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injection molding process


1
The Complete Injection Molding Process

As summarized in Fig. 1-2, injection molding is an important plastic processing method. This chapter provides an introduction and The

figure shows the necessary components overview of the injection molding machine for the injection molder to be successful and (IMM) process. It provides text with pictorial profitable. Recognize that the first to market reviews. Details on the important informa- with a new product captures 80% of martion pertaining to IMM and reviewed in this ket share. The young tree cannot grow if it chapter are provided in the other chapters. is in the shadow of another tree or if it does Figure 1-1provides an overview that basi- not keep up with competition. You need to cally summarizes what should be considered be at the top of the tree looking over the to ensure that the molded product meets per- other trees. Factors such as good engineerformance requirements and provides a good ing and process control are very important return on investment to produce all types and but only represent pieces of the pie. Without proper marketinghales you are literally out shapes of products for all types of markets. Injection molding is a major part of the of business. This diagram is basically a philoplastics industry and is a big business world- sophical approach to the overall industry in wide, consuming approximately 32 wt% of that it provides examples of all aspects of all plastics. It is in second place to extrusion, the technology and business that range from which consumes approximately 36 wt% (1, local to global competition. The old adage 3, 7). 11: the United States alone there are about the better mousetrap is no longer comabout 80,000 IMMs and about 18,000extrud- pletely true, since you need factors such as the ers operating to process all the many differ- support services from the “tree” to achieve ent types of plastics. In the industry an IMM commercial success and meet product design is not regarded as an extruder; however, it requirements (Chap. 5) (1,499). There are many different types of IMMs is basically a noncontinuous extruder and in some operations is even operated continu- that permit molding many different prodously (Chap. 15). IMMs have a screw plas- ucts, based on factors such as quantities, ticator, also called a screw extruder, that pre- sizes, shapes, product performance, or ecopares the melt (3). nomics. These different types of IMMs are

Introduction

1

2

1 The Complete Injection Molding Process

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COMPLETE CONTROL for MANAGEMENT IndividualCONTROL for each aperatton, from sorimre to hardware
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--ANALYSIS approach to meet dryer,

Integrate a11 indindual operations

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OPERATION

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Immediately afier part is in production--next step IMPORTANT STEP -- is to produce part to meet same requirements but produced at a lower cost

MOLDED
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Use FALLO approach. Reevaluate all parameters used from part design (use less plastic), use lower cost plastic with similiar processing cost (or plastic with higher cost, but faster process, results in lower total cost), check hardware performance, & other parameters described in the IM HANDBOOK.
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Set UP TESTINQ I QUALITY CONTROL Characterize properties, mechanical, physical, chemical, thermal, etc
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FALLO Follow ALL Opportunities

Set up practical i useful TROUBLESHOOTING C/ GUIDE based on causes & remedies of potential faults.
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Fig. 1-1 The FALLO approach: Follow ALL Opportunities.

reviewed throughout this book, particularly in Chap. 15. Small- and large-size IMMs both have their advantages. For example, if several small machines are used rather than one large one, a machine breakdown or shutdown for routine maintenance will have less effect on production rates. However, the larger machine is usually much more profitable while it is running. Because there are fewer cavities in molds for the small machines, they may permit closer control of the molding variables in the individual cavities. The two most popular kinds of IMM are the single-stage and the two-stage; there are also molding units with three or more stages. The single-stage IMM is also known as the reciprocating-screw IMM. The two-stage IMM also has other names, such as the piggyback IMM. It is comparable in some ways to a continuous extruder. The IMM has three basic components: the injection unit, the mold, and the clamping system. The injection unit, also called the plasticator, prepares the proper plastic melt

and via the injection unit transfers the melt into the next component that is the mold. The clamping system closes and opens the mold. These machines all perform certain essential functions: (1) plasticizing: heating and melting of the plastic in the plasticator, (2) injection: injecting from the plasticator under pressure a controlled-volume shot of melt into a closed mold, with solidification of the plastics beginning on the mold's cavity wall, (3) afterjiilling: maintaining the injected material under pressure for a specified time to prevent back flow of melt and to compensate for the decrease in volume of melt during solidification, (4) cooling: cooling the thermoplastic (TP) molded part in the mold until it is sufficiently rigid to be ejected, or heating:heating the thermoset (TS) molded part in the mold until it is sufficiently rigid to be ejected, and (5) molded-part release: opening the mold, ejecting the part, and closing the mold so it is ready to start the next cycle with a shot of melt.

1 The Complete Injection Molding Process

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Selecting Material Proce Evaluate I DetractorsConstraints Cost Analysis Alloys

Compression Reinforced Plastics

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First to market with a new product captures 80% of market share.The young tree cannot grow if it is in the shadow of another tree or if it does not keep up with the competition.You need to be at the top of the tree looking over the other trees.
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Fig. 1-2 Plastic product growth compared to tree growth.

