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Injection Mold Design Flipbook PDF

Injection Mold Design

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10

Injection Mold Design

This chapter provides practical information for planning the construction of a thermoplastic injection production mold. It discusses the function of the molds, categories of molds, mold bases, types of steels used in molds, common types of mold designs, the effects caused by the product design on the injection molding process, mold speciﬁcations, mold design check lists, and the basic mold design principles. The thermoplastic injection molds may seem to be a very expensive piece of steel, with the cost of some molds higher than those for an injection molding machine. However in the long run, the mold represents only a very small fraction of the part cost. Good planning during the construction of the molds involves a lot of communication between the product designer, tooling engineer, process engineer, mold designer, mold builder, and resin supplier’s technical representative. The thermoplastic injection molding process obviously includes the use of the mold necessary to produce the end product. It is important to have some understanding of the product design, the thermoplastic resin, the injection molding process, and the technical information required to design and build the mold. What is a thermoplastic injection mold? Simply stated, it is a collection of steel plates and other mold components which, when properly assembled and installed within the injection molding machine, is capable of producing the required molded products from a given thermoplastic material. The majority of thermoplastic injection molds are made using high quality tool steel, designed and built by qualiﬁed mold makers, to exacting standards, capable of withstanding the high injection pressures and elevated temperatures of the process together with the usually fast cycles, which make injection molding an economically feasible manufacturing process. Thermoplastic injection molds require a sprue bushing to provide entry of the viscous melt into the cavity. Because the mold must be mounted into an injection molding machine, provision must be made for proper clamping of the mold to the machine platens and for the ejection of the molded part. Figure 10-1 shows a typical thermoplastic injection mold, together with the most common standard mold components.

10.1

Classiﬁcation of Injection Molds

The plastic industry classiﬁes thermoplastic injection molds in three general categories: prototype molds (25 to 1,000 parts), production molds (low volume from 1,000 to 10,000 parts) and high volume production molds (10,000 to 2,000,000 parts). Prototype Molds A prototype thermoplastic injection mold represents a preliminary step required in the development process of a new product. Prototype molding is used to investigate the molding characteristics of the resin, mold shrinkage, gating,

Figure 10-1 Typical mold for injection molding of thermoplastics

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10 Injection Mold Design dimensional control of the molded part, molding process conditions, and molding cycle. The prototype molded parts are used for product quality control tests and occasionally can be used to meet initial market testing requirements. In simulating the production part, prototype molds become a relatively inexpensive learning device that the part designer can use to pinpoint and correct potential product design problems or material selection problems before expending on a production mold. A prototype mold may consist of an existing mold frame, interchangeable soft cavity inserts, simple cooling and ejection systems, manual insert loading and removal. Production Molds A production mold is built using a standard low cost mold base for the hardened tool steel cavities; this mold should be able to produce parts on demand at established production rates. The mold should allow easy repair and facilitate mold venting of the cavities to allow entrapped air and melt volatiles to escape during the molding cycle. The production mold must also have an automatic ejection system and a mold temperature control for consistent cooling of the thermoplastic melt to ensure minimum cycle time, lowest cost, and consistent quality. High Volume Production Molds These types of molds should have all the qualities of production tools, have multiple cavities and have fully interchangeable mold components. A high volume production mold should be designed to overcome any adverse outside force, with easy maintenance design. For example, how often do you start to take a mold apart only to ﬁnd a multitude of inserts without numbering or position marking? How about pry slots? How about jack screw holes for cavity removal? These obstacles can be overcome through a good planning process for the design and construction of the mold, a customized prevention maintenance program, and a protective surface coating on the steel to prevent corrosion and erosion.

10.2

Effects of Product Design on the Injection Molding Process

The product designer must have knowledge of the part geometry, because it may create problems during the molding process; he must also be familiar with the properties of thermoplastic materials, the injection molding process in general, mold design, and the quality of mold construction and product design required to produce functional thermoplastic molded parts. Producing quality thermoplastic parts requires converting the functional requirement of the application into a design. The product design’s geometrical conﬁgurations should not only satisfy functionally, but it also has to meet the conditions required by mold design and construction and operation of the mold in order to produce quality parts and guarantee efﬁcient molding operation. When designing injection molded thermoplastic parts, the product designer must also be aware of some part conﬁgurations that pose potential problems during the injection molding process. The part design requirements include uniform wall thickness, parting line location to balance the heat removal from both sides of the cavities, smooth internal corners, draft walls (to facilitate part

10.2 Effects of Product Design on the Injection Molding Process removal from the cavity), elimination of feather edges, elimination of fragile deep pockets (long thin cores), provide location for the gate, allow large permissible surface area for ejection, specify typical part dimension tolerances for plastics, and avoid the use of high-gloss surface ﬁnishing for the product.

10.2.1

Uniform Wall Thickness

Products that incorporate abrupt changes in wall thickness will create major mold design problems regarding the temperature control system of the mold. Abrupt changes in wall thickness make it difﬁcult to maintain a uniform temperature throughout the mold cavities during the molding cycle. After the thermoplastic melt has been injected, variations in part wall thickness do not allow the walls to cool at consistent rates. Thick walls will shrink more than thin walls, causing part warpage, voids on thicker wall cross sections, poor dimensional control, long cycle times, poor surface ﬁnish, and structural defects.

10.2.2

Balance Geometrical Conﬁguration

The positioning of the cavity should be balanced on both sides of the mold parting line. Both halves of the cavity should be subjected to the same volume of polymer melt for uniform cooling in the mold cavity. If one side of the cavity is injected with more melt than the other side, this side will become hotter. The hotter side of the cavity will have the tendency to stick on the deep hot spots, causing warpage, poor surface ﬁnish of the molded part, and long cycle times.

10.2.3

Smooth Internal Sharp Corners

Sharp corners create high stress concentrations on the thermoplastic part; they are also stress concentrators within the mold cavity. These sharp corner areas fail under high loads. Internal radii of at least 0.031 in should replace sharp corners in thermoplastic part design wherever possible. If a sharp corner is unavoidable, reduce the radii and polish this surface area; in addition, these mold cavity areas should be designed with removable inserts to facilitate ease of repair.

10.2.4

Draft Walls

Thermoplastic parts should be designed with positive draft walls. Minimum positive draft is required on all walls in the direction of mold opening or core pulling. Without draft, thermoplastic molded parts adhere to the mold cavity surface, causing drag marks and surface ﬁnishing defects. In many cases, the part will not be fully ejected so that the mold may close on it and cause damage. Lack of positive draft also increases cycle time and molding costs.

10.2.5

Feather Edges

Avoid the use of feather-shaped edges that require thin and fragile steel. Within the mold cavity, feather edges tend to break and chip, resulting in mold maintenance and downtime. Undetected broken and chipped feather edges will cause ﬂashing problems as the thermoplastic melt ﬁlls into the mold vents. Feather edges become extremely hot and take longer to cool because cooling

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10 Injection Mold Design water channels cannot be brought to the feather edge, thus increasing cycle time.

10.2.6

Proportional Boss Geometries

Avoid the use of long narrow cores. The height of the unsupported core should not exceed four times the core base thickness. During the molding process, the injection pressure will deﬂect long narrow cores, because they act as cantilever beams, causing parting line openings and possible early failure of the mold core insert. In critical cases, a structural analysis of the mold can be made based on expected forces and allowable deﬂection. Cores of greater height must be fully supported using core inserts to decrease the chance of failure and to ease repair.

10.2.7

Gate Type and Location

The gate is an important component in the injection molding process. The gate inﬂuences the type of mold needed for the application (two-plate, three-plate, hot runner, and automation). The location of the gate determines the mold shrinkage, the melt ﬂow, part dimension, warpage, and weld line strength. The gate functions as a thermo-valve between the runner and the cavity. The temperature is increased around the gate area by the melt injection speed, pressure, and temperature. The hot gate allows the melt to enter the cavity without shearing off the polymer; the gate cools off when the melt stops moving, closing the gate while the melt inside the cavity cools off under packing pressure.

10.2.8

Molded Product Ejection Surface Area

The mold ejection system automatically provides a uniform force to extract the molded product from the cavities. The ejection force breaks the vacuum between the internal surface wall of the part and the cavity core and ejects the parts from the mold. The ejection surface area of the product should be located in the direction of the moving half of the mold, where the ejector system is generally placed. The product designer should specify large ejection surface areas with heavy cross sections that are not critical for the functionality of the product. The type of ejection system depends on the molded product’s geometrical conﬁguration and the permissible ejection surface area wall thickness, the stiffness, crystallinity rate, and melt temperature of the thermoplastic resin. For example, punctuation holes or indentation defect marks on the external surface of a molded product may be produced by small diameter ejector pins pushing against a ﬂexible and thin-walled cross section of a thermoplastic molded product during the ejection cycle.

10.2.9

Molded Product Tolerances

A realistic view of the cost of tolerances often helps avoid high molding costs without affecting the performance of the part. It may be unreasonable to specify close production tolerances on a part when it is designed to operate within a wide range of environmental conditions. Temperature-induced dimensional changes alone can be three to four times as great as the speciﬁed tolerances.

10.3 Effects of Mold Design on the Injection Molding Process The tolerances for injection molded thermoplastic parts have been developed by the plastic molding industry. The purpose of these speciﬁcations is to assist the part designer in obtaining a quick and preliminary analysis for the different molding tolerance factors found in a generic injection molded thermoplastic part. Understanding the limitations of this process and knowing how to control ﬁne tolerances are the result of applying thermoplastic part design principles, working with molds and mold designers, being aware of the governing rules in polymer technology, and applying the latest technologies available for the injection molding process.

10.2.10 Surface Finish of Molded Product Surface ﬁnish affects part quality, mold cost, mold cycle, and delivery time. Surface ﬁnishing is used to enhance surface clarity for appearance of the molded product. The standard steel ﬁnishes range from a number one (mirror ﬁnish) to a number six (grit blast ﬁnish). Any ﬁnish speciﬁcations on the part print must reference the molded product and not the mold itself. Specifying the mold surface ﬁnish does not necessarily produce the expected result on the ﬁnished molded product. Although a requirement for a part with a high-gloss ﬁnish requires a high-gloss ﬁnish on the mold cavity, other factors, such as resin, gating, melt and mold temperature, injection speed, and mold venting affect the surface ﬁnishing of the part. For extremely high-gloss ﬁnishing, the types of steel used in the cavities may need to be speciﬁed to ensure reasonable life of the polished cavity in production.

10.3

Effects of Mold Design on the Injection Molding Process

Mold design is an important consideration in the injection molding process, the following are general concepts for mold design, more detailed information is presented later in this chapter.

10.3.1

Runner System

Good runner design includes not only the correct geometry, size, and layout of the runner, but cooling, ejectability, and minimizing regrinds. A balanced runner system is required for ﬁlling all cavities at the same time; this minimizes cycle time and gives the best dimensional integrity to the molded product. Long and skinny or half-moon runners require higher injection pressures so the mold does not cool off prematurely causing incomplete parts. Long and thick runners increase the amount of regrinds, decreasing the efﬁciency of the molding process. Ejector pins should be provided at the intersection of cold runners to provide sufﬁcient force to eject the runner. Runners should be placed on the ejector half of the mold, so that they can be pushed out by the ejector system.

10.3.2

Mold Cooling System

Mold cooling is one of the most important parameters for controlling dimensional integrity, physical properties, surface ﬁnishing, warpage, weld line strength,

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10 Injection Mold Design and cycle time. A series of cooling channels along the length of a long cavity provides poor warpage control. All core pins should be cooled, especially if the projected length to core diameter ratio exceeds four. Hot core pins cause surface defects and longer molding cycles. A cooled pin is a more efﬁcient system for extracting heat from the encapsulated surface area. Using water in contact with pins is far better than transferring heat across an air layer. Flexible resins need the ejector pins to be cooled for ejection. Temperature control of the sprue puller pin area reduces mold cycle time and ejection interruptions. High volume and turbulent ﬂow of the cooling liquid is critical to maintain good temperature control in the mold. To control the corrosion inside the water lines stainless steel mold plates are used; Corrosion can also be controlled by plating the cooling channel or by the use of rust inhibitors in the water. The mold plates should be thick enough to provide room for the proper size of the cooling channels.

10.3.3

Ejector System

Uniform ejection is critical to control part warpage. Ejector pins, sleeves, rings, or plates must operate without obstructions. A guided ejector system allows ejector pins and cores to be precision aligned and will also bear the weight of the plates so that the pins do not wear and misalign. Early return systems should be provided as a safety feature. They drive all ejector pins to their seated positions before the mold closes, which could collide with pins that did not fully retract. A protector pin should be provided on all molds where the ejector pins or sleeves are under any slides. This locks the ejection plate in its retracted position to prevent collisions between ejector components and slides. For ﬂexible, thinwalled, deep and box parts that are difﬁcult to eject, special ejection systems should be used.

10.3.4

Mold Venting

Mold venting of cavities and runner systems to remove the entrapped air and polymer melt volatiles is a must. A mold without venting causes many processing problems.

10.3.5

Other Mold Devices

Other mold design devices can also increase the cost of the mold, as well as create difﬁculty in injection molding the thermoplastic product. One example is the mechanically activated side cores or slides. Although sometimes necessary, side cores are expensive mold components and will have high maintenance costs.

