the hobbed insert and finally to give the impression a light polish before mounting the insert in its bolster. The hobbing process is particularly applicable for the production of cavities on multi-impression moulds. One master hob can often be used to produce a number of cavities, thereby saving considerable machining time. Providing the hob is highly polished, the hobbings too will have a good surface finish, which saves bench finishing and polishing time.
22
PRESSURE CASTING
In general a male hob is easier to manufacture than a corresponding cavity of the same form; however some shapes are very difficult to hob and careful consideration must be given to a project before attempting to use this technique (Bebb). For ease of bobbing, a low-carbon steel is usually used for the bobbing blank. As stated above, after the bobbing operation the bobbed insert is case-hardened. This means that the outside surface (including the cavity form) is extremely hard whereas the internal core of the steel remains relatively soft. Thus if in production a localised high force is developed there is a possibility that the surface of the impression may be depressed.
1.6 PRESSURE CASTING Beryllium-copper is a material which is increasingly being used in mould construction because it possesses several desirable characteristics. In particular it has a high thermal conductivity combined with a reasonable hardness (Brinell Hardness Number of about 250) which makes it suitable for certain types of cavity and core, and for other mould parts, such as hot-runner unit secondary nozzles. Its high thermal conductivity means that when beryllium-copper is used for a cavity or a core the heat from the melt will be transferred away from the impression faster than if a corresponuing steel cavity and core are used, and this often results in a shorter moulding cycle . . Beryllium-copper can be machined, in which case the conventional machine tools are used, and it can be cold-bobbed, hot-bobbed or pressure-cast. The last technique offers certain advantages over the bobbing methods, in that cold or hot bobbing of beryllium-copper tends to work harden the material which results in the development of stress concentrations. Pressure casting (or liquid bobbing) is used mainly for the production of cavities but it can be used, where applicable, for the production of the cores as well. As the terms suggest, it is basically a process which combines the casting and bobbing techniques. The basic principle of the pressure casting process is illustrated in Figure 1.15. (a) A master hob is made from a good-quality steel. Beryllium-copper has a shrinkage of approximately 0;004 mm/mm (in/in), therefore the hob is manufactured oversize to allow for this. (b) The master hob is attached to a plate and mounted in a chase. The assembly is preheated and then fitted on to parallels on the lower platen of the bobbing press. (c) Molten beryllium-copper is then poured into the recess formed by the hob and the chase. A shield is often used to protect the hob during the pouring stage. (d) A plunger attached to the moving platen of the bobbing press is brought down on top of the molten beryllium-copper and a force applied. The plunger is a good slide fit in the chase.
23
MOULD MAKING MASTER HOB
(a)
Be-Cu
(c)
HOVING Pl AIE H
(d)
fiXED PLATEN
(e) (f)
Figure 1.15-Stages is manufacture of cavity insert by pressure casting technique
(e) When the beryllium-copper has solidified the plunger is withdrawn, and the bobbing and hob is removed from the chase. Subsequently the hob is extracted from the bobbing, which is then machined externally to suit the bolster. (f) The bobbing is then annealed, hardened, lightly polished and finally fitted into the bolster.
24
SPARK MACHINING
Tt:te advantages of this technique are basically the same as those described for the cold hobbing of steel. However there is an important difference. Beryllium-copper is poured around the master hob in the pressure casting process, whereas in the cold hobbing process the master hob is forced into the steel. More intricate and delicate forms can therefore be produced with the pressure casting technique, without the risk of hob failure. It should be noted, however, that the major advantage is not so much the mould making process as the properties obtainable from bery Ilium-copper. The main limitation is in the size of hobbing obtainable. This is controlled by the size of hobbing press available and the material melting capacity.
1.7 SPARK MACHINING This is one of the more recent additions to mould making methods and, strictly speaking, it should come under the machine tool section. However, as the principle of operation is different from that of all other basic machine tools it is preferable to discuss this technique separately. Spark machining is a process in which steel, or other metals, can be machined by the application of an electrical discharge spark. The spark is localised and metal is progressively removed in small quantities over a period of time. First consider the spark erosion machine. This is illustrated diagrammatically in Figure 1.16. The workpiece, which in our case is a mould insert blank, is mounted on a platen and submerged in a tank containing a dielectric fluid (normally paraffin). The tank is mounted on the machine base. The tool, which has the required complementary form of the cavity (i.e. similar to a hob), is mounted in a chuck which is attached to a vertical rack. A servomotor (not shown) actuates the rack via the pinion. Thus the tool can be moved in the vertical direction with respect to the workpiece. Both the tool and the workpiece are connected to an electrical supply, and the tool becomes a negative electrode and the workpiece a positive electrode. The sequence in the machining of a mould insert is shown in Figure 1.17. This diagram illustrates a cross-section through a tool and workpiece. (a) The rack of the machine is moved downwards by the servomotor until a specific distance between the tool and the workpiece is reached at which point the dielectric separating the two electrodes breaks down and a spark occurs. This spark results in a small particle of the workpiece being eroded away. At the same time a similar but less severe erosion takes place from the tool. (b) A jet of dielectric fluid is directed at the workpiece via the hose (Figure 1.16) and the eroded particles are washed away as the tool is momentarily lifted. The tool again descends but, this time, because of the erosion, the tool descends a minute amount more than the previous 25
MOULD MAKING PINION DRIVEN BY SERVO MOTOR
PINION------
CHUCK
TOOL
-}ElECTRICAl 1- SUPPLY
HOSE
PARAFFIN
Figure 1.16-Spark erosion machine
SPARK (0)
SPARK
lb)
~
SPARK
SPARK
(C)
ldl
Figure 1.17-Stages in manufacture of cavity by spark erosion technique
stroke. Again the spark occurs, but in a different place, and another particle of the workpiece is eroded away. (c) So it continues, the tool lifts, the eroded particles are washed away, the tool descends and another spark occurs at the point of minimum gap. (d) We have mentioned previously that not only is the workpiece eroded, but so is the tool. This means that for a relatively deep cavity several tools will be required. The first two or three tools will perform the roughing out operation and the final tool will perform the finishing operation. The last sketch shows the final tool at the maximum depth required. The dielectric fluid is constantly being circulated; the fluid containing the eroded sludge is withdrawn from the base of the tank, passed through filters and pumped back to the tank via the hose.
26
BENCH FITIING
J-lardened steel can be machined by this technique so that intricately shaped hardened cavities (which might be impracticable by conventional machine-tool methods because of possible distortion during hardening) cao be formed. Spark machining finds many applications in the repairing and modifications of hardened cavities and cores. For instance, a small pocket can be machined into the mould plate and a suitable insert fitted, which saves the necessity of softening the mould plate with the possibility of distortion, etc. The major limitation of the process is that several tools are required to produce one cavity. If the cavity form is complex, the cost of machining these tools may make the use of this technique uneconomic.
1.8 BENCH FITTING Irrespective of the machine tool or technique used to manufacture the various parts of the mould, the final responsibility for the finishing of the individual parts and for fitting them together lies with the bench fitter. The mould finishing and assembly procedure adopted by the bench fitter varies from toolroom to toolroom and quite often between individual toolmakers working in the same toolroom; it is therefore impossible to set down a standard pattern for the work. In consequence, we intend only to indicate the general approach to this problem without going into details. We will do this by considering the various stages in the bench fitting involved in the manufacture of a simple mould. The various stages are illustrated in Figure 1.18. 1.8.1 Stage I. Finishing the impression
When the mould plates are received from the machine tool section, the impression form (on both plates) is in the rough machined state. Cutter marks, burrs, etc., are very apparent on the surface. The bench fitter's first job is to produce a cavity and core free of machine marks and to the shape and dimensions specified on the mould detail drawing. Basic hand tools, such as files, scrapers and chisels are used for this operation, various sizes, grades and shapes being used as and when applicable. In addition, wherever possible, power driven flexible shaft equipment is used to speed up this operation. This equipment incorporates various heads which accommodate special needle files or scrapers. The heads can have a rotary or a reciprocating motion. Once the cavity (or core) is free of machine marks the next stage is to remove the marks left by the file and the scraper. This is achieved by one or more of several techniques depending on the shape of the cavity (or core). These techniques are honing, lapping (including diamond lapping) and emery-cloth finishing. The last is the most common method used for simple forms. A medium emery cloth is used initially with a suitable backing tool to remove the deep scratches left by the file and scraper. A slightly finer grade is then used in a different direction of motion to 27
MOULD MAKING FIIIISH SUitfAC£ WITH FIL£,
SCIAPU, DIEIY PAPU
AU8II TWO IW.VES. 1011£ HOlES. FIT GUIOE PILLAIIS AltO lUSHES
CHISEL SHAIIP COMUS
STAGE I
11£0 00'1111 TWO IW.VES
FIT WATER COHHECTIOHS
STAGE Sa !ORE AND RUH EJECTOR PIN HOLES
SPOT THROUGH TO EJECTOR RETAINING PLATE
Figure /.IS-Stages in bench filling of simple mould
remove the scratches left by the preceding emery cloth. This procedure is continued, using progressively finer grades of emergy cloths and emery papers until, finally, a scratch-free surface is obtained. At a somewhat later stage, after all other work on the cavity or core is complete, the impression form is polished. This is accomplished with polishing cloths, mops or bobs, in conjunction with a polishing compound such as polishing soap or rouge. Polishing, which is a lengthy, time-consuming operation, must generally continue until the impression form has a mirror-like finish. Whereas a considerable amount of the bench fitter's time is spent on finishing the impression when supplied with a mould plate or insert which has been produced by the machine tool technique, far less time is required if certain of the other mould making techniques are used, such as electro-deposition or bobbing, etc. If the cavity (or core) plate is in the form of a built -up assembly (Chapter 2) then the insert and bolster must be drilled, tapped and counterbored (where applicable) to permit socket-headed screws to be
28
BENCH FITTING
ASSEMBlE TH£ EJECTOR ASS£MBlY
ASS£MBLE THE MOVING HALF REGISTER RING
~:t-~!!oor--SI'RU£ BUSH CLAMP HOLES
STAG£ 7 POLISH, HARDEN, REASSEMBLE TRY OUT
Figure 1.18-Stages in bench fitting of simple mould (contd)
fitted so that the assembly of parts, in fact, functions as a single mould plate.
1.8.2 Stage 2. Aligning cavity and core Once the cavity and core have been semi-finished, the next operation is to align the two parts with respect to each other so that the moulding produced will have the correct wall section. This is achieved by using packing pieces between the cavity and core. The two mould plates are clamped together and returned to the milling or jig boring machine to have guide holes bored through both plates. When this operation is complete, the clamps are removed, the mould plates separated and the guide pillars and guide bushes fitted. (For a cross-sectional view see Figure 2.24.) The two mould plates are again brought together and checked to ensure that the core is in alignment with the cavity. A dummy moulding is often made at this.stage, using wax, so that the wall section of the product can be checked. Any slight inaccurracies need to be corrected, of course.
1.8.3 Stage 3. Bedding down The next stage is to bed down the two mould halves. This is the process of 'marrying' the two opposing mould halves together to prevent the 29
MOULD MAKING
plastic material escaping between the two surfaces when the material is injected into the impression. Basically the process of bedding-down is simple. One surface (the core plate in our example) is given a very fine coating of toolmaker's blue. The two plates are then momentarily brought together and where there are high spots on the second mould plate (the cavity plate in our example), blue will be picked up. These high spots are removed by scraping and filing. This procedure is repeated until an even film of blue is transferred from one plate to the other. (See Chapter 5 for further details on parting surfaces.) 1.8.4 Stage 4. Water cooling circuit The holes drilled for the water circulation in the mould plates are tapped and plugs, baffles, or connectors fitted as appropriate. The circuit is checked to ensure that the flow is unidirectional and that no leakage occurs. (For further details on mould cooling see Chapter 6.) 1.8.5 Stage 5. Fitting ejector system (a) The holes to accommodate the ejector pins and push-back pins are marked out on the mould plate and subsequently bored and reamed. (b) The retaining plate is nominally clamped in position below the mould plate. The ejector holes, etc., are spotted through to this plate. (c) The retaining plate is drilled and counterbored to accommodate the ejector pins and push-back pins. The ejector plate and retaining plate are marked out, drilled, counterbored, and tapped where appropriate to permit the two plates to be held rigidly together by socket-headed screws. Assemble the ejector plate assembly. (d) The ejection half of the mould consisting of the mould plate, support blocks and back plate are marked out according to the mould detail drawing, drilled, counterbored and tapped where specified. The entire moving half of the mould is assembled. (For nomenclature and further details about ejector systems see Chapter 3.) 1.8.6 Stage 6. Fitting sprue bush and register ring Turning to the fixed mould half, the sprue bush and the register ring are located and fitted. (For a cross-sectional view see Figure 2.4.). Clamping holes are marked out (with respect to the register ring), drilled and tapped. 1.8.7 Stage 7. Polishing, hardening and try-out The mould is disassembled and the cavity and core form polished (see Stage 1). All parts which require heat treatment are sent for hardening. When this operation is complete the mould is reassembled and the cavity and core form given a final polish. The mould is then sent for try-out on an injection machine to produce a sample moulding. This is checked and, if necessary, adjustments are made. The mould is now ready for production. 30
2
General mould construction
2.1 BASIC TERMINOLOGY 2.1.1 Impression The injection mould is an assembly of parts contammg within it an 'impression' into which plastic material is injected and cooled. It is the impression which gives the moulding its form. The impression may, therefore, be defined as that part of the mould which imparts shape to the moulding. The impression is formed by two mould members: (i) The cavity, which is the female portion of the mould, gives the moulding its external form. (ii) The core, which is the male portion of the mould, forms the internal shape of the moulding. 2.1.2 Cavity and core plates This is illustrated for a simple hexagonal container in Figure 2.1. The basic mould in this case consists of two plates. Into one plate is sunk the cavity which shapes the outside form of the moulding and is therefore known as the cavity plate. Similarly, the core which projects from the core plate forms the inside shape of the moulding. When the mould is closed, the two plates come together forming a space between the cavity and core which is the impression. 2.1.3 Sprue bush During the injection process plastic material is delivered to the nozzle of the machine as a melt; it is then transferred to the impression through a passage. In the simplest case this passage is a tapered hole within a bush as shown in Figure 2.2. The material in this passage is termed the sprue, and the bush is called a sprue bush. FOOTNOTE. Various standard part suppliers are mentioned in this chapter. An asterisk, following a company's name, indicates that the name has been abbreviated. The company's full title and address can be found in the Appendix.
31
GENERAL MOULD CONSTRUCfiON
HOULDIHG
(b)
(a)
(C)
Figure 2.1- Basic mould consisting of cavity and core plate SPRUE &USH
SPRUE
OF IIIJECTKIII MACHINE
Figure 2.2- Feed system for single-impression mould
2.1.4 Runner and gate systems The material may be directly injected into the impression through the sprue bush (Figure 2.2) or for moulds containing several impressions (multi-impression moulds) it may pass from the sprue bush hole through a runner and gate system (Figure 2.3) before entering the impression. 2.1.5 Register ring If the material is to pass without hindrance into the mould the nozzle and
sprue must be correctly aligned. To ensure that this is so the mould must be central to the machine and this can be achieved by including a register ring (Figure 2.4). 2.1.6 Guide pillars and bushes To mould an even-walled article it is necessary to ensure that the cavity and core are kept in alignment. This is done by incorporating guide pillars on one mould plate which then enter corresponding guide bushes in the 32
BASIC TERMINOLOGY
Figure 2.3- Feed system for multi-impression mould
1
POSITION OF
~STATIONARY /
/] '
PLATEN
I
REGISTER RING
GUIDE PILLAR
GUIDE BUSH
HOVING HALF
FIXED HALF
Figure 2.4-Basic mould incorporating sprue bush, register ring, guide pillars, guide bushes
other mould plate as the mould closes. An example with guide pillars mounted on the core side and corresponding guide bushes in the cavity side is shown in Figure 2.4. The size of the guide pillars should be such that they maintain alignment irrespective of the applied moulding force; this they are normally able to do (for exceptions see Section 2.4.1). All the constituent parts of the basic mould have now been described and a cross-section drawing of the assembled mould illustrated (Figure 2.4).
33
GENERAL MOULD CONSTRUCI'ION
2.1. 7 Fixed half and moving half It can be seen (Figure 2.4) that the various mould parts fall naturally into two sections or halves. Hence, that half attached to the stationary platen of the machine (indicated by the chain dotted line) is termed the fixed half. The other half of the mould attached to the moving platen of the machine is known simply as the moving half. Now it has to be decided in which of the two halves the cavity or core is to be situated. Generally the core is situated in the moving half and the overriding reason why this is so, is as follows: The moulding, as it cools, will shrink on to the core and remain with it as the mould opens. This will occur irrespective of whether the core is in the fixed half or the moving half. However, this shrinkage on to the core means that some form of ejector system is almost certainly necessary. Motivation for this ejector system is easily provided if the core is in the moving half. (See Chapter 3.) Moreover, in the case of our single-impression basic mould, where a direct sprue feed to the underside of the moulding is desired the cavity must be in the fixed half and the core in the moving half (Figure 2.4).
2.1.8 Methods of incorporating cavity and core We have now seen that in general the core is incorporated in the moving half and the cavity in the fixed half. However, there are various methods by which the cavity and core can be incorporated in their respective halves of the mould. These represent two basic alternatives: (a) the integer method where the cavity and core can be machined from steel plates which become part of the structural build-up of the mould, or (b) the cavity and core can be machined from small blocks of steel, termed inserts, and subsequently bolstered. The choice between these alternatives constitutes an important decision on the part of the mould designer. The final result, nevertheless, will be the same whichever method of manufacture is chosen. In either design the plate or assembly which contains the core is termed the core plate and the plate or assembly which contains the cavity is termed the cavity plate. 2.2 MOULD CAVITIES AND CORES So far we have discussed the formation of the mould impression from the relative positions of the cavity and core. These give the moulding its external and internal shapes respectively, the impression imparting the whole of the form to the moulding. We then proceeded to indicate alternative ways by which the cavity and core could be incorporated into the mould and found that these alternatives fell under two main headings, namely the integer method and the insert method. Another method by which the cavity can be incorporated is by means of split inserts or splits. This has not been mentioned previously but is a variant of the insert method and is discussed in Chapter 8. We now go on to discuss the integer and insert methods separately in detail. 34
MOULD CAVITIES AND CORES
2.2. I Integer cavity and core plates When the cavity or core is machined from a large plate or block of steel, or is cast in on1 piece, and used without bolstering as one of the mould plates, it is termed an integer cavity plate or integer core plate. This design is preferred for single-impression moulds because of the strength, smaller size and lower cost characteristics. It is not used as much for multi-impression moulds as there are other factors such as alignment which must be taken into consideration. Typical mould designs which incorporate an integer cavity and core are shown in Figure 2.5.
I
PLATE
MOULD
e CIRCULAR 81 H MOULD
$
I
SQUARE CONTAINER MOULD
TRAY MOULD
J> STEERING-WHEEL MOULD
I
DISPLAY SIGN MOULD
Figure 2.5- Examples of integer type moulds
35
GENERAL MOULD CONSTRUCfiON
MANUFACTURE OF INTEGER CAVITY AND CORE. Of the many manufacturing processes available for preparing moulds only two are normally used in this case. These are (a) a direct machining operation on a· rough steel forgi~~ or _blan~ using t~e conve~tiona! mac_hine tools, or (b) th~ 'prectswn' mvestinent castmg techmque m whtch a master pattern ts made of the cavity and core. The pattern is then used to prepare a casting of the cavity or core by a special process (Section 1.3). A 4~% nickel-chrome-molybdenum steel (BS 970-835 M30) is normally specified for integer mould plates which are to be made by the direct machining method. The precision investment casting method usually utilises a high-chrome steel. usE OF LOCAL INSERTS. These may be incorporated in the integer block in order to simplify the process of mould making. If they are used, a recess or hole is made in the cavity or core plate to accommodate the insert which is then securely fitted into position. Some examples of the judicious use of local inserts in the integer type of mould are given below.