This cycle is more complex than that other processes such as extrusion in that it involves moving the melt into the mold and stopping lw it, rather than having a continuous f o of melt. The injection molding process is, however, extremely useful, since it permits the manufacture of a great variety of shapes, from

simple ones to intricate three-dimensional (3-D) ones, and from extremely small to large ones. When required, these products can be molded to extremely very tight tolerances, very thin, and in weights down to fractions of a gram. The process needs to be thoroughly understood in order to maximize its

4

1 The Complete Injection Molding Process clamping force required is determined. To provide a safety factor, 10 to 20% should be added. Molding Plastics

performance and mold products at the least cost, meeting performance requirements, and with ease (see the section on Molding Tolerances in Chap. 5). Machine Characteristics IMMs are characterized by their shot capacity. A shot represents the maximum volume of melt that is injected into the mold. It is usually about 30 to 70% of the actual available volume in the plasticator. The difference basically relates to the plastic material’s melt behavior, and provides a safety factor to meet different mold packing conditions. Shot size capacity may be given in terms of the maximum weight that can be injected into one or more mold cavities, usually quoted in ounces or grams of generalpurpose polystyrene (GPPS). Since plastics have different densities, a better way to express shot size is in terms of the volume of melt that can be injected into a mold at a specific pressure. The rate of injecting the shot is related to the IMM’s speed and also the process control capability for cycling the melt into the mold cavity or cavities (fast-slowfast, slow-fast, etc.). The injection pressure in the barrel can range from 2,000 to at least 30,000 psi (14 to 205 MPa). The characteristics of the plastic being processed determine what pressure is required in the mold to obtain good products. Given a required cavity pressure, the barrel pressure has to be high enough to meet pressure flow restrictions going from the plasticator into the mold cavity or cavities. The clamping force on the mold halves required in the IMM also depends on the plastic being processed. A specified clamping force is required to retain the pressure in the mold cavity or cavities. It also depends on the crosssectional area of any melt located on the parting line of the mold, including any cavities and mold runner(s) that are located on the parting line. (If a TP hot-melt runner is located within the mold half, its cross-sectional area is not included in the parting-line area.) By multiplying the pressure required on the melt and the melt cross-sectional area, the

Most of the literature on injection molding processing refers entirely or primarily to TPs; very little, if any at all, refers to thermoset TS plastics. At least 90 wt% of all injection-molded plastics are TPs. Injectionmolded parts can, however, include combinations of TPs and TSs as well as rigid and flexible TPs, reinforced plastics, TP and TS elastomers, etc. (Chap. 6). During injection molding the TPs reach maximum temperature during plastication before entering the mold. The TS plastics reach maximum temperature in the heated molds. Molding Basics and Overview The following information provides a complete overview of the process of IM (Figs. 1-3 to 1-10).Continually required is better understanding and improving the relationship of process-plastic-product and controlling the complete process. Injection molding is a repetitive process in which melted (plasticized) plastic is injected (forced) into a mold cavity or cavities, where it is held under pressure until it is removed in a solid state, basically duplicating the cavity of the mold (Fig. 1-11).The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels, or runners, which direct the flow of the melt to the individual cavities (Fig. 1-12). Three basic operations take place: (1) heating the plastic in the injection or plasticizing unit so that it will flow under pressure, (2) allowing the plastic melt to solidify in the mold, and (3) opening the mold to eject the molded product. These three steps are the operations in which the mechanical and thermal inputs of the injection equipment must be coordinated with the fundamental properties and behavior of the plastic being processed; different plastics tend to have different

1 The Complete Injection Molding Process

5

Fig. 1-3 View of an injection molding machine.

-Clamping cylinder

Injection unit

Heating

Injecting

Molding

Fig. 1-4 Basic elements of injection molding.

Fig. 1-5 The basic cycle.

melting characteristics, with some being extremely different. They are also the prime determinants of the productivity of the process, since the manufacturing speed or cycle time (Fig. 1-13) will depend on how fast the material can be heated, injected, solidified, and ejected. Depending on shot size and/or wall thicknesses, cycle times range from fractions of a second to many minutes. Other important operations in the injection process include feeding the IMM, usually gravimetrically through a hopper, and controlling the plasticator barrel’s thermal profile to ensure high product quality (Fig. 1-14). An example of complete injection molding operation is shown in Fig. 1-1.This block diagram basically summarizes what should be considered to ensure a good return on in-

vestment to produce all types and shapes of molded products. The block diagram meets the objective in bringing you up to date on today’s technology as well as what is ahead. These important steps must come together properly to produce products consistently meeting performance requirements at the lowest cost. Basically, the approach is to: (1) design a mold around the product to be molded, (2) put the proper auxiliary equipment around the mold, and (3) set up the necessary fabricating process such as quality controls, troubleshooting guides, preventative maintenance, and operational safety procedures. To be effective, the evaluation of a product should proceed according to a logical step-by-step process (Fig. 1-15). The result is to target for zero defects.

6

1 The Complete Injection Molding Process

Fig. 1-6 Schematic of plastic material flow through hopper and screw to the mold cavity.

People and Productivity

The recipe for productivity includes a list of ingredients such as R&D, new technologies, updated equipment, computer automation systems, and adequate modern facilities. But the one ingredient that ties the recipe together is people. None of the ingredients have much use without the right people. As an example, computer software (CAD, CAM, Plastic Materials CIIM, etc.) have their place together with the systems hardware. However, while the Many thousands of different plastics (also software and hardware all provide impor- called polymers, resins, reinforced plastics, tant resources for automating the manufac- elastomers, etc.) are processed (Chap. 6). turing line, to have the line run efficiently re- Each of the plastics has different melt bequires people to use these resources properly. havior, product performance (Figs. 1-16 and Equipment and plastic materials are not per- 1-17), and cost. To ensure that the quality of the different fect, so that they require the human touch to ensure their repeatability, etc. (see the sub- plastics meets requirements, tests are consection on Plastic Material and Equipment ducted on melts as well as molded products. Variables in Chap. 11.). There are many different tests to provide Achievable processing plans begin with all kinds of information. Important tests on the recognition that smooth does not mean molded products are mechanical tests such as perfect. Perfection basically is an unrealis- those shown in Fig. 1-18, the main one being tic ideal, however one strives to approach it. the tensile test (Chap. 12). The expectation of perfection can block genThere are basically two types of plastic mauine communication between workers, de- terials molded. Thermoplastics (TPs), which partments, management, customers and ven- are predominantly used, can go through dors (see the section on Perfection in Chap. repeated cycles of heating/melting [usu5 ) . A smooth run program can be defined ally at least to 260°C (500"F)I and coolas one that creates a product meeting fac- ing/solidification. The different TPs have diftors such as performance specification and ferent practical limitations on the number

delivery time and that falls within budget. It can be said that perfection is never reached; there is always room for more development and/or improvement. As has been stated throughout history, to live is to change, and to approach perfection is to have changed often (in the right direction).