10.4

Design Considerations for Injection Molds

The main parameters for the design of a thermoplastic injection mold are: type, size, number of cavities, tolerances, runner layout, gating, venting, parting line, ejection system, surface ﬁnishing, steel hardness, and mold cooling among others. The geometrical design of an injection mold, the type of resin, the dimensional tolerances of the product, the part quality, and the volume of part production inﬂuence the selection of the steel for the mold because of considerations involving cavity forming and difﬁculties in heat treatment. Too great a difference in the wall thicknesses of mold cavity inserts necessitates greater care during heat

10.4 Design Considerations for Injection Molds treatment, because the time to heat the thickest wall section uniformly may result in over heating the thinnest wall section. Quenching from high temperatures, cracking, or distortion may occur where a thin section adjoins a thick section. Injection mold design is also important from the standpoint of service performance. If mold walls are made too thin, excessive elastic deﬂection and even cracking can result from service stresses. Overcoming excessive mold deﬂection that results in ﬂashing problems, dimensional control problems, poor part surface ﬁnishing, and incomplete molded parts can only be accomplished by increasing the wall section size of the mold. It is of extreme importance that any injection mold be designed and built by a qualiﬁed mold maker to the exacting standards demanded by the industry. Typical mold development procedures are as follows: • The product design has been completed and approved for molding • Mold planning process, where the product designer, tooling engineer, process engineer, mold designer, mold builder, and resin supplier’s technical representative review the needs of the product and provide recommendations for the design and construction of the mold • Development of a preliminary mold design proposal is the responsibility of the mold designer • Review of the mold layout proposal by the product designer, tooling engineer, process engineer, mold designer, and purchasing, recommending mold changes if needed and providing authorization to ﬁnalize the mold design • Completing the mold design details • Reviewing the ﬁnal mold design by the product designer, tooling engineer, process engineer, mold designer, and mold builder • Construction of the mold • First molding run evaluation to produce samples at the mold maker’s shop, end user tool engineer and process engineer should be present. • Inspection and documentation of all product samples whether they meet print dimensions (end user) • Debugging of the mold if needed • Approval of molded products and pre-production run (mold maker’s shop) • Surface treatment and polishing of the cavities and cores.

10.4.1

Preliminary Mold Design

Preliminary mold design begins from the product dimensional drawing. This represents the part geometry and required dimensions after the thermoplastic injection molding process, when the molded part has completed the mold shrinkage process and has been stabilized at room temperature for at least 24 hours. The preliminary mold layout requirements are the following: • Mold base size, thickness of plates, and ejection traveling distance • Placement, number, and size of cavity insert blocks

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10 Injection Mold Design • Location, type, and size of water cooling systems • Runner layout and type of gates • Position and number of support pillars • Ejector pins sizes and locations • Parting lines layout • Position of injection machine tie bars • Injection machine stroke and shut height positions • Automation requirements • Hot runner mold feasibility After a review of the preliminary mold layout proposal, authorization is given for detailed mold design.

10.4.2

Detailed Mold Design

The cavity details are designed in accordance with the part geometry and the thermoplastic injection mold design, based on good mold manufacturing practices. The critical ﬁnal mold design requirements include: general mold layout, runner and gate system, cooling system, ejector system, cavity ﬁnishing and venting. General Mold Layout • The mold base speciﬁcations for optimizing the injection molding machine cost rates and cycle times • Weak, delicate cavities that wear should be designed with a cavity block for strength and cooling using segmented cavity inserts so that mold maintenance costs can be optimized • In a multi-cavity mold, the cavities should be positioned to balance the system, reducing the runner length. • Allow between 0.003 and 0.005 in projected height beyond the cavity plates on each cavity half, so that good contact of the cavity halves can be made under clamping pressure. • Eye bolt holes should be provided on both the top and back side of each half of the mold to ease mounting the mold into the injection molding machine • The appropriate type steels should be used for each mold component based on the function, machinability, dimensional stability, tolerances, wear resistance, toughness, and strength required for the mold • Mold components such as sprue bushing, core pins, inserts, and cavities should be keyed to prevent incorrect placement in the mold; movement of these components could cause problems in the process. • Mold mounting/removal should be easy so that the mold setup time can be minimized • Cavity inserts should be thick enough with smooth radii so that they are strong enough to survive during molding production

10.5 Types of Steels Required for Injection Molds • One leader pin and one return pin on the ejector system should be offset to prevent a 180° reversal of the mold halves’ orientation • Mold plates should be strong enough and thick enough for the cavity insert block size and cooling system • Sufﬁcient support pillars should not interfere with the ejector pins. Location under the cavity is the primary position to avoid deﬂections of the cavity during the injection of the melt inside the cavity • The ejector system should be designed so that travel is sufﬁcient. Too much travel increases the cycle time, wear of the ejector pins and sleeves. Sufﬁcient travel is such that the part can fall free from the mold • The mold must be vented properly to allow entrapped air and melt volatiles to escape during injection. Otherwise, the steel of the cavity’s deep pockets becomes corroded. The vent channels should be properly located to optimize venting without allowing ﬂash. The dimensions of the vents are determined by the viscosity of the thermoplastic material used in the molding process • The ﬁxed half side of the mold (top clamping plate and “A” cavity plate) need to be thick enough to support leader pins and to provide room for the water cooling lines and other needs.

10.5

Types of Steels Required for Injection Molds

This section provides information on different steel compositions available to manufacture molds used to produce thermoplastic injection molded products. These steels constitute the most commonly used types of materials in the construction mold industry. Other materials such as beryllium-copper, cast aluminum alloy, forged aluminum alloy, cobalt-nickel alloy, and kirksite are also used, but to a lesser extent. Steels are the workhorse of materials used in molds. No other materials offer comparable versatility for product applications. Steels are produced in the greatest variety of forms and ﬁnishes, have strengths ranging from 30,000 psi to over 300,000 psi, and can withstand a range of temperatures from cryogenic up to 2,000 °F.

10.5.1

Major Steel Families

Because of the great range of steel types, properties and applications, steels are categorized into many families based on the chemical composition, heat treatment, surface ﬁnishing, critical properties (mechanical, thermal, corrosion resistance, electrical, etc.), typical processing characteristics, end use applications, and other factors. The major families of steel are the following: Low Carbon Steels SAE 1008, 1010, 1015, and 1025 are the lowest carbon steels selected when cold forming ability is the primary prerequisite. These steels have relatively low tensile strength values. Strength and hardness increase with carbon addition and/or with cold work, but a decrease in toughness or the ability to withstand

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10 Injection Mold Design cold deformation is created. Low carbon steels are nearly pure iron in structure, they are readily welded, but do not machine freely, producing poor smooth ﬁnishes. SAE 1016 to 1025 provide increased strength and hardness and reduced cold forming ability. Carburizing or case hardening is possible in some grades. Increase in carbon gives greater core hardness in thicker sections. Increase in manganese improves the hardening ability and also improves machining. SAE 1025 is used for larger sections or where greater core hardness is needed. All of these steels may be readily welded or brazed. SAE 1020 is frequently used for welded tubing. These steels are used for forged parts; they usually machine better in the “as forged” condition without annealing, or after normalizing. Medium Carbon Steels The medium carbon steels, SAE 1030 to 1052, are selected for uses where higher mechanical properties are needed and frequent further hardening and strengthening by heat treatment or by cold work is done. The carbon and manganese levels selected increase the mechanical properties required in section thickness or in depth of hardening. The heat treatment preferred for any of the grades over 0.30% carbon allows selective hardening by induction or ﬂame methods. All of these groups of steels are used for forging, the section being governed by the section size and the physical properties desired after heat treatment. Medium carbon steels are popular for forging and general uses requiring greater strength than low carbon steels. High Carbon Steels Steels SAE 1055 to 1095 are the high carbon types, having more carbon than is required to achieve maximum as quenched hardness. They are used for applications where the higher carbon is needed to improve wear resistance, higher strength characteristics for cutting edges, for springs, and for special purposes. Selection of a particular grade is affected by the nature of the part, its end use, and the manufacturing methods available. Cold forming is not always suitable and most parts are heat treated before use. Free Machining Carbon Steels Low and medium carbon steels, in which sulfur, sulfur-phosphorus combinations, and/or lead are purposely added to improve machinability, are termed free-machining carbon steels. Their designations are SAE 1108 to 1151 for resulfurized grades and SAE 1211 to 1215 for rephosphorized and resulfurized grades. Leaded grades are indicated by the letter “L” with the number. Sulfur and phosphorus additions result in some sacriﬁce in cold forming ability, weldability, and forging ability. Lead additions have little effect on forming ability and forging ability, but impair weldability. Carburizing Carbon Steels This term is sometimes given to standard carbon steels, primarily low carbon grades, which are case hardened by various carburizing methods.

10.5 Types of Steels Required for Injection Molds Hardening Carbon Steels These heat treating grades are medium and high carbon steels, hardened by heat treatment before use. They also are referred to as water or oil hardening grades, depending on the quenching media used during heat treatment. Carbon Spring Steels This is a common term for medium and high carbon steels (SAE 1050 to 1095) used in spring applications. Annealed and pretempered strips and wires are the common conditions and forms. These steels also are used for music and piano wire, rope wire, and saw blades. Low Temperature Carbon Steels These are low carbon (0.20 to 0.30%), high manganese (0.70 to 1.60%), silicon (0.15 to 0.60%) steels produced to a ﬁne grained structure with uniform carbide dispersion achieved through careful composition control and heat treatment. The steels feature relatively high strength and toughness combinations with ductility transition temperatures as low as 130 °F. High Strength Low Alloy Steels These are low carbon, manganese steels containing alloying elements, such as chromium, columbium, copper, molybdenum, nickel, titanium, vanadium, and zirconium. Their mechanical properties and corrosion resistance are superior to the more widely used structural carbon steels. These steels usually are used without heat treatment although annealing, normalizing, and stress relieving is required. Carburizing Alloy Steels These steels, which include all the low carbon standard alloy steels, are widely used for carburized parts. Low alloy grades (e.g., AISI 4023, 4118, 5015) are used for parts requiring better core properties than are obtainable from carburizing grades of carbon steels. The higher alloy grades (e.g., AISI 3120, 4320, 5120, 8620) are used for better case and core properties. Steel selection among the higher alloy grades depends primarily on hardening ability required to obtain the desired set of properties for the speciﬁc conditions of size and heat requirements. Alloy steels, direct hardening grades quenched and tempered to speciﬁc strength and toughness levels, include most of the standard AISI and SAE alloy steels (13XX, 31XX, 40XX, 41XX, 43XX, 46XX, 50XX, 51XX, 5XXX, 61XX, 86XX, 87XX, 92XX, 98XX); they are by far the most widely used alloy steels. They are also classiﬁed by SAE by the carbon content and as low, medium, and high hardening ability grades. Other designations include water or oil hardening types, or simply quenched and tempered steels. H-Alloy Steels They are standard direct hardening alloy steels that meet speciﬁc hardening ability limits as determined by end quench tests. The steels carry the letter “H” following their conventional numerical designations (e.g., AISI 3120H, 4340H, 8740H).

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10 Injection Mold Design Boron Alloy Steels Boron steels are direct hardening alloy steels containing very small amounts of boron to intensify hardening ability. They are also H-alloy steels and carry the letter “B” after the ﬁrst two digits of their designations (e.g., AISI 10B18, 50B46, 94B17). Nitriding Steels These are low and medium carbon/chromium, chromium/molybdenum, or chromium/aluminum steels that are case hardened by nitriding. They are referred to as low carbon, quenched and tempered, constructional alloy steels to distinguish them from the higher carbon quenched and tempered constructional alloy steels. They are noted for high strength, toughness, and weldability and for greater corrosion resistance than structural carbon steels. Their carbon content ranges from 0.10 to 0.20%, the typical alloying elements are chromium, molybdenum, nickel, copper, vanadium, titanium, boron, and zirconium. Low Temperature Alloy Steels These are primarily low carbon, nickel steels of relatively high strength and very high toughness at temperatures of 75 to 320 °F. The three most common grades have a maximum carbon content of 0.13 or 0.20% and nominal nickel contents of 2.25, 3.50 and 9%. Tool Steels Tool steels are medium and high carbon alloy steels noted primarily for their high hardness, abrasion resistance, and resistance to softening at elevated temperatures. Low carbon and relatively low alloy grades also are available. Tool steels are further classiﬁed as prehardened, carburizing, water, oil, and air hardening grades, hot and cold work grades, high speed grades, shock resisting grades, and by other designations. Despite their name, their primary use is for injection mold components and for non-tooling applications. Electrical Steels Electrical steels are alloy steels containing from 0.50 to 5.0% silicon and featuring relatively high permeability, high electrical resistance, and low hysteresis loss. They also are classiﬁed as grain-oriented and non-oriented types with the latter type further classiﬁed as low, medium, and high silicon grades. AISI designates the steels by the preﬁx letter “M” (for magnetic material) followed by a number that originally was approximately equal to 10 times the core loss in watts/seconds (for 29 gage sheet at 15 kilogausses and 60 cycles). Stainless Steels These are alloy steels that have superior corrosion resistance than carbon and conventional alloy steels as a result of moderate to high additions of chromium. Most grades also are noted for heat and oxidation resistance and some stainless steels have very high strength. These materials can be grouped into six major types: austenitic, martensitic, ferritic, age-hardenable austenitic, age-hardenable semi-austenitic, and age-hardenable martensitic. • Austenitic stainless steels 200 and 300 series designations are non-magnetic, non-hardenable by heat treatment

10.6 Steels for Thermoplastic Injection Molds • Ferritic stainless steels 400 series are magnetic and non-hardenable by heat treatment. • Martensitic stainless steels 400 and 500 series designations are magnetic, can be hardened by quenching and tempering. • Age-hardenable austenitic stainless steels (e.g., A-286) are strengthened by heating at moderate temperatures. • Semi-austenitic stainless steels (e.g., 17-7PH) are austenitic in the annealed condition and martensitic in the hardened condition. • Age-hardenable martensitic stainless steels (e.g., 17-4PH) are strengthened by aging reactions during tempering. High Temperature Steels Steels for high temperature service can be put into two categories: those primarily used for heat or oxidation resistance and those used for structural requirements. The most common heat resistant steels are the austenitic stainless steels that can be used up to about 2,000 °F. However, numerous other steels, such as the martensitic stainless steels, aluminized, and chromized carbon steels also fall into this category. High temperature (700 to 1,200 °F) structural steels include low alloy martensitic steels, Cr-Mo-V medium alloy air hardening steels, martensitic stainless steels, precipitation hardening stainless steels, and Cr-Ni-Mo alloy steels. Both categories of high temperature steels are classiﬁed under the AISI 600 series of high temperature and high strength alloys. Ultra High Strength Steels These are the highest strength steels produced. The designation is somewhat arbitrary, but generally refers to steels having yield strengths above 160,000 psi. The major types of ultra high strength steels are: medium carbon, low alloy, quenched, and tempered steels such as AISI 4130 and AISI 4340, 5Cr-Mo-V medium alloy air hardening steels, martensitic stainless steels, cold rolled austenitic stainless steels, precipitation hardening stainless steels, 12 and 18% nickel maraging steels, and 9Ni-4Co quenched and tempered steels.

