1 Introduction to Injection Molding

1.1 Introduction to Plastics

Material characteristics and the injection-moldingprocess interactively affect the quality of the result-ing molded part. Fortunately, the overall process isgoverned by thermodynamic principles, making itpossible to manage the characteristics of the finalproduct. In this chapter, the variables that influencefinal part size are identified and explained in thecontext of the fundamental notions of interactivityand thermodynamics. Thermodynamics is the phys-ical science that studies the effects of temperaturevariations and heat transfer on a material, especiallyas the bodies change state as from solid to liquid andback again, as plastics do during plastic processingand as heat is transferred from the injection-moldedplastic part into the cooling system of the mold.

This section of the book is not intended to bea comprehensive discussion of the injection-moldingprocess, but rather an overview of the process.

1.2 Interactivity Basics

Two general types of plastic materials arecommonly used in injection molding. These arereferred to as thermoset and thermoplastic, reflectingtheir manufactured part properties.

A thermoset plastic is one in which cross-linking isstopped early in the reaction. The reaction either willnot continue, or will continue at a very slow rate, undernormal conditions. At temperatures above about 93 �C(200 �F), the material is a viscous fluid that can beforced into a mold. At temperatures of 150e175 �C(300e350 �F), the cross-linking reaction proceeds ata rapid rate until the reaction is complete and essen-tially all possible cross-links are established. (For thinparts, the reaction is complete in a matter of a fewseconds. Thicker wall parts might require severalminutes or more). Once the reaction is complete, thematerial will not again soften to allowmolding. This isa onetime process and is irreversible (although somemolders grindup sprues, runners, and badparts and adda small percentage of the resulting particles as filler to

Handbook of Molded Part Shrinkage and Warpage. http://dx.doi.org/10.10

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the unprocessed raw material for subsequent moldingof additional parts). Thermosetmaterials includemanytypes of rubber (silicone, polyurethane, naturalrubber),melamine formaldehyde (dinnerware), phenolformaldehyde (panhandles and many electrical prod-ucts), the allyl family, which includes diallyl phthalate(mostly electrical parts), and epoxy.

A thermoplastic material also softens to a viscousfluid when heated; however, few, if any, cross-linksare established during processing. A thermoplastichardens to a useful condition when cooled. Whilesoft, the material can be forced into a mold to assumethe shape of the mold. This cycle can be repeatedmany times because the finished part can be groundup and reprocessed. There is no significant chemicalreaction during the processing of thermoplasticsother than some degradation of the physical proper-ties. Although properly processed, there is littledegradation, but if the material requires drying and isnot properly dried before processing, or if excessiveheat is used during processing, significant degrada-tion will occur. The processing temperaturesrequired for thermoplastics differ according to theirmelting temperature. Some plastics can be processedat 205 �C (400 �F) or less. Other high-performancethermoplastics might require processing tempera-tures of 315 �C (600 �F) or more.

Common thermoplastics are encountered ineveryday life. Most jars and bottles containing liquidsor medicines are thermoplastics such as polyethyleneterephthalate, polyethylene, or polystyrene. Furni-ture, carpets, and floors are usually partially orwholly made of one of several thermoplastic resins,such as vinyl or thermoplastic polyester. Electricalwall outlets can be either an engineering grade ofthermoplastic or thermoset. Telephones, computers,television housings, and other electronic devicesare molded of one or more thermoplastic resins,most commonly the thermoplastic alloy known asacrylonitrile-butadiene-styrene. The foamcushions inchairs and beds are often thermoset polyurethane.Many shoes are thermoplastic, including the soles.Most of the storage containers used in the kitchen are

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2 HANDBOOK OF MOLDED PART SHRINKAGE AND WARPAGE

thermoplastic, usually polyethylene. Much of a car’sinterior ismade of thermoplastic, as aremany externalsurfaces.Medicine has undergone a revolution by virtue of

plastics. Most medical devices are now discardedrather than being (imperfectly) sterilized. Manydevices that are being implanted in the human bodyare made of, or contain some form of, thermoplastic.There are thermoplastics that act like rubber,

others that act like glass, and still others that mimicsome metals. Practically all toys are molded ofthermoplastics. Milk and soda containers are made ofthermoplastic resins. Most tools have housings orhandles comprised of thermoplastics. Plastics ingeneral, and thermoplastics in particular, havebecome so pervasive in our world that it is unlikelythat an individual can do anything without direct orindirect contact with plastic.There are many more molders processing ther-

moplastics than are processing thermosets. Thermo-sets seem to be more stable than thermoplastics, andthe challenge of controlling shrink and warp is less.Therefore, this book focuses on the injection moldingof two main types of thermoplastics: crystalline andamorphous. Crystalline plastics form crystals whenthey cool, but do not totally crystallize. They formislands of crystals surrounded by amorphous mate-rial; see Fig. 1.1.1 The crystalline areas are truesolids; thus they tend to be more rigid than amor-phous plastics. Amorphous plastics, on the otherhand, never form crystals and really never solidify.