1 The Complete Injection Molding Process
CLAMPOPEN
MOLD INJECT I O N INJECTION

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I N J E C T I O N T I M E S OUT

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P A R T I S EJECTED FROM MOLD

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RECYCLE I M E R T I M E S OUT T

S T A R T CYCLE ETC.

Fig. 1-7 Molding-machine functions.

8

1 The Complete Injection Molding Process

Products Properties

Density Melt Index Mol. Wt. Distribution Additives
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Resin

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Process Temperature Pressure

Cycle
Mold & Process Design
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Fig. 1-8 Interrelation of product, resin, and process.

of heating-cooling cycles before appearance and/or properties are affected. Thermosets (TSs), upon their final heating [usually at least to 120°C (248"F)], become permanently insoluble and infusible. During heating they undergo a chemical (cross-linking) change. Certain plastics require higher melt temperatures, some as high as 400°C (752°F) (see section on Recycling in Chap. 6). Extensive compounding of different amounts and combinations of additives (colorants, flame retardants, heat and light stabilizers, etc.), fillers (calcium carbonate, etc.), and reinforcements (glass fibers, glass flakes, graphite fibers, whiskers, etc.) are used

with plastics. Compounding also embraces the mixing (alloying, blending, etc.) of two or more plastics that may be miscible or immiscible, with or without additives. With TPs, the mold initially is kept at as low a temperature as possible, below the melting point of the plastic melt. This approach causes the injected hot melt to initiate surface freezing on the cavity wall, followed by formation of the solid product. After a sufficient cooling time, the mold opens and the part(s) are ejected. When processing TSs [from the injection unit (plasticizer)], the hot melt entering the heated mold initially remains below the temperature that would cause premature solidification due to its exothermic reaction.

PROCESS

PRODUCT

Fig. 1-9 Simplified processing steps.

1 The Complete Injection Molding Process

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1

Performance Requirements

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Morphology and Performance

.......;.

The processability and performance of TPs, such as meeting product tolerance requirements and mechanical properties, are influenced by factors such as molecule I Material Selection I size and weight, molecular distribution, and shapes or structures of individual molecules. TPs are formed by combining into long chains of molecules, or molecules with branches (lateral connections) to form complex molecular shapes. All these forms exist in either two or Ideal choiceiCompromise three dimensions, Because of their geomeF g 1-10 Flow diagram for setting up the selec- try (morphology), some of these molecules i. tion procedure. can come closer together than others. These are identified as crystalline (such as PE, PP, After properly filling the cavity or cavities, and PA); the others are amorphous (such as the mold’s higher temperature causes the PMMA, PS, SAN, and ABS). Morphology melt to undergo its final chemical cross- pertains to TPs but not TSs. When TSs are linking action resulting in solidification. processed, their individual chain segments
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I

The Complete Injection Molding Process

are strongly bonded together during a chemical reaction that is irreversible. Plastics are either truly homogeneous, -PLASTIC COOLING IN MOLD TRAVEL amorphous solids or heterogeneous, semicrystalline solids. There are no purely crystalline plastics; so-called crystalline materials also contain different amounts of amorphous SHRINKAGE CCCCRS material. The term semicrystalline is techni30 sec 5 sec cally more accurate, but seldom used. VariFig. 1-13 Example of an injection molding cycle. ous methods of characterizing and evaluating
INJECTION

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HIGH-QUALITY MOLDED PARTS

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F g 1-15 Overall product approach. i.

1 The Complete Injection Molding Process
TOUGH

11

The rheology of plastics, particularly TPs, is complex but manageable. These materials combine the properties of an ideal viscous liquid (pure shear deformations) with those POLYPROPYLENE VINYL of an ideal elastic solid (pure elastic deformation). Plastics are therefore said to be viscoelastic. The mechanical behavior of plastics is dominated by the viscoelastic parameters such as tensile strength, elongation at break, BRITTLE and rupture energy. The viscous attributes of melt flows are very important considerations Note: With formulation changes (via addltlver, flllen,relnduring any processing system (see section on lorcements. alloylng, etc.) porltlon 01 plastlc can moue practically any place in the "pie." Molding Thin Walls in Chap. 7). Fig. 1-16 Range of properties. Viscosity is a material's resistance to viscous deformation (flow). Quantitatively it plastics are used, such as their molecu- is expressed by the modulus of elasticity E lar weight distribution (MWD). A narrow (Chap. 12). MWD enhances the performance of plastic Plastics undergo non-Newtonian flow: the products. MWD affects melt flow behavior curve of pressure vs. flow rate for the melt (Chap. 6). is not a straight line. By contrast, the flow of water is nearly Newtonian. Not only are there these two classes of deformation; there are also two modes in Melt Flow and Rheology which deformation can be produced: simple Rheology is the science that deals with the shear and simple tension. The actual behadeformation and flow of matter under various vior during melting, as in a screw plasticator conditions. An example is plastic melt flow. (injection unit), is extremely complex,
NATURAL GAS PETROLEUM COAL AGRICULTURE

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Fig. 1-17 Raw materials to products.