Example 1. The cavity form for a bucket which has a rim at the base to stand on. In this case the narrow groove in the base of the cavity, necessary for forming this rim, represents a reasonably difficult machining problem. This difficulty can be overcome by making the base of the cavity in the form of a local insert, as shown in Figure 2.6. This method has the additional advantage that the cavity can be formed by a straight-through machining operation which cuts down the overall machining time. The local insert is machined separately and then supported at the base by a subsidiary backing plate. Example 2. A local variation in an otherwise constant form is required in a moulding. The example chosen here is of a bath with carrying handles, one of which is shown in Figure 2.7a. The inside form of the bath rim
INTEGER CAVITY
LOCAL INSERT
SUBSIDIARY BACKING PLATE
RIM Of BUCKET
Figure 2.6- Local insert filled to an integer cavity to facilitate machining
36
MOULD CAVITIES AND CORES
requires a complementary male form around the top of the cavity (b)_ Now, while most of it can be produced by a simple turning operation, the presence of· the handle projection prevents any simple machining operation if a wholly integer mould is attempted. The general form of the rim and the impression is shown at (c) and the local section at the handle (d)_ Without a local insert all of the projecting male form on the cavity side will have to be made by copy milling which is time-consuming and costly. However, by incorporating the local insert shown at (e) the rim can be turned and a recess made for the insert by a simple milling operation. The local insert, incorporating the male form for the handle is then fitted into the recess in the cavity plate. So far as the core side is concerned, it is not necessary to use an insert. In this case the external mould form for the handle is made by removing metal from the general form of the core plate and not, as in the previous case, by adding metal to it.
Example 3. A mould contains slender projections which may get damaged and require replacement. Any small projection that forms a moulding recess or hole and which is liable to damage because of its proportions relative to the rest of the mould should be incorporated as a local insert. This will allow easy replacement of the damaged part. An example is illustrated in Figure 2.8. This shows a sketch and part section
X
{b)
(o)
LOCAL INSERT
(c)
(d)
(e)
Figure 2.7-Illustrating desirability of fitting local insert into an integer cavity: (a) part of component (a bath)-note projecting handle; (b) sketch of corre~ponding portion of cavity-note projection to form handle; (c) cross-section through mould at 'X-X; (d) cross-section through mould at 'Y-Y' without local insert; (e) cross-section through mould at 'Y- Y' with local insert
37
GENERAL MOULD CONSTRUCTION
THREADED PLUG
Figure 2.8- Local insert forming hole in toothbrush stock
of a mould for a toothbrush stock. A hole is required in the handle of the component. The core which forms this is a slender rectangular projection, and while this can be made by machining from the solid cavity plate it is more practical to let in a local insert as shown in the sectional drawing. The local insert is held in position by a threaded plug. Example 4. A round hole is required in the moulding. The reasons for adopting a local insert in this case will be similar to the last example if the size of the hole necessitates a slender core. However, in general, the reason for using local inserts is that the male form required to mould the hole is round and a round, local insert, core pin is very much simpler to produce than the corresponding pin machined from the solid. When round holes are specified in the component form, the mould designer should always consider the use o( local inserts. These round pins fit into holes machined into the mould member (cavity or core plate). The various methods of securing circular local inserts to a mould plate are shown in Figure 2.9. Relatively large-diameter local inserts can be secured either by a flange fitting (a) or by the screw-down method (c). For small-diameter local inserts the shoulder method is always adopted (b). Note that the overall lengths of the local inserts in the latter design have been shortened for ease of manufacture. Example 5. An engraving is to be included in the impression. This is usually best incorporated as a local insert pad to allow for change of engraving if this is required. There are other reasons however. First, if the engraving were to be made at the bottom of a deep cavity, the engraving operation would be extremely difficult. Second, as engraving machines are
(a)
(b)
(c)
Figure 2. 9- Various methods for securing local inserts
38
MOULD CAVITIES AND CORES
ENGRAVE 0 PAD
Figure 2.10-Engraved (local insert) pad fitted to integer cavity
usually of light construction, they will not accommodate large mould plates. An engraved insert pad is shown in Figure 2.10. In this case the pad is incorporated in a pocket machined in the bottom of the cavity, and secured with socket-headed screws.
2.2.2 Inserts: cavity and core For moulds containing intricate impressions, and for multi-impression moulds, it is not satisfactory to attempt to machine the cavity and core plates from single blocks of steel as with integer moulds. The machining sequences and operation would be altogether too complicated and costly. The insert-bolster assembly method is therefore used instead. The method consists in machining the impression out of small blocks of steel. These small blocks of steel are known, after machining, as inserts, and the one which forms the male part is termed the core insert and, conversely, the one which forms the female part the cavity insert. These are then inserted and securely fitted into holes in a substantial block or plate of steel called a bolster. These holes are either sunk part way or are machined right through the bolster plate. In the latter case there will be a plate fastened behind the bolster and this secures the insert in position. To simplify machining the designer should make the insert either circular or rectangular in shape. Which of these two shapes is to be used depends on the shape of the moulding. It is convenient to make circular or near circular mouldings in correspondingly shaped inserts and all other shaped mouldings in rectangular inserts. Examples are illustrated in Figure 2.11. Circular inserts are fitted into holes in the bolster (Figure 2.12). A bolster for a multi-impression mould will, therefore, have a number of holes either in lines or on a pitch circle diameter. The latter system is illustrated in Figure 2.13. An important economic aspect of a multi-impression mould containing circular inserts is that the manufacture of the inserts, the turning and the machining of the bolster holes by boring arc both cheap machining operations. As previously stated, for components with shapes other than circular it is generally desirable to make the insert rectangular. The reason for this is best explained by an example. Consider the case of a long narrow component, say a box. If a circular insert is used, the diameter of the insert will be related to the length of the component, so that the circular insert will be relatively large. If a multi-impression mould is required, the mould
SHAPE AND TYPE OF INSERT.
39
GENERAL MOULD CONSTRUCfiON
(e)
(f)
(9)
{h)
Figure 2.1 i- Types of cavity and core inserts: (a, b) rectangular, screw-down; (c, d) rectangular, flanged; (e, f) circular, screw-down; (g, h) circular, flanged
-k I
L--
TO SUIT PLATE
0.
~5mm
SUIT PLATE
r-
0101.1.
(d + 5 mml
L-
I
f
d 01 SLIDE
i
3 mm OIA DOWEL HOLE
Figure 2.12-Fitting details for small core insert
40
d DIA
MOULD CAVITIES AND CORES
Figure 2.13-Circu/ar core inserrs (flanged) filled ro frame-rype bolsrer
will also have to be large to accommodate these inserts. This will offset the economic advantages gained by using circular inserts in the first place_ Rectangular inserts are fitted into a multi-impression mould in a different manner to the circular inserts just mentioned. In general, they are placed together to form one large insert (Figure 2.14). This illustrates the core plate of a six-impression mould in which ~ach core has been machined as a separate insert. However, the rectangular inserts have been placed side by side in two sets of three and between the two sets is inserted a central block of steel called a bridge piece. This central block is incorporated to allow one large pocket to be machined out of the bolster plate rather than two smaller pockets each to contain three inserts only. It may be felt that the bridge piece could be dispensed with by making the inserts larger and meet, in fact, on the mould's centre line. However, this creates difficulties for both the sprue bush and the feed system. Without the bridge piece the sprue bush would enter the mould on a line where the edges of two inserts meet and the main runner would be along the line also. If there is a slight gap between the inserts the material will creep down and tend to force the inserts apart. Although in many cases the multi-impression mould will have the same impression repeated several times, there are examples when it will contain a number of differently shaped impressions. This happens quite frequently in the toy industry, where all the component parts of a toy are
01010,, OIOIO~ Rf:CTAIIGUI.All INSERTS
IRIOGE PI£CE
Figure 2.14·- Recrangular core inserrs (screw-down) filled
10
solid bolsrer
41
GENERAL MOULD CONSTRUCfiON
incorporated in the same mould. One moulding shot then gives all the components necessary to complete the article. The dimensions adopted for each insert will depend primarily on the size and shape of the moulding. However, the individual insert dimensions are adjusted so that the final, overall, form of the complete insert assembly is rectilinear. This procedure facilitates the machining of the bolster and in fitting the inserts to the bolster. Local inserts can again be used in cavity and core inserts. Previously we saw their use in integer cavities and cores, and their function here is similar. Another variation in the make-up of a cavity or core insert is that either one or both can be made in more than one piece. An example (Figure 2.15) is of a component called a rocker arm (a). The slender wings on the component make the machining of a one-piece cavity insert difficult. By making the cavity insert in two parts (b) the machining is simplified. The outside form of the composite insert is made a convenient shape for bolstering purposes. METHODS OF FITTING INSERTS. There are tWO methods by which inserts can be securely fitted into a bolster. Which method is used will depend on the type of insert and the problem is outlined below. Method (i) uses the screw-down technique. In this case the insert fits into a blind recess in the bolster and is secured by socket-headed screws from the underside through holes bored in the bolster. Note that the threaded holes in the insert which receive the screws should not be drilled right through the insert, because the resultant gap left above the end of the screw may well become a material trap for any flash (Figure 2.16a). Similarly, if the insert is secured in the reverse manner by screws from the top face of the insert (Figure 2.16b) then an undesirable crevice is again created, around the head of the screw, which again may act as a material trap. The correct technique is illustrated in Figure 2.16c. Rectangular inserts (Figure 2.1la, b) are normally fitted by this method because they are usually sufficiently large to accommodate screws between the impression and the edge of the insert. Small circular inserts on the other hand generally cannot accommodate screws and for this reason, as well as others, are usually fitted by method (ii).
(a)
(b)
Figure 2.15-Composite rectangular cavity insert: (a) component (rocker arm); (b) composite cavity insert
42
MOULD CAVITIES AND CORES HATE RIAL TRAP
(a)
(b)
(c)
Figure 2.16-fl/ustrating correct and incorrect methods of al/aching screw-down type insert to bolster. Methods (a) and (b) both result in crevices in which plastic material may subsequently be trapped. Method (c) is correct design
Method (ii) utilises a flange which is incorporated in the insert design (Figure 2.1lg, h) and which fits into a mating recess in the bolster. In this case a hole is made right through the bolster, the mating recess being on the underside, the insert is located so that the flange sits in the mating recess and is secured by a backing plate screwed on to the underside of the bolster (Figure 2.13). One advantage of this method is that it is simpler to machine a circular hole right through the bolster than a blind recess. Another advantage is that it is relatively simple to machine a flange and mating recess for a circular insert, which is not the case -for a rectangular insert. However, the overriding advantage of this method lies in the case of a multi-impression mould where inserts are used in both halves. The fact that the holes for the inserts go right through the bolster in each half allows both bolsters to be machined together' if corresponding cavity and core inserts can be made of the same size. Perfect alignment of cavities and cores should now result from simply fitting the inserts into the two bolsters. In practice this is found to apply particularly to circular inserts because, for reasons to be given, rectangular inserts are rarely flanged. The disadvantages in using flange-type rectangular inserts (Figure 2.llc, d) are: (i) extra machining is necessary to incorporate the flange on the insert, and (ii) the recess in the bolster constitutes more work and increases the cost. This objection would only be overruled either because of the increased ease of alignment or in the case of small rectangular inserts because there is insufficient steel for the screw-down technique to be used.
2.2.3 For and against integer and insert-bolster methods Both the integer and the insert-bolster methods have their advantages depending upon the size, the shape of the moulding, the complexity of the mould, whether a single impression or a multi-impression mould is desired, the cost of making the mould, etc. It can therefore be said that in general, once the characteristics of the mould required to do a particular job have been weighed up, the decision as to which design to adopt can be made.
43
GENERAL MOULD CONSTRUCTION
Some of these considerations have already been discussed under various broad headings, but to enable the reader to weigh them up more easily. when faced with a particular problem, the comparison of the relative advantages of each system is discussed under a number of headings. (i) Cost. The total cost is derived from (a) the cost of the mould material and (b) the cost of machining and fitting. (a) The integer method requires the whole mould plate to be made of expensive mould steel, whereas the insert-bolster method needs only that part which forms the impression to be made of mould steel, the bolster being of the considerably cheaper. mild steel. In material cost, then, the integer mould would usually be more expensive. (b) The machining and fitting of a single-impression integer type of mould is less costly in time and in the number of operations as compared with the insert-bolster combination. However, for multiimpression moulds there are other factors to be considered. (ii) Number of impressions. The difficulty in machining and aligning the cavities and cores in an integer type mould increases with the number of impressions the mould contains. Therefore for multi-impression moulds it is usually preferable to use the insert-bolster system. (iii) Multi-impression mould alignment. The ease with which adjustments to the cavity and core positioning can be made is of particular significance in the making of multi-impression moulds. For example, after the completed mould is tried out and the mouldings are examined, some non-uniformity in wall section may be detected in certain mouldings. While this can usually be rectified in an insert-bolster assembly, it is far more difficult in an integer mould. (iv) Mould size. The very fact that the integer mould is a whole unit whereas the insert-bolster mould is an assembly means that the overall size of an integer mould will be smaller than a corresponding insert-bolster mould. However, if we consider the individual impressions, by confining the moulding form to inserts, the difficulties in handling and in machining the blocks are minimised. The integer mould requires that quite heavy steel blocks are handled during the manufacturing stage. This not only increases the difficulties of manipulation of these blocks but also increases the capital cost of the machinery used. (v) Heat treatment. It is often desirable to heat-treat that part of the mould which contains the impression to give a hard, wear-resisting surface. During this heat treatment the possibility exists that the steel may distort. The smaller the block of steel the less likely is this distortion to occur. Thus, from the hardening stand-point, the insert method is preferred to the integer mould. (vi) Replacement of damaged parts. With the insert-bolster system it is possible to repair a damaged impression while continuing to operate the 44
llOLSTERS
mould with the remaining impressions. the runner feeding the damaged impression being suitably blocked with the result that there is a minimum interruption to production. (vii) Cooling system. This is usually far simpler to design for an integer cavity or core plate because the designer can place his cooling system close to the cavity walls without the sealing complications that arise when attempting to cool cavity and core inserts. (See Chapter 6.) (viii) Conclusions. Unquestionably for single impression moulds the integer design is to be preferred irrespective of whether the component form is a simple or a complex one. The resulting mould will be stronger, smaller. less costly, and generally incorporate a less elaborate cooling system than the insert-bolster design. It should be borne in mind that local inserts can be judiciously used to simplify the general manufacture of the mould impression. For multi-impression moulds the choice is not so clear-cut. In the majority of cases the insert-bolster method of construction is used, the ease of manufacture, mould alignment, and resulting lower mould costs being the overriding factors affecting the choice. For components of very simple form it is often advantageous to use one design for one of the mould plates and the alternative design for the other. For example, consider a multi-impression mould for a box-type component. The cavity plate could be of the integer design to gain the advantages of strength, thereby allowing a smaller mould plate, while the core plate could be of the insert-bolster design which will simplify machining of the plate and allow for adjustments for mould alignment. 2.3 BOLSTERS We have seen that when it is decided to incorporate the cavity and core into a mould design as inserts, they must be securely retained in the mould. This is achieved by fitting the inserts into a bolster, which, when fitted with suitable guiding arrangements, ensures that alignment of the cavity and core is maintained. The fundamental requirements of a bolster can be summed up as follows: (i) It must provide a suitable pocket into which the insert can be fitted. (ii) It must provide some means for securing the insert after it is fitted in position. (iii) It must have sufficient strength to withstand the applied moulding forces.
Bolster material. The bolster is normally made from mild steel plate to the BS 970-040 AIS specification. In certain cases, however, a medium carbon steel (BS 970-m\0 M40) is to be preferred. as. for instance, in the case of small area inserts where the moulding has a projected area which is relatively large compared with the base of the insert. In such a case the
45
GENERAL MOULD CONSTRUCITON
pressure developed in the impression by the melt will tend to 'hob' the insert into the bolster. The use of a better-quality bolster steel would obviously be an advantage in such a case, as it minimises this tendency. Type of bolster. Various types of bolster have been evolved by designers in attempts to simplify either the fitting of the insert or the machining of the bolster. There are five main types of bolster to consider, as follows: (i) Solid bolster (Figure 2.17). This is suitable for use with both rectangular and circular inserts. (ii) Strip-type bolster (Figure 2.18a). Suitable only for rectangular inserts. (iii) Frame-type bolster (Figure 2.18b, c). Although this can be used for both types of inserts, it is particularly suitable for circular inserts.
(a)
(b)
(c)
Figure 2.17-Solid bolsters: (a) basic rectangular pocket type; (b) pocket made by slotting technique; (c) basic circular pocket type
Figure 2.18-Alternative types of bolster: (a) strip type; (b) rectangular frame type; (c) circular frame type; (d) open channel type; (e) enclosed chase bolster; (f) bolster plate
46
BOLSTERS
(iv) Chase-bolster (Figure 2.18d, e). This type is used in conjunction with 'splits' (split inserts). (v) Bolster plate (Figure 2.18f). This is used in particular circumstances with certain types of both rectangular and circular inserts.
2.3.1 Solid Bolster (Figure 2.17) This bolster is made by squaring up a block of suitable steel; then, by a direct machining operation, a pocket is sunk into the top surface to a predetermined depth. The shape of the pocket is either rectangular or circular to suit the shape of the mould inserts. The circular pocket is the simplest to manufacture; straight-forward boring and grinding operations provide a pocket into which the circular insert is easily fitted, thus providing accurate positioning in the mould. A typical solid bolster suitable for circular inserts is shown in Figure 2.17c. The inserts are retained by suitable screws from the underside of the bolster and, as can be seen, this particular example has been designed as a two-impression mould. A pocket to suit the rectangular insert is more difficult and expensive to machine in the bolster than that described above. In this case the bolster block is mounted on to a vertical milling machine, and the rectangular form is sunk by means of an end mill type of cutter as shown in Figure 2.19. The circular form of the end mill, however, leaves a radius in each corner of the cavity. Because of this the designer should stipulate as large a radius as is practicable in these corners to permit large-diameter end mills to be used for the cutting operation. The reason for this is that the smaller the radius, the smaller is the end mill required and hence the higher the cost of the operation because of the longer machining time necessary. The rectangular type of mould insert will have square corners resulting from its basic manufacture and it is necessary, therefore, to decide whether to modify the insert to suit the radius in each corner of the bolster pocket or vice versa. Both ways will now be considered, the first
80LSTER"
'
jt Figure 2.19- End-milling pocket in solid bolster
47
GENERAL MOULD CONSTRUCfiON
two methods being modifications of the insert and the last two modifications of the bolster pocket: (i) Radii are incorporated on each corner of the insert which is then accurately fitted to the bolster. This method provides a smooth unbroken mould surface which is desirable but, because of the accurate fitting required, is expensive (Figure 2.20a). (ii) The corners of the insert can be chamfered at 45° (Figure 2.20b). This obviates the necessity of accurately matching the radius as described for (i) and it is cheaper to make. (iii) The bolster cavity can be recessed at the corners to accommodate the square-cornered insert (Figure 2.20c). This involves quite a simple operation, the end mill used to machine out the pocket being sunk in to each corner to machine away the unwanted fillet. Methods (ii) and (iii), while cheaper than (i), suffer from the disadvantage than they result in open crevices which may become material traps if the mould flashes. Build-up of plastic material in these traps is difficult to remove and can, if allowed to become excessive, damage the opposite face of the mould. All three designs have a common disadvantage in that a simple grinding operation of the sides of the pocket is impossible. Due to this, more bench work is necessary in fitting the insert to the bolster. (iv) Figure 2.17b shows a method of machining a bolster that allows the sides of the pocket to be ground and also provides square corners for the pocket so that the rectangular insert is easily fitted. The bolster block is initially slotted, as shown, on a horizontal milling machine using a side and face cutter. The centre portion is then removed as before by an end milling operation. An advantage of method (iv) is that the sides can be ground with a saucer-shaped grinding wheel which passes through the slots. However, this bolster suffers the same disadvantage as (ii) and (iii) in that the slots may become material traps unless they are fitted with small strips of mild steel. While the illustrations show the outside form of the bolsters as rectangular they could equally well be made circular. BOlSTER
BOlSTER
INSERT
(a)
(b)
t
BOLSTER
f
INSERT
(c)
Figure 2.20-A/temative methods of fitting rectangular insert to solid bolster
48
BOLSTERS
2.3.2 Strip-type bolster
This is an alternative method for making a bolster to suit rectangular inserts and it overcomes some of the disadvantages listed for the solid bolster. The pocket is made by machining a slot completely through the bolster block. Steel strips are then fitted at either end of the slot to complete a frame for the inserts as shown (Figure 2.18a). To prevent the strips from moving under possible side thrust, a projection extends from the underside of the strip and this fits into a mating recess in the bolster. The strips are also securely bolted to the bolster with socket-headed screws. The advantage of this method is that both the sides and base of the pocket can be ground as also can the inner edge of the strips. This means that all the important surfaces are ground and the subsequent fitting of the rectangular insert is simplified. However, this bolster is not as strong as the solid bolster type as the supporting walls are non-continuous. The proportions of the strip should be such that it has a width to depth ratio of 3:2. For example a cavity insert 50 mm (2 in) deep will have a strip 75 mm (3 in) wide. This may well be a disadvantage when designing a bolster to accommodate a deep insert in that the overall length of the mould (when viewed in plan) may become excessive. 2.3.3 Frame-type bolster
This bolster consists of two parts, namely a frame and a backing plate. The frame is made by machining an aperture of the required shape completely through the bolster plate as illustrated (Figure 2.18b, c) for use with both rectangular- and circular-shaped inserts, respectively. The bottom of the insert is supported by a backing plate secured to the frame with a number of socket-headed screws. The inserts themselves may be secured either in the same manner by screws through the backing plate or alternatively (and this is the more usual) by the usc of flanged inserts (see Figure 2.13). In the latter case a recess is machined at the bottom of the aperture in the bolster plate, and the insert is fitted and secured in place by the backing plate. This type of bolster is particularly useful with small inserts where there is often insufficient room in which to position screws. It is also usually included in the design of multi-impression moulds containing circular inserts. Note that the backing plate, which is ground to ensure a perfect s~ating, must be of adequate thickness to withstand the force transmitted to it by the insert during the injection of the melt. 2.3.4 Chase-bolster
When splits (split inserts) are to be incorporated in the mould design it is necessary for one of the bolsters to lock the splits in their closed position. Although this topic as a whole is discussed in Chapter 8, the various designs of chase-bolster are included here for completeness.