10.6

Steels for Thermoplastic Injection Molds

The selection of a thermoplastic injection mold material depends on a number of factors, including such considerations as product design (geometry, part dimensional control, and tolerances), application end use requirements, thermoplastic material, number of mold cavities, expected production life of the mold, injection molding process conditions, and mold design and construction. The particular usefulness of steels is based on the unique combination of properties achievable through alloying (chemical composition) and uniform heat treatment process. Speciﬁc properties of steels that can be modiﬁed to achieve desirable characteristics are machinability, polishing ability, surface hardness and depth hardening, wear resistance, corrosion resistance, dimensional stability, thermal conductivity, mechanical strength, toughness, and cost. No other class of mold material is as versatile as steel.

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10 Injection Mold Design The main factors to be considered in selecting the optimum steels for making a thermoplastic injection mold are: • Product design geometry, tolerances, end use requirements • Molded product manufacturing costs • Type of thermoplastic (properties and process characteristics) • Injection molding process conditions • Number of molded parts to be produced with the mold • Surface ﬁnish of the molded parts • Number of cavities and mold size • Mold cooling and venting design • Type and size of runner layout (cold, balanced, or hot runner) • Geometry, location, and number of gates (two plate, three plate mold, or hot runner drops) • Cavity forming methods (machined, hobbing, EDM) composed of several assembled inserts or a single unit • Heat treatment method.

10.6.1

General Steel Selection Procedures

The following recommended tools are used to assist in the selection of optimum steels for thermoplastic injection mold components: • Material selection guide publication by the American Society for Metals (ASM) Metals Handbook, Volume 1, “Materials for Plastic Molds” • Technical information provided in this section, “Types of Steels Required for Injection Molds” • Comparative tested properties provided by steel suppliers design handbooks of candidate steel compositions • Mold designers’ and mold manufacturers’ recommendations of the types of steels with proven performance in similar injection mold applications and the years of experience working with these materials. The principal kinds of steel used in making thermoplastic injection molds are selected by their chemical composition, nitriding, prehardened, carbonizing, oil hardening, air hardening, and stainless properties. The principal methods used for forming cavities in steel molds are conventional machining, hobbing, and electrical discharge machining (EDM). Heat treatment is part of the mold making process, unless a prehardened type of steel is selected. Finishing of the mold surface cavity is usually accomplished by grinding and polishing. It is expected that the information provided in this section will help in selecting a suitable mold steel and mold making process for economic production of a thermoplastic injection molded part. Such decisions are generally considered to be the responsibility of mold designers, although they may receive assistance from others such as mold makers, product designers, process engineers, tool engineers, and resin supplier technical engineers.

10.6 Steels for Thermoplastic Injection Molds

10.6.2

Properties and Characteristics of Tool Steels

Uniformity of mechanical properties in the annealed as well as heat treated condition is considered important for tool steels. This requires attainment of a uniform micro structure throughout the mold component section after annealing and after heat treating, or, in a case hardened zone, after carbonizing and heat treating. Good machinability results from a uniform micro structure together with a relatively low hardness level in the annealed condition. There is a direct correlation between annealed hardness and hobbability. Working with soft steels, hobbability improves dramatically as hardness decreases. A uniform micro structure also facilitates machining steels in the prehardened condition. Low distortion in heat treatment is dependent on uniformity of the annealed micro structure, chemical composition, slow heating, and a controlled quenching rate. Good polishing ability is dependent on steel quality, hardness, and the type and uniformity of micro structure of the hardened cavity surface. Abrasion resistance, including the ability to retain a satisfactory polished surface under service conditions, depends on hardness level and micro structure. The presence of ﬁne excess alloy carbides in a high carbon martensitic matrix results in high abrasion resistance and good polishing ability. Higher corrosion resistance is attained with stainless mold steels or by chrome or nickel plating of the nonstainless steels. Commercially available high quality tool steel, alloy steels, and stainless are used in the construction of thermoplastic injection molds. Extensive precautions are generally taken in melting, processing, and inspection to prevent the occurrence of defective conditions. For example, using ultrasonic quality control inspections to detect defects is routine practice in the steel making process.

10.6.3

Effects of Alloying Elements on Tool Steel Properties

The steels used for thermoplastic injection molds range in composition from the relatively low alloy oil hardening types containing less than 0.50% total alloying elements through the medium alloy air hardening types up to the most highly alloyed steels that contain up to 21.00% of alloying elements by weight. Alloying elements are added to steels to achieve certain desirable properties or characteristics that would otherwise be unattainable. Some alloying elements, either alone or in combination, enable heat treatments to be carried out which alter the micro structure providing the desirable tool steel properties. The addition of more than one alloying element to a steel often produces a synergistic effect. Thus, the combined effects of two or more alloying elements may be greater than the sum of the individual effects of each alloying element.

10.6.4

Chemical Composition of Steels Used for Molds

Typical chemical compositions of commonly used steels used to fabricate the mold components are given in Table 10-1.

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10 Injection Mold Design Table 10-1 Chemical Composition of Steels Used for Molds

Steel Type

C

Mn

Si

Cr

Mo

V

P

S

Ni

Al

SAE-1015

0.15

0.30

–

–

–

–

0.04

0.05

–

–

SAE-1018

0.18

0.60

–

–

–

–

0.04

0.05

–

–

SAE-1020

0.20

0.90

0.20

–

–

–

0.04

0.05

–

–

SAE-1030

0.30

0.75

0.25

–

–

–

0.04

0.05

–

–

AISI-4130

0.30

0.48

0.20

0.95

0.20

–

0.15

0.15

–

–

AISI-4140

0.43

0.75

0.15

0.80

0.15

–

0.15

0.15

–

–

AISI-4150

0.50

0.90

0.30

0.95

0.20

–

–

–

–

–

SAE-6145

0.45

0.70

0.20

0.80

–

0.15

0.04

0.04

–

–

SAE-A2

1.00

0.70

0.30

5.00

1.00

–

–

–

–

–

SAE-A4

1.00

2.00

0.35

1.00

1.00

–

–

–

–

–

SAE-A6

0.70

2.00

0.30

1.00

1.25

–

–

–

–

–

SAE-D2

1.50

0.30

0.30 12.00 1.00

1.00

–

–

–

–

SAE-H13

0.35

0.40

1.10

5.00

1.50

1.10

–

–

–

–

SAE-O2

0.90

1.60

0.25

–

–

–

–

–

–

–

SAE-P2

0.07

0.30

0.15

2.00

0.20

–

–

–

0.50

–

SAE-P6

0.10

0.50

0.25

1.50

–

–

–

–

3.50

–

SAE-P20

0.35

0.90

0.50

1.70

0.40

–

–

–

–

–

SAE-P21

0.20

0.30

0.30

0.25

–

0.20

–

–

0.40

1.20

SAE-S7

0.50

0.70

0.25

3.25

1.40

–

–

–

–

–

Stainless 410

0.15

1.00

1.00 12.25

–

–

–

–

–

–

Stainless 420

0.15

1.00

1.00 13.00

–

–

–

–

–

–

Stainless 440C

1.05

1.00

1.00 17.00 0.75

–

–

–

–

–

10.6.5

Effects of Alloying on Tool Steels

The contribution of alloying elements on the mechanical properties and the characteristics of tool steels can be summarized as follows: Carbon (C) Carbon is the most influential element in controlling hardness, depth of hardening, and strength. Raising the carbon content by different amounts will increase the hardness, depth of hardness, strength, and abrasion resistance achievable after heat treatment and reduce ductility and toughness. Carbon combines with the carbide forming elements (Fe, Mn, Si, Cr, Mo, W, V, P, S, Ni and Al) to produce hard carbide particles that contribute signiﬁcantly to wear resistance. The amount of carbon in tool steels is speciﬁed for attaining certain properties (such as in the water hardening category, where higher carbon content may be chosen to improve wear resistance, although to the detriment of toughness) or, in the alloyed types of tool steels, in conformance with the other constituents for producing well balanced metallurgical and performance properties.

10.6 Steels for Thermoplastic Injection Molds Manganese (Mn) The principal function of manganese is to combine with free sulfur to form discrete sulﬁde inclusions and thus to improve hot working ability. Manganese is also a deoxidizing agent. In smaller amounts (0.60%), manganese is added for reducing brittleness and to improve forging ability. Larger amounts of manganese improve hardening ability, permitting oil quenching for non-alloyed carbon steels, thus reducing deformation and toughness. Molybdenum (Mo) Molybdenum is a strong promoter of certain metallurgical properties of alloy steels, such as deep hardening and toughness and is used for this purpose in most of the thermoplastic injection mold steels. Molybdenum contributes to secondary hardening on tempering when added in amounts from 0.20% to 1.50% or higher. Secondary hardening enables exceptionally high tempering temperatures to be used to obtain a given hardness and strength level. The higher tempering temperatures increase elevated temperature stability and strength and result in more complete relief of residual stresses for greater dimensional stability. It is used often in larger amounts in certain high speed tool steels to replace tungsten, primarily for economic reasons, often with nearly equivalent results. Vanadium (V) This element contributes to the reﬁnement of the carbide structure, improving the forging ability of alloy tool steels. Vanadium has a very strong tendency to form a hard carbide, which improves both the hardness and the wear properties of tool steels; however, a large amount of vanadium carbide makes grinding very difﬁcult.Vanadium is a relatively expensive alloying element and a strong carbide former that is usually added to control grain size and to increase wear resistance. Vanadium combined with carbon produces one of the hardest alloy steels. Aluminum (Al) Aluminum combines with nickel to form an intermetallic alloy steel where its hardness is controlled by the cooling rate. Aluminum is used to produce tool steels such as SAE-P21 (thermoplastic injection mold steel). Aluminum and manganese are the principal elements contributing to hardening ability so that full heat treated hardness can be obtained by air cooling. Distortion during heat treatment is lessened as the required quenching rate is reduced. Silicon (Si) Silicon is used in most tool steels in quantities from 0.15% to 1.10%. The principal function of silicon is as a deoxidizing agent during melting and to improve the hot forming properties of the steel. In combination with certain alloying elements, the silicon content is raised for increasing the strength and toughness of steels used for tools which have to sustain shock loads. Silicon increases hardening ability slightly and, in higher quantities, retards tempering reactions, thus allowing the use of higher tempering and operating temperatures. Chromium (Cr) Chromium is a carbide forming element with a dual function. When present as carbides, all or some may dissolve on hardening and reprecipitate on tempering, it contributes strongly to hardening ability, abrasion resistance, and toughness. When additional chromium, more than what can be utilized by the carbon to

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10 Injection Mold Design form carbides, is added, this chromium remains in solution and contributes to corrosion resistance. However, high chromium levels raise the hardening temperature of the tool steel, causing hardening deformation problems. A high percentage of chromium also affects the grinding ability of the tool steel. Nickel (Ni) Nickel in combination with other alloying elements is used to improve the toughness and the wear resistance of tool steels. Nickel is added to increase the hardening ability of alloy steels. In SAE-P21 tool steel, nickel combines with aluminum to form an intermetallic compound on aging to increase hardness and strength. Large amounts of nickel are needed to ensure the formation of martensite without carbon. Because of its tendency to promote higher annealed hardness and lower machinability, nickel improves aging, providing strength and hardness. Tungsten (W) Tungsten is one of the important alloying elements of tool steels. Tungsten improves the hot hardness, or the resistance to the softening effect at elevated temperature; tungsten forms hard, abrasion resistant carbides, thus improving the wear resistant properties of the tool steels. Cobalt (Co) A cobalt alloying element is used in applications where an increase in hot hardness is needed for the tool steels. Substantial addition of cobalt, however, raises the critical quenching temperature of the tool steel with a tendency to increase the decarburization of the surface and the reduction in toughness.