12

1 The Complete Injection Molding Process

Tensile loud

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Resistance to bending

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Fig. 1-18 Examples of mechanical tests.

displaying many types of shear-tension relationships. Together with the screw design, the deformation determines the pumping efficiency of the plasticator and controls the relationship between output rate and pressure drop through the melt flow to solidification in the mold cavity(s).

Plasticating
Plasticating is the process that melts the plastics. Different methods are used. The most common are the single-stage (recip-

rocating screw) and the two-stage. In Fig. 1-19, (a) and (b) show the ram (also called plunger) systems used in the original IMMs since the 1870s, and now used mainly to process plastics with very little melt flow, such as ultrahigh-molecular-weight polyethylene. They use a piston, with or without a torpedo, for plastication. Part (c) shows the singlestage reciprocating screw plasticator, and (d) the two-stage screw plasticator. There are different IMM operating designs in use: all-hydraulic, all-electrical, and hybrid (combination of hydraulic and electrical). Each design provides different

1 The Complete Injection Molding Process

13

MOLD

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SINGLE-STAGE RAM INJECTION

MOLD

W
MOLD

SINGLE-STAGE OR RECIPROCATING SCREW

SCREW PLASTICATOR

W
MOLD

TWO-STAGE OR PREPLASTICKING SCREW

Fig. 1-19 Examples of different plasticating systems.

advantages such as reducing product weight (reducing plastic consumption), eliminating or minimizing molded-in stresses, molding extremely small to very large products, and/or improving performance. There are also IMMs that perform specialty molding operations. An example is the gas-injection molding machine (GIMM) systems. They basically involve the injection of an inert gas, usually nitrogen, into the melt as it enters

the mold. The gas forms a series of interconnecting hollow channels within the melt. The gas pressure at about 4,300 psi (30 MPa) is maintained through the cooling cycle. In effect the gas packs the plastic against the cavity (Chap. 15). Another design is injection-compression molding, also called injection stamping or more often coining. It uses a compression type mold having a male plug that fits into

I4

1 The Complete Injection Molding Process

Fig. 1-20 Sections of a screw.

a female cavity. After a short shot enters the mold (which has been previously opened and closed so that it is unpressurized), the stress-free melt is compressed to mold the finished product. Other systems include coinjection, two-color injection molding, counterflow injection molding, multi-live injection molding, oscillatory injection molding, reaction injection molding, liquid injection molding, foam injection molding, fusible- and soluble-core injection molding, tandem injection molding, injection blow molding, injection molding with rotation, continuous injection molding (Velcro strips, etc.), metalplastic injection molding, and vacuum injection molding (Chap. 15)
Screw Designs

The primary purpose for using a screw located in the plasticator barrel is to take advantage of its mixing action. The motion of the screw is controlled to keep the IMM’s process controls operating at their set points. The usual variation in melt temperature, melt uniformity, and melt output is kept to a mini-

mum prior to entering the mold. Heat is supplied by heater bands around the barrel and by the mixing action that occurs when the plastic is moved by the screw. Both conduction heating and mechanical friction heating of the plastic occur during screw rotation. The different controls used during injection molding, such as back pressure and screw rotational speed, influence the melt characteristics (Chap. 3). Most IMMs use a single constant-pitch, metering-type screw for handling the plastics. The screw has three sections, for feed, melting (transition), and metering (Fig. 1-20).The feed section, which is at the back end of the screw (where plastic first enters), can occupy from very little to 75% of the screw length, usually 50 to 75%. Its length essentially depends upon how much heat has to be added to the plastic that enters the hopper, where it may be preheated. The melting (transition) section is where the softening of the plastic occurs; the plastic is transformed into a continuous melt. It can occupy from 5 to 50% of the screw length. This section, usually called the compression zone, has to be sufficiently long to make

1 The Complete Injection Molding Process sure that the plastic is melted. A straight compression-type screw is one having no feed or metering section. For certain plastics, particularly TSs, there tends to be no compression zone, since overheating and solidification of the melt could occur between the screw and barrel. In the metering section, the plastic is smeared and sheared to give the melt its final uniform composition and temperature for delivery to the mold. As high shear action will tend to increase the melt’s temperature, the length of the metering section is dependent upon the plastic’s heat sensitivity and whether any additional mixing is required. For certain heat-sensitive plastics very little or no metering action can be tolerated. For other plastics it averages about 20 to 25% of the screw length. Both the feed and metering sections usually have a constant cross section (zero compression ratio). However, the depth of flight in the feed section is greater than that in the metering section. The screw’s compression ratio can be determined by dividing the flight depth in the feed section by that in the metering section. Depending on the plastic processed, ratios usually range from 0 to 4.