Figure 1.1 A representation of a semicrystallineplastic at room temperature. Area A represents a crys-talline area, whereas area B represents an amorphousarea.1 Courtesy of Quantum Chemical Corporation.

Amorphous materials are more subject to creep. (Forcomparison, consider glass, which is also an amor-phous material. Glass that has been in a window formany years is measurably thicker at the bottom thanat the top). Crystals lock portions of a moleculerelative to other molecules. The amorphous areasaround the islands of crystals are the only areas thatallow limited creep and only until the moleculesbetween the crystals are fully extended. In general, itcan be said that polymers with molecules having veryuniform linear shapes can crystallize, while polymerswith molecules having irregular shapes, with perhapsmany branches, are not likely to crystallize. Theyform amorphous masses when they cool from themolten condition. It can be argued that although thepolymers used in commodity and engineeringmaterials creep they do not creep indefinitely andthey gradually recover completely when the stress isremoved.2 From a theoretical standpoint, thiscontention may be true for semicrystalline materials;however, it is extremely rare in practice that a stressis removed. More often than not, loaded plasticdeforms under excessive stress until other factorsreduce the stress to a “bearable” level or until theapplication fails. Herein is an essential challenge topart designers and molders working with plastics.The injection molding of thermoplastics is a form

of processing in which highly complex physicalprocesses take place. Each molding compound reactsdifferently as it is heated to a temperature suitable formolding and as it cools within the mold. The moldingcompound first has to be melted, and then injected athigh pressure into a “cold” mold. Since the mold iscooler than the compound, the shaped plastic partsolidifies rapidly and can then be removed from themold. Each step of the injecting and cooling processaffects the quality of the subsequent molded part, asshown in Fig. 1.2: each of the factors on the leftaffects filling and/or cooling to varying degrees.Note that the temperature control system of the

mold plays a central role in the quality and cost-efficiency of injection-molded parts. It decisivelyinfluences quality features such as surface appear-ance and warpage. Efficient mold-temperaturecontrol also helps to save costs, since the coolingtime, and hence the cycle time, can be optimized.Cooling, that is too aggressive, can cause post-molding problems, such as excessive size change;these can occur days, weeks, or even months later.Control systems are examined in more detail inChapters 6 and 8.

Figure 1.2 The diagram shows the primary variables that affect the final size of a molded part. These variablesare the focus of discussion in the chapter sections noted.

Figure 1.3 A typical injection-molding machine. The raw material hopper is at the far right end over the heatingcylinder and injection unit.3 Courtesy of Toyo Machinery & Metal Co., Ltd.

1: INTRODUCTION TO INJECTION MOLDING 3

The thermodynamic processes that prevail duringeach step of the injection-molding process aredescribed in the following section.

1.3 Thermal Principles GoverningInjection Molding

When thermoplastics are injection-molded ina machine resembling the one depicted in Fig. 1.3,3

granules of plastic are melted inside a heated barrel(tube). In the barrel, a screw conveys the plasticforward along the screw into a holding spacewhile thepreviously injected part cools. The plastic granulesare brought from room temperature to a molten statein amatter of few seconds. Themoltenmaterial is thenstored, which develops a “heat history” until it leavesthe barrel one or more cycles later as previously

molded parts are removed from the mold. It mayrequire molding two or more parts before the heatedmaterial leaves the barrel.

1.3.1 Filling

Figure 1.4 shows a schematic diagramof the actionstaking place in the injection-molding machine.4

Before thematerial is injected, themold is closedwithpressure adequate to resist opening under the injectionpressure. Once the injection signal has been given,the screw moves forward and presses the moltenplastic through both the machine nozzle and therunner/gate system into the cavity. At this point, theplasticmelt andmoldmay see pressures in the range of1360 bar (20,000 psi). The filling process frequentlyimposes a high level of mechanical and thermal stresson the melt. The chief parameters affecting this stress

Figure 1.4 Schematic of a typical injection-molding machine.4 Reprinted with permission of Voridian, Divisionof Eastman Chemical Company.

4 HANDBOOK OF MOLDED PART SHRINKAGE AND WARPAGE

are the nozzle/runner/gate geometry, wall thickness ofthe molded part, filling rate, molding compoundtemperature, and mold-wall temperature.The pressure acting on the melt as it moves

through the system causes internal friction as thematerial flows through restrictions and aroundcorners. This friction adds heat to the molten mass.Experimental tests have shown that the meantemperature increase of the plastic material due tofriction is approximately equal to the energy givenup in pressure loss as the plastic flows into the mold.