49
GENERAL MOULD CONSTRUCfiON
The open channel. This is used for shallow rectangular splits and is made by machining a channel across the width of the bolster plate. The sides of the channel are sloped or angled as illustrated (Figure 2.18d). The machining of this bolster is fairly simple and grinding is possible on all faces. The sloping sides or wedge faces of the channel are extremely susceptible to wear. For this reason they are usually faced with hardened 'wear strips', made from a low-carbon steel suitably carburised to give a hard wear-resisting surface. This permits the use of a lower-quality steel for the main bolster than would otherwise be practicable. Enclosed chase-bolster. For deep splits the chase-bolster is normally of the enclosed type (Figure 2.18e) as against the open-channel type just discussed. It is machined from a solid block and the pocket which is to accommodate the splits may be of a tapered circular or a tapered rectangular form. From the cost standpoint the circular form is to be preferred, but its use is normally limited to single-impression moulds. (Plate 8.) 2.3.5 Bolster plate Inserts can be mounted directly on to a plain bolster plate. This system provides no side support or location and the walls of the inserts must therefore be of sufficient thickness to withstand the applied moulding pressure without undue deflection. The insert must also be securely screwed and dowelled in position to prevent misalignment. Figure 2.18f shows the bolster plate with an insert indicated in position by a chain dotted line. Generally this design is only used for a limited pre-production run to prove a component design prior to a more extensive tooling programme being embarked upon.
2.3.6 Bolsters-comparative survey The comparative table below (Table 2.1) indicates the relative merits of each bolster type, with regard to the ease with which it is machined, ground and the insert fitted. The relative strengths are also indicated. The rating values ranging from 1 to 10, a rating of 1 indicating great difficulty in machining, grinding and fitting inserts, respectively, and with regard to strength - that it is extremely poor. The opposite will be indicated, of course, for a rating of 10. It can easily be seen that whereas a bolster type may even have the top rating from some considerations, from others it may have the lowest. In fact, no bolster type is perfect in all respects, and the designer has to decide which is the most suitable for the particular application in hand. 2.4 ANCILLARY ITEMS In addition to the main constructional parts of the mould discussed in the previous sections there are certain essential ancillary items that are vital if
50
ANCILLARY ITEMS TABLE 2.1 Bolster-comparative survey
Type
Fig. No.
Ease of machining
Ease of grinding
Ease of fitting
Relative strength
1. Basic solid rectangular
2.17a
4
1
4
10
2. Basic solid circular
2.17c
8
6
8
10
3. Slotted type
2.17b
5
6
6
5
4. Strip type
2.18a
6
9
6
6
5. Rectangular frame type
2.18b
5
4
4
8
6. Circular frame type
2.18c
7
8
8
8
7. Open channel type
2.18d
7
9
7
6
8. Enclosed chase bolster (circular)
2.18e
8
6
8
10
9. Bolster plate
2.18f
10
10
10
1
the mould is to function satisfactorily. These ancillary parts ensure that the two mould halves remain in correct alignment, that a path for the plastic material is provided from the machine nozzle to the mould face, and that the mould is accurately positioned on the machine. These ancillary items are: (i) the guide pillars and bushes, (ii) the sprue bush, (iii) the register ring and (iv) the mould plate fastening. 2.4.1 Guide bushes and guide pillars
A description of a guide bush and guide pillar, and their use, has already been given in Section 2.1.6. The purpose of this current sub-section is to discuss these mould components in more detail. GUIDE BUSHES. A guide bush is incorporated in a mould to provide a suitable wear-resisting surface for the guide pillar and to permit replacement in the event of wear and damage. A typical guide bush is shown (Figure 2.21b ). Its internal bore is designed as a slide fit on the adjacent guide pillar, while the external diameter is a press fit into the mould plate. A radius is made at the front end of the bore (as shown) to provide a lead-in for the guide pillar. The rear end of the bush is often counterbored to a greater diameter than the working diameter (Figure 2.21b). On each stroke the guide pillar should ideally pass through the working diameter of the bush, its end passing well into the counterbore (Figure 2.24). If the counterbore is not incorporated, the pillar will operate over a limited part of the iAternal bore and cause uneven wear which, on very long production runs, may cause a ridge to occur inside the bush. Figure 2.21b shows the guide bush with the normal flange type of fitting.
51
GENERAL MOULD CONSTRUCTION SLIGHT UNDERCUT TO ENSURE SHARP CORNERS
5 mm
""3 mm RAD
(a)
1.5 mm RAD
(b)
Figure 2.21-Guide pillar and guide bush design
Various other designs of guide bush are used in practice and these will be discussed in the next sub-section with respect to specific guide pillar designs. There are five basic types of guide pillar in current use and these are designated: (i) leader pins: (ii) standard; (iii) spigotted: (iv) surface fitting and (v) pull-back. As the fitting arrangements are different for the various types, they will be discussed separately. Each guide pillar (except the first design) has a specific guide bush design associated with it. A number of companies produce a range of standard parts for moulds incluuing guide pillars and bushes, and a list of the companies who provide a service in the UK is given in Chapter 7 for reference. Note, however, that the complete range is not necessarily available from one particular standard parts manufacturer.
GUIDE PILLARS.
Leader Pins. During the early evolutionary period of the mould design for polymers, the mould consisted simply of two plates, a cavity plate and a core plate. The alignment between the two plates was achieved by incorporating shouldered pins in one half and by machining accommodating holes in the other half (Figure 2.22). These pins were subsequently called 'leader pins' (1) (this early term has been retained by DME* for the whole of its guide pillar range).
Figure 2.22-A non-stepped guide pillar, known as a 'leader pin'
52
ANCILLARY ITEMS
(a)
(b)
Figure 2.23-l/lustrating the advantage of using stepped guide pillar: (a) bent constant-diameter guide pillar, difficult to remove; (b) bent stepped guide pillar, relatively simple to remove
The basic design of leader pin fitting into a machined hole suffers two major disadvantages: (i) once the hole in the mould plate becomes enlarged due to wear, the positive location between the two halves is lost. This problem is easily overcome by fitting guide bushes into the one mould half and which can be replaced should wear deem this necessary; (ii) should the leader pin become bent it is difficult to remove from the mould plate without damaging its accommodating hole (Figure 2.23a). However, if the pin is stepped, as shown in Figure 2.23b, removal of this pin will present no difficulties. The stepped leader pin subsequently becal)1e known as a guide pillar (or guide pin) and, even more recently, as a 'standard guide pillar' to differentiate it from other developing types. STANDARD GUIDE PILLAR AND GUIDE BUSH. A typical design for a standard guide pillar and guide bush is shown in Figure 2.21. It will be noted that the guide pillar (Figure 2.21a) is designed such that the working diameter dis smaller than the fitting diameter D by a minimum of 7 mm 0 in). This introduces a step on to the pillar where it emerges from the mould plate, and while this is initially more costly to machine than a leader pin, it nevertheless has certain advantages. The fitting diameter of the guide pillar can be made the same as the guide bush, thus the same diameter can be bored and ground through both mould plates when clamped together. This allows perfect alignment to be achieved and also facilitates the fitting of both component parts (Figure 2.24). Note that modern precision jig boring and grinding techniques allows the holes for the guide pillars and bushes to be bored and ground to within very close tolerances. This avoids the necessity of boring and grinding the plates when clamped together to achieve the perfect alignment required, and lends itself to an interchangeable mould plate system. The second advantage, which was discussed above, is that the design simplifies the removal of guide pillars, should they become damaged. It is essential that the guide pillar is securely held in the mould plate and diameter D (Figure 2.21) is a press fit. While, in theory, a contant diameter fitting shank could be used there is a danger that the guide pillar may be pulled from its seating as the mould opens. To overcome this possibility, a flange is normally incorporated at the bottom of the guide pillar and this is accommodated in a recess in the mould plate as shown in the assembly drawing (Figure 2.24). The guide bush incorporates a similar
53
GENERAL MOULD CONSTRUCfiON
Figure 2.24-JI/ustrating the fitting of guide pillar and guide bush to two-plate mould
shoulded portion. A clearance is permissible around the flange to facilitate fitting. The general design for these components is very similar to the standard guide pillar and guide bush designs except that in this design an additional 'spigot' is incorporated on botlJ component parts. With reference to Figure 2.25 note that the guide pillar spigot (1) is fitted into an accommodating hole in the backing plate (2), while the guide bush spigot (3) is fitted into a complementary hole in the second backing plate (4). Thus, with this system, the guide pillar and guide bush, in addition to providing the basic guidance system between the two mould halves, additionally provides an alternative to the use of dowels for the alignment of the respective mould plate assemblies.
SPIGOTIED GUIDE PILLAR AND GUIDE BUSH.
An alternative method of fitting the guide pillar and guide bush is to fit both of these components from the parting surface side of the mould plate. Note that the standard design (Figure 2.24) requires that both the guide pillar and guide bush are
SURFACE FITIING GUIDE PILLAR AND GUIDE BUSH.
Figure 2.25- A spigotted guide pillar and a spigotted guide bush
54
ANCILLARY ITEMS GRUBSCREW CIRCLIP
(a)
(b)
Figure 2.26-Methods for securing guide pillars and guide bushes in deep mould plates
inserted from the rear side of the respective mould plates, and each is secured in position by via the flange. The surface fitting guide pillar may, or may not, incorporate a flange on the parting surface depending upon the design adopted. For example in Figure 2.26a the shank of the guide pillar is fitted into a blind ended hole and it is secured in position by a grub screw from the side of the mould plate. However, this method of fitting does not conveniently permit the respective guide pillar and guide bush holes to be bored and ground at one setting. The alternative design in which a flange is incorporated at the parting surface and a continuous hole provided in the mould plate is to be preferred (see next sub-section). The surface fitting guide bush normally incorporates a flange as shown in Figure 2.26b. This permits the guide bush to be secured in position by a circlip. PULL-BACK TYPE GUIDE PILLAR AND GUIDE BUSH. This is a patented type of guide pillar and guide bush known variously as 'Europa' (a standard part available from Uddform*), or 'Euro' (a standard part available from Desoutter*). The principle of the design, which is a variation of the surface fitting guide pillar and guide bush design discussed above, is that both component parts incorporate a flange at the operating end which permits them to be fitted from the mould parting surface (I) as illustrated in Figure 2.27. The major differences with the surface fitting design is that both the guide pillar (2) and guide bush (3) incorporate a central threaded hole. This permits both component parts to be positively secured in position by screws (4) mounted in a securing bush (5) fitted in the rear side of the relevant mould plate. Note that both the guide pillar and the guide bush are 'pulled-back' into their respective holes by the screw. Positive location of individual mould plate assemblies may be achieved, quite conveniently, by extending the length of the guide pillar bush as shown. Unfortunately, for the beginner, this guide pillar and guide bush design tends to make the final drawing appear more complex than it actually is. Compare the two worked examples shown in Figures 16.72 and 16.78, and in particular refer to the lower part of each cross-sectional view showing the mould guiding arrangements.
55
GENERAL MOULD CONSTRUCTION
Figure 2.27- Pull-back type guide pillar and guide bush
The advantage claimed for the above design is that it eliminates the need for separate screws to hold the various mould plates together. This is a definite advantage in that it reduces the number of holes bored in the respective plates. However, as only four guide pillars are normally fitted to each mould plate, this means that the number of screws securing the mould plates together is limited to four. Therefore for all but the smallest types of mould- say, 300 X 300 mm (12 x 12 in), additional securing screws must be fitted to ensure an adequate clamping-together force is applied. The main disadvantage with the system is that the guide bush does not have a straight through bored hole. Many designers prefer to allow the working diameter of the guide pillar to extend completely through the guide bush, in order that constant wear to the surface of the guide bush is achieved (see note above, under 'guide bush'). The normal size range of guide pillars obtainable as standard parts is between 10 mm (a in) and 50 mm (2 in) working diameter. Some very large moulds may require guide pillars outside this range. The decision as to which size to use depends upon the size of mould and whether or not a side force is likely to be exerted on the guide pillar. A guide to the guide pillar diameter for a given mould size is shown in Figure 2.28. If large side forces are likely to occur then the next larger size of guide pillar would be appropriate. (The guide pillar sizes given in Figure 2.28 are based upon the DME* range).
GUIDE PILLAR AND GUIDE BUSH SIZE RANGE.
The surface of the guide pillar and guide bush must be hard and wear resisting. This is achieved by machining the components from a low-carbon steel (BS 970-080 MIS) which is subsequently case hardened. This process gives a surface which resists pick-up and scoring as the guide pillar continually enters and leaves the guide bush. If the guide pillar is likely to be subject to bending forces, the use of a carburising nickel-chrome steel (BS 970 835 Ml5) is to be preferred.
STEEL FOR GUIDE PILLARS AND GUIDE BUSHES.
56
ANCILLARY ITEMS
'@'
600
X
600
1.00
X
400
200
X
200
160
X
160
120
X
120
100
X
100
r·"· ~
20 16 12
GUIDE PILLAR DIAMETER Cmm) Figure 2.28-A guide to the guide pillar diameter for a given mould plate size, based upon the DME* standard guide pillar range
FUNCTION oF GUIDE PILLARS. The design of mould, as well as the size and shape of the moulding, will affect the size, number and disposition of guide pillars on the mould plate. Although the guide pillar system is primarily concerned with alignment of the mould faces as they close during the moulding cycle, the pillars have also the subsidiary functions of protecting the core and acting as locating pins when the mould is being assembled. Guide pillars are usually necessary to ensure that both halves of the mould are kept in alignment while the mould is closing, though the necessity for this will depend on the design of the component. For instance the impression can, as in case (i), be wholly in one mould half with no actual core (Figure 2.29). In this case only nominal location of the two halves is required, as relative movement between the two plates will not affect the dimensions of the moulding. On the other hand, as in case (ii), where the impression is formed by cavities in both halves of the mould (Figure 2.30), a good alignment
Figure 2.29- When impression is in one mould plate, only nominal alignment between mould plates is required
57
GENERAL MOULD CONSTRUCTION
IMPRESSION
~
MOULDING
Figure 2.30- When impression is formed by both mould plates, accurate alignment is essential. Note resulting component from inaccurate alignment CORE
CAVITY
~ESSION
Figure 2.31- Misalignment between the cavity and core for box-shaped component; note resulting undesirable unequal wall section
between the two halves is essential. Even slight misalignment produces a moulding that is 'out of mitre' as shown. In the more usual case (iii), a cavity and core form the impression. Any misalignment (Figure 2.31) here is particularly unfortunate as the resulting moulding will have one wall thinner than the other. In addition to this, material entering the mould will take the easiest route and flow down the thicker section first. This will tend to move the core further out of alignment, and result in unacceptable mouldings. An extension of this case to large mouldings is considered in the next section. On two-plate moulds the guide pillars are normally fitted on the moving half so that they provide some protection for the core when the mould is off the machine. However, care must be taken in their positioning to ensure that they do not restrict the fall of the moulding after ejection. The alternative method is to fit the guide pillars on the fixed mould plate which, irrespective of their position, allows a free fall-away path for the moulding. This arrangement is adopted on standard mould systems-see Chapter 7. In either design the length of the guide pillar should be such that both mould halves are positively aligned before the core enters the cavity. Without this precaution any slight misalignment, perhaps due to wear of the platen bushes, may cause the core to strike the cavity wall with disastrous results (Figure 2.32). To safeguard against this possibility the guide pillar should be of sufficient length to enter the guide bush before the core enters the cavity (Figure 2.32). Note that the taper on the guide pillar tip provides a lead-in into the bush. 58
ANCILLARY ITEMS CUIDE PILLAR
CORE HISALIGNHENT
(0 )
GUIDE &USH
CAVITY
(b)
Figure 2.32-/ncorrect (a) and correct (b) length for guide pillar
For most small moulds the guide pillars are normally able to maintain alignment of the two halves, within reasonably close tolerances, irrespective of the force applied during injection. However, moulds for thin-walled articles require special treatment as even a slight discrepancy cannot be tolerated in this case. With some very large mouldings the size of pillar necessary to maintain alignment becomes excessively large. Therefore, in either case, the designer must consider means of maintaining alignment without relying entirely on guide pillars. A satisfactory way of achieving this is by the incorporation of a tapered location, normally in addition to the guide pillars. Consider, for example, a mould for a thin-walled bucket. If the design (Figure 2.33a) relies entirely on guide pillars for alignment, difficulties may arise due to the melt tending to flow down one side of the impression first. This will occur if the dimensions of the impression vary due to a slight discrepancy in the shape of the cavity or core. The melt will then flow first through that part of the impression which has the thickest cross-section. The result of this is that a differential force will then be applied on face A which is resisted by the guide pillars B. The forces involved in the moulding of such an article are extremely large and will result in increased misalignment which will, in turn, cause variation in wall thickness of the moulding. One method of holding the core in alignment in the case of such a large moulding is to provide a recess in the cavity plate into which fits a tapered location turned from the core plate at C (Figure 2.33b ). The taper ensures that the two surfaces do not rub and cause wear during the normal operating stroke of the mould. The guide pillars, which are still incorporated, provide approximate alignment while the tapered location assures final alignment. This method of incorporating a tapered location, however, has two disadvantages: (i) it necessitates machining the cavity from a deeper block of steel to accommodate the depth of the recess, (ii) the applied internal forces may force the cavity to expand and hence registering between both GUIDE PILLARS REINFORCED BY A TAPERED LOCATION.