15

leased to prevent melt burning and the formation of voids in the product. With TPs, temperature-controlled water (with ethylene glycol if the water has to operate below its freezing point) circulates in the mold to remove heat; with TSs, electrical heaters are usually used within the mold to provide the additional heat required to solidify the plastic melt in the cavity. The mold basically consists of a sprue, a runner, a cavity gate, and a cavity. The sprue is the channel located in the stationary platen that transports the melt from the plasticator nozzle to the runner. In turn, melt flows through the runner and gate and into the cavity. With a single-cavity mold, usually no runner is used, so melt goes from the sprue to the gate. Different runner systems are in use to meet different processing requirements. The most popular are cold and hot runners. With a TP cold runner, the melt flowing from the sprue to the gate solidifies by the cooling action of the mold as the melt in the cavity or cavities solidifies. With a TP hot runner the sprue to the gate is insulated from the chilled cavity or cavities and remains hot, so that the melt never cools; the next shot starts from the gate, rather than from the nozzle as in a cold runner. With a TS hot runner, the melt in the Molds runner solidifies. The TS cold runner keeps the plastic melted by using a cooled insulated The mold is the most important part of the manifold; its next shot starts from the gate, IMM. It is a controllable, complex, and ex- rather than from the nozzle as in a TP hot pensive device. If not properly designed, op- runner. erated, handled, and maintained, its operaMolds are provided with different means, tion will be a costly and inefficient. such as sliders, unscrewing devices, undercuts Under pressure, hot melt moves rapidly (Fig. 1-21), and knockout systems, to eject through the mold. During the injection into products as well as solidified runners at the the mold, air in the cavity or cavities is re- proper time. These basic operations in turn
Nominal thickness should be maintained throughout part

deeper hole

intersecting side walls

Fig. 1-21 Methods of molding holes or openings in side walls without undercutting mold movements.

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1 The Complete Injection Molding Process

Processing
Processing steps are summarized in Figs. 1-9, 1-10, and 1-24 to 1-27. Different machine requirements and material conditions DEPTH OF DRAW are considered in choosing the most efficient injection molding process. It is important to understand and properly operate the basic DIMENSION IMM as well as its auxiliary equipment. In DIFFERENCE F g 1-22 Example of mold-cavity draft angle re- particular, in practically all operations the i. quired to ensure removal of molded product dur- screws must not be damaged or worn and the plastic must be properly dried. Special ing its mold ejection action. dryers and/or vented barrels are required for drying hygroscopic T P materials such as PC, require control of various parameters such PMMA, PUR, and PET (Chap. 10). Use of TP regrind may have little effect as fill time and hold pressure (Chap. 4). To simplify molding, whenever possible on product performance (appearance, color, one should design the product with fea- strength, etc.). However, reduction in perfortures that simplify the mold-cavity melt filling mance can occur with certain TPs after even operation. Many such features can improve one passage through the IMM. Granulated the product’s performance and/or reduce TSs cannot be remelted but can be used as cost. An example is choosing the mold-cavity additives or fillers in plastics. Many TPs can be recycled indefinitely by draft angle according to the plastic being processed, tolerance requirements, etc. (Fig. granulating scrap, defective products, and so 1-22). Figure 1-23 shows a situation where on. During these cycles, however, the plasit is possible to eliminate or significantly re- tic develops a “time-to-heat” history or resduce shrinkage, sink marks, and other defects idence time. This phenomenon can significantly compromise processing advantages (Chap. 8).

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POOR DESIGN SINK MARKS
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SUGGESTED ALTERNATIVES

CORE

BOTH SIDES IF POSSIBLE
v2

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POOR RECTANGULAR PART WITH ROUND HOLES

MATCH OUTSIDE CONFIGURATION TO INSIDE CORES

F g 1-23 Example of coring in molds to eliminate or reduce shrinkage and sink marks. i.

1 The Complete Injection Molding Process

17

Fig. 1-24 Relationship between manufacturing process and properties of products.

Bulk density Tablet density Tablet height

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Feeding ease Feeding accurcy
~ ~~

Pre-heating Process: Compression molding transfer molding injection molding (temperature, time) (time, pressure) (temperature, time)

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Melt flow behavior Curing characteristic Water content

pre-heating mold filling curing Dimensional stability Demoldlng behavior Mold life Machine wear

Shrinkage characteristic Water content Stickiness (adhesion)

Fig. 1-25 Processing behavior.

PROCESS ANALYSIS

PARAMETERS

PRODUCT REQUIREMENTS

t

FILLING PHASE PROCESS MODEL

FOLLOW-UP

CLOSEDLOOP

1

PROCESS COMPUTER MACHINE PARAMETER

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FINAL MACHINE SEllINGS WITH OPERATING RANGES
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INTERFERENCE MAGNITUDES

Fig. 1-26 Process control model.

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1 The Complete Injection Molding Process

Preimpregnated

Post impregnation

Fig. 1-27 Processing steps via a fishbone diagram.

and properties, requiring compensation in the product design or process setup, and/or material modification by incorporating additives, fillers, and/or reinforcements. For all types of plastics, injection molding troubleshooting guides have been written to allow fast corrective action when products do not meet their performance requirements. Examples of errors in the mold and product design with possible negative consequences during processing and/or product performance are presented throughout this book. Troubleshooting guides can be incorporated in process control systems (Chap. 11).An example is checking dryer performance as summarized in Table 1.1.

Process Controls
Proper injection of plastic melt into the mold is influenced by several process control conditions (Chap. 7). Any one or combination of these can affect various performance parameters, such as the rate of which the raw

material is fed into the IMM (Fig. 1-28), flow of melt, packing of mold cavity or cavities and cycle time, which in turn affect product performance (Chap. 8). As an example, parameters that influence product tolerances involve (1) product design, (2) plastics used, (3) mold design, (4) IMM capability, and ( 5 ) molding cycle time. Different types of machine process controls (PCs) can be used to meet different requirements based on the molder’s needs. PC systems range from simple monitors (alarm buzzers, flashing lights, etc.) to very sophisticated program controllers [personal computers (PCs) interrelate different IMM functions and melt process variables]. (Note that PC has two meanings; see Appendix 1,Abbreviations.) Knowledge of the machine and plastic capabilities is needed before an intelligent PC program can be developed (Chap. 9). The use of PC or SPC (statistical PC) software requires continual study of the endless new computer technology as it applies to basically melting plastic (Chap. 13).