10.6.6

Effects of Heat Treatment on Tool Steel Properties

Heat treatment of thermoplastic injection molds is carried out to achieve satisfactory combinations of abrasion resistance, strength, and toughness in both carburizing and deep hardening types of mold steels. When a mold is to be heat treated, precautions should be taken to prevent surface carburization or decarburization as well as distortion and cracking. The thermoplastic injection mold cavities need to be heat treated by methods that introduce residual stresses; such a condition may result in distortion during subsequent heat treatment. Therefore, a stress relief treatment at about 1,200 °F is recommended if considerable machining has been done. To allow for distortion on subsequent heat treatment, thermoplastic injection mold components are generally rough machined within 0.125 to 0.25 in of ﬁnal dimensions before stress relieving. By a “trueing up” machining operation after the stress relief treatment, more reliable allowances for distortion can be made to limit the amount of grinding necessary after heat treatment. The typical methods for heat treatment of thermoplastic injection mold components are the following: Flame Hardening This is probably the oldest method used to increase wear resistance of the steel materials. AISI 4140 steel is usually purchased by screw manufacturers in the heat treated condition. Normally, this condition is 28–32 Rc, which provides

10.6 Steels for Thermoplastic Injection Molds good mechanical strength of approx. 100,000 psi tensile yield. The steel is still readily machinable in this condition. The mold component can be further ﬂame hardened to approx. 48–55 Rc. Of course, this cannot be done with low carbon steels. Approximately 0.40% carbon content is needed to achieve this result on a practical basis. The process employs an open gas/oxygen ﬂame followed by rapid quenching. The usual depth of hardness is approx. 0.125 in, but the hardness tapers off as you go deeper from the outside. Induction Hardening This process gives the same result as ﬂame hardening, but uses induction heat created by magnetic ﬂux reversals rather than a ﬂame. Nitriding A hard outside case can be obtained by subjecting the mold components to a high nitrogen atmosphere (ammonia gas) at elevated temperatures of approx. 950 °F. This comparatively low temperature causes minimal distortion but provides a very high case hardness of 60–70 Rc. The depth of case ranges from 0.020 to 0.024 in. This thin case diminishes in hardness from the outside, causing the mold components to wear rapidly once any signiﬁcant wear has occurred. Allowance for 0.0005 to 0.001 in of growth must be considered when designing mold components before the nitriding process. The high hardness is caused by the formation of metallic nitrides. A proper nitriding steel, such as Crucible Nitriding 135 or Ryerson Nitralloy 135M should be used to develop maximum hardness from the process. These steels are similar to AISI 4140 in their chemistry, but they have 0.95% to 1.30% aluminum added to form very hard aluminum nitrides. Alloy steels, such as AISI 4140 can be nitrided, but they exhibit a slightly lower hardness with a little increased depth of case. Nitriding is usually done over the entire mold component. This improves wear resistance to abrasion by ﬁber glass or abrasive mineral reinforced thermoplastic resins. Ion nitriding is a process superior to gas nitriding, which is described above. Ion nitriding is more expensive, but causes less distortion because of lower processing temperatures. The hardness, depth of case, and wear properties are very similar in either process. Precipitation Hardening Precipitation hardening is a low temperature process used to harden certain grades of stainless steels. 17-4PH stainless steel is an example of this type, where the PH stands for precipitation hardening. These grades are usually supplied in condition “A” (solution treated), which is similar to being annealed. The stainless steel component is then machined and hard-surfaced before precipitation hardening. Heat Treatment Process Rapid heating of a thermoplastic injection mold component to the austenitizing temperature and inadequate support of the mold component in the furnace can result in unacceptable distortion. Bolting the mold component to a support can prevent sagging. A support may also be necessary to prevent distortion of a large mold component during its removal from a furnace before quenching. Since the greatest shape distortion generally occurs during quenching, the objective should be to cool all sections of the mold component at the same rate. A mold

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10 Injection Mold Design component with uniform wall thicknesses on all sides will cool more uniformly than a mold component with both thick and thin walled cross sections. It is desirable to use a quenching rate that is not much faster than necessary to achieve the desired degree of hardening. Lower distortion can be achieved by air hardening or precipitation hardening of the steel. In carrying out heat treating operations, such as stress relieving and austenitizing, the holding time at temperature is determined by the maximum cross section wall thickness of the mold component. Tempering temperatures should be selected to achieve a satisfactory combination of hardness and toughness. Shape distortions on tempering can be minimized by heating the mold component slowly to the tempering temperature starting with a cold furnace, tempering as required by the desired properties and cross section wall thickness, followed by furnace cooling. The use of a protective furnace atmosphere, a vacuum furnace, or salt bath is generally necessary to prevent excessive oxidation and carburization or decarburization during austenitizing that could adversely affect the surface ﬁnish and surface properties. Deterioration and nonuniformity may also result from failure to clean all the oil or grease from a mold component surface before hardening by using carburizing compound for case hardening or by not removing any copper that may be on the surface.

10.6.7

Prehardened Tool Steels

The most commonly used prehardened steels for making thermoplastic injection mold components are SAE P20, P21 and other medium carbon low alloy grades of suitable quality. Prehardened SAE H13 tool steel is available in rounds from a limited number of suppliers. Stainless steel 420 is also available in the prehardened condition. The use of prehardened steels eliminates the possibility of cracking, distortion, scaling, and pitting that can occur during heat treatment of a mold component that has been machined in the annealed condition. Prehardened steels with hardnesses in the range of 28 to 32 Rc are used mainly for injection mold plates. The upper limit of this hardness range corresponds to about the upper limit of adequate machinability using conventional metal cutting tools. A stress relief at 100 °F below the tempering temperature of these steels is usually carried out after rough machining the mold cavity to avoid excessive residual stress that might result in distortion or possible cracking during subsequent grinding. Prehardened SAE P20 Tool Steel Prehardened SAE P20 tool steel is a 0.35% carbon, 1.70% chromium, 0.40% molybdenum, 0.90% manganese, and 0.50% silicon tool steel available in the heat treated condition at 28–32 Rc as a result of prior hardening and tempering. This steel has excellent dimensional stability, good machinability, polishing ability and toughness, but fair wear resistance. Eliminating a second machining operation normally required by steels that must be heat treated is considered suitable for various sizes of molding machines and types of injection molds. To obtain greater surface strength, carburizing is followed by conventional hardening and tempering treatments to obtain adequate hardness and strength. Prehardened SAE P21 Tool Steel Prehardened SAE P21 is mainly used for injection molds; its chemical composition is 0.20% carbon, 0.40% nickel, 0.25% chromium, 0.02% vanadium, 0.30%

10.6 Steels for Thermoplastic Injection Molds manganese, 0.30% silicon, and 1.20% aluminum; available in the precipitation hardened condition at 32–39 Rc as a result of prior solution treating and aging. This steel exhibits high strength, excellent dimensional stability, good machinability, fair toughness and wear resistance. Cavities and cores are cut directly from this steel. Uniformity of hardness and strength can be attained in a relatively large section, provided cooling is at a sufﬁcient rate from the solution treating temperature to prevent softening as a result of aging. Prehardened SAE H13 Tool Steel Prehardened SAE H13 steel is a 0.35% carbon, 0.40% manganese, 5.00% chromium, 1.50% molybdenum, 1.10% silicon, and 1.10% vanadium tool steel available in the heat treated condition at 30–36 Rc as a result of prior hardening and tempering (65–74 Rc) This steel has optimum dimensional stability, excellent toughness and wear resistance. This tool steel is primarily used for protection from abrasion, for injection molds requiring high thermal shock resistance, and good polishing ability. A drawback to this tool steel is the difﬁculty of machining or rebuilding by hard-surfaced welding. It is also difﬁcult to straighten by conventional methods A free machining grade of SAE H13 tool steel is produced by use of carefully controlled and evenly dispersed sulﬁde additions in the melting operation. Such a steel can be prehardened to 50–52 Rc and still possess adequate machinability. This has the advantage of higher strength although the polishing ability may be adversely affected by the increased sulﬁde content. Prehardened AISI 4130 Alloy Steel Medium carbon alloy steel AISI 4130 prehardened is a 0.30% carbon, 0.95% chromium, 0.48% manganese, 0.20% silicon, and 0.20% molybdenum alloy steel available in the heat treated condition at 28–32 Rc. The hardening ability is low and it is quenched in water. This steel has optimum dimensional stability and machinability, excellent toughness and adequate wear resistance. This steel is advantageous for injection molds because of high toughness, moderate strength, and high thermal shock resistance. Prehardened AISI 4140 Alloy Steel Medium carbon alloy steel AISI 4140 prehardened is a 0.40% carbon, 0.80% chromium, 0.75% manganese, 0.15% silicon, and 0.15% molybdenum alloy steel available in the heat treated condition at 28–32 Rc and stress relieved conditions. It has good strength and can be ﬂame hardened or rebuild by hardsurfaced welding. The hardening ability is medium and it is quenched in oil. This steel has optimum dimensional stability and machinability, good toughness, strength and wear resistance. It is important to make sure that special, easy to machine grades are not selected. These grades usually contain either lead or are resulphurized. This makes the mold component impossible to rebuild by hard-surfaced welding due to extreme porosity and stress cracks. Prehardened AISI 4150 Alloy Steel Medium carbon alloy steel AISI 4150 prehardened is a 0.50% carbon, 0.95% chromium, 0.90% manganese, 0.30% silicon, and 0.20% molybdenum alloy steel available in the heat treated condition at 30–36 Rc. The hardening ability is high

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10 Injection Mold Design and it is quenched in oil. This steel has high strength, very good dimensional stability and machinability, good wear resistance and moderate toughness. AISI 4340 Alloy Steel This alloy steel is similar to AISI 4140, but includes nickel as an alloying element plus a greater percentage of molybdenum. This provides a slightly higher strength, but the major difference is the greater penetration of heat treatment providing superior mechanical properties at the core of the mold component. Nitralloy 135M Alloy Steel A number of steel manufacturers produce a product similar or identical to Nitralloy 135M, but this is the name best known. Nitralloy is similar to AISI 4140, but is slightly lower in physical properties. The major difference is the inclusion of a small portion of aluminum. CPM-10V Tool Steel This material is a totally new concept for tool steels based on particle metallurgy. CPM stands for “Crucible Particle Metallurgy”. This process enables the incorporation of very high amounts of carbon, chromium, and vanadium not possible by conventional methods used to manufacture steels. CPM-10V steel is intermediate between tool steels and cemented carbides, yet it has excellent strength. It also has much higher toughness than conventional tool steels, if heat treated properly. It provides superb wear resistance in ﬁber glass reinforced thermoplastic processing components such as injection screws, check valves, barrels, nozzles, mold cavity inserts, and gate inserts. CPM-10V large bar stock is very expensive, but in smaller sizes (2.50 in) is fairly competitive. The material is probably the best choice for mold components subjected to more than 10% glass ﬁbers or abrasive mineral reinforced thermoplastic resins, especially if these polymers have high melt ﬂow rates.

10.6.8

Carburizing Tool Steels

The two most commonly used carburizing steels for making thermoplastic injection molds are SAE P2 and SAE P6. Hobbed or machined injection molds made from these steels are case hardened by a carburizing treatment followed by hardening and tempering treatments. The aim of case hardening is to attain an adequate combination of abrasion resistance and toughness for a particular application. Carburizing steels are used for injection molds when a higher hardness is required than can be achieved with prehardened steels. Selection of the carburized case depth depends on the service conditions to which the mold component will be subjected. The deeper and harder the case hardened zone, the greater the resistance to abrasion. The stronger and tougher the core zone, the shallower is the required case depth. However, a large cavity usually requires a greater case depth along with a tougher core than a small cavity. A greater case depth is also required if a mold is subjected to a high injection molding pressure or if a cavity is shallow compared to other dimensions. Carburizing SAE P2 Tool Steel SAE P2 steel is a 0.07% carbon, 2.00% chromium, 0.50% nickel, 0.15% silicon, and 0.30% molybdenum tool steel available in the annealed condition. This steel has fair dimensional stability, good toughness, and very good wear resistance.

10.6 Steels for Thermoplastic Injection Molds After a mold cavity is formed from SAE P2 tool steel by hobbing, the mold cavity is carburized to a case depth of 0.05 to 0.06 in and heat treated to a case hardness of 60–64 Rc with an internal core hardness of 14–18 Rc. Due to the high strength and abrasion resistance at the surface, carburized SAE P2 tool steel’s main advantages are that it can be easily cold hobbed in the annealed condition and it can subsequently be polished to a high luster in the carburized condition. Carburizing SAE P6 Tool Steel SAE P6 steel is a 0.10% carbon, 3.50% nickel, 1.50% chromium, 0.25% silicon, and 0.50% molybdenum tool steel available in the annealed condition. This steel has fair dimensional stability, good toughness and machinability, and very good wear resistance. After a mold cavity is formed from SAE P6 tool steel by machine cutting, the mold cavity is carburized and heat treated to a case hardness of 58–61 Rc with an internal core hardness of 26–28 Rc. SAE P6 tool steel is comparable to SAE P2 with regard to surface strength, abrasion resistance, and polishing ability. Due to the higher internal core hardness, SAE P6 mold cavities are capable of withstanding higher injection pressures than SAE P2 mold cavities.

10.6.9

Oil and Air Hardening Tool Steels

The most commonly used oil and air hardened tool steels for making thermoplastic injection mold components are SAE O2, S7, H13, A2, A4, A6, and D2. These tool steels offer suitable combinations of high abrasion resistance, high strength, and high polishing ability required for thermoplastic injection mold components. To achieve a mirror ﬁnish on the cavity surfaces it is necessary to attain a hardness greater than 54 Rc. Because of its lower hardness in the annealed condition, machinability is superior to that of the prehardened steels. Oil Hardening SAE O2 Tool Steel SAE O2 tool steel is a high carbon oil hardened tool steel available in the spheroidized/annealed condition. SAE O2 chemical composition is a 0.90% carbon, 1.60% manganese, and 0.25% silicone tool steel. It is capable of attaining surface hardness up to 63 Rc by heat treatment. This steel has optimum machinability, adequate dimensional stability, toughness, and wear resistance. Air Hardening SAE S7 Tool Steel SAE S7 tool steel is an air hardening 0.50% carbon, 0.70% manganese, 0.25% silicon, 3.25% chromium, and 1.40% molybdenum shock resisting tool steel that is available in the spheroidized/annealed condition. The suggested hardness range of 52–58 Rc can be attained by air hardening and tempering. This steel has excellent toughness, very good machinability, dimensional stability, and wear resistance. SAE S7 tool steel is suitable for injection mold components when high strength, toughness and thermal shock resistance are required. Air Hardening SAE A2, A4 and A6 Tool Steels These are commercially medium alloy air hardened tool steels in the spheroidized/annealed condition. Hardness in the range of 58–60 Rc can be attained by heat treatment. This hardness range is suitable for injection molds. SAE A2 steel is a 1.00% carbon, 0.70% manganese, 0.30% silicon, 5.00% chromium, and 1.00% molybdenum tool steel that has the highest abrasion resistance of the group.

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10 Injection Mold Design SAE A4 steel is a 1.00% carbon, 2.00% manganese, 0.35% silicon, 1.00% chromium, and 1.00% molybdenum produced as a free machining grade tool steel. SAE A4 tool steel has the advantage of requiring a lower hardening temperature than SAE A2. SAE A6 is a 0.70% carbon, 2.00% manganese, 0.30% silicon, 1.00% chromium, and 1.25% molybdenum produced as a free machining grade tool steel. SAE A6 tool steel has the higher toughness of the group and the advantage of requiring a lower hardening temperature than SAE A2. Air Hardening SAE D2 Tool Steel SAE D2 tool steel is a 1.50% carbon, 0.30% manganese, 12.00% chromium, 1.00% molybdenum, 0.30% silicon, and 1.00% vanadium air hardened tool steel available in the spheroidized/annealed condition. A hardness of 58–60 Rc can be achieved by heat treatment. This tool steel has excellent dimensional stability and wear resistance, but poor toughness. SAE D2 tool steel is the most abrasion resistant of the deep hardening steels listed in Table 10-1. It has been primarily used for applications requiring abrasion and corrosion resistance. A drawback to this tool steel is the difﬁculty of machining or rebuilding by hard-surfaced welding; it is difﬁcult to straighten by conventional methods.