DvM ¼ Dp=ðr � cpÞ (1.1)

Figure 1.5 A typical temperature profile of a flowingplastic melt. The shape and magnitude of thetemperature variation will differ depending on mate-rial and flow rates.

DvM¼mean temperature increase in the meltDp¼ pressure differential in a flow section of the

distribution systemr¼melt densitycp¼ specific heat capacityNote that this equation does not allow for the

exchange of heat with the cooler mold cavity. It doesdescribe the process independently of the partgeometry.5

As the plastic flows into the mold, it comes incontact with the walls of the mold and starts to coolimmediately. The thickness of the cooled and rela-tively stable plastic against the wall depends on therate of flow of the plastic past the wall and thetemperature of the wall. The faster the plastic flowsand the hotter the wall, the thinner the solidifiedplastic wall and the more friction heat generated.There can actually be a rise in the plastic temperaturenext to the stable wall due to frictional heating, sothat there is a temperature peak adjacent to the stablesolidified wall of plastic that is higher than the coretemperature of the flowing plastic. See Fig. 1.5.5

Another phenomenon taking place during themold-filling process is that the polymer moleculesare partially oriented and stretched in the direction offlow. The molecules try to relax from this stretchedcondition. (Their natural condition is to be morerandomly oriented, like a length of string stuffed ina cup.) This orientation can cause greater shrinkageof unfilled materials in the direction of flow.

1: INTRODUCTION TO INJECTION MOLDING 5

Amorphous materials shrink slightly less whencooled rapidly than when cooled more slowly.However, time and exposure to heat will encourageadditional shrinkage. Over time, especially atelevated temperatures, the ultimate size change isnearly the same. Amorphous materials behave likea box full of corn chips. Their shape is such that theywill not nestle closely together. When shakenviolently and suddenly stilled, the apparent volumein the box is greater than if the shaking is graduallydiminished, allowing the chips to nestle moreclosely. Amorphous materials, of course, do not“nestle” as intimately as crystalline structures.

In the case of semicrystalline plastics, themolecular chain is often folded back upon itself ina nested or layered condition as it attempts to formcrystals. Other molecular chains are normallyincorporated so that any crystal contains fragmentsof many different molecules. The crystallizationprocess tends to pack the long-chain molecules sideby side, causing a more compact structure across thedirection of flow than along it. This sometimesresults in greater cross-flow shrinkage than longi-tudinal shrinkage.

Crystalline molecules cool to a certain point, andthen begin to consolidate into crystals. Since theformation of crystals starts in a multitude of placesmore-or-less simultaneously, the various crystalscannot mesh to form a single large crystal that isthe shape of the part. As crystals form, they releasea lot of heat; therefore, the material temperaturechanges very little as the crystals form. When thecrystals begin to abut one another, the remainingvolume within the part forms an amorphous massas the material temperature again begins to drop.This mixture of amorphous and crystalline volumes(semicrystalline) results in much more shrinkagethan in pure amorphous materials because thecrystalline structure is much denser. The amount oftime available to create the crystalline structuresalso affects the percentage of the volume that iscrystalline. Thus, hotter mold surfaces or thickersections tend to allow a greater percentage ofcrystalline formation than cold molds and thin-walled parts.

Semicrystalline materials act like toothpicks. Ifa container with a large quantity of toothpicks in it isshaken violently (equivalent to heating the plastic),the toothpicks are randomly oriented. If the shakingof the container suddenly stops (rapid cooling),the toothpicks are mostly still randomized, but if the

magnitude of shaking diminishes gradually, thetoothpicks will nestle together in clusters andbecome more organized (crystallized). The degree oforganization depends in part on the rate of reductionof the shaking (rate of cooling).

1.3.2 Holding

Once the mold is completely filled with plastic, theholding phase of the cycle begins. Pressure ismaintained on the plastic in the cavity until the gatefreezes or until pressure is released on the plastic stillin the barrel of the machine. During this phase,a small amount of plastic will flow into the mold asthe plastic in the mold cools and shrinks. Thisholding time and holding pressure have a significanteffect on in-mold shrinkage.

Figure 1.6 represents the volume vs. temperaturerelationship of amorphous and crystalline materials.5

The crosshatched area represents the space betweenmolecules. When amorphous or crystalline materialsare in the fluid state, there is no crystalline structureand a significant amount of free space exists aroundthe molecules. The hotter the temperature, the morethe space is and the more space the individualmolecules occupy.