1 The Complete Injection Molding Process
Table 1-1 Trouble shooting dehumidifier dryer performance
Symptom Possible Cause(s) Heater failure. Cure

19

1. Cannot attain desired air inlet temperature.

Hose leakages and excessive length on air inlet side. Line, hopper, or filter blockage.

2. Dewpoint as measured at air inlet to the hopper is unacceptable.

Loss of regeneration heaters in one or both beds or line fuses. Loss of timer or clock motor ability to switch from one head to the other, Le., continuous operation on only one desiccant bed.

Check process air or afterheatersregeneration heaters play no part in this aspect of operation. Locate and repair-if the hose is old and brittle, replace. Shorten all hose to minimum lengths. Check for collapsed or pinched lines, valves that are closed (some makes have airflow valves located on the air inlet side of the hopper). Filters should be changed or cleaned frequentlya good trial period is every four weeks until experience dictates a shorter or longer period. These can be checked with a voltmeter at the control panel. Check clock motor for movement by observing either function indicators or valve-shifting mechanisms. Note that loss of regeneration heaters may occur if the clock motor or shifting mechanism malfunctions. Most manufacturers suggest checking the desiccant annually and replacing when it does not meet test criteria. Typically two to three years is a reasonable interval, depending upon the severity of service. During regeneration cycle, exterior of the desiccant bed should be hot to touch. Check contacts on relays or printed circuit board for flaws; check line fuses if so equipped. Replace. Tighten. Change. Correct and relieve restrictions. Use of a pressure gauge or flowmeter is suggested. Proper rotation is that at which the highest flow is indicated.

Desiccant has deteriorated or been contaminated.

Loss of power to one or both desiccant beds.

3. Airflow low or nonexistent.

Fan motor burned out. Loose fan on motor shaft. Clogged filter(s). Restricted or collapsed air lines. Blower motor is reversed.

20

I

The Complete Injection Molding Process

4

\

Fig. 1-28 Hopper feed control unit.

Control Guides
Adequate PC and its associated instrumentation are essential for product quality control (QC). The goal in some cases is precise adherence to a control point. In other cases, maintaining the temperature within comparatively small range is all that is necessary for effortless control (of temperature, time, pressure, melt flow, rate, etc.) that will produce the desired results (Chaps. 7 , 9 , and 13).

Regardless of the type of controls available, the processor setting up a machine uses a systematic approach that should be outlined in the machine and/or control operating manuals. Once the machine is operating, the operator methodically targets one change at a time to achieve maximum injection molding efficiency. With injection molding, as with all types of plastics processing, troubleshooting guides are established to take fast corrective action

1 The Complete Injection Molding Process

21

when parts do not meet their performance re- (2) an increase in annual production volquirements (Chap. 11). This problem-solving ume, (3) a reduction of demolding time, and approach fits into the overall PC and fabri- (4) a shortening of transit time if additional activities can be carried out within the procating interface. Control systems for units with complex grammed cycle time. The profitability of a flexibly automated processes such as injection molding are becoming increasingly common. Such systems injection molding plant is influenced by consist mostly of control chains and circuitry (1) increased capital cost, (2) reduced perthat are often coupled in their functions, as sonnel costs due to fewer personnel required, well as the corresponding exchange of data. and (3) changes in energy costs and the In a broad sense, the control systems serve mold-cost structure. With automation, new the purpose of cost reduction by monitoring goals can be met through plant flexibility, quality and establishing high line efficiency, such as (1) improved delivery consistency, in addition to the reduction of raw mate- (2) greatest possible preparedness for meetrial consumption and labor costs. A control ing delivery dates, (3) large range of products, system contributes in different ways, partic- and (4) short job processing time. There are ularly in controlling the flow of plastic melt. also quality-related effects that result in imIt can function by itself and fulfill the duties proved quality assurance and a reduced numassigned to it, often resulting in product im- ber of rejects. Work environment changes ocprovement. cur in (1) psychological and physical stresses Since the 1960s, a procedure to influence on staff, (2) qualification requirements from important properties of the final product has staff, (3) social welfare of staff employed on been developed. The solutions, when intro- the injection molding machine, and (4) the acduced into practice, served first of all to cident risk situation. An evaluation of the utiimprove the product line in different manu- lization efficiency serves for assessing the crifacturing plants. However, initially these sys- teria that cannot be quantified in monetary tems established themselves in only relatively terms. An established utilization efficiency small niches of the commercial market. Later value can be taken as a decision aid, which in many more came aboard. conjunction with the investment calculation The use of flexibly automated injection will allow a better selection of alternatives molding controls and systems definitely de- under consideration. pends on the tasks the machine has to perform and the production sequences required. Automation is one possibility for Art of Processing putting in-house aims into practice and/or Processing of plastic is an art of detail. The meeting market-dictated demands such as (1) production-cost reduction, (2) short job more you pay attention to details, the fewer processing time, (3) low expenditure on hassles you will get from the process. If a prosetup, (4) greatest possible preparedness cess has been running well, it will continue for meeting delivery dates, (5) large product running well unless a change occurs. Correct range, and (6) improved delivery consistency. the problem; do not compensate. That may In order to utilize the advantages of flexi- not be an easy task, but understanding your bly automated injection molding cells, a con- equipment, material, environment, and peosiderably larger capital investment is nec- ple can make it possible. essary than with other choices of systems, which are less automated and flexible. This increases the investment risk, so that the Fine Tuning question of the profitability of such systems becomes more urgent. The following are A computer-integrated injection molding examples of productivity-increasing effects: (CIIM) system makes it possible to target (1) an increase in the annual utilization time, for: (1) approaching a completely automated