10.6.10 Stainless Steels Stainless steel types 410, 420 and 440C are martensitic stainless steels commonly used for thermoplastic injection molds. Their high corrosion resistance is of advantage in molding relatively corrosive thermoplastics or molding under relatively corrosive atmospheric conditions. Stainless Steel 410 Stainless steel 410 is a 0.15% carbon, 1.00% manganese, 1.00% silicon, and 12.25% chromium steel available in the annealed condition. Suitable heat treatment can produce a hardness of 38–41 Rc. This stainless steel has excellent polishing ability eliminating the need for chrome plating. The toughness is excellent, dimensional stability and wear resistance properties are good, but the machinability is fair. It has slightly better corrosion resistance than stainless steel 420. Stainless Steel 420 Stainless steel 420 is a 0.15% carbon, 1.00% manganese, 1.00% silicon, and 13.00% chromium steel available in the annealed condition. By heat treatment, a hardness of 50–54 Rc is attained, which results in higher strength and abrasion resistance than stainless steel 410. This stainless steel has excellent polishing ability, eliminating the need for chrome plating. The toughness, dimensional stability, and wear resistance properties are good, but the machinability is fair. Stainless Steel 440C Stainless steel 440C is a 1.05% carbon, 1.00% manganese, 1.00% silicon, 17.00% chromium, and 0.75% molybdenum steel available in the annealed condition. By heat treatment, a hardness of 56–61 Rc can be attained, which results in higher strength and abrasion resistance than stainless steel 410 and 420. This stainless steel has excellent polishing ability, eliminating the need for chrome plating. The toughness is poor, dimensional stability and wear resistance properties are very good, but the machinability is fair.

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10.6 Steels for Thermoplastic Injection Molds

10.6.11 Steels Used in Thermoplastic Injection Mold Components Prehardened, carburizing, and deep hardening steels are all used for general purpose thermoplastic injection molds because of their service record, cost, and ready availability. Prehardened steels of moderate strength and wear resistance are widely used, but as operating conditions become more severe, high alloy tool steels become more common. Carburized tool steels are used for high ﬂow, ﬁber glass, and abrasive mineral reinforced injection molding thermoplastics. Heat resistant steels are used for injection molding thermoplastics requiring high mold temperatures. Stainless steels, as well as chrome and nickel plated prehardened or deep hardened steels, are used for injection molding corrosive thermoplastic resins. The types of steels used in the construction of the mold components can be divided into two groups, typical commercial mold base combinations of steels as shown in Figure 10-2 and the steels used for the remaining mold components. The hardness of a mold cavity is usually in the range between 50 and 60 Rc and it may be chromium- or nickel-plated with plating layers usually under 0.002 to 0.005 in thick. Care must be taken when chromium-plating. The plated components must be treated to prevent hydrogen embrittlement where the hydrogen gas generated in plating can cause loss of steel toughness. The steels listed in Table 10-2 can be used for making thermoplastic injection mold components. The selection of steels depends on the mold component’s operating functions, type of thermoplastic resin to be molded, number of parts to be produced, and the method of forming the mold cavity. There are other materials used in mold components, depending on the application: Aluminum is used for prototype molds. The types used are 6061 and 7075 series, with tensile strengths over 75,000 psi and relatively good scratch resistance. Copper, aluminum, or beryllium copper alloys are used in molds, they have high thermal conductivity and can be hardened to 45 Rc. Their problems are poor machinability; they are expensive and have low strength and stiffness, so core deﬂection can be a problem. Bronze is used in some cases for wear resistance in mold mechanisms and occasionally for cavity inserts.

Steel “A” Steel “A” Steel “A” Steel “A” Steel “A” Steel “A” Economical grade Steel “B” Steel “B” Steel “B” Steel “B” Steel “A” Steel “A” General grade Steel “B” Steel “C” Steel “C” Steel “B” Steel “A” Steel “A” Performance grade

Steel “A”: SAE 1015, 1018, 1020, 1030. Low carbon hot rolled steel with good tensile strength properties, machines easily, allowing faster and smoother cuts. Steel “B”: AISI 4130, 4140, 4150. Preheat treated medium carbon alloy steels (28–36 Rc), high strength, machinability, and toughness, with good wear resistance properties. Steel “C”: SAE P20, P21. Medium hardening ability preheat treated tool steels (28–39 Rc), excellent dimensional stability and surface ﬁnishing, high strength, good machinability and toughness, but low wear resistance properties. Steel “D”: SAE A4, A6, O2, S7, H13, P6, P20. Preheat treated medium carbon tool steels, chrome and nickle plated, excellent dimensional stability and surface ﬁnishing, high strength. Steel “E”: Stainless Steel 410, 420, 440C High corrosion resistance by heat treatment to 38–54 Rc, high strength, surface ﬁnishing, and wear resistance, but hard to machine

Steel “D” Steel “E” Steel “E” Steel “D” Steel “D” Steel “D” Corrosion resistance

Figure 10-2 Typical commercial mold base combinations of steels

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10 Injection Mold Design Table 10-2 Steels Used in Thermoplastic Injection Mold Components

Mold components

Types of steels used

Locating ring

SAE 6521; 6524 (28–32 Rc)

Extension nozzle bushing

AISI 4140 (28–32 Rc)

Sprue bushing

SAE 6145; Ampco 940/stainless steel

Sprue puller pin

Nitrided H13 (65–74 Rc)

Ejector pin

Nitrided H13 (65–74 Rc); CM 50 (60–65 Rc)

Ejector sleeve

Nitrided H13 (65–74 Rc)

Ejector plate

SAE 1020 (30–35 Rc)

Ejector retainer

SAE 1020 (30–35 Rc)

Leader pin

SAE 1018; 8020 (30–35 Rc)

Return pin

Nitrided H13 (65–74 Rc)

Ejector stripper plate

Prehardened P20; H13; S7; A2; A4; A6

Cavity block

Prehardened or precipitation hardening P20; P21; H13; S7; A2; A4; A6; P2; P6; Stainless steel 420 (28–50 Rc)

Cavity insert

Prehardened or precipitation hardening H13; D2; S7; A2; A4; A6; P2; P6; stainless steel 420; 440C (50–64 Rc)

Gate insert

D2; CPM 10V; CPM 9V (50–60 Rc)

Core block

Prehardened or precipitation hardening P20; P21; H13; S7; A2; A4; A6; P2; P6; stainless steel 420 (28–50 Rc)

Core insert

Prehardened or precipitation hardening H13; S7; P2; stainless steel 420, 440C; beryllium copper; Ampco 940

Slide

Carburized or nitrided P20; nitrided P21; O2; A2; A6; P2

Wear plate

SAE A2; A6; P2 bronze-plated

Angle pin

Nitrided H13 (65–74 Rc)

Knockout rod

SAE 1020 (30–35 Rc)

Support pillar

SAE 1040 (28–32 Rc)

Stop pin

SAE 1040 (28–32 Rc)

Parting line interlock (male)

SAE 8620 (50–55 Rc)

Parting line interlock (female)

SAE 8620 (55–60 Rc)

Insulator sheet

Asbestos-free glass reinforced polymer composite

Zinc alloys are too soft and not strong enough for production molds, unless the injection pressure and melt temperature are very low, and the molded product does not require good dimensional tolerances. “Kirksite” is used for prototype molds, but “Kirksite” has higher thermal conductivity than steel and will produce molded parts with different shrinkage than parts from a production steel mold. Electro-deposited cavities are used for prototype molds and for small specialized cavities used in production steel molds.

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10.7 Mold Cavity Surface Finishing

10.7

Mold Cavity Surface Finishing

The cavity surface ﬁnishing, runner and gating system, mold temperature, the type of thermoplastic resin, and the process molding conditions will determine the quality surface ﬁnish that will be obtained on the molded thermoplastic parts. The type of tool steel, its quality, structure, and heat treatment process can also affect the polishing ability of the cavity and the surface ﬁnish obtained. Several new technical innovations or improvements by various leading steel mills are producing better quality tool steels as a result of alloying compositions, modern manufacturing processes, and metallurgical quality control. The product design surface ﬁnish speciﬁcations may require the use of a speciﬁc type of thermoplastic resin. The product surface ﬁnish must be studied in great detail before proceeding with the design and manufacture of the mold. The difference between a number three cavity surface ﬁnishing (good mold ﬁnish) and a number one surface ﬁnishing of optical quality (mirror ﬁnish) can double the price of the mold. When the mold design is more complicated, the tool steel for the mold cavity is more expensive, and the heat treating method is more difﬁcult; machining, grinding, and polishing represent the effort of many man hours reﬂecting the price increase. Processing with a mirror ﬁnish mold requires a better quality thermoplastic resin that is more expensive. The material handling efﬁciency for the resin is low, the resin must be dry before molding and the use of reground material (runners and rejected parts) is not allowed. The molding process efﬁciency is also low with long molding cycles and high rejection rates, which require extra quality control support. High maintenance costs, spare parts for inventory, and mold cavity insert replacements, requiring special equipment for repairs, which are also time consuming. The life expectancy of such a mold is typically low because of complications and a new spare mold will probably be needed for the molding process. The surface ﬁnish characteristics of the thermoplastic resins are important parameters for the injection molding process. Unreinforced amorphous resins have better surface ﬁnish characteristics than glass- or mineral-reinforced semicrystalline resins. For example, injection molded products (lenses, tail lights, or dial faces) from transparent materials such as acrylic and PC will require the highest quality ﬁnish. Other materials, such as low density polyethylene or high impact polystyrene, which have a natural shine, do not reproduce the high level ﬁnish that is generated with a diamond polished surface (No.1 surface ﬁnish). The Society of the Plastics Industry and the Society of Plastics Engineers developed mold steel surface ﬁnishing standards, which provide an excellent visual comparison. The standards consist of a group of six pieces of steel, each ﬁnished to a different level and used as a comparison guide in mold ﬁnishing. Finish

Process parameters

Number 1

8,000 Grit (0.0 to 3.0 diamond compound micro range)

Number 2

1,200 Grit (up to 15.0 diamond compound micro range)

Number 3

320 Grit, abrasive cloth

Number 4

280 Grit, abrasive stone

Number 5

240 Grit, dry blast (5.0 in distance at 100.0 psi pressure)

Number 6

24 Grit, dry blast (3.0 in distance at 100.0 psi pressure)

Number 4

Number 5

Number 1

Number 6

Number 2

Number 3

Figure 10-3 SPI-SPE mold steel surface ﬁnishing comparison kit

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10 Injection Mold Design The grades range from number six, which is a 24 grit dry blast ﬁnish, to number one, which reﬂects a diamond polish with an 8,000 grit compound. The ofﬁcial SPI-SPE mold steel surface ﬁnish standard comparison kit and the descriptions are shown in Figure 10-3. There are other, more sophisticated, methods of measuring surface characteristics. One such method is the use of an instrument called a proﬁlometer. It measures the surface “roughness” of the metal. The stylus of this instrument is moved over the surface of the steel; the peaks and valleys of the surface ﬁnish are fed into the analyzer and the surface roughness is charted in terms of micro inches. By deﬁnition, a micro inch is a millionth of an inch (0.000001 in). The surface of any material is not truly “smooth” when examined under the microscope, but rather generally covered with ridges and scratches. The proﬁlometer measures the depth of the scratches and the height of the ridges and reports them as maximum peak-to-valley height, along with the average deviation from the mean surface. The average deviation from the mean surface is called the “Root Mean Square” and is known as RMS. A micro inch is the most often used value for specifying surface ﬁnishes. Mold steel surface ﬁnishing comparison standards have established the mold steel surface ﬁnish assigned number versus the RMS relative value in micro inches. Table 10-3 shows the equivalent RMS values for the SPI-SPE mold steel surface ﬁnishes. Table 10-3 Equivalent RMS Values of Standard Mold Steel Surface Finish

Mold ﬁnish

RMS value range

Mold ﬁnish

RMS value range

Number 1

0.50 to 1.00

Number 4

12.00 to 15.00

Number 2

1.00 to 2.00

Number 5

26.00 to 32.00

Number 3

7.00 to 7.50

Number 6

160.00 to 190.00

Table 10.4 provides a reference for the ﬁnishing process required to produce the desired RMS surface ﬁnish classiﬁcation. Table 10-4 RMS Surface Classiﬁcations Based on the Finishing Process Used

RMS surface range

Finishing process parameters

0.00 to 10.00 RMS

Fine abrasive hand polish, coarse to ﬁne diamond polish, ultra-ﬁne hand-rubbed diamond ﬁnish.

10.00 to 20.00 RMS

Fine cylindrical grind, smooth ream, ﬁne surface grind, medium grind hand polish.

20.00 to 40.00 RMS

Fine machine grind, medium surface grind, rough abrasive polish, ream.

40.00 to 100.00 RMS

High grade machine ﬁnish, coarse surface grind, smooth cylindrical grind, smooth hand ﬁle.

100.00 to 250.00 RMS

Medium machine cuts, coarse surface grind, smooth disc ﬁnish grind, medium ﬁle.

250.00 to 500.00 RMS

Heavy machine cuts, rough ﬁling, rough disc grinding, sand cast surfaces.