59
GENERAL MOULD CONSTRUCTION A
(b)
(c)
(d)
Figure 2.33- Tapered location: (a) unbalanced force on core of large mould being resisted by guide pillars only; (b) tapered location recess in cavity plate; (c) tapered location recess in core plate; (d) enlarged view of tapered location
mould halves is immediately lost. The preferred method is to provide a tapered location on the cavity side, fitting into a recess on the core side (Figure 2.33c). This achieves the same result as in the previous case but overcomes both disadvantages. It also has the additional advantage of acting as a form of 'chase' for the cavity, helping it to resist the expansion forces. An enlarged view of the tapered location is shown (Figure 2.33d). The example given was for a circular component which permitted a circular tapered location to be used. Providing that the length - breadth dimensions do not vary too much, the circular tapered location can be adopted for rectangular components also. Where there is a wide variation of width - breadth dimensions a straight tapered location must be included on all sides. LOCATION UNITS An alternative to the tapered location method is by the use of a number of location units (also called 'tapered interlocks' and 'matchloks' - refer to standard part catalogues). Each unit consists of a pair of circular matched members which arc incorporated in the opposite faces of two mould halves as shown in Figure 2.34.
60
ANCILLARY ITEMS
Figure 2.34-A number of location units may be used as an alternative to the tapered location method
The male member of the two-part assembly incorporates a tapered projection which registers in a complimentary tapered recess in the second member. The included angle for this taper can lie between 20 and 30° respectively. Typical operating diameters which are available as standards parts are as follows: 7; 13; 16; 20 and 30 respectively. To achieve precise location it is advisable to bore and grind the accommodating holes in the respective guide plates when clamped together. Careful control of the functioning parts of the unit is essential to ensure that the precise location is achieved when the mould is just closed. Two to six pairs of location units are incorporated, the precise number depending upon the size of the mould. POSITIONING OF GUIDE PILLARS. The number of guide pillars incorporated in a mould varies from two (Figure 2.35a), for the simplest type, to four (Figure 2.35c), which is the preferred number for rectangular moulds. Some circular moulds have three guide pillars (Figure 2.35b). In addition to aligning the two mould halves, the guide pillar is used to prevent the mould from being assembled in the wrong way, and in this sense acts as a locating pin. The design of the two-pillar system (Figure 2.35a) is such that one guide pillar is made larger than the other. The three- and four-pillar system however is designed so that one or more of the pillars are offset. Figure 2. 35b shows a three-pillar system in which two pillar holes are offset by 7 mm (! in) from the basic 120° marking plane, whereas with the rectangular bolster (Figure 2.35c) the two bottom holes are set 13 mm 0 in) further in than the top two. All these methods guarantee that the mould cannot be assembled incorrectly. An alternative arrangement to the above, and the method adopted by all of the standard mould unit suppliers, is to use a symmetrical layout for the guide pillars but, this time, incorporate one guide pillar smaller than
61
GENERAL MOULD CONSTRUCfiON A
DIAMETER
A + 7 mm DIAH£TER
~*~r
i--x-13 mm --1
(b)
(c)
~7mm
(a)
•
7 mm
Figure 2.35-Guide pillar positioning. Positioning details for two-(a), three-(b), and four(c) guide pillar systems
-~=
+-----~--
1
---1-1
2
Figure 2.36-Guide pillar positioning. The diameter of one guide pillar is specified to be smaller than the others, in order to prevent the mould halves from being assembled incorrectly
the rest. Thus, for example, one 15 mm diameter pillar (1) would be used in association with three 16 mm diameter pillars (2) (refer to Figure
2.36). 2.4.2 Sprue bush The sprue bush (also called sprue bushing) is defined as that part of the mould in which the sprue is formed. In practice the sprue bush is the connecting member between the machine nozzle and the mould face, and
62
ANCILLARY ITEMS
provides a suitable aperture through which the material can travel on its way to the impressions or to the start of the runner system in 'multiimpression' moulds. The spr!Je bush is fairly highly stressed in some applications and should therefore be made from a H% nickel chrome steel (BS 970-817 M40) and should always be hardened. Note that while initially the nozzle may have been set up correctly (according to the machine maker's instructions), an increase in the injection cylinder temperature will cause a fairly high force to be applied to the sprue bush because of the expansion of the injection cylinder. The internal aperture of the sprue bush has between 2° and 4° included taper, which facilitates removal of the sprue from the mould at the end of the moulding cycle. For reference, 2; 2.6; 3 and 4 are the included tapers adopted by various standard parts suppliers. There are two basic designs of sprue bush which differ only with respect to the form of seating between the sprue bush and the nozzle of the machine. Both designs are shown (Figure 2.37a). The first of these is a sprue bush with a spherical recess which is used in conjunction with a spherical front ended nozzle as illustrated (Figure 2.38a). The second has a perfectly flat rear face, the seating between it and its corresponding nozzle being shown (Figure 2.38c). Providing the alignment between the nozzle and the bush apertures is perfect, a leak-free joint is achieved with the spherical seating. It should be noted that the radius on the nozzle is slightly Jess than that in the bush, which ensures both parts are in physical contact at the apertures. However, if the nozzle and sprue bush are slightly out of line (due perhaps to wear on platen bushes) a gap will result and leakage behind the mould will develop (Figure 2.38b). This drawback is not present if slight misalignment occurs at the seating of the sprue bush and flat-faced nozzle (Figure 2.38d). As can be seen, no leakage can occur if the two apertures are slightly out of line and no restriction to flow will occur either, providing that the sprue bush has an aperture slightly larger than that of the nozzle. An allowance of 0.8 mm (-bin) on diameter is usually made. With reference to Figure 2.37, the range of d/D ratio of sizes available as standard parts, are as follows: 12/25, 12/28, 18/38, 24/48 and 25/48. (The figures given are in mm). Both spherical recess and flat rear face types of sprue bush are available, in addition to blanks (no internal bore incorporated). 3-5° INCLUSIVE ANGLE
NOZZLE APERTURE
3 mm RADIUS (a)
+
0.8 mm
(b)
Figure 2.37-Sprue bushes: (a) spherical seating; {b) fiat seating
63
GENERAL MOULD CONSTRUCfiON
(a)
NOZZLE
HISI.LIGHHEHT
(c)
(d)
Figure 2.38-1/lustrating effect of misalignment between sprue bush and nozzle
2.4.3 Register ring
The register ring (also called locating ring or location ring) is a circular . member fitted on to the front face (and often also to the rear face) of the mould. Its purpose is to register (or locate) the mould in its correct position on the injection machine. When the mould is mounted on the machine the front mounted register ring fits into a circular hole which is accurately machined in the injection platen on the cylinder-nozzle axis. This ensures that the small aperture in the nozzle is in direct alignment with the sprue bush hole. Now, since the sprue bush is the connecting member between the machine nozzle and the mould face, this alignment of nozzle aperture and sprue bush hole permits an uninterrupted flow of material from the cylinder, through the nozzle and sprue hole into the mould runner system. The register ring, in fact, forms a direct connection between the sprue bush and the hole in the injection platen of the machine. There are several variations in the design of the register ring, and these have been necessitated primarily by standardisation requirements. The primary classification for this component is whether the register ring is to be mounted on the outside diameter of the sprue bush or whether it is to be mounted in a recess in the front plate of the mould. The sprue bush mounted register ring is illustrated in Figure 2.39. The inner bore of the register ring is accurately located on the external diameter of the sprue bush, while the outer diameter of the register ring is located in the bore of the injection machine platen. Positive alignment of the mould and injection machine nozzle is thereby achieved. The register
TYPES OF REGISTER RING.
64
ANCILLARY ITEMS
SECURIN; SCREW
(!OFF)
I SPlUE BUSH
I R£GISTER RING
MOULD PLATE
Figure 2.39-Location of mould on platen by register ring
ring is attached to the face of the front plate by screws as illustrated (Figure 2.39) .. If the mould is subsequently to be set up on another injection machine which has a different size of aperture, then this is simply a matter of changing to the appropriate size of register ring. The front plate. mounted register ring is illustrated in Figure 2.40. The plate is recessed, as shown, to accommodate the outer diameter of the register ring. The depth of the register ring is such that it projects from the front mould surface and registers in the accommodating mould platen aperture. Once again the register plate is attached to the front plate by screws. The front plate mounted register ring design is adopted by all of the standard parts manufacturers as it lends itself to standardisation. A specific diameter is specified for the recess, and the size adopted is normally one of the following: 75 mm (2.95 in); 90 mm (3.54 in) or 100 mm (3.94 in) diameter. Figure 2.41 illustrates the types of register ring which are readily available as standards, although there are variations in detail between the various manufacturers. (1) Reduced diameter type. This design is used when the injection machine's platen bore is smaller than the mould plate recess diameter (Figure 2.41a). (2) Constant diameter type. This design is adopted when the platen bore and mould recess diameters are identical (Figure 2.41 b). (3) Increased diameter type. This design is used when the injection machine's platen aperture is larger than mould's recess diameter (Figure 2.41c).
65
GENERAL MOULD CONSTRUCTION
Figure 2.40-Front plate mounted register ring
(a)
(b)
(C)
(d)
Figure 2.41- Type of register ring which are available as standard parts: (a) reduced diameter type; (b) constant tliameter type; (c) increased diameter type; (d) increased depth type
66
ANCILLARY ITEMS
(4) Increased depth type. This design is similar to '3' above except that the depth of the mould fitting diameter is increased in order to accommodate a sheet of insulating material adjacent to the front plate as shown (Figure 2,4ld). Note that all of the above designs incorporate a 90 degree inclusive internal tapered entry. In designs where the register ring is mounted on the sprue bush (in addition to locating in the front plate recess) the depth of the register varies between 1 and 5 mm (0.040 and 0.2 ins). The external diameter of the register ring incorporates a 2 mm by 45 degrees chamfer to facilitate mould setting. The sprue bush in the above designs, may be quite independent of the register ring as shown in Figure 2.41b, or alternatively, either the sprue bush may be stepped to fit into the register ring's internal bore (Figure 2.42a), or the internal bore of the register ring may be stepped to accommodate the sprue bush (Figure 2.42b). All of the above designs
(a)
(b)
Figure 2.42-A/ternative methods for nominally securing the sprue bush in the axial direction by the register ring
67
GENERAL MOULD CONSTRUCTION
nominally secure the sprue bush in the axial direction by virtue of the register ring attachment screws. When a direct feed into the base of a moulding is contemplated then it is advisable to adopt a more positive fixing method for the sprue bush than that described above. Note that in these cases the melt pressure (p) acts directly upon the face of the sprue bush tending to move it out of position. One convenient method for small moulds is to adopt the design shown in Figure 2.43. The applied force (F) tending to displace the sprue bush is transmitted directly to the register ring and thereby to the injection machine platen. Thus the securing screws in this design are not subject to large forces. Note that this design is basically the same as Figure 2.41a; however, when the design is used on large platen aperture injection machines, the designer cannot rely upon having a suitable standard part available. The front plate mounted register ring is always specified for use in association with long-reach or extended nozzles, the sprue bush mounted
Figure 2.43- Positive method for securing the sprue bush in the axial direction by the register ring
Figure 2.44-Sprue bush register ring arrangement for use with long-reach nozzle
68
ANCILLARY ITEMS
register plate design not being a practicable proposition in this case. A typical design is illustrated in Figure 2.44. Moving half register ring. As noted above, register (location) rings are also used for registering (locating) the moving mould half on the moving half platen. The injection machine manufacturer provides a complimentary aperture in the platen for this purpose. The register ring, in this case, is always mounted in a recess in the back plate, and the same designs of register ring, as shown in Figure 2.41 for the front mounted designs, are normally adopted. Naturally the sprue bush detail should be neglected in these drawing. Register rings without a central hole are also available as standard parts. 2.4.4 Mould plate fastening
Each mould half consists of a number of individual mould plates which must be securely attached together. If one plate should move with respect to another during production, the mould impressions and guidance systems would be seriously damaged and a very expensive repair or replacement would result. The novice should therefore give very careful attention to this aspect of the design. As noted previously for drawing convenience most of the illustrations in this book utilise a symbol 'S' to indicate those parts of the mould which must be positively attached together by screws and dowels. While this method is acceptable for illustrative work and for general arrangement designs (including the worked evamples for students, given in Chapter 16), for drawings which are required for mould production, detailed information relating to mould plate fastening must be given. There are three methods used to fasten mould plates together: the first is the basic screw and dowel method, the second utilises screws and spigotted guide pillars and bushes, while the third utilises the pull-back type guide pillar and guide bush. The last two methods may necessitate the use of additional tubular dowels. Screws and dowels. This is the basic design used in many engineering applications for locating and fastening plates together. The design incorporates a number of round dowels which are fitted into accommodating drilled and reamed holes in the mould plates as shown at 'A' in Figure 2.45. This feature ensures that the mould plates are always assembled and reassembled in their correct position in relation to each other. The complete mould plate assembly is clamped together by a number of socket headed screws (see 'B'). While four screws may be adequate for a small mould, say 200 x 200 mm (8 x 8 in), this number should be progressively increased in direct relationship to the mould size, to ensure that an adequate clamping-together force is obtained. Careful positioning of the screws and dowe"ls is necessary to ensure that they do not foul, or are dangerously close to, waterway holes.
69
GENERAL MOULD CONSTRUCfiON
suitable centres through this portion. The projection may be produced either by machining from the solid (as shown) or, when a two-plate construction ~s practicable, the rearmost plate is extended. A similar design is shown at (b), the only difference being that slots are provided through the projection instead of holes. Slots facilitate the setting of the mould on the platen, and this is particularly important for shallow-type moulds. With the mould closed there may be insufficient space left between the projections of both mould halves to permit a bolt to be inserted into a hole, whereas the bolt can normally be fitted, relatively easily, into a slot. It is worth noting that it is generally more convenient to set up a closed mould which has its own alignment system than set up the mould halves separately and align them afterwards. To save a considerable amount of machining on deep mould plates, the design shown at (c), in which local bolting recesses only are provided, is often used. The size of the recess, naturally, must be sufficient to permit the use of a suitable spanner for inserting and removing the bolt. Mould platens often incorporate T-slots in addition to, or instead of, threaded holes for mould clamping purposes. This design necessitates the use of a complimentary headed bolt which can be moved to the required mould bolt-hole position. 2.5.2 Indirect bolting method
With this design the attachment of the mould to the machine is by means of a clamp plate. It is used when it is not possible to use the direct bolting method which is generally to be preferred. For example it is often impracticable to incorporate direct bolting due to the inept positioning of tapped holes in the platen. The indirect bolting assembly consists of three parts (Figure 2.49), namely the clamp plate, the bolt and the packing piece. Two alternative designs are given. At (a) the front plate (or back plate) incorporates a projection (or flange) and a clamping force is applied to this by the bolt via the clamp plate. Alternatively, a slot can be machined through the mould plate (as (b)) and the clamping force applied in identical manner. PACKING PIECE
MOULD
PLATEN
PLATEN
(a)
(b)
Figure 2.49-Attachment of mould to platen; indirect bolting arrangements
72
A'ITACHMENT OF MOULD TO PLATEN
In practice the packing piece often forms an integral part of the clamp plate, and the unit can be obtained as a casting. Mould clamps are available as standards, and one based upon a DME* design is illustrated in Figure 2.50a. The clamp plate (1) incorporates a central slot as shown, for adjustment purposes. Tightening the bolt (2) applies the required clamping force via the bridge piece (3). The packing piece, in this design, is replaced by an adjusting screw (4) which can be set, by the mould setter, to the appropriate front (or back) plate thickness size. A similar design may be adopted for those platens which incorporate the T-slot clamping arrangement. A complementary T-shaped headed bolt with associated nut replaces the standard bolting system shown. An example of this system is illustrated in Figure 2.50b. The T-headed bolt (1) is fitted into the platen's T-slot (indicated by chain dotted lines) adjacent to the moult plate to be clamped. The mould clamp assembly consists of the clamp plate (2) which is pivoted on a pivot pin (3) which in turn is mounted in the packing plate (4). A nut (5) and olive washer (6) completes the assembly. The above design is based upon a Hasco®* mould clamp and it is, therefore available as a standard part.
4
··-~-·4
(a)
't.;}
(b)
Figure 2.50- Typical standard type mould clamp plates: (a) design based upon the DME mould clamp; (b)design based upon the Hasco® mould clamp
73
3 Ejection
3.1 GENERAL The previous chapter dealt with the basic two-part mould in which a moulding is formed by injecting a plastic melt, under pressure, into an impression via a feed system. The two parts by themselves, however, do not constitute an efficient design as no means are incorporated for removing the moulding once it is made. It must therefore be removed manually. Furthermore, all thermoplastic materials contract as they solidify, which means that the moulding will shrink on to the core which forms it. This shrinkage makes the moulding difficult to remove. It is normal practice, therefore, to provide some means by which the moulded part can be positively ejected from the core, and this chapter deals with the various methods which are used. Facilities are provided on the injection machine for automatic actuation of an ejector system, and this is situated behind the moving platen. Because of this, the mould's ejector system will be most effectively operated if placed in the moving half of the mould, i.e. the half attached to the moving platen. We have stated previously that we need to eject the moulding from the core and it therefore follows that the core, too, will most satisfactorily be located in the moving half. The ejector'system in a mould will be discussed under three headings, namely: (i) the ejector grid; (ii) the ejector plate assembly; and (iii) the method of ejection. 3.2 EJECTOR GRID The ejector grid is that part of the mould which supports the mould plate and provides a space into which the ejector plate assembly can be fitted and operated. The grid normally consists of a back plate (clamp plate) on to which is mounted a number of conveniently shaped 'support blocks'.
FOOTNOTE. Various standard part suppliers are mentioned in this chapter. An asterisk following a company's name. indicates that the name has been abbreviated. The company's full title and address can be found in the Appendix.
74
EJECTOR GRID
There are three alternative designs: (i) The in-line ejector grid (Section 3.2.1) (ii) The frame-type ejector grid (Section 3.2.2) (iii) The circular support block grid (Section 3.2.3) 3.2.1 In-line ejector grid (Figure 3.1) This consists of two rectangular support blocks (risers) mounted on a back plate. The ejector plate assembly, shown in chain-dotted lines, is accommodated in the parallel space between the two support blocks. A cross-section through the ejector grid is shown in Figure 3.1b. The position of the mould plate is also indicated in chain-dotted lines for reference. The design as illustrated is quite suitable for small types of mould where the overall size of the ejector plate assembly does not necessitate the support blocks being fitted a great distance apart. When this situation does arise, however, unless the mould plate is made reasonably thick there is the probability that the mould plate will be distorted by the injection force (see Figure 3.2a). To avoid the necessity of incorporating a thick, and therefore heavy, mould plate, extra support blocks are often added in the central region of the mould (Figure 3.2b ). The extra support can take the form of an additional rectangular support block (or blocks) fitted parallel to the outer pair (Figure 3.3). The ejector assembly used in conjunction with this type of ejector grid is shown in chain-dotted lines. It consists essentially of bars (rectangular cross-section) which extend completely across the mould and which are coupled together by a cross-bar at either end. An alternative support arrangement is shown in Figure 3.4. In this system additional local support pillars (support blocks) are incorporated in judicious positions to provide the required additional support. These SUPPORT BLOCK
BACK PLATE
EJECTOI'l PLATE ASSEMBLY
I I !
-~ SUI'POIIT aLOCK
(a)
l10ULD PLATE
(b) Figure 3.1- In-line ejector grid
75
EJECTION
(o)
(b)
Figure 3.2-Mould plate distortion is likely when support blocks are far apart (a), extra support blocks fitted close to centre can avoid this hazard (b)
~.==~
I :
~...~·I
.i
~~~J
Figure 3.3-Multiple in-line ejector grid, used in conjunction with ejector-bar system
support pillars are made from mild steel bar and are held in position by a single screw from the underside of the back plate. The ejector plate assembly (shown dotted) naturally must incorporate holes bored in positions corresponding to those of the support pillars. Because of the last point, the positioning of these support pillars is always delayed until after the position of the ejector element (i.e. ejector pin, ejector sleeve, etc.) has been decided upon. All of the mould systems which are available as standards are based upon the in-line arrangement. Now while relatively thick mould plates are incorporated in these mould units, it is essential to incorpcrate extra local support pillars if the applied injection force to be encounted is likely to be excessive. Note that for practical reasons the standard mould unit manufacturers space the support blocks (risers) relatively wide apart in order to encompass the greatest effective ejection area. Support pillars are available as standards in the UK in the following outside-diameter sizes (the dimensions given are in mm while the 76
EJECfOR GRID SOCKET- HEADED SCREW
~LOCAL
SUPPORT
BLOCK
Figure 3.4-Extra support for mould plate can be obtained by judiciously positioning local support blocks
bracketed dimensions give the Imperial equivalent): 30(1.2); 32(1.3); 40(1.6); 50(2.0); 60(2.4); 63(2.5); 70(2.8); 80(3.2); 90(3.5); 100(4.0); 120( 4. 7). A number of alternative lengths are available for each diameter. (Note that the above range of diameters is not available from one particular supplier). Standard support pillars are available with either a central threaded hole (Figure 3.4), a blind-ended dowel hole (this is often provided at the opposite end to the tapped hole) or a straight-through central hole. This later design allows the pillar to be clamped between the mould plate and back plate by directly bolting these two plates together \>Vith a socket headed cap screw.