22

1 The Complete Injection Molding Process functions repeat. The IMM stops only in the event of a malfunction or if it is manually interrupted. Machinery and mechanisms are self-controlled so that manual input is not necessary during operation. The continuing development of more sophisticated processing equipment in turn allows the development of more integrated processing equipment. This action results in many improvements, such as (1) increased operating efficiency through reducing scrap and/or rejects, (2) improved quality through uniform, repeatable manufacturing procedures, (3) decision making and record keeping by converting data to information, (4) access to manufacturing information by supervisors and management, and ( 5 )process control and process management. Automation level The automation level is the degree to which a process operates automatically. The choice of level must take into account the ability of the system to diagnose problems in operation, the ability of the system to recover from error or fault, the ability of a system to start up and shut down without human intervention, and the like. Automated vision Vision automation provides a means to achieve automatic equipment operation by adaptive part removal. It provides the capability of detecting a variety of part problems or defects by critical part inspection. Semiautomatic

injection molding system, (2) simultaneously achieving high quality (zero defects), ( 3 ) increasing productivity, and (4) minimizing cost. It does this in several ways, basically by enabling the molder to fine-tune all the relationships that exist among the many machine settings and properties of the plastic melt. These systems, when properly used, readily adapt to enhanced processing capabilities. Once processing variables (machine and plastic) are optimized through computer simulation (rather than the usual trial-and-error method), these values are entered in computer programs in the form of a rather large number of machine settings. Establishing the initial settings during startup can be inherently complex and time-consuming. Regardless, the many benefits of these systems are well recognized and accepted. However, it is evident that self-regulation of injection molding can be effective only when the design of the product and the mold are optimized with the correct processing conditions. Otherwise, a self-regulating IMM is confused and can issue conflicting instructions. The results can be disastrous, including damage to the machine and/or the mold as well as safety hazards. Therefore, the efficient utilization of microprocessor control systems depends on the success of utilizing correct and optimum programs with knowledgeable people (Chap. 9). On the horizon is the potential for fuzzy control to provide an important aid to optimizing process control performance. As reviewed in Chap. 7, fuzzy logic, since its inception in 1981, has striven with increasing success to mimic the control actions of a human operator. Molding Operations The following modes of operations typify injection molding operations. Automatic A machine operating automatically will perform a molding cycle where programmed

A semiautomatic machine will perform a complete cycle of programmed molding functions automatically and then stop. It will then require an operator to start another cycle manually.
Manual It is an operation in which each function and the timing of each function is controlled manually by an operator.

1 The Complete Injection Molding Process

23

Primary

There are a wide variety of tasks requiring the use of auxiliary equipment that inIdentifies the main molding operation cludes warehousing to handling materials. As equipment to fabricate products namely the reviewed throughout this book, performance requirements are important for the successful injection molding machine (Chap. 2). operation of the IMM and auxiliary equipment. They usually require raw materials, additives, spare parts, molds, tools, molded Secondary products, and so on to be stored and handled After fabricating (primary) molded prod- safely and economically. Various systems are ucts, secondary operations may be required available to meet different needs in wareto produce the final finished product. These housing. They can implement schemes for operations can occur online or offline. They integrating the inward and outward flow of include any one or a combination of opera- goods, order picking and transportation, factions such as the following: annealing (to re- tory administration, and process control for lieve or remove residual stresses and strains), warehousing. postcuring (to improve performance); plating; joining and assembling (adhesive, ultrasonic welding, vibration welding, heat weld- Processors ing, etc.);drilling; cutting; finishing; polishing; labeling; and decorating/printing. The type of There are basically three types of procesoperation to be used depends on the type of sor: captive, custom, and proprietary. plastic used. As an example, decorating or bonding certain plastics is easy, while others require special surface treatments for those Capfive purposes (Chap. 10).

Purchasing and Handling Plastics
On the average, raw materials and their handling services incur at least half of the costs in plastic injection molding. Wages, utilities, overhead, and capital equipment costs account for the rest. All costs are important to evaluate and justify. As an example in a highproduction injection molding line, equipment costs may represent less than 5% of the total cost of production. Nevertheless, economy and rationality are worthy aims when purchasing equipment (Chap. 14). It is obviously important to at least purchase the raw materials at favorable prices. One must see that they are delivered punctually bust in time (JIT) or otherwise], provide the required handling systems, use as little as possible (design minimum wall thicknesses of products, do not overpack in cavity, etc.), and ensure that material conforms to the required specification(s). Action is usually required to check materials received.

Captive processors, also called captive fabricators, are in-house facilities of companies that have acquired plastics processing equipment to make parts they need for the product they manufacture. For example, a electrical connector manufacturer may acquire an IMM to produce connectors. Generally speaking, these manufacturers will install a captive operation when their component requirements are large enough to make it economical or they have a secret product or process. Some manufacturers that run their own plastics fabricating lines will nevertheless place a portion of their requirements with outside vendors to keep their own capital investment down, to avoid internal single-source supply, to maintain contact with the outside world and the pricing intelligence it provides, and so on. The vendor may be a custom processor or have a captive operation for their requirements. A problem with some captive operations is that they do not keep up with new developments, some of which may be critical.