As the fluid cools, the amorphous material coolsand contracts along the solid line above the hatchedarea. Because amorphous materials do not formcrystals, free spaces always remain between themolecules. Crystalline materials rarely form single-crystal structures, so there is usually some free spacebetween the crystals. The lower solid line in thefigure represents the volume occupied by a fullycrystallized material.

Semicrystalline materials fall somewhere inbetween the two lines. There are amorphous regionsbetween small crystalline regions. The amorphousregions contain some free space, so these types ofmaterials never reach their theoretical maximumdensity. The faster the semicrystalline plastics cool,the smaller the crystalline regions and the larger theamorphous regions. This uncertainty accounts fora significant amount of the unpredictability of plasticshrinkage. Unfortunately, even after the semi-crystalline materials cool to room temperature, theymay continue to slowly increase the percentage ofcrystallization and thus continue to shrink. The solidline within the hatched area of the figure representsone possible temperature vs. volume curve fora semicrystalline material.

Figure 1.6 A volume vs. temperature chart.5 Courtesy of Bayer.

Figure 1.7 Temperature profiles through the partwall at different times after the cavity fills.5

Courtesy of Bayer.

6 HANDBOOK OF MOLDED PART SHRINKAGE AND WARPAGE

1.3.3 Cooling

After the holding phase, the plastic continues tocool until it reaches a temperature at which it is rigidenough to be removed from the mold and remainadequately stable. Too short a cooling time results ina part with excessive shrinkage or warpage. Too longa cooling time results in excessive molded-in stresses(and possible breakage), as well as an uneconomicalcycle time.The temperature of the plastic is not uniform when

it is removed from the mold. The temperature profileacross the wall of the molded part is represented inFig. 1.7 as a function of time after the mold fills.5

Plastic is a poor conductor of heat. The temperatureof the core of the plastic part when it is removed fromthe mold is higher than the surface temperature. Thecore takes longer to cool and shrink than the surface.There are always some molded-in stresses as a resultof this differential cooling. The greater the part wallthickness, the greater the differential cooling andstress. For very thick walls, the core temperature canbe so high that even though the part looks all rightwhen it is removed from the mold, the heat from thecore material can remelt the surface and cause allsorts of difficulties. For this reason, it is sometimesappropriate to place thick-walled parts into a cooling

fluid to keep the surfaces rigid until the core is fullycooled.It should be apparent, then, that a mold has several

functions. It provides an appropriate shape for theplastic part and necessary strength to resist theextremely high injection pressures (which can beover 1350 bar, 20,000 psi). A mold also functions toefficiently and uniformly remove heat from theplastic part, and therefore serves as a heat exchanger.

1: INTRODUCTION TO INJECTION MOLDING 7

While the part is cooling, amorphous materialsbehave differently than crystalline materials. Amor-phous molecules gradually form friction bonds, asopposed to crystalline bonds, with adjacent mole-cules, and the mass becomes progressively moreviscous until it is rigid enough to retain the desiredshape. At this point it is removed from the mold.Continued cooling causes it to become more rigiduntil it reaches its maximum strength and rigidity.

Plastic shrinkage after the part is removed from themold is more complicated than simple thermalcontraction. Simple thermal contraction does notinclude excursions into the molten condition, as doesthe molding process. Furthermore, most materials donot have the longmolecular chain structure that manyplastics have. This structure encourages additionalstress relaxation and crystallization at the tempera-ture at which the molded parts are normally used.

Amorphous materials change very little beyondgradual stress relaxation after they are cooled toroom temperature. Semicrystalline materials, on theother hand, continue to build the crystalline structurefor a while after the part is removed from the mold.The change in structure from room temperature outof the mold to 48 h later is not nearly as great as thechange that occurs during the molding process, but

must be considered. Some semicrystalline materialssuch as nylon are hygroscopic and must be thor-oughly dried before molding. After molding, theywill absorb moisture from their surroundings untilthey are “saturated”. This changes the size andphysical characteristics of the material. For example,dry nylon is much more brittle than saturated nylon,while nylon with a modest amount of absorbedmoisture is quite tough.

References

1. Polyolefin injection molding: an operatingmanual, USI [supplier brochure].

2. McCrum N, Buckley C, Bucknall C. Principles ofpolymer engineering. Oxford Science Publ.;1988.

3. Plastar plastic injection molding machine, ToyoMachinery and Metal Co., LTD [advertisingbrochure].

4. Injection molding lids from tenite polyethylene,Eastman [supplier brochure].

5. Zollner O. Process variables as production costfactors in the injection moulding of thermoplas-tics, ATI 916e, Bayer Application TechnologyInformation.