10.7 Mold Cavity Surface Finishing

10.7.1

Mold Surface Finishing Process Procedures

The various surface textures for mold cavity and core surface ﬁnishing levels required in the thermoplastics injection molding industry are more demanding than the conventional molds built for other plastic processes, such as blow molding or thermoforming. The mold cavity surfaces must be prepared to accept the ﬁnal polish, whether it is a 320 grit cloth ﬁnish or a number one (1) diamond polish mirror ﬁnish. The polished surface must be free of waves, ripples, distortions, pits, scratches, or orange peel. Machining Machining is the ﬁrst step in the manufacturing process of a mold component. It determines the tools to be used, the surface condition obtained during the machining operation, and the methods of polishing and ﬁnishing. When the mold components are machined according to the size and geometry of the mold design that has already accounted for the mold shrinkage of the thermoplastic part, the cavity dimensions are left with about 0.001 to 0.003 in of stock for subsequent ﬁnishing operations. When the mold cavity and core inserts are to be heat treated, the details are machined to within 0.0003 to 0.0005 in of the ﬁnal surface ﬁnish. Mold components that come off the duplicating machine will have the characteristic ridges and valleys resulting from this particular type of machining. The ridges on either side of the valley must be leveled to expose an even surface for the ﬁnal ﬁnishing. The metal removal operation can be aided by the use of a metal dye painted over the surface to serve as a guide in metal removal. It is often advisable to machine cavities between 0.002 and 0.003 in deeper than necessary and remove the proper amount from the top surface, after the mold has been tested in the injection molding machine. Electrical Discharge Machining (EDM) Electrical erosion is a metal removal process using a master electrode that is electrically conductive. Copper alloys are generally used to make the master electrode because hardness is not a requirement. Cast zinc and machined graphite are also used in some instances. The principle of spark erosion is used by the mold making industry. The gap between the master and the cavity insert is quite uniform and small. As the master descends, small intense sparks are generated wherever the gap is reduced. Erosion occurs on both master and cavity inserts, the master’s negative polarity erodes only at 1/4 to 1/10 of the speed at which the cavity insert erodes at positive polarity. The cavity insert may be hardened before the EDM process begins so that distortion due to heat treatment is eliminated. The dielectric ﬂuid must circulate continuously to remove the minute particles that are formed between the master and the cavity insert. Electrical erosion is slow compared to mechanical cutting of soft steels, but for certain conditions, such as narrow deep slots, it offers great advantages. To utilize the EDM process, the mold maker roughs out stock by machining wherever possible and then sends the cavity insert to the EDM processor. Grinding To remove ridges and rough cut marks, a hand or ﬂexible shaft grinder is ﬁtted with a grinding wheel or abrasive disc. Extreme care must be exercised to prevent the grinder from following the ridges and removing more material than necessary.

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10 Injection Mold Design Grinding strokes should follow the crest of the ridges until the mean surface is reasonably constant. A more recently developed tool, the portable belt sander, can also be used in this “roughing” phase of the operation. Discs or abrasive belts have the advantage of being able to span a greater area, but care must be taken because they can also cut very quickly. Grits in the range between 60 and 320 are used. Filing provides ample control and keeps the surface quite even. The grade of ﬁle used will be determined by the amount of metal to be removed. Rough grinding, or coarse ﬁling, can leave the metal surface full of tears and waves. Finer wheels, drums, and ﬁles may be used to improve the surface texture and bring the cavity or core closer to print dimensions. After all the metal has been removed to make the part to print (plus the allowance for ﬁnishing stock), the surface should be examined to determine if it is ready for stoning. The surfaces of heat treated thermoplastic injection molds are ground before polishing. Improper grinding can generate very high temperatures and precautions should be taken to avoid the following defective surface conditions: • Softening from over tempering • Embrittlement due to rehardening • Distortion and unfavorable residual stress • Cracking Avoiding these problems requires grinding with a proper wheel, at its recommended rotational speed, at an adequately low rate of metal removal, and with an adequate coolant. If residual stresses imposed by grinding are not relieved, pitting during subsequent polishing may result. Adequate relief of residual grinding stresses can usually be accomplished by heating to about 100 °F below the tempering temperature. Stoning Choosing the initial grit of stone depends on the degree of ﬁnish left by the machining, grinding, or ﬁling operation. Machining or duplicating usually results in a coarser ﬁnish than grinding; therefore, a coarser grit stone would normally be used. Preliminary stoning may be done with a 240 grit stone to remove ﬁnal dips, depressions, waves, or other imperfections to obtain a ﬂat or properly contoured surface. If defects are not too great, a 320 grit stone will be sufﬁcient. The stone should be moved back and forth (medium pressure applied), over the surface in a 90° direction from the direction made by the last operation. Before they are used, the stones should be soaked in a contaminantfree oil base lubricant. The recommended stoning procedure for a cavity surface is as follows: • Stone with 240 grit • Stone at 90° to previous scratches with 320 grit until the previous scratches are removed • Stone at 90° to previous scratches with 400 grit until these scratches are removed • Stone at 90° to previous scratches with 900 grit

10.7 Mold Cavity Surface Finishing Polishing is the process of producing a series of overlapping “scratches” that get progressively ﬁner. To accomplish this, it is important that for each ﬁner grade stone used, the angle (direction) is changed relative to the marks made by the preceding coarser stone. After each grit ﬁnish is completed, the entire workplace must be thoroughly washed with clean stoning oil and wiped with a clean tissue to remove all particles of the grit remaining on the surface. This is necessary to ensure that none of the particles of the coarser grit will be picked up at a later time by a ﬁner grit stone, causing deeper scratches. Here are a few hints pertaining to the use of polishing stones: • Do not use a stone that is too coarse • Always dress the polishing stone with a grinding wheel or coarse paper to provide the maximum contact with the work surface • Use care when dressing the polishing stone • Use sufﬁcient stoning oil to prevent the stone from loading • Hold the polishing stone ﬁrmly for directional control, but press only hard enough to make the stone cut • Make sure the stone marks from previous grit size are all crossed out • Change stoning direction with each successive grit • Clean the work area thoroughly between each change of grit • Keep each grit of polishing stone in a separate stoning oil can • Exercise utmost care when stoning at an edge (parting line). If the mold cavities and cores need heat treatment, the question often arises as to how far the polishing should proceed before the part is heat treated. After the heat treating process, the knock out holes (or any other holes in the work area) must be covered or ﬁlled to avoid “dishing” (rounding off sharp edges) as the ﬁnishing continues. A 240 or 320 grit stone will remove the scale developed during the heat treatment. Make sure to “cross” stone the last marks until they are removed. Then proceed to ﬁner stones, such as 500, 600, or 900 grit. Surface ﬁnishes from stoning are often sufﬁcient after using a 600 to 900 grit stone. While the mold cavity surface exhibits a soft, “matte” ﬁnish, it will be smooth and ﬂat enough to allow easy ejection of the thermoplastic molded parts. Luster or shine can be developed on this type of surface with some 500 to 600 grit paper that will remove the ﬁne stone marks. Other methods for developing the ﬁnal “shine” on the surface include the use of diamond compound with brushes and felt bobs. Orange peel, the lightly dimpled ﬁnish that often appears during the ﬁnal bufﬁng operations of polishing, can be caused by several conditions. If the cavity insert steel has been overheated during heat treatment, the steel structure is changed resulting in an inconsistent surface hardness. This permits the softer surfaces to be deformed or abraded away more quickly than the harder surfaces. Consequently, care should be taken during heat treatment to ensure uniform surface hardness. If polishing pressure is too high, this softer area can be torn from the surface,

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10 Injection Mold Design leaving pits. By looking at the pits through a magnifying glass, a little “tail” at the edge of the pit following the direction of polishing can be seen. The following factors may contribute to the formation of orange peel and pits in polished mold cavity inserts: • Low mold cavity surface hardness due either to an incorrect tool steel composition or improper heat treatment • Presence of excessive nonmetallic particles • Presence of internal porosity, particularly at the center regions of mold cavity inserts of relatively large cross section • Retained austenite on the mold cavity surface usually caused by overheating during hardening or carburizing. Retained austenite has lower strength than martensite; therefore, it may cause deformation and breakage • Clustering of carbides causes overheating, nonuniform hardness, and low strength areas and the removal of the carbide particles can result in scratches during the carburizing process. If orange peel or pitting occurs, it may be possible to repair the mold cavity surface by ﬁrst removing the defective condition by hand polishing with a ﬁne stone, stress relieving at 100 °F below the tempering temperature, and hand polishing with diamond paste using light pressure and appropriate passes. If orange peel or pitting is caused by excessive retained austenite, this condition can usually be alleviated by transforming the retained austenite using additional tempering treatments. Using a second additional tempering treatment allows tempering of the brittle martensite formed during the ﬁrst tempering treatment. Another method of reducing the retained austenite content is to cool the mold cavity insert to a temperature between 100° and 150 °F. A subsequent tempering treatment of the brittle martensite formed during this cold treatment is advisable. Over-carburized or pack-hardened steel can also produce similar defects. In these instances, the orange peel is caused by a similar inconsistent hardness of the surface, but the pits are the result of very hard particles of iron carbide being pulled out of the surface. Most of these pits will have no tails. Orange peel can also appear on the surface of properly heat treated steel that has been ﬁnished with powered, mechanical devices. In this case, the surface defect is caused by excessive pressure of the polishing implement (brush, felt, etc.) against the steel, or by the over-polishing of the mold cavity surface. The harder the tool steel, the better the “polish” that can be achieved. Since orange peel occurs when the tool steel is stressed above its yield point (the yield point of steel can be increased by hardening or nitrating), it follows that the softer tool steels are more prone to orange peel. Remember, over polishing and polishing with too much pressure are the causes of pitting and orange peel. Diamond Polishing Polishing of thermoplastic injection mold cavity surfaces with abrasives is carried out either by some type of rotating machine or by hand. Machine polishing is more economical, but avoiding over-polishing caused by excessive pressure and/ or speed is more difﬁcult. Hand polishing usually involves preliminary work with a series of silicon carbide stones (240 to 900 grit) and a ﬁnishing operation with

10.7 Mold Cavity Surface Finishing diamond pastes from 15.0 to 2.0 micron. There is less chance of over-polishing if light pressures are used. Severe bufﬁng or the use of loaded stones may result in severe residual stress and pitting. The ﬁnal diamond polish is the last step in the process of polishing; if one of the previous stoning steps has not been done properly, the ﬁnal smooth luster surface will not be satisfactory. If mistakes have been made earlier during the ﬁnishing process, they will certainly show up on the ﬁnal surfaces. Stone ﬁnishing the mold cavity surfaces involves the preparation of the tool steel surface by producing a series of crisscrossing “scratches” that will get ﬁner and ﬁner. Coarse grits remove more metal and cut deeper. As the surface gets smoother and the surface begins to develop, it means that the peaks are being lowered to the depths of the valleys. In other words, the RMS measurement of the surface irregularity is becoming more minute. Since there is a practical limit to the size of the “scratch” that can be achieved with conventional abrasives, very small grit sizes of diamond must be used in order to obtain an optical quality polish or a Number One (1) ﬁnish. There is also another fundamental difference between the ﬁnishing process that has been done up to now and the ﬁnal diamond polish. The stoning process prepares the surface for the ﬁnal polish or the honing operation, in which the stone has the abrasive ability to cut the metal. On the other hand, diamond polishing is more a lapping operation. Fine diamond compounds remove very little metal while developing the ﬁnal luster of the ﬁnish. There are more than a dozen grades of diamond compounds ranging from 120 for fast cutting to 14,000 for super ﬁnishing, which are used to polish the mold cavity surfaces. The starting point of diamond polishing will depend, to some degree, on the sequence of stones that have been used to prepare the mold cavity surface. Beginning with a coarser diamond grade, a small amount of the compound is applied directly to the surface being worked. Then, by means of a bristle, brass, or steel brush, the compound is swirled over the surface using a rotary tool at slow speed. A speed of 500 rpm for roughing and 5,000 to 10,000 rpm maximum for ﬁnal polishing is a good general rule. Using light to moderate pressure, care must be taken to keep the brush ﬂat to the mold cavity surface to avoid cutting deep swirl marks. A “crisscrossing” action should be employed when using diamond compounds. The compound in use will become darker, indicating that metal is being removed and mixed with the compound. The surface should be brushed until all that is visible are ﬁne swirly marks left by the brush’s rotary action. There should be no stoning marks visible at all. The next step, the removal of the swirly marks left by the bristle brush, is accomplished with a felt product. Felt bobs are available in various degrees of hardness, preassembled in a shanked nylon holder. Mounted in a rotary tool and using light to moderate pressure, the surface is polished with diamond compound until all that is visible are felt swirls. The cavity surface should be thoroughly cleaned to remove all residual particles of the previous grade, before applying a ﬁner diamond compound. The ﬁnal step in polishing with diamond compound is a hand operation. Depending on the various conﬁgurations of the mold cavity surface, ﬁne tissue paper, felt sticks or cotton swabs may be used with an ultra ﬁne grade of compound to arrive at the ﬁnal high gloss luster.

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10 Injection Mold Design A better polishing surface is found in the ﬁber direction than across the direction of the ﬁber. If the ﬁber direction can be made to correspond to the largest cavity surface, less difﬁculty from pitting during polishing will be encountered. For ﬁnishing a stainless steel cavity surface, it is important to use polishing and bufﬁng compounds without iron oxide. If iron oxide particles are impregnated into the surface of a stainless steel, rust spots and surface pits will develop. The following diamond polishing guidelines should be used: • Apply a small amount of diamond compound at ﬁrst, then add more if required • Do not mix the diamond compound grades • If the diamond compound gets dry or hard, add a clean diamond thinner or lubricant • If the ﬁrst grade of diamond compound used does not remove marks from the last stoning operation, STOP! These marks must be removed with a coarser grade of compound or you will end up with highly polished, shiny scratches • Clean mold cavity surfaces thoroughly before progressing to a ﬁner grade of diamond compound • Do not use more than one grade of diamond compound on the same brush, felt, or lap • Be sure that each step completely removes marks left from the previous step • Never use an abrasive stick or a piece of abrasive cloth because the grit could become embedded in the lap and cause serious scratches on the mold cavity surface.

10.8

Thermoplastic Injection Mold Bases

The majority of the thermoplastic injection mold designs falls within the twoplate mold categories. However, several other mold designs are also used by the plastics industry. These molds are known as two-plate, interchangeable cavity inserts, vertical encapsulation, lost core, three-plate, hot runner, insulated runnerless, and stacked types of molds.