24 Custom

1 The Complete Injection Molding Process checklist. Times on cutting tools include basics in equipment and their control operations (2000 h), lathe (800), milling (lOOO), grinder (lOOO), chrome plating (loo), jig bore (700), honing (loo), EDM (300), inspection tools (loo), and so on. The list of postsecondary schools devoting a significant portion of their funds to moldmaking and related programs is growing rapidly. As the industry continues to review the labor pool and come up short, and as undergraduate institutions fight over a shrinking market, education-and-industry partnering is increasing in urgency. As an example, the Moraine Park Technical College of Southeastern Wisconsin, an internationally known facility of the machine tooling industry, is a well-established school with a reput able program that, in conjunction with other area schools, has provided local industries a highly trained workforce for decades (410).

The custom processor’s facilities,like those in the metal-working field, may be called job shops. They process plastics into products or components used in other industries. For example, a manufacturer of injection-molded bottles may retain a custom processor to mold preforms. Custom processors typically have a close relationship with the companies for whom they work. They may be involved (to varying degrees) in the design of the product and the mold, they may have a voice in material selection, and in general they assume responsibility for the work they turn out.

Custom-contract There is a subgroup of custom processors known as contract fabricators. They have little involvement in the business of their customers. In effect, they just sell machine time. Proprietary

Processor Certifications
A proprietary operation is one where the processor makes a product for sale directly to the public or to other companies. It usually has its own trade name. National skills certification programs by different organizations are in existence worldwide to certify the skills and knowledge of plastics-industry processor machine operators. Action by the different organizations continues to provide methods of improving these programs. As an example, the Society of Plastics Industry’s Industries National Certification in Plastics (NCP) program has as its purposes: (1) to identify job-related knowledge, skills, and abilities, ( 2 ) to establish a productive performance standard, (3) to assess and recognize employees who meet the standard, and (4) to promote careers in the plastics industries. The examination includes basic process control; prevention and corrective action on primary and secondary equipment; handling, storage, packaging, and delivery of plastic materials; quality assurance; safety; tools and equipment; and general knowledge. The Society of Plastics Engineers’ Plastics Technology Certification was for plastics professionals who have the knowledge and ability to apply mathematics, the physical

Training Programs
Various training programs and seminars for processors and mold manufacturers are available worldwide. Information concerning processors’ training programs is reviewed in Chaps. 2,9, and 12 as well as other chapters. A tooling example is the apprentice training programs of the USA Tooling & Manufacturing Association (Park Ridge, IL). Their effective programs are based on well-planned services that involve properly supervised onthe-job training and classroom instruction. Such programs start with the development of a policy manual. One of TMA’s most effective trainers is Northwestern Tool and Die Manufacturing Corp. (Skokie, IL). Each training module includes a practical experience checklist, material checklist, practical experience record of hours, and safety

1

The Complete

Injection

Molding

Process

25

SERVICES
ConsultantsDesigners -Process Engineering -ISO -Education -LegalAccounllng -FlnanclalMarketing -Advertising -PublIshing -Training

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I Household I Business I GovernmentI Export I

Fig. 1-29 The plastics industry.

26

1 The Complete Injection Molding Process

Fig. 1-31 This eight-station rotary IMM from EPCO has a shot size of 36 oz using a 150-ton clamping press.

sciences, and engineering principles and reviewed throughout this book and particumethods to technological problem solving. larly in Chap. 2, the basic IMMs must meet Due to the lack of industry response and the many different performance requirements SPE's financial constraints, this program was for molding. closed as of May 1,1999. However, the SPE Figure 1-31 shows an example of a rotary stated that it remains an important concern IMM. and should eventually be reinstated.
Summary
Plastics Machinery Industry

In addition to the injection molding process, the plastics industry is characterized by a wide variety of processing methods for fabricating many different plastic materials into many different products. Figures 1-29 and 1-30 provide a summary of the interrelations of plastics, processing, and products (221). The different processes each have their area of capabilities, at times competing. As

Injection molding (like other plastics fabricating processes) provides the world with useful and/or required products, consuming about 32 wt% of all plastics. With new developments in equipment and materials, the processor is required to keep up to date and determine when changes are to be made, taking advantage of the continuing new developments. Factors such as energy conservation and expanding the use of reinforced

1 The Complete Injection Molding Process

27

plastics (RPs) provide more potential pro- practices not properly updated. A technical cost modeling (TCM) system can be used duct growth. Already injection molding is the highest- for analyzing the economics of alternative volume method for RPs processed using injection molding methods and other promilled or short glass fibers. Long-fiber mate- cesses without the prohibitive economic burrials such as bulk molding compounds have den of trial-and-error innovation and process been used for about half a century using optimization. Cost variations are analyzed by stuffer-ram feeders with ram and/or screw setting up differing (1)performance requireIMM plasticators. With in-mold layups of re- ments, (2) part design, (3) plastic selection, inforcements, RPs’ high-performance direc- (4) hardware selection, and ( 5 ) testing, quality control, and troubleshooting factors tional properties are achievable (1,18). Although considerable talent can be (Chap. 14). Any design choice for injection molding brought to bear on processing and engineering aspects, selecting the best process (or any other process) is a balance between technique and plastic material also involves gains and losses. A gain in one area can comeconomic and legal concerns (Chaps. 14, promise product performance, cost, and/or 16). Cost problems are particularly acute other factors. However, with people workwhen the technology that will be employed ing smarter, using the F A L L 0 approach is not fully understood and much of the (Fig. 1-l),analyzing failures or limitations, cost analysis is based on historical data, and innovating, you can expand your target past experience, and individual accounting and meet future product requirements.


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