3.2.2 Frame-type ejector grid Some frame-type ejector grid designs are illustrated in Figure 3.5. The most common type encountered is the rectangular frame (a) constructed of four support blocks suitably mounted on a back plate. This design is favoured by many mould designers for the following reasons: (i) it is simple and cheap to manufacture; (ii) it provides good support to the mould plate on a small mould; (iii) it allows for the use of a conveniently shaped (rectangular) ejector plate assembly and (iv) the ejector plate assembly is completely enclosed, thereby preventing foreign bodies entering the system. When the outside shape of the mould plate is circular it is often convenient to design a correspondingly shaped ejector grid. A typical design is illustrated at (b). It consists of a circular support frame mounted on to the back plate. The circular support frame, being machined from the solid block of steel, makes the design slightly more expensive to produce than the rectangular design.
77
EJECTION SUPPORT BLOCK
(d)
(b)
(c)
Figure 3.5-Frame-type ejector grid: (a-c) various alternative designs; (d) general cross-section
We stated previously that the ejector grid must provide adequate support for the mould plate. Now, as the size of the mould plate increases (and assuming that the ejector plate assembly correspondingly increases in size), the effective support provided by either of the above ejector grid designs progressively decreases. One method of improving this situation is to incorporate additional local support pillars in judicious positions in a manner similar to that described for the in-line ejector grid system (Section 3.2.1; Figure 3.4). It is often possible, however, to obtain additional support for the mould plate by designing the ejector grid of a shape other than the basic 78
EJECfOR GRID
rectangular or circular. The precise shape is dependent primarily upon the positioning of the ejector elements, which in turn determines which part of the ejector plate assembly can be machined away to permit additional support to be incorporated in the design. One example is illustrated at (c). In this case greater support is achieved at each corner by a simple modification to the ejector plate design (i.e. the corners of a rectangular ejector plate are removed). Even more irregularly shaped frames are designed when warranted to give maximum possible support. A general cross-section taken through any of the above frames (a, b or c) is shown at (d). The mould plate and the ejector assembly are shown in chain-dotted lines. Note that certain screws are used simply to attach the support block to the back plate, whereas other screws pass completely through the support block and are used to attach the mould plate to the ejector grid assembly. By undoing these latter screws the ejector grid can be removed from the mould as a unit. This feature facilitates repairs, etc. 3.2.3 Circular support pillar grid
In this design, circular support pillars are used to support the mould plate only, the rectangular outer support blocks of certain of the previous systems being dispensed with altogether. This system is used for large moulds when it is felt that no extra support would be gained by including rectangular blocks as well. A typical support pillar grid system is shown in Figure 3.6. The design simply consists of a number of circular support pillars judiciously positioned on the back plate of the mould. The grid is attached to the mo~ld plate by socket-headed screws. The ejector assembly (chain-dotted) BACK PlATE SUPPORT BlOCK
SOCKET- HEADED /~
SCRE'tl
HETAl PLATE
Figure 3.6-Circular support-block f(Tid
79
EJECfiON
can move freely as with previous designs, holes being bored through it to receive the circular support pillars. To prevent foreign matter getting into the ejector system it is desirable to attach thin metal plates to enclose the grid completely. 3.3 EJECTOR PLATE ASSEMBLY The ejector plate assembly is that part of the mould to which the ejector element is attached. The assembly is contained in a pocket, formed by the ejector grid, directly behind the mould plate. This is illustrated in Figure 3.7. The assembly consists of an ejector plate, a retaining plate and an ejector rod. One end of this latter member is threaded and it is screwed into the ejector plate (see cross-section view (b) ). In this particular design the ejector rod functions not only as an actuating member but also as a method of guiding the assembly. Note that the parallel portion of the ejector rod passes through an ejector rod bush fitted in the back plate of the mould. Before proceeding to discuss the individual parts in more detail, let us consider how this assembly is actuated. A cross-section through the moving half of a typical mould is shown in Figure 3.8. (The core and ejector elements are excluded for clarity.) The mould is mounted on the moving platen of the injection machine. To the left of the moving platen is the machine's actuating rod. This member can be adjusted to allow for various alternative 'ejector strokes'. When the moving platen is caused to move to the left, and the mould opens, the mould's ejector rod at some point of the stroke strikes the actuating rod. The entire ejector plate assembly is arrested as shown at (b). The remainder of the moving half (i.e. the mould plate and the ejector grid) continues to move to the left until the opening stroke is complete (c). This relative movement between the ejector plate assembly and the mould plate is necessary to operate the ejector element. In the above illustration the machine's actuator rod is shown passing through the centre of the moving platen. This is the normal arrangement
r· 1__
EJECTOR ROO
i
EJECTOR /-iPLATE
-~
' EJECTOR ROD
I .-.--,
LJ ___l___
(a) Figure 3. 7- ejector plate assembly
80
(b)
'RETAINING
_j
PLATE
EJECTOR PLATE ASSEMBLY fiXED PLATEN
EJECTOR ROD
PLATEN
ACTUATING ROD
(a)
MOVI~
MOULD HALF
FIXED MOULD HALF
(b)
I (c) Figure 3.8-As moving platen moves left, ejector plate assembly is actuated by machine's actuator rod
for the smaller types of injection machine. However, on larger machines several actuator rods are normally incorporated so that a balanced force can be applied to the ejector plate. Such a system is illustrated in Figure 3.9. A view of the moving platen of the machine without the mould is shown at (a). In this example four actuator rods are incorporated and these pass through suitable clearance holes in the moving platen. The method of actuation is identical to that described above for central actuation except that in this case the actuator rods push directly on to the ejector plate as shown at (b). If the ejector rod and ejector rod bush are not incorporated (as in this design) then a separate method of guiding and
81
EJECfiON HOVING PLATEN
ACTUATOR ROD
'----
MACHINE's COLUMNS
(a)
(b)
Figure 3. 9- Direct actuation of ejector plate assembly by machine's actuator rods
supporting the ejector plate assembly must be incorporated (Section 3.3.3). In addition to the fixed actuator rods, many injection machine manufacturers incorporate a hydraulic actuator system to facilitate the ejection function. This feature permits the ejection system to be operated (and returned, if required) at any point in the ejection phase of the machine's cycle of operation. The hydraulic actuator may be centrally mounted as shown in the schematic illustration (Figure 3.10). When moulding ejection is required, the hydraulic ram ( 1) is activated via the machine's hydraulic control system and this forward movement operates the mould's ejector system via the ejector rod (2). Alternatively the ejector rod may be coupled directly to the hydraulic ram so that forward and rearward movement of the ejection system may be achieved. Standard automatic couplings are available which facilitate the setting-up of the mould. A typical standard assembly (illustrated in Figure 3.11) consists of two primary parts, the nipple (1) which is screwed into the end of the hydraulic actuator rod, and the coupling body (2) which is fitted into the ejector plate. A key (not shown) is required to prevent the coupling body from rotating during production. For reasons of clarity Figure 3.11 shows the assembly prior to the two parts being coupled together. A variation of the forward acting hydraulic actuator discussed above, is for the injection machine manufacturer to mount the nydraulic actuator (1) facing the opposite direction. This time the hydraulic ram (2) is coupled to a cross-head (3) as shown in Figure 3 .12. A number of actuator rods ( 4) arc attached to this cross-head, and these rods pass through suitable drillings in the moving platen (5), and the mould's back plate (6).
82
EJECTOR PLATE ASSEMBLY
i
.
,------,
HYDRAULIC ACTUATOR
r _L _{
J.;Jl
I:'f·H
II::
rzzr-r--=BIIf.Tl
L----·-tJ---MOVING
PLATENJ,
L _____ j
2
Fjgure 3.10-0peration of ejector assembly by machine's hydraulic actuator
I
L _____ J
2
Figure 3.11-Standard coupling for automatic assembly of ejector rod to hydraulic actuator ram
Thus the ejector plate (7) may be operated directly by this means, that is, without an ejector rod assembly. 3.3.1 Ejector plate
The purpose of this member is to transmit the ejector force from the actuating system of the injection machine to the moulding via an ejector element (Section 3.4).
83
EJECI10N
Figure 3.12-0peration of ejector assembly by a number of actuator rods (hydraulic operation)
The force required to eject a moulding is appreciable, particular with those mouldings which are deep and which incorporate little draft. Most ejector plates which fail in operation do so in fact because too thin a plate is specified in the design. The ejector plate must be sufficiently thick not to deflect to any significant extent. Deflection tends to occur at the beginning of the ejector stroke when there is maximum adhesion between the moulding and the core. The deflection of any beam is inversely proportional to the cube of its depth and, therefore, a relatively small increase in plate thickness will decrease deflection of the plate. If an ejector plate does deflect to any extent, side forces are applied to the ejector elements which result in increased wear in the mould plate holes, bent ejector pins and, in extreme cases, in the complete seizure of the system. During the injection part of the cycle, with certain pin and sleeve type ejector systems (see Section 3.4), the melt pressure acts directly on to the ejector element (Figures 3.25, 3.40). To prevent the ejector elements being bobbed into the ejector plate by the applied force, a reasonably tough steel must be specified for this member. A general purpose medium-carbon steel (BS 970-080 M40) is suitable. The overall size in plan view of the ejector plate is dependent primarily upon the positioning of the ejector elements. For example, consider the plan view of the mould plate for a rectangular box (Figure 3.13). Suppose we decide that four ejector pins are sufficient to eject the moulding. They may be arranged either as at (a) in the figure or as at (b). The ejector plate must back up all the elements in either case, so it is apparent that method (b) permits a smaller ejector plate (shown by inner dotted lines) to be used. It must be remembered that the smaller the ejector plate the greater the support one can obtain from the ejector grid system. For example, compare the support (indicated in both drawings by the outer dotted line) 84
EJECfOR PLATE ASSEMBLY
(a)
(b)
Figure 3.13-Alternative ways of arranging four ejector pin elements. Note that method (a) results in larger ejector plate assembly than does method (b)
provided by the in-line ejector grid (a) with that provided by the frame-type ejector grid at (b). A typical rectangular type of ejector plate which may be used in conjunction with either an in-line or a frame-type ejector grid is shown in Figure 3.7. For the circular support block ejector grid system a similar rectangular ejector plate assembly design is used, but in this case holes are bored through the ejector plate (and retaining plate) to provide clearance for the columns. This type of ejector plate is illustrated in Figure 3.14. The mould plate and ejector grid system is shown in chain-dotted lines in the cross-sectional view (b). Finally we turn to the ejector bar system. This is used where relatively few ejector elements are incorporated in straight lines (or comparatively straight lines) on large moulds. In this system (Figure 3.15) individual bars are used instead of a plate, and the bars are joined together normally at the outer ends by cross-bars. 3.3.2 Retaining plate This member is securely attached to the ejector plate by screws (Figure 3.7). Its purpose is to retain the ejector element (or elements) and
]
r·:
I
i
!
/ ' :~.L. J! (b)
Figure 3.14- Ejector plate assembly for use in conjunction with circular support grid (see Figure 3.6)
85
EJECfiON EJECTOR 8AR CROSS-BAR
RETAINING PLATES
Figure 3.15- Ejector bar system
particular examples are illustrated in Figure 3.32 for pin-type ejection, and in Figure 3.40 for sleeve-type ejection. The thickness of the plate is governed by the depth of the head of the ejector clement it retains. In general, retaining plates are within the 7 mm (! in) to 13 mm in) thickness range. For small moulds the retaining plate is made to the same general dimensions (plan view) as the ejector plate (Figure 3.7). For larger moulds, however, it is convenient from the mould making viewpoint to incorporate local retaining plates (i.e. small blocks of steel) in judicial positions to accommodate one or a small number of ejector clements only. For a similar reason, local retaining plates arc normally fitted on ejector bar systems (Figure 3.15). However, for commercial standard systems. the size of the retaining plate is always supplied to the same dimensions as that of the ejector plate. Retaining plates are normally made from a mild steel (BS 970-040 A15).
n
3.3.3 Guiding and supporting ejector plate assembly This assembly must be guided and supported if there is any possibility of undue strain being applied to any ejector element. The type of guide system used will depend largely upon the size of the mould. We have previously noted that, for the smaller type of mould, the ejector plate incorporates an ejector rod which slides within an ejector rod bush which, in turn, is securely fitted into the back plate of the mould (Figure 3. 7). This system very conveniently maintains alignment and provides support for the ejector plate assembly. An alternative method for aligning and supporting the ejector assembly is shown in Figure 3.16. Bushes arc incorporated within the ejector assembly and these slide on hardened steel columns attached to the back plate. These columns arc normally also used as support pillars. For heavy types of ejector plate or bar assemblies, the plate (or bar) may be supported on its bottom edge as illustrated in Figure 3.17. In this design support strips arc attached to the lower support block. The support
86
EJECTOR PLATE ASSEMBLY lUSH
STEEL COLUMN
lACK PLATE
EJECTOR ASSEMBLY
Figure 3.16-Guiding and supporting ejector plate assembly
EJECTOR PLATE ASSEMILY
SUPPOII T ILOCK SUPPORT STRIPS
Figure 3.17-Method of guiding and supporting heavy type of ejector plate
strips are of either hardened steel or phosphor bronze. An alignment feature may be incorporated if desired in which case T-section support strips are used as illustrated. The projecting portion is a slide fit in a mating recess in the ejector plate assembly. It is common practice, however, on heavy moulds to usc hardened steel columns for the main alignment, and incorporate strips purely for the purpose of supporting the member.
87
EJECfiON
3.3.4 Ejector rod and ejector rod bush There are two alternative designs which may be adopted for this assembly. The first is the older conventional design which may be adopted when when standard parts are not readily available, and the second is the standard part design. (i) Conventional design. A typical design for this assembly is shown in Figure 3.18. The dimensions given apply to a relatively small mould. The assembly details are as follows: The ejector rod is attached to the ejector plate by means of a thread as shown at A. To ensure concentricity a small parallel length of a slightly larger diameter than the thread is provided at B on both the ejector rod and the ejector plate. The threaded hole may either extend completely through the ejector plate as shown, or it may be blind. If the former design is adopted it is normal practice to make the end of the ejector rod at C level with the ejector plate surface. This is particularly desirable when a central sprue puller is used. Spanner flats are provided on the ejector rod at D. The position of these flats must be such that at no time do they enter the ejector rod bush. (If the top end of the ejector rod is damaged by unskilled usc of the spanner, and this is then forced into the ejector rod bush, a seizure is likely.) The dimension specified for the distance between the end of the ejector rod bush and the ejector rod flats (see diagram) should therefore exceed the maximum ejector plate movement by at least 7 mm (t in). The ejector rod bush is normally made a press fit into the back plate of the mould. Some designers, however, prefer to extend the flange (E) and positively secure the member to the back plate with screws. Both the ejector rod and the ejector rod bush arc normally made in a low-carbon steel (BS 970-080 M 15) and suitably case-hardened. This gives both members a wear resisting surface. B
A
c
0
1I
~
G'
45 mm 35 mm 25 mm OIA OIA OIA
~
~
J.
l,m
· B.S.W
l!f-6
Fig11re 3./8-Ejector rod and ejector rod b11sh assembly (dimensions given are Sllitable for a small type of mo11ld)
88
EJECfOR PLATE ASSEMBLY
/ 3
2
1
6
7
5
I.
Figure 3.19-Standard part ejector rod and ejector rod bush assembly
(ii) Standard part design. An illustrative design of the assembly is shown in Figure 3.19. This design, or similar designs with minor variations, are available from all standard parts manufacturers. With reference to Figure 3.19 the assembly consists of a plain diameter ejector rod (1), to which an ejector rod cap (2) is attached by means of a socket headed cap screw (3). The attachment of the ejector rod to the ejector plate (4) is either by means of a projecting integral threaded member (as for the conventional design shown in Figure 3.18) or by fitting a suitable diameter grub screw (5) into the front end of the ejector rod to produce the same result. The range of diameters available as standards are as follows: 10(0.4), 14(0.56), 16(0.63), 18(0.7), 20(0.79), 24(0.94), 30(1.2) and 34(1.3). (The above diameters are in mm and the adjacent bracketed terms give approximate Imperial equivalents. Note that the complete range of sizes, as listed, is not available from one particular supplier.) A compression spring (6) may be fitted on the ejector rod, as shown, as an ejector plate return device, see sub-section 3.3.5. The standard ejector rod bush (7) is shorter than its non-standard counterpart. The bush is fitted into a suitably stepped aperture in the back plate (8) as shown. Note that if a moving half register or locating ring (9) in incorporated in the design (as illustrated), this feature nominally secures the bush in position. 3.3.5 Ejector plate assembly return systems We saw, earlier in this chapter, the mechanism by which the ejector plate assembly is moved forward relative to the remainder of the moving half (Figure 3.8). We must now consider how we are to return the ejector plate assembly to its rear position in preparation for the next shot, when the mould closes. 89
EJECTION
Certain ejection techniques provide for the positive return of the ejector assembly by virtue of the mould geometry. The stripper plate design is a good example of this (Figure 3.57). In this design the stripper plate is directly connected to the ejector plate by tie-rods. When the mould closes the stripper plate strikes the cavity mould plate thereby causing the stripper plate and the ejector plate to be returned to their rear positions. However, the ejector pin and ejector sleeve ejection techniques do not have a large surface contact with the fixed mould half and these techniques require, therefore, the use of a special system to return the ejector plate. Two systems in common use are (i) the push-back return system and (ii) the spring return system. (i) Push-back return system. 'Push-back pins' (return pins) are basically large-diameter ejector pins fitted close to the four corners of the ejector plate back pin is shown in Figure 3.20. In the moulding position as shown
PUSH-BACK PIN
(b)
(a)
Figure 3.20-'Push-back' ejector plate return system
90
EJECTOR PLATE ASSEMBLY
at (a) the push-back pins are flush with the mould plate surface. In the ejected position the push-backs protrude beyond the mould plate surface (b). Thus, when the mould is in the process of being closed, the push-back pins strike the fixed mould plate and progressively return the ejector plate assembly to the rear position (a). The shouldered head design illustrated is adopted for push-back pins under 13 mm (!in) working diameter. For large pins an alternative design is sometimes adopted. In this, the push-back pin is secured to the ejector plate assembly by a shouldered, headed capscrew (Figure 3.21). Suitable sizes of push-back pins (return pins) may be selected from a standard ejector pin range. Certain manufacturers will supply a standard mould unit with a puch-back system already fitted. (ii) Spring return systems. For small moulds, where the ejector assembly is of light construction, a spring or a stack of 'Belleville' washers can be used to return the ejector plate assembly. A typical arrangement of the former actuating method is illustrated in Figure 3.19. In this design the spring is fitted on the ejector rod. A cap is attached to the end of the ejector rod to hold the spring in position under slight compression. In operation, when the ejector assembly is actuated, the spring is compressed further. Immediately the mould closing stroke commences, however, the spring applies a force to return the ejector assembly to its rear position. An alternative design, used for heavier ejector assemblies, is to incorporate a multiple spring system between the retaining plate and the rear face of the mould plate. These springs are often fitted on local circular support blocks. When a mould is fitted to a machine which incorporates a hydraulic actuating system, providing the ejector rod is positively coupled to the hydraulic ram, the return of the ejector plate assembly follows automatically. However, even in these cases, many designers prefer to incorporate a push back return system as a safety feature, to ensure that the ejector system is fully returned prior to the melt injection phase.