10.8.1

Standard Mold Base Components

Because of the similarities in the two-plate mold construction, it is desirable to have some standard mold base available to permit the thermoplastic injection molds to be produced in quantity, with a short delivery time, thereby reducing manufacturing costs. Logically, it is advantageous for the mold makers to purchase a mold base quickly at reasonable cost, rather than expending engineering mold design time, steel selection, construction, and assembly of the mold base components. The mold base and component manufacturers have developed tool technologies for the new engineering thermoplastic polymers, producing a range of standard high quality products that are available, ready to use in the designs and manufacture of thermoplastic injection molds.

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10.8 Thermoplastic Injection Mold Bases

Top clamping plate

Locating ring Sprue bushing

"A" cavity plates

"B" cavity plates Support plate

Leader pin Shoulder bushing

Ejector retainer plate Ejector plate

Ejector pin

Sprue puller pin Ejector housing

Figure 10-4 Standard two-plate mold base components

A thermoplastic injection mold base may be deﬁned as an assembly of mold components that conforms to an accepted structural shape and size. The proper type of mold base is speciﬁed by the number of plates, premachining of the plates (if desired), steel selection, dimensions, ejection travel distance, size and type of locating ring and sprue bushing required by the injection molding machine. The remaining mold base components are suitably attached together and a guidance system incorporated. Of course, the mold base does not include runners, cavities, cooling, venting, hardness, ﬁnishing etc.; these aspects of mold manufacturing must be left to a specialized mold maker. The thermoplastic injection two-plate mold has been adopted as the standard mold base by mold component manufacturers because this particular mold construction is the most widely used design in industrial practice. Mold bases in a wide range of sizes made to suit a variety of purposes are produced by a number of manufacturers. These standard mold bases need only to be machined to make the mold components for a particular part. It is necessary to know the terminology and function of the components that make up the mold base. Figure 10-4 shows the location and nomenclature of the basic thermoplastic injection two-plate mold base.

10.8.2

Functions of the Mold Base Components

The most important features of standard thermoplastic injection mold bases are reviewed here to provide the reader with a working knowledge of the basic components and their functions.

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10 Injection Mold Design Top Clamping Plate The top clamping plate supports the “A” cavity plate, locating ring, and sprue bushing. In some mold designs, these two plates may be combined into a single, thicker plate, which serves both functions. It holds the stationary half of the mold to the stationary platen of the injection molding machine. Locating Ring The locating ring is that portion of the mold that is ﬁtted into the stationary platen of the injection machine. Its intended purpose is to properly situate the mold in relation to the injection nozzle of the machine. Most standard injection machines use different diameters with a variety of design features to accommodate the platen entry for the nozzle. The correct sizes of locating rings are provided in the molds to ﬁt a particular type of injection machine. Where relatively thin-walled parts are being molded and injection pressures may be high, the locating ring may be required to retain the sprue bushing within the mold, so the nozzle and sprue bushing are aligned. Sprue Bushing The sprue bushing seals off the melt from the injection nozzle conveying the melt through a conical shaped internal channel and forcing it into the sprue puller, runner, gate, and cavity conﬁnes of the mold itself. The sprue bushing has a tapered internal round channel that can vary in size as needed. In proper mold design, the sprue is made as short as possible, consistent with a given part design, thereby reducing the injection pressure drop in the runner system. Both, the locating ring and the sprue bushing, are usually supported by the top clamping plate, which is used to support the stationary half of the mold. “A” Cavity Plate The “A” cavity plate contains and supports the cavity or cavities or the core insert, sprue bushing, and the runners for the parts to be molded. In some cases, the cavity may be cut directly into the solid steel plate, while in others the cavities can be constructed separately and inserted into pockets within the cavity plate. The “A” cavity plate is part of the stationary section of the mold half; this plate is where the leader pins are mounted. “B” Cavity Plate The “B” cavity plate contains or supports the other half of the cavity or a core section of the molded part and also contains the leader pin bushings. The plane between these two plates is the normal parting line of the mold, which separates the two halves of the tool. The “B” cavity plate is the top plate of the movable section of the mold half. It is used to hold the sprue puller and ejector pins as well as the core inserts, or the cavity inserts. Support Plate The “B” cavity plate is mounted on top of the support plate. The support plate is used to provide strength to the cavities to avoid deﬂection during melt injection inside the cavities. Ejector Housing The ejector housing parallel blocks are added to provide the height required for the movement of the ejector system. The base plate of the ejector housing is used

10.8 Thermoplastic Injection Mold Bases for clamping the moving half of the mold to the moving platen of the machine. The ejector housing is a single unit for the ejection system. The injection mold base is manufactured with the parallels (vertical supports) welded to the bottom clamping plate. Bottom Clamping Plate A bottom clamping plate secures the movable half of the mold to the movable platen of the injection molding machine. If the mold is exceptionally large, the ejector system may require additional support, provided by the insertion of support pillars that bear the load between the bottom support plate and bottom clamping plate. Ejector Retainer Plate Mounted on top of the ejector plate, this plate retains the ejector head pins, ejector return pins, and sprue puller pin through counter bored holes. Ejector Plate The ejector plate is bolted together with the ejector retainer plate to form a unit. It acts as a back support plate for the ejector pins, return pins, and the knockout bar. These pins pass through drilled holes in the “B” cavity plate, insert cavity, and support plate. Stop Pins The stop pins are mounted on top of the bottom clamping plate; they are used as stops for the ejector housing when the ejector system returns as the mold closes. Support Pillars The support pillars are round bars placed between the support plate and the bottom clamping plate; they have the same height as the parallels. Bolted to the bottom clamping plate, they are used as additional support to avoid deﬂection of the “B” cavity plate. Sprue Puller Pin Pin located below the main runner, directly under the large diameter of the sprue channel. It is used to pull the solid sprue out of the bushing automatically when the mold opens and the molded parts and runner system are ejected. Ejector Pins The ejector pins enter the cavity to make contact with the molded part. Return Pins The return pins contact the stationary cavity plate and prompt the movement of the ejector plates back to the normal position prior to the next injection shot (not identiﬁed in Figure 10-4). Leader Pins The leader pins, used to align the plates on the closing of the mold, are hardened and ground steel pins mounted into one of the mold halves. One of the leader pins is offset so that the mold halves can only be closed when the leader pins are in the correct relative position.

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10 Injection Mold Design Shoulder Bushings Hardened and ground steel bushings are mounted into the other half of the mold, in-line with the leader pins. They serve as bearing surfaces for the leader pins.

10.8.3

Types of Standard Mold Bases

Figure 10-5 shows two basic styles of mold bases; one has a rectangular cross section (this is the most common form), the other is round. The rectangular cross section mold bases are commercially available in various standard conﬁgurations as shown in Figure 10-6. It is important for the mold designer to appreciate that the majority of mold designs can be broadly classiﬁed within one of these mold base types. For example, a slide-type of mold can be described as a two-plate mold with the addition of a slide assembly. The mold designer may have to accept a compromise in mold size; mold base types, and the range of sizes; for certain molds, some modiﬁcation to the mold base is necessary to accommodate a speciﬁc feature. Considered overall, however, the advantages of using standard mold bases outweigh the disadvantages and for a large number of mold designs, the standard mold base can be beneﬁcially used by the mold designer, mold maker, and end user. Nevertheless, on occasion it is necessary to have a “made-to-measure mold” so a special feature may be incorporated or a speciﬁc mold cooling system be adopted, or simply because a standard mold base of a suitable shape or size is not available.

Figure 10-5 Rectangular and round standard mold bases

Economical series standard mold base

Most frequently used standard mold base

Five plate reverse mold base with stripper plate

Six plate reverse mold base with stripper plate

Standard mold base with one floating plate

Standard mold base with two floating plates

Figure 10-6 Types of standard commercial mold bases (rectangular)

10.9 Types of Thermoplastic Injection Molds Advantages of Standard Mold Bases • Design drawing for individual unit sizes, reduces mold design time • Less steel needs to be carried in stock, investment is reduced • Buying and stock controls are simpliﬁed • Mold base price is known, estimating the cost of the mold is easier • Waiting time for steel blanks, etc., is avoided • Shaping, planing, and drilling of steel plates and blocks is avoided • Machining, grinding, hardening, ﬁtting of pins and sleeves is avoided • The ejector plate is already in place • The individual mold plates are screwed and doweled together • Machining time and labor costs are reduced • Design work on the insert cavities can usually begin immediately • Mold base components are standard; if a component is damaged during manufacture or in production, the part can be quickly replaced • Mold maintenance and down time is reduced • Mold delivery time is reduced • Efﬁcient mold maker’s engineering and use of production labor, provide mold cost reductions, satisﬁed customers and higher proﬁts. Disadvantages of Standard Mold Bases • Number of mold base sizes available is limited • Large size mold bases are limited • Maximum depth of mold base plates is also relatively small • The ejector system travel may be larger than is actually required • Cooling channels in a desired mold location are difﬁcult to obtain because of conﬂict with other mold base components • Extra number of support pillars positioned relatively far apart are needed to avoid deﬂection of the mold “B” cavity plate • The support plate cannot be unscrewed independently to expose the ejector assembly; the mold’s moving half must be disassembled • Limited number of special tool steel plates for the standard mold bases.

10.9

Types of Thermoplastic Injection Molds

Designs for injection molds differ depending on the product design geometry, end use applications, product tolerances (type of gate and location), product life, allowable product cost, molding production requirements, efﬁciency (maximum economy), amount of mechanization, factors that will dictate the size of the mold and the type of thermoplastic material being molded. The most common types of injection mold designs are the following:

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10 Injection Mold Design

10.9.1

Two-Plate Molds

Figure 10-7 shows a so called two-plate mold that certainly has more than two plates. It is the common name for a thermoplastic injection mold with a single parting line. The parting line of a mold can best be deﬁned as that surface where both halves of the mold separate to permit the injection molded parts and runners to be ejected from the mold. One side of the cavity is mounted in the “A” cavity plate with the locating ring and sprue bushing assembled into the stationary half of the mold. The “B” cavity plate is part of the moving half of the mold and contains the cores, the runner systems, and the ejector system, that are operated by the knockout bar that is attached to the machine actuator and the ejector plate assembly, moving the ejector pins and sprue puller pin forward to eject the molded parts and the runner system when the mold opens. The molten thermoplastic is fed directly from the machine nozzle through the sprue bushing that is connected to the runner and edge gates, ﬁlling the cavities with melt. Then the mold cavities cool off, forming the ﬁnal solid product (small containers).

Locating ring Sprue bushing Top clamping plate "A" cavity plate Cavity insert Shoulder bushing

f hal ary n tio Sta

t Par

Support plate

Leader pin

Ejector retainer plate Ejector pin Ejector plate Sprue puller pin

Ejector housing

Knock-out bar

Figure 10-7 Two-plate mold and components (cross section)

alf

gh

vin Mo

line ne

g li

tin Par

"B" cavity plate Core insert

ing

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10.9 Types of Thermoplastic Injection Molds

10.9.2

Round Mate® Interchangeable Insert Molds

The Round Mate® System consists of a circular master frame with easily removable interchangeable round inserts, as shown in Figure 10-8. For high injection molding production operations, or to produce different parts at the same time, the use of multi-cavity inserts and a hot runner system is available in all sizes. The Round Mate® System provides the ﬂexibility and ease of operation to change inserts in the injection molding machine in just ten minutes. This Round Mate® System saves needless hours of mold making time. Using the universal Round Mate® Frame, standard machining and chase work are done once. For new injection molded parts, only the individual, interchangeable inserts are needed at a reduced cost of a complete, traditional mold. The Armoloy plated, AISI 4140 tool steel master frame is completely rust and corrosion resistant. The design of the inserts allows the cooling system to be wiped free of all mineral deposits and corrosion. Maintenance is reduced to a minimum. Cavities and runner systems can be cut directly into the solid insert. Inserts are available in a choice of tool steels and can be heat treated or plated. SAE P20 (prehardened) and SAE H13 (heat treatable) are in stock for immediate shipment. Other tool steels, such as stainless steel 420, SAE S7, SAE A6, are available as special order. Master frames and inserts are available in 2.50, 4.00, 6.00 and 8.00 in diameters. The Round Mate® system has several advantages:

Fixed frame half

Fixed insert

Moving frame half

Moving insert

Figure 10-8 Round Mate® frame and interchangeable cavity inserts

• Fast standard mold delivery time • Reduced mold costs. Design and labor time are reduced to a minimum because the master frame and mold insert blanks are standard. Normal chase work is already completed. The Round Mate® system has a unique water cooling jacket, cavity pockets, guided ejection system, and many other features already incorporated. • Fast insert changeover. Simple hex-key wrenches are used to change the mold insert in the injection molding machine in less than ten minutes. • Improved part quality. The beneﬁts of a round shape include more uniform clamping pressure in the mold, balanced heat transfer and cooling, completely self-centering, more efﬁcient runner layout, better part quality, and faster molding cycles.

10.9.3

Master Unit Die Interchangeable Insert Molds

MUD frame

With the advent of the standard mold base, Master Unit Die Products, Inc. has developed a system for the quick replacement of inserts, which are easily interchangeable within a single frame for all injection molding machine applications. This approach was certainly a major breakthrough in reducing mold costs for the plastics industry. By combining a standard mold frame with interchangeable mold inserts, this concept not only cuts mold building costs and reduces mold delivery time, it also increases productivity by reducing the injection molding machine down time signiﬁcantly. Master Unit Die Products, Inc. offers frames in four basic styles and companion inserts in two basic styles. These frames and their companion inserts are in use throughout the world. A typical MUD mold is shown in Figure 10-9.