Figure 3.21-Alternaiive method of attaching push-back pin to ejector plate assembly
91
EJECfiON
3.3.6 Stop pins With a large ejector plate or large ejector bar system, it is often preferable to incorporate stop pins on the underside of the ejector plate. This design drastically reduces the effective seating area. In so doing, it diminishes the possibility of the ejector elements remaining slightly proud of their correct position due to foreign matter being trapped behind the ejector plate. Four such stop-pins are normally fitted directly below the push back pins as shown in Figure 3.22. The heads of the stop-pins should be of a relatively large diameter to prevent the possibility of their being hobbed into the back plate. Stop-pins of 16 mm (~ in) and 25 mm (1 in) are available as standard parts. Stop-discs (plain discs with central countersunk hole for a screw) are also available in two diameters, namely 18 mm (0.7 in) and 28 mm (1.1 in) respectively. In the case of both stop-pins and stop discs, the effective depth is in the 3-4.5 mm (0.1-0.17 in) range. 3.4 EJECfiON TECHNIQUES When a moulding cools, it contracts by an amount depending on the material being processed. For a moulding which has no internal form, for example a solid rectangular block (Figure 3.23a), the moulding will shrink away from the cavity walls as shown, thereby permitting a simple ejection technique to be adopted (for example, perhaps, a jet of air). However, when the moulding has internal form, the moulding, as it cools, will shrink onto the core and some positive type of ejection is necessary (Figure 3.23b). The designer has several ejection techniques from which to chose but, in general, the choice will be restricted depending upon the shape of the moulding. The basic ejection techniques are as follows: (i) pin ejection; (ii) sleeve ejection; (iii) bar ejection; (iv) blade ejection; (v) air ejection and (vi) stripper plate ejection. Certain of the ejector elements used in the above techniques are illustrated in Figure 3.24.
5TOP PIN
hJ.:IIrOtOGtltBulktd'"atftlt
R-.nuret~bfldaQft
~deop.-d.n.
"'""""'~ Cnadtttltf~
loblkdOitloo\noklr.on
Sp&nor041Y..,
f>oartoondei"An~
Figure 7.8- DM E catalogue page (ii): other standard component parts associated with the 156 x 246system
closed (i.e. 1625) to identify a particular catalogue page. (The relevant pages from the DME catalogue for the 1625 series are reproduced as Figures 7.7 and 7.8 The third and fourth variables, being the depths of mould plates, are
239
STANDARD MOULD SYSTEMS
shown as dimension T in box 'NlO'. The final variable, that of the ejector stroke is shown in Box 'N30'. Note that the height 'H' of the support block (riser) is made the pertinent dimension (i.e. the one used for ordering purposes) but the stroke (S) is also shown for reference purposes. All other plates have fixed depths and are therefore indirect variables. With reference to Figure 7. 7, note that thumb-nail sketches of the five alternative ranges of mould systems are illustrated in a column on this catalogue page for quick reference purposes. A cross-sectional drawing, with relevant dimensions, of each of the mould plates is also given on this page. Each drawing is enclosed in a 'box' which also lists the depth, or depths of plate available, together with alternative steels, where appropriate. From this particular mould system series (Figure 7.7), six alternative depth of mould plate are available (see box NlO), which when coupled with the three alternative ejector strokes (see box N30), means that a total of 108 mould systems are available in this particular mould series, i.e. 6 X 6 X 3. The facing catalogue page (Figure 7.8), lists the available alternative sizes of the minor component parts which are associated with this particular series. Additional ordering options include the following: (i) Extra wide overhang front and back plates. (ii) Stripper plate with guide pillars mounted in the core plate. (iii) A guided ejector plate system. (iv) Clamp slots for use with flush-type front and back plates.
7.2.4 Hasco® standard mould system A completely modular system is adopted by Hasco, and the term 'K-Standard Elements' is used to designate their standard system. Mould plates are supplied, bored and ground to accommodate guide pillars, guide bushes, etc. Note that 'Hasco' does not market an assembled standard mould unit. Each individual plate must be ordered separately. Because the above mould system is only supplied in kit form, there is no reason to have a standard mould as a reference base. However, a diagram is useful for part identification and ordering purposes, and Hasco have adopted a three-part. design with this objective in mind. A thumb nail-size drawing of this basic three-part mould is repeated nine times on each pair of catalogue pages. (A typical matching pair of catalogue pages for the 196 196 series is illustrated in Figures 7.9 and 7.1 0, for reference). Each drawing identifies a specific plate by the usc of colour (shown black in this illustration). A correspondingly small dimensioned detailed drawing is shown adjacent to the assembly drawing, together with a list of alternative depths which are available for that specific plate. · The catalogue page is identified by a number which represents the width and length dimensions (in mm) of the mould plate (i.e. W x L).
240
STANDARD TWO-PART MOULD SYSTEMS
Hasco also offer an optional standard mould plate in which a grinding allowance has been left, in all of the bored holes. This is useful if the plate is to be subsequently heat treated or if the plate is subject to machining stresses which may have to be relieved. In either case slight movement of the position of the bored holes is a possibility and which is catered for in the above design. Three or four alternative strokes are available depending upon the specific series chosen. The stroke cannot be specified directly with this system and it is achieved by a combination of the support block height (designated K40) and the combined thicknesses of the ejector plate (K70) and retaining plate (K60). An assembled ejector plate/retaining plate combination, fitted with stop discs is also available (Reference K60/70). An interesting alternative support block system is offered, in which the support blocks are split into two (K41) in order to accommodate a crosstype ejector plate assembly (K61/71). This later design can be useful for many applications. Note that three heights of support block are available for the 196 196 series-Figure 7. 9. The front plate KlO, K12 or K15), back plate (Kll, K13 or K16), and core backing plate (K30) are available in some of the series in two thicknesses. This is the case with respect to the 196 196 series illustrated in Figure 7.9. The reason why there are three numbers designated to the front and back plates is that these may be ordered to overhang the remainder of the rr:ould either longitudinally or laterally, or be flush with the remainder of the mould. A very large number of alternative assemblies can be produced with this system. For example, for the 196 196 series the number of possible arrangements for a two plate mould is 2400 (10 x 10 x 3 X 2 x 2 x 2). This figure does not include whether or not the front and back plates overhang, or that a cross-type ejector plate assembly is available. The lower part of the right catalogue page (Figure 7.10) lists the alternative sizes of the various standard parts such as guide pillars, guide bushes, hollow dowels, cap screws etc., which are associated with the 196 196 mould plate series. Hasco also offers a range of seventeen alternative circular mould systems which can be extremely useful for certain applications. 7.2.5 Desoutter standard mould series Two alternative ranges of mould systems are available from Desoutter, as follows: (i) A kit of component parts, all parts being ordered separately. (ii) An assembled mould unit, comprising mould plates, guide pillars, guide bushes, hollow dowels and cap screws etc. Register rings, sprue bush, sprue ejector pin, ejector rod and push back pins can also be fitted if required. The catalogue page incorporates a similar layout to that of Hasco, with the exception that an assembled standard mould unit is illustrated on the left page. 241
STANDARD MOULD SYSTEMS
,;
196196
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i
i
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Figure 7.9-Hasco® catalogue page (i): layout of variables for a 196 x 196 standard mould system
A number, preceeded by the letter K, is used to identify a specific plate in the assembly. The primary mould plates (designated K20 in the catalogue) are available in a number of thicknesses and the number available varies between
242
STANDARD TWO-PART MOULD SYSTEMS
' 6
K60
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:
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Figure 7.10-Hasco® catalogue page (ii): other standard component parts associated with the 196 x 196 system
six and ten, depending upon the particular series chosen. For example, with reference to the illustrated catalogue page for the 196 196 series (Figure 7. 9) ten different thicknesses are listed in a number of alternative materials.
243
STANDARD MOULD SYSTEMS
246X246sERIES2525 M12 ''---- I
j 246
l
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Figure 7.11- Desouuer catalogue page (i): Ia yow of variables for a 246X246 standard mould system
A thumbnail drawing of the assembled mould is repeated seven times on each double catalogue page, each drawing identifying a particular mould plate or part by the use of colour. The 'box' shown adjacent to each drawing specifies the relevant size of plate (width and length) together with its thickness, or range of thicknesses. 244
STANDARD TWO-PART MOULD SYSTEMS
SERIES2525
246 X246 COot! 198 -.C62556 76 198 - "62578
96 198 -
462596
116 198 -o4625116
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Sl)rue Bu&haa
Etectet Plna
Figure 7.12-Desoutter catalogue page (ii): other standard component parts associated with the 246 x 246 system
A typical pair of facing catalogue pages relating to the 246 x 246 mould plate size is given as 'series 2525' in Figures 7.11 and 7.12 respectively. The primary mould plates, designated by the letters C, S, D are available in a number of thicknesses ranging from four, for the smallest system, up to eight for the larger sizes. Note the catalogue code letters 245
STANDARD MOULD SYSTEMS
represent the following: Cavity plate (C); Stripper plate (S); Core plate (D); respectively. Eight thicknesses of mould plate are shown to be available in two alternative steels for the specific catalogue page given in Figure 7.11. As this is primarily a kit ordering system, the stroke is achieved by the combination of the support block (riser) height and (designated R) and the depth of the ejector plate assembly (designated ER and Er). Four alternative depths of support block are available for the majority of the range including series 2525. The front and back plates (designated TF, T A or TB depending upon whe~her they are flush or overlapping types), and the core support plate (J), each have two alternative sizes available. The total number of alternative assemblies which can be achieved for a two plate mould for the series 2525 is 1024 (8 x 8 x 4 x 2 x 2). The available size range of the associated mould component parts, such as guide pillars, guide bushes, hollow dowels, cap screws etc., are shown on the lower right catalogue page (Figure 7 .12). 7 .2.6 Uddform standard mould systems
This mould system is supplied in kit form only. Each plate, component parts and other accessories are supplied individually packed in polythene sheet. The catalogue page follows a similar pattern to that described for the previous two systems. Six thumbnail drawings of a standard two-plate mould system assembly are shown on the left page of each series number. An individual mould plate (or plates) is identified by being shown in black. Plan view of each of the relevant mould plates together with the available alternative plate thicknesses, are shown in adjacent 'boxes'. A typical pair of catalogue pages are reproduced as Figures 7.13 and 7.14 respectively. The width length dimensions (246 x 296 in this case) are rounded to the nearest em to give the series number-2530. Note that the 'E' following the series number indicates the spigotted guide pillar and bush system is provided with this particular mould system. (An 'S' following the series number would similarly indicate that standard guide pillars and bushes are provided. The latter components are used for all mould systems above the series size 346 x 346. The primary mould plates (cavity and core), together with the core backing plate are shown in a large box in a central position on the left page. Seven or eight alternative thicknesses are available for these plates in a number of alternative steels. In the 2530 series, for example, one of seven thicknesses of plate may be chosen in four different steels. For ordering purposes the plates in this section are designated by the letter 'K' followed by a number, e.g. 'K2201'. The last figure in this number changes with steel specification. One or two thicknesses of front and back plate are available depending upon the series chosen. The catalogue page illustrated (Figure 7.13) shows that one of two thicknesses may be ordered for the 2530 series.
246
DEVIATIONS FROM THE STANDARD MOULD
The plates are available either to fit flush with the other mould plates or as overhang plates. The final variable, that of the stroke, is once again achieved by a combination of support block height and ejector plate assembly thickness. Three to five alternative support block heights are available, together with two alternative ejector plate thicknesses. With respect to the 2530 series under discussion, one of three support block heights may be chosen. Therefore for this specific series a total of 4116 alternative assemblies can be achieved for a two plate mould system (7 x 7 x 7 x 2 x 2 x 3). The complementary right page of the catalogue (Figure 7 .14) in addition to showing a drawing of a two-plate assembly lists the associated mould parts including guide pillars, guide bushes, hollow dowels, cap screws etc. Summary From the preceding discussion it is evident that the standard mould system (SMS) may be used for most of the designs given in Part One-Elementary Mould Design. The only limiting factor is that of size of SMS available. At the time of writing the largest unit is 100 mm x 800 mm and the maximum plate depth if 196 mm. Thus, provided the size of the mould falls within specified limits, the following design features can be accommodated in SMS: pin ejection (Figure 3.25); D-pin ejection (Figure 3.37); blade ejection (Figure 3.44); valve ejection (Figure 3.50); stripper bar ejection (Figure 3.55); stripper ring ejection (Figure 3.68). In the last two designs the ejector retaining plate is not necessarily used. There are two items in the elementary design section which necessitate some deviation from the SMS: the sleeve ejection design (Figure 3.40) (this involves only a minor deviation) and the stripper plate design (Figure 3.57). While some of the mould designs discussed in Part Two, Intermediate Mould Design, can be accommodated in a SMS, in the majority of cases some modification is required. 7.3 DEVIATIONS fROM THE STANDARD MOULD SYSTEM Any deviation from the SMS simply involves the addition or subtraction of plates to achieve the required assembly. Guide pillars and guide bushes may have to be reversed and, in addition, the sprue puller may have to be removed. Mould designs necessitating a deviation from the SMS are discussed in turn below. In each illustration the SMS is shown as a reference base, and the required mould unit is shown below. Added plates are shown cross-hatched. Mould plates added to or subtracted from the SMS are marked 'IN' or 'OUT', respectively. 247
STANDARD MOULD SYSTEMS 2530E (246x29~,~~-=
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Figure 7.15-S/eeve ejection mould system SMS with additional core-retaining plate (CRP)
250
DEVIATIONS FROM THE STANDARD MOULD
7.3.2 Stripper plate mould system There are basically two types of stripper plate mould which may be classified as the standard design and the basic design. In the standard design, the stripper plate is incorporated between the fixed and moving halves of the mould. STRIPPER PLATE DESIGN. This mould system assembly is achieved, quite simply, by adding an extra plate (the stripper plate)
STANDARD
I
I A 8
II
I
II
I
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c J
SMS
SP
I
Figure 7.16-Stripper plate mould system (standard type). SMS with additional 'stripper plate' (SP) mounted between plates A and B. Reverse the guide pillar/guide bush assembly (not shown)
251
STANDARD MOULD SYSTEMS
between the two primary plates A and B. The stripper plate may be positively coupled to the ejector plate assembly by means of tie-rods (Figure 3.57) (the retaining plate being therefore unnecessary), or 'operating pins' (basically ejector pins) may be incorporated in the ejector plate assembly (Figure 3.58). The necessary SMS adaption and the final unit assembly as shown in Figure 7 .16. PLATE DESIGN. The directly operated strippe~ plate assembly of a standard mould system is theoretically achieved by: (i) removing the ejector plate assembly and ejector grid; (ii) fitting an extra
BASIC STRIPPER
I
I A
B
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I
I
I
I
I SMS
SP
'--
out
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Figure 7.17-Stripper plate mould system (basic type). SMS with additional 'stripper plate' (SP) mounted between plates A and B. Reverse the guide pillar/guide bush assembly. Remove the ejector grid and ejector assembly
252
TABLE 7.2 Comparative terminology for the various standard mould parts DMS Black and front plate Backing plate Cavity and core plate Ejector plate Guide pillar Guide bush Push-back pin Register ring Retaining plate Sleeve Sprue puller (pin) Standard mould system Stripper plate Sprue bush Support blocks Support·pillars N VI
w
Clamp plate Backing plate Cavity plate
DME Top and bottom clamping plate Support plate Mould plate
UDDFORM
HAS CO
DESOUTTER
Clamping plate
Clamping plate
Clamping plate
Support plate Cavity plate
Intermediate Mould plate
Backing Cavity plate
t:l trl
- 0
z
Cll
Ejector plate
Ejector plate
Ejector plate
Guide pillar Guide bush Return pin Location spigot Ejector pin retaining plate Sleeve ejector
Leader pin Leader pin bush Return pin Location ring Ejector retaining plate Ejector sleeve
Guide pillar Guide bush
Sprue ejector pin
Sprue puller pin
Standard mould system Stripper plate Sprue bush Spacer blocks Support pillars
Mould base
Mould set
Stripper plate Sprue bushing Risers Support pillars
Stripper plate Sprue bush Riser Support pillars
Locating ring Sleeve ejector pin
Ejector plate base Guide pin Guide bush Return pin Locating ring Ejector plate
Ejector plate Guide pillar Guide bush Return pin Locating ring Ejector plate
:;t1
0 E:::
;! trl
Cll
~
z
t:l
Sleeve ejector
:>
:;t1
Sprue puller K-Standard elements Stripper plate Sprue bush Supports Support pillars
'Tj
t:l
Mould set Stripper plate Sprue bush Risers
E::: 0 c r t:l
STANDARD MOULD SYSTEMS
plate (the stripper plate) between the two primary plates, A and B. Actuation of the stripper plate may be either by means of the injection machine actuator rods (Figure 3.1) or by some other means, such as length bolts (Figure 3.1) or chains (Figure 3.1), etc. The main deviations from the SMS are shown in Figure 7.17. Naturally, in practice, the relevant plates required are ordered as a kit, one doesn't have to physically remove the ejector plate and ejector grid from a standard mould unit. MORE COMPLEX ASSEMBLIES. Deviations from the standard mould system for all mould designs covered in Part 2 of this book are discussed under the respective chapter headings for these designs.
COMPARATIVE TERMINOLOGY The terminology used throughout this book for the various mould parts does not necessarily accord with that of the mould unit manufacturers. To avoid confUsion, the relevant terms used by the various manufacturers, are compared, in Table 5, to those adopted in this book.
254
PART TWO
Intermediate mould design
8 Splits
8.1 GENERAL The mould designer is frequently confronted with a component design that incorporates a recess or projection which prevents the simple removal of the moulding from the mould. The component designer, while endeavouring to produce in line of draw component designs, has often to include a recess. or a projection to perform a particular function or to satisfy an artistic requirement. A moulding which has a recess or projection is termed an undercut moulding. The mould design for this type of component is inevitably more complex than for the in line of draw component, as it necessitates the removal of that part of the impression which forms the undercut prior to ejection. 8.1.1 External undercut components
Any recess or projection on the outside surface of the component which prevents its removal from the cavity is termed an external undercut. Various components which incorporate external undercuts are shown in Figure 8.1. The arrows on each drawing indicate the position of the undercut. There are two forms of undercut to be considered. (i) The undercut may be local, in that the recess or projection occurs in one position only. The clip on a pen cap is an example of this (Figure 8.lj). (ii) The undercut may be a continuous recess or a projection on the periphery of the component. The water connector (Figure 8.la) has a number of such undercuts. In either case, it is necessary to split the cavity insert into parts and open these, generally at right angles to the line of draw, to relieve the undercut before the moulding is removed. Since the cavity .is in two pieces, a joint line will be visible on the finished product. Now this joint line, on an undercut component, is comparable to the parting line on an in line of draw component and the FOOTNOTE. Various standard part suppliers are mentioned in this chapter. An asterisk, following a company's name, indicates that the name has been abbreviated. The Company's full title and address can be found in the Appendix.
257
SPLITS
same careful consideration must be exercised in deciding its position before attempting to design the mould. Let us now consider the choice of joint line for specific components. The symmetrical component will present no difficulties as the joint line can be positioned on any centre line (Figure 8.la, b, e, f). Note, ALTERNATIVE POSITION FOR JOINT LINE
(c) (a)
JOINT LINE
--
(d) (~)
(f)
-11~1111~IC.(h)
Figure 8.1- External undercut components: (a) water connector; (b) spool; (c) threaded adaptor; (d) threaded ferrule; (e) connector; (f) pipe stem; (g) egg cup; (h) pulley; (j) pen cap. Arrows indicate position of undercut
258
GENERAL however, that as it is desirable to keep the splits movement to a minimum the joint line for regular rectangular components should be positioned on the longitudinal centre-line (for example, Figure 8.1g). For unsymmetrical components the choice of joint line is far more critical in that there is usually only one possible position. Consider, for example, the pen cap (Figure 8.lj). To extract this component from the mould, half the depth of the clip must be included in each split, therefore the joint line can only occur on the centre of this projection. For very complex components it is advantageous to have a model made to simplify the selection of the joint line. If an incorrect joint line is chosen, the moulding will tend to restrict the free opening movement of the splits which will result in scored or cracked mouldings. Externally threaded components (Figure 8.1c, d) may be included under the general undercut component category but as there are alternative methods for dealing with these specific components they are discussed separately in Chapter 11. The joint line should be as inconspicuous as possible on the moulding. To achieve this, the mould making must be excellent, the mould itself must be efficiently serviced during its life, and the correct conditions must be maintained during moulding. 8.1.2 Splits Let us now consider why it is necessary to make the cavity insert in two parts for mouldings which incorporate as undercut. Figure 8.2a shows a part section through a two-plate mould for the water connector. In this design the cavity form is machined directly into the cavity plate. We note that when the moutd is opened the core can be withdrawn, but the barbs on the component form undercuts which make its removal from the cavity impossible. In the second design (b), the cavity form for the same component is shown machined into two separate blocks of steel. These split cavity blocks are called splits. One half of the component's form is sunk into each split and, providing the splits can be opened (Figure 8.2c, lower drawing), the moulding can be extracted. The splits can be incorporated in the mould design in several ways. The simplest is by fitting the splits into a chase bolster. This method has a major disadvantage in that after each moulding operation the splits must be removed and opened prior to the mouldings being extracted. This manual operation, of necessity, lengthens the moulding cycle and therefore this design should be avoided except for moulds for prototype components. Let us now consider more complex systems where the splits are retained on the mould plate and actuated automatically. There are two basic designs: sliding splits and angled-lift splits. In both designs there are moving parts and it is necessary to arrange for (i) guiding the splits in the desired direction, (ii) actuating the splits, and (iii) securely locking the splits in position prior to the material being injected into the mould. The two basic designs will now be discussed in detail.
259
SPLITS
(a)
(b)
: :::::1 (c) Figure 8.2-(a) Impracticable cavity design; removal of moulding is impossible. (b & c) Practicable design using splits; drawmgs show closed and open positions. respectively
8.2 SLIDING SPLITS In this design, the splits are mounted in guides on a flat mould plate and they are actuated in one plane by mechanical or hydraulic means. The splits are positively locked in their closed position by heels which project from the other mould half. Sliding splits may be mounted on either the moving or the fixed mould plate, but as the former is the more general case, to prevent confusion, we will confine the discussion to this type only. The principle of the sliding action is illustrated in Figure 8.3. This shows the splits in their closed moulding position (a) and in their fully open position (b).
260
SLIDING SPLITS
Figure 8.3- Basic sliding split mould. Splits are shown in closed (a) and open (b) positions
8.2.1 Guiding and retention of splits
There are three main factors in the design of the guiding and retention system for a sliding splits type mould. (i) Side movement must be prevented to ensure that the split halves always come together in the same place. (ii) All parts of the guiding system must be of adequate strength to support the weight of the splits and to withstand the force applied to the splits by the operating mechanism. (iii) Two two split halves must have a smooth, unimpeded movement. Although this point appears obvious, it is easy for a beginner to position guide pillars, for example, i'1 the path of the splits. In most designs, the guiding function is accomplished by providing an accurately machined slot in the mould plate in which the splits can slide. Close tolerances on both members are essential to prevent side play. The splits retaining system usually adopted is based upon a T-design. Each split incorporates shoulders which are caused to slide in the Tshaped slot that extends across the mould plate (Figure 8.3). The depth of the shoulder and of its female counterpart in the mould plate must be carefully dimensioned to ensure the split has smooth, judder-free movement. By utilising the side walls of the T-slot for the guiding function as well, the mould design can often be simplified. MOULD PLATE DESIGNS. There are several ways of producing the Tshaped slot in the mould plate, and three examples are shown in Figure 8.4. (a) The form may be machined from a solid steel plate using aT-type of milling cutter. This system is seldom used, as it is comparatively difficult to grind the T-form to accurate dimensions. (b) Separate guide strips may be attached to a flat mould plate as shown. The guide strips must be screwed and dowelled to the mould plate to ensure rigidity.
261
SPLITS
X
(a)
(d)
(e)
(b)
(c)
(f)
Figure 8.4-Mou/d plate designs for use in conjunction with sliding splits: (a, b, c, f) various alternative designs; (d, e) fitting details
(c) In this design the required form is produced by machining a Ushaped slot across the face of the mould plate and then two flat steel strips are securely attached to the top surface. No grinding difficulties are associated with either of the last two designs. It is undesirable for the split to be in sliding contact with more than one pair of side faces of the T-form, as this makes the mould making unnecessarily difficult. Thus the actual guiding contact surface should either be as shown at X (Figure 8.4d) or as at Y (Figure 8.4e ). A definite clearance of 0. 75 mm (0.030 in) should be specified, as shown.
262
SLIDING SPLITS
(d) A guide block may be mounted on either side of the mould as illustrated in Figure 8.4f. The guide block (2) incorporates a groove, which accommodates the split shoulder (3). (The split is shown as a chain dotted line). Careful attention must be given to the fitting and securing of these guide blocks to the mould plate (1) to ensure that they cannot be displaced during production. The advantage in this type of design is that the splits extend right across the mould allowing greater space for the impressions. Hasco®* utilises the principle of this design for its standard 'Split mould kit'. The company manufactures the guide block in two parts, and subsequently attaches these together with screws and hollow dowels. This design approach facilitates the machining and grinding of an accurate groove. SPLIT DESIGN TYPE 1. The basic shouldered split design is shown in Figure 8.5. The only difference between its two variants is that (a) is made by machining the form from the solid block, while (b) is of two-part construction. While the economic advantage of the two-part construction for small splits is insignificant, some manufacturing time can be saved on larger splits. Note that if the latter alternative is adopted positive alignment between the two parts is essential and in the example given (b) a rectangular key is fitted for this purpose. The depth of the split's shoulder must be dimensioned to be a slide fit in the T-form of the guide (Figure 8.4d, e).
2. This design incorporates a male T-shaped projection below the base of the split (Figure 8.6a). The guiding arrangement can thereby be positioned directly below the split, which permits a narrower mould than is possible with the previous design. A general arrangement scrap section assembly drawing is shown (Figure 8.6b) which gives details of the fitting arrangements. Note particularly where clearances are specified. As the split's side face and the mould plate edge are in general alignment, a cam plate can be conveniently fitted close to the split for actuation purposes. A pictorial view of the splits and mould plate assembly is shown in Figure 8.14 .
SPLIT DESIGN TYPE
• (a)
~KEY (b)
Figure 8.5-Sp/it, design 1: (a) basic and (b) a/temative methods of manufacture
263
SPLITS MOUlD PLATE
0.38 mm
II
(0.015 in) --11-
(b)
(a)
I
.~ (C)
Figure 8.6-Sp/it, design 2: (a) design of split, (b) part cross-sectional view through split and mould plate; (c) design adopted by Uddform and DME
A variation of this design is adopted by both Uddform* and DME* for their standard sliding splits design. Uddform designates its design as a 'Standard sliding-core mould set' and DME uses the terminology 'Sliding core system'. The general principle of the design is illustrated in Figure 8.6c. The guide strips, shown chain dotted, (1) are of a right angle shape, which permits them to be secured to the corner of the mould plate, as shown. This design feature allows for greater rigidity to be achieved than is possible with the basic flat guide strip design. A slide (2) is attached to the underside of the strip (3) by means of screws and dowels. Note that this slide extends beyond the angled (locking) face of the split to provide an operating region for angled cams
264
SLIDING SPLITS
(see next sub-section). A drawing of a worked example utilising a standard split's type mould system is given in Figures 16.77, 16.78 and 16.79 respectively. 3. In this design (Figure 8.7), the splits are guided by a projection from the base of the split (b) which slides in a slot that extends across the face of the mould plate. The direct alignment between the split and the mould plate is advantageous when heavy splits must be guided and supported. General splits retention, as for the other designs, is by the split's shoulders, which fit into a T-shaped slot in the mould plate. An enlarged cross-sectional drawing (c) shows where specific clearances are required.
SPLIT DESIGN TYPE
8.2.2 Methods of operation
We will now consider the various methods which can be used to actuate the splits in relation to the mould plate. The most frequently used designs are based on various types. of cam. Under this heading we will discuss finger cam, dog-leg cam, and cam track methods of actuation. The basic operation with cam actuation is as follows. As the mould is opened, the cams attached to the fixed mould half cause the splits to slide across the moving mould plate. Conversely, when the mould halves are
(a) 0.75 mm (0.0 :::\:3 0 - · n ) 0.75 mm (0.030 in)
---4
~
'L~ !'--, ,~'""
SLIDE
(b)
(0.030 in) (c)
Figure 8. 7-Sp/it, design 3: (a) design of mould plate; (b) design of split; (c) part cross-sectional view through split and mould plate
265
SPLITS
brought together, the splits are progressively closed. The cams generally lose contact with the splits as the mould opens and should either split be moved out of position prior to the mould closing serious damage will occur. To obviate this danger some safety features are incorporated in the design. Another method of actuating the splits is by the use of compression springs, but as these can only be used to open the splits, the locking heels on the mould plate are utilised to fulfil the closing function. This system, while simple and cheap, has limited application as only relatively small split movement can be obtained. Most machine manufacturers incorporate facilities in the hydraulic circuit for the operation of additional actuators if required. Thus, providing an actuator can be accommodated in the design, this method of splits operation should not be overlooked. The actuators can be mounted on opposite sides of the mould and the rams coupled directly to the splits. The main advantage of this system is that large splits movements are practicable. The various actuating methods will now be discussed in detail. In this system, hardened, circular steel pins, termed finger cams, are mounted at an angle in the fixed mould plate (see qualifying note in Section 8.2). The splits, mounted in guides on the moving mould plate, have corresponding angled circular holes to accommodate these finger cams. A typical design is given in Figure 8.8, which is of a mould for a spool component, the splits being shown in the closed position at (a). As the mould opens, the finger cam forces the split to move outwards, sliding on the mould plate (b). Once contact with the finger cam is lost, the split's movement ceases immediately. Continued movement of the moving half causes the ejector system to operate and the moulding to be ejected (c). On closing the reverse action occurs. The finger cam re-enters the hole in the split and forces the split to move inwards. The final closing nip on the splits is achieved by the locking heels and not by the finger cams. The distance traversed by each split across the face of the mould plate is determined by the length and angle of the finger cam. The movement (Figure 8. 9) can be computed by the formula
FINGER CAM ACTUATION.
M
=
(L sin ¢) - (c/cos ¢)
(8.1)
As the required movement is known from the amount of component undercut, the following rearranged formula to determine the finger cam length is of greater use, apart from checking purposes L
where M lj>
L c
266
= splits
= (M/sin
¢)
+ (2c/sin 2¢)
movement,
= angle of finger cam,
= working length = clearance.
of finger cam,
(8.2)
SLIDING SPLITS
----MOULDING
FINGER CAH
SPLIT
(a)
(b) LOCKING HEElS
Figure 8.8-Finger-pin actuated mould: (a) splits in closed position; (b) intermediate position; (c) splits fully open; moulding ejected by sleeve
The designer must aim to keep the splits' movement down to a minimum, at the same time ensuring that the moulded part can be easily and quickly removed from the mould. The clearance c (Figure 8.9) serves a dual purpose. (i) It ensures that the force which is applied to the split during the injection phase is not transfer~ed to the relatively weak cam.
267
SPLITS
(ii) It permits the mould to open a predetermined amount before the splits are actuated; this movement can in certain circumstances be used to withdraw the core from the moulding. The amount of delay movement D before the splits are actuated is determined by the following relationship:
= c/sin
(8.3) 10° is a suitable angle for , but if the mould height has to be increased unduly to accommodate excessively long finger cams it is permissible to increase this angle up to a maximum of 25°. For actuating small splits, a finger cam diameter of 13 mm (0.5 in) is suitable, but for large splits or where a greater than 10° angle is specified the diameter should be increased accordingly. Note the lead-in angle at the front end of the finger cam (Figure 8.9). This facilitates the re-entry of the finger cam into the split as the mould is being closed. This angle is normally ( + 5) 0 • One or two finger cams are used to operate each split. When two are used (i.e. for splits greater than 76 mm (3 in) in width) it is essential that both function in unison. One method of fitting the finger cam into the mould plate is illustrated (Figure 8.8). A backing plate is not essential as the finger cam is supported by the machine platen when the mould is fitted to the injection machine. The injection platen must be checked, however, to ensure there are not any holes or depressions in the proposed finger cam position. The hole in the moving mould plate, positioned below the split to accommodate the projecting end of the finger cam, may be circular or link shaped in the plan view. It is important to note that the side of this hole (see A, Figure 8.8) is straight, i.e. the angled hole of the split is not continued through the mould plate. A typical finger-cam actuated design is shown in Plate 7. D
DOG-LEG CAM ACTUATION. This method of actuation is used where a greater splits delay is required than can be achieved by the finger cam
FIXED MOULD PLATE
I
Figure 8. 9-Split movement calculations (finger-pin actuation)
268
SLIDING SPLITS
method. The dog-leg cam (Figure 8.10), which is of a general rectangular section, is mounted in the fixed mould plate. Each split incorporates a rectangular hole, the operating face of which has a corresponding angle to that of the cam. The sequence of operation of this type of mould is shown in Figure 8.11. At (a) the mould is closed and the splits are locked together by the locking heels of the fixed mould plate. The feed to the impression is not shown. The splits do not immediately start to open when the mould halves are parted (b) because of the straight portion of the dog-leg cams. The moulding, which is encased within the splits, will thus be pulled from the stationary core. Further movement of the moving mould half causes actuation of the splits by the dog-leg cams, thereby releasing the moulding (c). The reverse action occurs when the mould is closed. The simple form of the component in Figure 8.11 permitted the mould to be designed without an ejector system. There is a tendency for the moulding to be left in one of the split halves and it is desirable, therefore, to arrange for the splits to start opening before the moulding completely leaves the core; this holds the moulding central. Another example is shown in Figure 8.12. This design is for a circular component which necessitates positive ejection. To make the problem a little more difficult let us assume that no joint lines are permissible on the major diameter (top and bottom). This means that we can only use the splits for the central region, the remainder of the moulding's exterior being formed in cavity inserts. The component design necessitates two cores which meet at Z. Now, in this example, it is desirable to pull that part of the moulding which is formed in the fixed mould plate completely clear of the impression before the splits are opened. Thus the delay dimension D of the cam must be sufficient to accomplish this. After actuation of the splits by the dog-leg cams the component is ejected by the sleeve. (Note: it would be necessary with this design to limit the forward movement of the sleeve to prevent fouling by the splits as they are closed, or to provide a mechanism to ensure the ejector system is returned before the splits are actuated).
Figure 8.10-Method of a/laching dog-leg cam to mould plate
269
SPLITS SPLITS
LOCKIIIG HE£LS
FIXED MOULD PLATE
STATIOIIARY CORE
(c)
Figure 8.11-Dog-leg cam actuated mould: (a) splits closed; (b) intermediate position (note splits still closed, moulding pulled from core); (c) splits opened by cam
270
SLIDING SPLITS
-Z
CORE
CORE
- FIXED MOUlD PlATE CAVITY INSERT
SPliTS
CAVITY INSEI\T
Figure 8.12- Typical dog-leg cam actuated mould
The relevant formula for calculating the opening movement, the length of cam, and the delay period (Figure 8.12) are given by
M La D
where M
= movement
= La tan ljJ - c = (M + c)/tan ljJ = (L, - e) + (c/tan l/J)
(8.4) (8.5) (8.6)
of each split,
La = angled length of cam, Ls = straight length of cam ljJ = cam angle, c = clearance, e
= length of straight
D = delay, portion of the hole.
Typical cross-section dimensions of a dog-leg cam for a small mould are 13 mm (4 in) by 18 mm U in). The angle l/J, ideally, is 10°, but here again this may be increased to 25° if by so doing the overall height of the mould can be reduced. A lead-in at the front end of the cam should be provided to facilitate the re-entry of the cam into the split as the mound closes. This lead-in may either take the form of a 10° taper or a generous radius. One method of attaching the dog-leg cam to the mould plate is shown in Figure 8.10. The link shape at the lower end of the cam allows the complementary hole in the mould plate to be machined by a simple endmilling operation. The cam is held back on to the mould plate by a socket-headed screw. When a back plate is incorporated in the design (Figure 8.12), the link,-shaped portion of the cam may extend completely 271
SPLITS
through the mould plate and be secured as shown. For splits under 100 mm (4 in) in width, one cam is used, for splits of greater width two cams are preferable. The clearance hole in the moving mould plate below the split (A, Figure 8.12) can either circular or rectangular with radiused corners. The latter design is generally preferred as it results in less of the mould plate, which supports the split, being machined away. This method of actuation utilises a cam track machined into a steel plate attached to the fixed mould half. A boss fitted to both sides of the split, runs in this track. The movement of the splits can thus be accurately controlled by specific cam track design. Figure 8.13 illustrates two typical cam track plates; they differ only in respect of whether the cam track is machined part way through (b) or completely through the plate (a). Design (b) is the stronger but to maintain the same-depth of boss contact as (a) the plate must of necessity be thicker. To ensure smooth operation a generous radius should be incorporated at each point where the cam track form changes. A radius or taper should also be included at the entrance to form a lead-in for the boss as it re-enters the track. Figure 8.14 shows a simplified pictorial view of a moving mould half incorporating splits used in conjunction with this actuating arrangement. It will be noted that the width of the splits in this design (Section 8.2.1) corresponds to the width of the mould plate. This is necessary so that the cam plates can be in the close proximity to the splits in order to actuate them. The drawing also shows the bosses which protrude from the side faces of the splits. Corresponding bosses protrude from the lower faces but are not visible.
CAM TRACK ACTUATION.
Figure 8. I 3- Typical cam track plate designs
272
SLIDING SPLITS
Figure 8.14-Sliding split mould (cam track plate actuated)
Details of the operation and of the mould assembly are given in Figure 8.15. The bottom drawing shows a typical cross-section through this type of mould. The splits are mounted on a mould plate. The bosses, screwed into the side faces of the split, protrude into the cam track plates. The latter are securely attached to the side faces of the fixed mould plate. Note that a small clearance of 1.5 mm (is in) is provided between the cam track plate and the moving mould half. The mould operation is illustrated in the top drawing. At (a) the mould is shown in the closed position. As the mould opens the bosses follow the cam track and thereby cause the splits to open. An intermediate mould open position is illustrated at (b), which shows the boss on the point of leaving the cam track. Note that at this point the splits are fully open. When the mould is closed the boss re-enters the cam track and the splits are progressively closed. The relevant formulae for calculating the distance traversed by each split, the length of cam track, and the delay period (Figure 8.16) are as follows:
M = La tan - c M
(8.7)
+c
(8.8)
tan
D = Ls +
ta~ + '(ta~
1 - sin )
(8.9)
where M movement of each split, La = angled length of cam track, L, = straight length of cam track, = cam track angle, c = clearance, D =delay, r = radius of boss.
273
SPLITS
X
.t
X
(a)
I
I I I
-
I I
•
,;
SPLITS
HOULO PLATE
00
I
,
,) ;
0 0
(b)
BOSSES
FIXED HOULO PLATE
(c)
CAM TRACK PLATES
1.5 mm (1/16 in)
Figure 8.15-Cam track actllation: (a) mould closed; (b) intermediate position; (c) section X-X
There is a greater range of permissible operating angle with this design than with other methods because the cam plate is more rigid. The angle used is normally between 10° and 40°. This design, which obviates the use of cams altogether, incorporates compression springs to force the splits apart and
SPRING ACTUATION.
274
SLIDING SPLITS
utilises the angled faces of the chase bolster to close them. The outward splits movement must therefore be limited so that they will re-enter the chase bolster as the mould is closed. This design is limited to mouldings which incorporate relatively shallow undercuts. A typical basic design is shown in Figure 8.17. The splits are mounted on the mound plate and retained by guide strips. Studs project from the base of the splits into a slot machined in the mould plate. The outward movement of each split is therefore controlled by the length of this slot. A compression spring is fitted between the studs in a link-shaped pocket situated in the lower mould plate. The splits are held closed by the chase bolster. The sequence of operations is shown in Figure 8.18. (a) The splits are held closed by the chase bolster during the injection phase.