Interchangeable companion insert

Figure 10-9 MUD mold frame and interchangeable cavity inserts

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10 Injection Mold Design MUD systems ﬁnd applications in different areas: • Prototype parts. The MUD system is ideal for prototype parts. Fast mold design is achieved by not using the entire mold assembly. Prototype part mold costs can be reduced to an absolute minimum. • Short production runs. The almost instant interchangeability of MUD inserts is a big advantage when a variety of parts are scheduled for production. Production time can be gained during changeovers. MUD systems offer several advantages: • Lower mold costs. The concept makes the inserts easily interchangeable within a single frame. Mold costs are reduced because only the insert is replaced, not an entire standard mold base • Quicker delivery. The simplicity of the system means less time is required for mold fabrication. Standard blank inserts are usually available from stock within ten days • Faster setup. The Master Unit Die frame remains in the injection molding machine with the cooling lines operational when inserts are interchanged. Initial production setup is simply a matter of sliding and clamping the insert in position (leader pins and bushings in the core and cavity plates assure alignment) • Instant changeover. Inserts are easily removed and interchanged by loosening four clamps, disconnecting the cooling lines, removing one insert and sliding in another, then simply reversing the process, the complete change taking normally less than ten minutes • Minimum purging. The injection molding machine’s plastifying unit is less likely to overheat, requiring purging, because of instant changeover of the MUD inserts (material and production savings). • Easier maintenance and repair. The MUD inserts are easier to remove and reinstall, they are lighter, smaller, easier to handle and store, which is a big plus when maintenance or repair is required • Greater ﬂexibility. Most Master Unit Die frames will accommodate two or more inserts. When single molding applications are scheduled, a blank insert can be installed in the other section of the frame. • Maximum versatility. Inserts are available in “T” and standard styles. Mold design latitude is almost unlimited because inserts can be engineered for parts requiring stripper plates, sleeve ejection, single or double cam action, hydraulic, mechanical or pneumatic powered cylinders, and any feature desired including three or four plate molds.

10.9.4

Three-Plate Mold Cold Runner System

The three-plate mold design is used where a center pin point gate is required on a multi-cavity mold or when the molded part geometry requires a stripper plate ejection. This three-plate mold system operates as follows: The mold is opened at the parting line and the sprue is pulled immediately by the sprue puller pin. The entire three-plate runner system thereby moves back with the moving cavity

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10.9 Types of Thermoplastic Injection Molds

Figure 10-10 Three-plate mold cold runner system

plate, the runner is withdrawn from the sucker pin by the stripper plate. When the ﬂoating distance of the sucker pin has been taken up, the three-plate runner system with the pin point gate breaks off from the cavity as a result of the pull-off force produced by the sucker pin. The continuous movement of the “A” cavity plate causes the sucker pin to shear off from the three-plate runner and frees itself from the cavities ready for ejection. When the mold is closed, the plates are progressively returned to their original positions. An illustration of a three-plate mold system, cold runner, and a pin point gate is shown in Figure 10-10.

10.9.5

Vertical Insert Mold for Thermoplastic Encapsulations

The vertical mold clamp and horizontal injection system for encapsulating inserts was developed to produce electrical and electronic thermoplastic products such as electric cord plugs, transformers, motor housings, automotive speed and temperature sensors, etc. Figure 10-11 shows a thermoplastic encapsulated automotive ABS sensor and a fuel injector produced by this molding process. The properties of special thermoplastic injection molding polymers are ideal for encapsulating various types of inserts by using vertical clamp and horizontal injection molds. These polymers have excellent electrical properties, high end use temperatures, and good chemical resistance. Some of these thermoplastic resins are also “wire friendly” for encapsulating delicate electrical wires and leads. Figure 10-12 shows a typical vertical clamp and horizontal injection mold for encapsulating wire wound bobbins and magnet inserts for the production of electronic ABS sensors. This type of mold may look similar to a two-plate mold, but it operates completely differently. The upper half of the mold (top clamping and “A” cavity plates) is mounted to the moving vertical platen of the machine, but it does not have a sprue bushing and a locating ring. The lower half of the mold is mounted to the ﬁxed platen, shuttle platen, or rotating table of the machine, depending on

Figure 10-11 Thermoplastic encapsulated ABS sensor and fuel injector

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10 Injection Mold Design

Figure 10-12 Vertical clamp, horizontal injection unit

the application. The ejection system is located in this lower half and connected to the knock-out bar that is operated by a hydraulic actuator. The injection unit is in-line with the mold parting line; the melt is injected into the split sprue bushing (mounted in each half of the mold) that is connected directly to the main cold runner and sub runners, distributing the melt through the gates to the cavities. The inserts are loaded horizontally in the lower cavities, on top of the support pins. When the mold is closed, the spring loaded upper support pins lock the inserts in position, preventing the inserts from moving during the melt injection in the cavities. The melt shrinks around the inserts forming a good mechanical bond between the inserts and the thermoplastic during the mold cavity cooling. The molded parts and the runner system remain in the lower half of the mold, when the mold opens, the ejector pins push the molded parts out from the lower cavities and the operator removes the parts and loads the inserts to start a new molding cycle.

10.9.6

Hot Runner Molding Systems

In a hot runner system the melt directly ﬁlls the cavity from which the solidiﬁed thermoplastic part is ejected when the mold opens. The melt is conducted through heated channels through the runnerless manifold where the melt is proportionally subdivided, depending on the number of cavities and injected to the cavities through the temperature controlled gates or drops. Hot runner molding can result in as much as a 50% cost savings, part quality is improved, cycle time is faster, and since parts are ﬁnished when they leave the mold, it is not necessary to remove sprues or runners. Hot runner molding systems eliminate the use of cold runners, the melt is maintained at a constant temperature until it reaches the cavities, providing a uniform melt viscosity and better injection pressure control reducing the melt shot capacity. The hot runner molding systems are more expensive, they require an additional temperature controller, higher maintenance, and operational training. Figure 10-13 shows a typical hot runner mold cross section illustrating the operational sequences of this molding process.

10.9 Types of Thermoplastic Injection Molds

Figure 10-13 Typical hot runner mold system (cross section)

10.9.7

Hot Runner Mold Temperature Control Systems

Second only to gating, the most common source of problems in hot runner molds is temperature control throughout the system. A high degree of thermal uniformity in the manifold and from drop to drop is essential to avoid degradation of resin and to obtain uniform ﬁlling, packing, shrinkage, appearance, etc. in all cavities. Each drop has a different temperature proﬁle. The temperature setting for drops may be substantially higher than in the manifold. Manifold temperature should not exceed the process melt temperature. However, lower manifold temperatures will minimize minor streamlining ﬂaws with thermally sensitive resins. If residence time in the manifold is short, a low manifold temperature will have little effect on the temperature of the ﬂowing melt. Some of the devices used to maintain uniform temperature control are the following: Cartridge Heaters The runnerless manifold temperature is controlled with cartridge heaters. The temperature is dependent on the cartridge heater design, wattage, location, support, and installation tolerances of heaters, thermocouples, thermal conductivity of metals used and independent zone controls. Serious problems may result when a single cartridge heater fails, causing the controller to increase heat from other cartridges in the same zone. Temperature uniformity will be upset by the development of hot or cold areas, increasing the overall manifold temperature. Loose ﬁtting cartridges also tend to burn out more frequently. Cartridge heaters are also used as internal heaters inside the manifold runner channels or as torpedoes for drops. Different installation tolerances will cause temperature variations because the thermocouple in the cartridge heater is insensitive to ﬁt problems. Tubular Heaters Tubular heaters, similar to those used to heat the oven of an electric range, are also used to heat hot runner manifolds. They are pressed into grooves on both

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10 Injection Mold Design sides of the manifold plate. They may be shaped and conﬁgured to follow the melt ﬂow paths around the connecting area of each drop. They also provide a more uniform heat input along the length of the heater and are more resistant to burnout because fewer individual heaters are used. Cast-in Heaters Thermally conductive metals such as copper and copper alloys are often used in hot sprue bushings, portions of manifolds, and drops to improve heat distribution and uniformity. Permanent heater elements are often cast into these metals to improve durability. Although failure is much less frequent, they are more expensive. Heat Pipes Kona Corporation has patents on the use of heat pipes to obtain uniform temperatures in manifolds and drops. These devices not only conduct heat rapidly from one area to another, but they tend to equalize temperature along their path by releasing more heat to colder areas. This is a natural result of conducting heat by vaporizing and condensing a liquid in the heat pipe. The colder the area, the more liquid will condense and the more heat will be released. Heat pipes require external heater bands located only at the end of the bushing. They also make it possible to reduce manifold temperature below that needed to keep gates operating, which is an important consideration for heat sensitive resins. Bands and Coil Heaters These are most often used in sprue bushing, pipe connected manifolds and drops. Coil heaters lend themselves to the necessary wattage distribution to avoid overheating the center section of a bushing in an effort to provide enough heat to keep the gate open. Placement of band heaters close to the gate area and use of conductive metal in drops or sleeves can also control the temperature distribution. Torpedo Heaters They are used in sprue bushings, drops, and internally heated manifolds. These torpedo heaters are helpful in distributing wattage effectively in drops and internally heated manifolds. Manufacturers of torpedo heaters have made signiﬁcant progress in overcoming the inherent lack of temperature uniformity of heated manifolds or torpedoes supported at one or both ends in cold steel mold plates.

10.9.8

Hot Runner Mold Gates (Drops)

The most critical hot runner mold design parameter is the gate. The thermoplastic melt must be kept in a ﬂuid condition up to the point of separation. The gate area must freeze rapidly and without serious ﬂaws. Control of gate freezing along with gate vestige, part quality, and appearance are not only functions of temperature control, but of the speciﬁc gate design details needed to avoid serious molding problems. A small insulating ring of titanium (very low heat conducting metal) between the bushing and cavity is used to maintain the temperature differential at the gate. The types of gates used with the hot runner molds are the following:

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10.9 Types of Thermoplastic Injection Molds Sprue

Thermocouple

Hot bushing Coil heater

Hot bushing

Hot bushing

Thermocouple

Coil heater

Air gap

Top insert

Insert Insert

Straight sprue gate

Molten insulation

Reverse taper sprue gate

Figure 10-14 Two sprue gates (drops), straight and reverse taper

Straight Sprue Gate An open gate is a simple, restricted opening to the cavity. It is usually difﬁcult to keep this gate from freezing with semi-crystalline and high melt temperature resins, which leave a long vestige. A straight sprue gate is shown in Figure 10-14, left illustration.

Straight sprue insulated gate Hotbushing

Straight Sprue Molten Insulated Gate Figure 10-15 shows these types of gates; they are straight sprue gates with molten insulated traps for melt surrounding the gate close to the cavity. These traps are intended to provide an impediment to heat transfer away from the gate. Much of the thermoplastic trapped in this insulated melt may freeze, but portions closest to the gate may be hot, which constitutes a serious hold-up spot for heat sensitive resins. Tit Edge Gate The concept of a hot bushing drop feeding two or four side tit edge gates is an attractive way to simplify runnerless molds for small parts. Current designs do not appear to be well suited to control gate freezing and separation for semicrystalline and high temperature resins. Tit edge gates tend to shear ﬂush with the gate opening and leave a frozen stub in the gate, which either freezes solidly preventing subsequent shots or is injected into the next shot as a cold slug. Figure 10-16 shows these types of gates.

Thermocouple

Coil heater

Top insert Pin hole

Molten insulation

Insulated insert

Insulated pin point gate

Reverse Taper Sprue Gate The reverse taper gate is a modiﬁcation of the straight sprue gate. A reverse taper sprue gate land between 0.25 and 0.75 in moves the separation point away from the cold cavity and into the bushing where it can be controlled better, making this gate more suitable for engineering polymers. The resulting long gate vestige on the part is a serious disadvantage. A reverse taper sprue gate is shown in Figure 10-14, right illustration.

Insulated insert

Figure 10-15 Two insulated gates, straight sprue and pin point

Main sprue

Cavity Air gap

Sub sprue

Sprue and tw o horizontal tit edge gate s

Hot bushing Coil heater

Hot Tip Spreader Insert Gate The hot tip spreader is a small, conical insert in the gate end of a hot bushing drop made of copper alloys to aid heat transfer. The spreader’s internal tip contacts the inside diameter of the hot bushing and draws heat from the melt and bushing heater. Several streamlined passages around the cone allow melt to pass into the cavity. The purpose of this spreader is to deliver heat to the gate separation point (prevent freezing) and to establish the exact location of the separation point.

Gate inserts

Sprue and angled tit edge gates Figure 10-16 Hot runner drops, main and sub sprues tit edge gates

Next

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10 Injection Mold Design

Figure 10-17 Hot runner drop, hot tip spreader gate

The external spreader tip is very close to the cavity surface and it tends to limit gate vestige. Figure 10-17 shows this type of gate. Reciprocating Pin Valve Gate A valve gate is an open gate with a reciprocating valve pin. The valve is opened and closed during each cycle by means of a hydraulic, pneumatic, or spring mechanism. Valve pin gates provide maximum control of gate vestige and cosmetics, but they add complexity and introduce serious wear problems for glass or mineral reinforced thermoplastic resins. Valve gates also tend to push cold slugs into the gate area of the part, causing lower impact resistance or appearance problems. Figure 10-18 shows a pin valve gate (top illustration) and a guided pin valve gate (bottom illustration).

Figure 10-18 Hot runner drops, reciprocating pin valve gates (Courtesy: Mold Master)

Hot Tip Fixed and Valve Torpedo Gates These types of gate are established, when an internally heated hot tip torpedo is used in the drop. The hot tip torpedo approaches or enters the gate area. The reciprocating hot tip torpedo works as a valve pin controlling melt ﬂow at the gate. The torpedo is forced to retract from the gate by the injection force of the melt on the torpedo tip (gate open), just before the melt is injected. When injection is complete, Belleville springs above the torpedo ﬂange support, which were compressed during injection, cause the torpedo to close the gate. Figure 10-19 shows three types of hot tip torpedo gates. Melt

Insert

Torpedo down

Torpedo up

Gate close

Fixed hot tip torpedo and water heated insert

Melt

Hot bushing

Water

Water

"O" ring

Air gap

Gate open

Hot tip valve torpedo and water heated insert

Figure 10-19 Hot runner drops, hot tip ﬁxed and valve torpedo gates

Fixed hot tip torpedo and hot bushing