Tl M
Figure 8.16-Split movement calculations (cam track actuation) Y
STUDS
SPLITS I
CH~SE
-~~-rll I
BOLSTER
COMPRESSION ~PRIHG
I
LOWER MOULD
MoULD PLATE
PL~TE
Figure 8.17-Spring actuation
275
SPLITS
(b) Immediately the mould begins to open, the compression springs exerts a force to part the split halves. (c) The splits movement is stopped by the stud reaching the end of the slot in the mould plate. Continued movement of the moving mould half operates the ejector system (not shown). During the closing stroke the splits re-enter the chase bolster and are progressively closed. Calculations for this arrangement are limited to those for the splits opening movement (Figure 8.19). M
=
~H
tan
cp
(8.10)
where M = movement of each split, H = height of locking heel. ¢ = angle of locking heel. A suitable angle for the locking heel is between 20° and 25°, and therefore approximately M
(a)
= 0.2
(b)
H
(8.11)
(c)
Figure 8.18-Mould opening sequence of spring-actuated split mould
1-'-'~~:..:....:...~ _j_ M
I'I
·--·-r
Figure 8.19-Split movement calculation (spring actuation)
276
SLIDING SPLITS
The splits open immediately the mould parts and no delay is possible; the moulding must therefore be biased to remain in the moving half so that it can be positively ejected. For splits under 76 mm (3 in) in width, one stud per split is suitable. for those over this width two studs should be used. The studs can be flattened at Y (Figure 8.17) to provide a seating for the spring. In operation, spring failure may result in the splits being damaged by ejector system. Careful selection of the spring and regular mould maintenance are therefore essential. HYDRAULIC ACTUATION. In this design the splits are actuated hydraulically and, unlike the previous systems, it is not dependent on the opening movement of the mould. The splits can be operated automatically at any specific time by including this function in the operating programme of the machine. On machines which do not programme for auxiliary cylinder control it is necessary to add a separate hydraulic operating circuit to the existing system. The designer does not generally rely on the locking force which can be applied hydraulically to keep the splits closed during the injection phase, as this would mean fitting large-diameter and, therefore, heavy cylinders. However, if the total projected area of the mouldings is relatively small this method should not be overlooked as the mould design is considerably simplified. (See Chapter 9, which deals with hydraulicallyactuated side cores.) In all other cases, the conventional locking heel system (Section 8.2.3) is used. A general design for shallow splits is shown in Figure 8.20. The splits incorporate a projection on the underside to which the ram of the hydraulic actuator is attached. The projection is free to move in a slot which extends part-way through the mould plate (see also section A-A). The hydraulic actuator is fitted on a mounting plate which is securely attached to the side wall of the mould plate. the splits are shown in the closed position, and are held by the locking heels of the chase bolster. For deeper splits, a similar arrangement is adopted, except that the ram is attached to the angled face of the split (Figure 8.21). In this design it is necessary to provide a slot through each looking heel (see part-section X-X) to clear the ram. A foot-mounted hydraulic actuator is shown fitted to an angle-plate, which in turn is securely attached to the mould plate wall. In either design, the splits can be actuated at any time after they are clear of the locking heels, even in the extreme case when the mould is fully open. However, to reduce the cycle time to a minimum it is desirable to operate the splits while the mould is opening. On the return stroke it is essential the splits are closed before they re-enter the chase bolster. From the above comments it will be apparent that large delay movements and large split movements can be achieved with this design. Against these advantages, the fact that the mould is more bulky as compared with the other designs makes the mould setting more difficult
277
SPLITS
MOUNTING PLATE
CHASE BOLSTER
SPLITS
MOULD PLATE
RAH
SECTION
A-A
Figure 8.20- Hydraulic aclllation of small splits
and, of course, the hydraulic system has to be connected each time the mould is set up.
8.2.3 Splits locking method It is essential that the splits are held rigidly during the actual injection phase, as high pressures developed within the impression will tend to force them apart. To resist these pressures some means of locking the splits must be provided. Sliding splits can be conveniently locked in
278
SLIDING SPLITS
I
PART-SECTION
X-X
Figure 8.21- Hydraulic actuation of large splits (guide strips are not shown in this drawing, for reasons of clarity)
position by the use of a chase-bolster. When the mould is closed the chase-bolster encloses and clamps the splits (Figure 8.8). Each split will have a sloping or angled face accurately matching the complementary angled face of the chase-bolster. Where the cam method of actuation is/adopted this locking angle must be at least so greater than the cam operating angle to ensure the splits do not foul the chase bolster heels as the splits open. There are two basic designs of chase-bolster, namely the open-channel type and the enclosed-channel type. Both types are used for sliding and angle-lift split designs. However, as the basic mould assembly is different for each method the following discussion refers specifically to the sliding split design. The basic design of the chase-bolster is illustrated in Figure 8.22. It is made by machining a channel with angled
OPEN-CHANNEL CHASE-BOLSTER.
279
SPLITS
side across the width of a steel plate. The resultant projections on either side of the plate are termed locking heels. To resist wear the locking heels are faced with wear plates which are secured with socket headed screws as shown. The height of the locking heel (depth of the channel) should be at least ~ the depth of the split (Figure 8.23). Thus the gap between the chasebolster and the moving mould plate depends on the height of the split (compare drawings (a) and (b)). If we now refer back to Figure 8.3 we will note that with this particular design of split mould plate assembly the guides extend down either side of the mould plate and are accommodated in the gap between the mould plate and the chase-bolster. Therefore, if the depth of the split is less than a specified minimum value (e.g. 16 mm in) for a 11 mm (is in) deep guide strip) the chase-bolster design must be modified. Figure 8.24 shows the alternative design. The locking heels
a
,_
X
,.-
C>
C>
~
C>
' ' '
l1'.:' ~
:~+~' SECTION
,_j
X-X
X
Figure 8.22-0pen-channe/ chase-bolster
HOVING HOULO PLATE
(a)
SPLIT
CHASE-&oLSTER
(b)
Figure 8.23- Depth of locking heel of chase-bolster
280
SLIDING SPLITS
are reduced in width to permit the guides to straddle the locking heels when the mould is closed. The centre sectional view of the closed mould illustrates the position of the locking heel (shown dotted) in relation to the guides. This alternative design is not necessary when the guiding arrangement is mounted below the split. The open-channel design can be made as an assembly. The heel blocks are individually attached to a flat plate (Figure 8.25). Projections are incorporated on the underside of the heel blocks to resist applied forces. These projections accurately fit into recesses machined in the mould plate. This method of construction economises in steel though it is not as strong as the solid designs. ENCLOSED CHASE-BOLSTER. For deep splits, the enclosed chase-bolster design is preferred as it results in a more rigid structure than can be obtained with the open-channel design. The chase-bolster is made by machining a pocket, which may be of tapered circular or taper~d rectangular form, into a solid steel block (Figures 8.26 and 8.27 respectively).
Figure 8.24-0pen-channe/ chase-bolster design for shallow splits
Figure 8.25-0pen-channel design made as an assembly. Note method of fitting heel blocks
281
SPLITS
Figure 8.26-Tapered cylindrical enclosed chase-bolster
Figure 8.27- Tapered rectangular enclosed chase-bolster
An example of the tapered circular design is illustrated in Plate 8 which shows a sliding split mould for a jug top (the moulding is in the foreground). The left-hand view illustrates the fixed mould half which consists of the enclosed chase-bolster, the core (fixed half), dog-leg cams, guide pillars and lifting eyebolt. The moving half (right-hand view) consists of the tapered circular splits mounted on the moving mould plate being retained by the guides. Also visible are the core (moving half) and the guide bushes. The sprocket and chain are part of a split balancing mechanism. The circular chase-bolster design is adopted in this case as the component is of a general circular form and a single impression only is required. If two impressions are required, or if the component is more rectangular in shape, rectangular splits fitting into a complementaryshaped pocket will be used. From the standpoint of cost, the circular form is preferred as this design allows for the tapered pocket and for the splits (external form) to be produced by a simple turning operation. The alternative rectangular design necessitates expensive milling operations. It should be noted . however, that with the rectangular pocket design wear strips can be fitted, thereby allowing a medium carbon steel (BS 970-080 M40) to be
282
SLIDING SPLITS
used, whereas a more expensive case-hardening nickel-chrome steel (BS 970-835 M15) is required for the circular design as wear strips are not easy to fit.
8.2.4 Splits safety arrangements It is necessary to incorporate certain safety features in moulds which utilise the cam method of actuation. As noted previously, the splits are not in contact with the cams when the mould is fully open. This means that they may be accidentally moved out of alignment by shock, by vibration or even by gravity. If this occurs, the splits and cams will certainly be damaged as the mould is closing. To safeguard against being opened by gravity, the splits should operate horizontally with respect to the machine. It is advisable to stipulate that the top of the mould is so marked to prevent the mould being fitted incorrectly on the machine (Figure 8.28). To prevent movement of the splits by shock or vibration, the splits must be retained nominally in the open position. There are several methods from which to choose, two of which are now given.
A spring-loaded plunger is fitted below the surface of the split (Figure 8.29). When the split is open the plunger is engaged in a shallow conical depression which retains the split nominally in that position. The distance M between the plunger and the depression must equal the calculated movement of the split. The plunger is located on, or as close as possible to, the centre-line of the split (plan view); the exact position depends on whether one or two cams are used.
SPRING-DETENT METHOD.
SPRING-LOADED METHOD (1). The splits may be individually spring loaded so that after they are actuated they remain in the open position. The
STAAf' 00 MARK Ia' ()'
~LD·
Figure 8.28-Sp/irs saf.:ry arrangements. The splits should operate horizontally as shown (b) otherwise I hey will be affec/ed by gravity (a)
283
SPLITS DEPRESSION
PLUNGER
Figure 8.29-Spring-detent method (Splits safety arrangement)
SPRING
SPLIT
PIN
MOULD PLATE
Figure 8.30-Spring-/oaded method (I) (Splits safety arrangement)
design is shown is Figure 8.30. A stud, fitted to the underside of the split, is free to move in a recess in the mould plate. A spring is fitted, between the stud and the end of the slot. The strength of the spring must be such that it nominally holds the splits open. Too strong a spring will put unnecessary loading on the cams during closing. To ensure that the cam re-enters the split correctly, the distance M must equal the calculated movement of the split. (2). An alternative spring-loaded design which is suitable for the larger type of split is shown in Figure 8.31. A bolt (or
SPRING-LOADED METHOD
284
SLIDING SPLITS
~Sf!W(;
II li II
~OAn
Figure 8.31-Spring-/oaded method (2) (Splits safety arrangement)
bolts) is fitted to the locking angle of the split. The bolt extends through a plate securely attached to the mould plate. A spring is mounted between the plate and the head of the bolt. Here again, the strength of the spring must be such that it nominally holds the splits open, and it does not put undue strain on the cams during the closing stroke. A stop-pin is used to control the maximum movement of the split. The stop-pin is essential with this design, so that in the open position the hole in the split is in alignment with the operating cam, in preparation for the closing stroke. For shallow type splits (Figure 8.31 for example) it is necessary to provide grooves or cut-away portions in the locking heels to provide a suitable clearance space within which the bolts can be accommodated. For deep splits there is usually sufficient space between the top of the locking heels and the moving mould plate to incorporate the bolts without modifications to the locking heels being necessary. It is a good design practice to incorporate a positive stop for the outward movement of the split. As will have been noticed in the previous section, a positive stop is essential for springloaded designs.
STOP-PINS AND STOP-PLATES.
285
SPLITS
b
Figure 8.32-Stop-pin and stop plates
The positive stop may take the form of a stepped pin, termed a stoppin (Figure 8.32a) or small plate, termed a 'stop-plate' (Figures 8.32b and c). The stop-pin is a light drive fit in a suitable hole machined in the mould plate as illustrated in Figure 8.31. Note that the hole is drilled completely through the mould plate for extraction purposes. The stop-plate is drilled and counterbored to enable the plates to be attached to the mould plate by screws. As the screws are on the outside surface, the stop-plates are easily removed when required. Note that the stop-pins or stop-plates must be in the split's path in order to function. It is essential, therefore, that these positive stops are easily removable to enable the split to be withdrawn for cleaning and maintenance purposes. 8.2.5 Splits-stripper plate design
It is sometimes necessary to incorporate stripper plate ejection for components which also necessitate splits. In this design instead of mounting the splits on the moving mould plate, the splits are mounted on the stripper plate. To ensure a positive action a latching system is necessary, either to hold the stripper plate in contact with the fixed mould plate while the core is pulled, or to hold the stripper plate in contact with the moving plate while the splits are actuated. Both designs will now be discussed. It is often desirable to pull the core through the stripper plate before opening the splits. This system is useful where the ejection area is particularly small. By keeping the splits closed the projections in the splits form an extra ejection surface. The latching arrangement required for this system is shown in Figure 8.33d. A spring plunger box holds the latch against a stop. A stud fitted to the stripper plate engages in the latch. When the mould opens, the cam plate lifts the latch at a predetermined point, thereby releasing the stripper plate. The opening sequence of the mould is as shown: (a) The mould is in the closed position. (b) Actuation of the moving plate with respect to the closed split allows the core to be withdrawn from the moulding. CORE PULLED FIRST.
286
SLIDING SPLITS
(c) (o)
(b) MOULD PLATt
STRIPPER PLATE
CAM PLATE
(d) Figure 8.33-Cvre pulled first, technique
(c) Subsequent to the latch being lifted the stripper plate is pulled back by length bolts which are attached to the moving plate thereby causing the splits to be actuated. (The splits actuating method is not shown.) The basic mould opening sequence and the general latching arrangement for this system are shown in Figure 8.34. The purpose of the latch is to hold the stripper and moving plates together during the initial period of the mould opening stroke to allow the splits to be actuated. Subsequent movement of tile moving mould plate causes ejection of the mouldings. The latching arrangement is shown at (d). The latch is pivoted on a shouldered pin which is fitted in the back plate. The latch is held against a stop by a spring loaded plunger. A stud fitted to the stripper plate engages in the latch. When the mould is opened the stripper plate is cause to move back with the moving mould plate thereby causing the splits to be opened (b). After the cam plate lifts the latch, the stripper plate's movement is arrested by length bolts. Continued movement of the moving half causes the core to be withdrawn through the stripper plate (c), and the moulding is extracted from the mould.
SPLITS OPEN FIRST.
287
SPLITS
(c) (a)
LENGTH BOLTS
(b)
SHOULDERED PIN
HOVING HOULD PLATE
STRIPPER PLATE
CAM PLAT£
(d) Figure 8.34-Splits open first, technique
8.3 ANGLED-LIFT SPLITS In this design the splits are mounted in a chase-bolster which forms part of the moving half of the mould. The splits are caused to move out with an angular motion, the outward component of which relieves the undercut portion of the moulding. The splits are normally actuated by the ejector system. A typical design is shown in Figure 8.35. This shows the moving half of an angled-lift split mould for producing a spool. It will be noted from this illustration that the guiding of the angled-lift is not as critical as for guiding the sliding splits. The alignment of the splits, when closed, is accomplished by their being seated in the chase-bolster. The main requirement of the guiding system is that the split must be restrained to move smoothly in the required plane. A substantial chase-bolster, which may be of the enclosed or open channel designs (see Section 8.2.3), locks the splits against the applied injection force. When the open channel design is used it is necessary to provide some means of alignment between the two split halves as the ends of the splits are not located. Alignment is normally accomplished by incorporating dowels in one split which fit into holes in the other (Figure 8.36). 288
ANGLED-LIFf SPLITS
SPLITS OPEN
SPLITS CLOSED
Figure 8.35- Angled-lift splits
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SECTION
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Figure 8.36- Alignment between angled-lift split halves
To simplify the discussion of sliding splits we separated the guiding, operating and locking functions and discussed these separately. However, with the angled-lift design we cannot easily separate these functions as they are closely interrelated. We will therefore discuss the various alternative systems with respect to particular actuating methods as follows: (i) Angled guide dowel actuating system. (ii) Cam track actuating system. (iii) Spring actuation. 8.3.1 Angled guide dowel actuating system A typical design of a mould of this type is illustrated in Figure 8.37. The
mould is shown closed (a) and fully open (b). In this design the ejector system is utilised to open the splits as illustrated. 289
SPLITS EJECTOR SYSTEH
GUIDE DOWELS
CHASE-BOlSTER
WEAR STRIP
(a)
SPRING
(b)
Figure 8.37-Ang/ed guide-dowel actuating method: (a) mould closed; (b) mould open, splits actuated by ejector system
Guide dowels are fitted at an angle to the underside of each split. These guide dowels pass through holes machined at an angle, in the enclosed chase-bolster. These holes are often bushed as shown. When the ejector system is actuated, the relative movement between the ejector plate and the enclosed chase-bolster causes the guide dowels to be moved forward, and, as these are fitted at an angle, the splits are caused to open (b). 290
ANGLED-LIFT SPLITS
A convenient angle for the guide dowel is 10° but this may be increased if a large opening movement is required. The actual opening movement of each split may be computed from: M
=
E tan cj>
(8.12)
where E = effective ejector plate movement, cj> = guide dowel angle. An important design point to note is that there is a transverse movement of the guide-dowel head with respect to the ejector plate as this latter member is actuated. For this reason the head of each guide dowel is domed and the working surface of the ejector plate is hardened. For the same reason, in an alternative design (Figure 8.38) the face of the ejector plate is angled as shown, the operating face being at right angles to the centre line of the guide-dowel. This permits flat-headed guide dowels to be used, which gives a larger contact surface than for the previous design. The operating movement of each split with this design, using the same symbols as before, can be computed from M = E sin 2cj>/2
(8.13)
Further examples of design features which allow for the relative movement between the guide dowels and the ejector plate are given in Section 10.2.2. The angle of the locking chase must exceed the operating angle cj> by at least 5° to prevent the splits fouling the chase as they are opened. Wear strips (Figure 8.37) are normally incorporated on the working faces of the chase-bolster as shown. Immediately the mould closing stroke commences the spring around the ejector rod causes the ejector plate to return to its rear position. The splits are returned progressively to their nest by the action of the mould closing. However, to minimise the possibility of damage occurring either
Figure 8.38-Alternative design of ejector plate for use in conjunction with angled guide-dowel actuating method
291
SPLITS
to the face of the split or to the face of the fixed mould plate, springs may be fitted around each guide dowel (Figure 8.38) to nominally return the splits before the mould is finally closed. 8.3.2 Cam track actuating system The opening movement of the splits in this design is controlled by a cam track. When the splits are actuated, studs fitted to each end of the split slide along this cam track. Actuation of the splits is by means of a conventional pin ejector system. Let us consider a typical design. Figure 8.39 is a composite drawing the top and bottom halves of which show the splits before and after actuation, respectively. The splits are fitted into an open channel type chasebolster which incorporates wear plates. Studs, screwed into the splits, protrude into the cam track machined in the cam track plate (see also lower cross-sectional drawing through A-A). This cam track plate is
STUDS
CAH TRACK PLAT£
I
{c)
SECTION A-A
Figure 8.39-Cam track actuating method: (a) split shown seated in channel bolster; (b) split shown in forward, ejected, position; (c) part-section A-A to show stud, cam plate arrangements
292
ANGLED-LIFf SPLITS
securely attached to the chase-bolster. A conventional pin ejector system, including push-back pins (not shown), is fitted below the chase-bolster. When the ejector system is actuated the ejector pins press against the splits which then move out at an angle. Note that a gap is left between the top of the ejector pin and the bottom of the split when the ejector system is fully returned. This ensures that there is no possibility of the split being held off its seating in the chase-bolster by the pins. The number of ejector pins used depends upon the size of the split. For small and medium-sized moulds, two to four ejector pins may be used. The pins should be at least 13 mm 0 in) diameter and hardened. This is essential as when the splits are actuated they slide across the top surface of the pins. The opening movement of each split can be calculated from the following relationship:
M where E