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TheInjectionMoldingTheIntroductionofMoldsThemoldisatthecoreofaplasticmanufacturingprocessbecauseitscavitygivesapartitsshape.Thismakesthemoldatleastascritical-andmanycasesmoreso-forthequalityoftheendproductas,forexample,theplasticitingunitorothercomponentsoftheprocessingequipment.MoldMaterialDependingontheprocessingparametersforthevariousprocessingmethodsaswellasthelengthoftheproductionrun,thenumberoffinishedproductstobeproduced,moldsforplasticsprocessingmustsatisfyagreatvarietyofrequirements.Itisthereforenotsurprisingthatmoldscanbemadefromaverybroadspectrumofmaterials,including-fromatechnicalstandpoint-suchexoticmaterialsaspapermatchedandplaster.However,becausemostprocessesrequirehighpressures,oftencombinedwithhightemperatures,metalsstillrepresentbyfarthemostimportantmaterialgroup,withsteelbeingthepredominantmetal.Itisinterestinginthisregardthat,inmanycases,theselectionofthemoldmaterialisnotonlyaquestionofmaterialpropertiesandanoptimumprice-to-performanceratiobutalsothatthemethodsusedtoproducethemold,andthustheentiredesign,canbeinfluenced.Atypicalexamplecanbeseeninthechoicebetweencastmetalmolds,withtheirverydifferentcoolingsystems,comparedtomachinedmolds.Inaddition,theproductiontechniquecanalsohaveaneffect;forinstance,itisoftenreportedthat,forthesakeofsimplicity,aprototypemoldisfrequentlymachinedfromsolidstockwiththeaidofthelatesttechnologysuchascomputer-aided(CAD)andcomputer-integratedmanufacturing(CIMS).Incontrasttothepreviouslyusedmethodsbasedontheuseofpatterns,theuseofCADandCAMoftenrepresentsthemoreeconomicalsolutiontoday,notonlybecausethisproductioncapabilityisavailablepin-housebutalsobecausewithanyothertechniqueanorderwouldhavetobeplacedwithanoutsidesupplier.Overall,althoughhigh-gradematerialsareoftenused,asarulestandardmaterialsareusedinmoldmaking.New,state-of-theart(high-performance)materials,suchasceramics,forinstance,arealmostcompletelyabsent.Thismayberelatedtothefactthattheirdesirablecharacteristics,suchasconstantpropertiesuptoveryhightemperatures,arenotrequiredonmolds,whereastheirnegativecharacteristics,e.g.lowtensilestrengthandpoorthermalconductivity,haveaclearlyrelatedtoceramics,suchassinteredmaterial,isfoundinmildmakingonlytoalimiteddegree.Thisreferslesstothemodernmaterialsandcomponentsproducedbypowdermetallurgy,andpossiblybyhotisocraticpressing,thantosinteredmetalsinthesenseofporous,air-permeablematerials.Removalofairfromthecavityofamoldisnecessarywithmanydifferentprocessingmethods,andithasbeenproposedmanytimesthatthiscanbeaccomplishedusingporousmetallicmaterials.Theadvantagesoverspeciallyfabricatedventingdevices,particularlyinareaswheremeltflowfrontsmeet,I,e,atweldlines,areasobviousasthepotentialproblemareas:ononehand,preventingthetextureofsuchsurfacesfrombecomingvisibleonthefinishedproduct,andontheotherhand,preventingthemicrosporesfromquicklybecomingcloggedwithresidues(brokenoffflash,depositsfromthemoldingmaterial,so-calledplateout,etc.).Itisalsointerestinginthiscasethatcompletelynewpossibilitieswithregardtomolddesignandprocessingtechniqueresultfromtheuseofsuchmaterials.DesignrulesTherearemanyrulesfordesigningmolds.Theserulesandstandardpracticesarebasedonlogic,pastexperience,convenience,andeconomy.Fordesigning,moldmaking,andmolding,itisusuallyofadvantagetofollowtherules.Butoccasionally,itmayworkoutbetterifaruleisignoredandanalternativewayisselected.Inthistext,themostcommonrulesarenoted,butthedesignerwilllearnonlyfromexperiencewhichwaytogo.Thedesignermusteverbeopentonewideasandmethods,tonewmoldingandmoldmaterialsthatmayaffecttheserules.ThebasicmoldMoldcavityspaceThemoldcavityspaceisashapeinsidethemold,“excavated”insuchamannerthatwhenthemoldingmaterialisforcedintothisspaceitwilltakeontheshapeofthecavityspaceand,therefore,thedesiredproduct.Theprincipleofamoldisalmostasoldashumancivilization.Moldshavemetalsintosandforms.Suchmolds,whicharestillusedtodayinfoundries,canbeusedonlyoncebecausethemoldisdestroyedtoreleasetheproductafterithassolidified.Today,wearelookingforpermanentmoldsthatcanbeusedoverandover.Nowmoldsaremadefromstrong,durablematerials,suchassteel,orfromsofteraluminumormetalalloysandevenfromcertainplasticswherealongmoldlifeisnotrequiredbecausetheplannedproductionissmall.Ininjectionmoldingtheplasticisinjectedintothecavityspacewithhighpressure,sothemoldmustbestrongenoughtoresisttheinjectionpressurewithoutdeforming.NumberofcavitiesManymolds,particularlymoldsforlargerproducts,arebuiltforonlycavityspace,butmanymolds,especiallylargeproductionmolds,arebuiltwith2ormorecavities.Thereasonforthisispurelyeconomical.Ittakesonlylittlemoretimetoinjectseveralcavitiesthantoinjectone.Forexample,a4-cavitymoldrequiresonlyone-fourthofthemachinetimeofasingle-cavitymold.Conversely,theproductionincreasesinproportiontothenumberofcavities.Amoldwithmorecavitiesismoreexpensivetobuildthanasingle-cavitymold,butnotnecessarily4timesasmuchasasingle-cavitymold.Butitmayalsorequirealargermachinewithlargerplatenareaandmoreclampingcapacity,andbecauseitwilluse4timestheamountofplastic,itmayneedalargeinjectionunit,sothemachinehourcostwillbehigherthanforamachinelargeenoughforthesmallermold.CavityshapeandshrinkageTheshapeofthecavityisessentiallythe“negative”oftheshapeofthedesiredproductwithdimensionalallowanceaddedtoallowforshrinkingoftheplastic.Theshapeofthecavityisusuallycreatedwithchip-removingmachinetools,orwithelectricdischargemachining,withchemicaletching,orbyanynewmethodthatmaybeavailabletoremovemetalorbuilditup,suchasgalvanicprocesses.Itmayalsobecreatedbycastingcertainmetalsinplastermoldscreatedfrommodelsoftheproducttobemade,orbycastingsomesuitablehardplastics.Thecavityshapecanbeeithercutdirectlyintothemoldplatesorformedbyputtinginsertsintotheplates.CavityandcoreByconvention,thehollowportionofthecavityspaceiscalledthecavity.Thematching,oftenraisedportionofthecavityspaceiscalledthecore.Mostplasticproductsarecup-shaped.Thisdoesnotmeanthattheylooklikeacup,buttheydohaveaninsideandanoutside.Theoutsideoftheproductisformedbythecavity,theinsidebythecore.Thealternativetothecupshapeistheflatshape.Inthiscase,thereisnospecificconvexportion,andsometimes,thecorelookslikeamirrorimageofthecavity.Typicalexamplesforthisareplasticknives,gamechips,orrounddiskssuchasrecords.Whiletheseitemsaresimpleinappearance,theyoftenpresentseriousmoldingproblemsforejectionoftheproduct.Thereasonforthisisthatallinjectionmoldingmachinesprovideanejectionmechanismonthemovingplatenandtheproductstendtoshrinkontoandclingtothecore,fromwheretheyarethenejected.Mostinjectionmoldingmachinesdonotprovideejectionmechanismsontheinjectionside.PolymerProcessingPolymerprocessing,initsmostgeneralcontext,involvesthetransformationofasolid(sometimesliquid)polymericresin,whichisinarandomform(e.g.,powder,pellets,beads),toasolidplasticsproductofspecifiedshape,dimensions,andproperties.Thisisachievedbymeansofatransformationprocess:extrusion,molding,calendaring,coating,thermoforming,etc.Theprocess,inordertoachievetheaboveobjective,usuallyinvolvesthefollowingoperations:solidtransport,compression,heating,melting,mixing,shaping,cooling,solidification,andfinishing.Obviously,theseoperationsdonotnecessarilyoccurinsequence,andmanyofthemtakeplacesimultaneously.Shapingisrequiredinordertoimparttothematerialthedesiredgeometryanddimensions.Itinvolvescombinationsofviscoelasticdeformationsandheattransfer,whicharegenerallyassociatedwithsolidificationoftheproductfromthemelt.Shapingincludes:two-dimensionaloperations,e.g.dieforming,calendaringandcoating;three-dimensionalmoldingandformingoperations.Two-dimensionalprocessesareeitherofthecontinuous,steadystatetype(e.g.filmandsheetextrusion,wirecoating,paperandsheetcoating,calendaring,fiberspinning,pipeandprofileextrusion,etc.)orintermittentasinthecaseofextrusionsassociatedwithintermittentextrusionblowmolding.Generally,moldingoperationsareintermittent,and,thus,theytendtoinvolveunsteadystateconditions.Thermoforming,vacuumforming,andsimilarprocessesmaybeconsideredassecondaryshapingoperations,sincetheyusuallyinvolvethereshapingofanalreadyshapedform.Insomecases,likeblowmolding,theprocessinvolvesprimaryshaping(pair-sonformation)andsecondaryshaping(pairsoninflation).Shapingoperationsinvolvesimultaneousorstaggeredfluidflowandheattransfer.Intwo-dimensionalprocesses,solidificationusuallyfollowstheshapingprocess,whereassolidificationandshapingtendtotakeplacesimultaneouslyinsidethemoldinthreedimensionalprocesses.Flowregimes,dependingonthenatureofthematerial,theequipment,andtheprocessingconditions,usuallyinvolvecombinationsofshear,extensional,andsqueezingflowsinconjunctionwithenclosed(contained)orfreesurfaceflows.Thethermo-mechanicalhistoryexperiencedbythepolymerduringflowandsolidificationresultsinthedevelopmentofmicrostructure(morphology,crystallinity,andorientationdistributions)inthemanufacturedarticle.Theultimatepropertiesofthearticlearecloselyrelatedtothemicrostructure.Therefore,thecontroloftheprocessandproductqualitymustbebasedonanunderstandingoftheinteractionsbetweenresinproperties,equipmentdesign,operatingconditions,thermo-mechanicalhistory,microstructure,andultimateproductproperties.Mathematicalmodelingandcomputersimulationhavebeenemployedtoobtainanunderstandingoftheseinteractions.Suchanapproachhasgainedmoreimportanceinviewoftheexpandingutilizationofcomputerdesign/computerassistedmanufacturing/computeraidedengineering(CAD/CAM/CAE)systemsinconjunctionwithplasticsprocessing.Itwillemphasizerecentdevelopmentsrelatingtotheanalysisandsimulationofsomeimportantcommercialprocess,withdueconsiderationtoelucidationofboththermo-mechanicalhistoryandmicrostructuredevelopment.Asmentionedabove,shapingoperationsinvolvecombinationsoffluidflowandheattransfer,withphasechange,ofavisco-elasticpolymermelt.Bothsteadyandunsteadystateprocessesareencountered.Ascientificanalysisofoperationsofthistyperequiressolvingtherelevantequationsofcontinuity,motion,andenergy(I.e.conservationequations).InjectionMoldingManydifferentprocessesareusedtotransformplasticgranules,powders,andliquidsintofinalproduct.Theplasticmaterialisinmoldableform,andisadaptabletovariousformingmethods.Inmostcasesthermoplasticmaterialsaresuitableforcertainprocesseswhilethermosettingmaterialsrequireothermethodsofforming.Thisisrecognizedbythefactthatthermoplasticsareusuallyheatedtoasoftstateandthenreshapedbeforecooling.Theromosets,ontheotherhandhavenotyetbeenpolymerizedbeforeprocessing,andthechemicalreactiontakesplaceduringtheprocess,usuallythroughheat,acatalyst,orpressure.Itisimportanttorememberthisconceptwhilestudyingtheplasticsmanufacturingprocessesandthepolymersused.Injectionmoldingisbyfarthemostwidelyusedprocessofformingthermoplasticmaterials.Itisalsooneoftheoldest.Currentlyinjectionmoldingaccountsfor30%ofallplasticsresinconsumption.Sincerawmaterialcanbeconvertedbyasingleprocedure,injectionmoldingissuitableformassproductionofplasticsarticlesandautomatedone-stepproductionofcomplexgeometries.Inmostcases,finishingisnotnecessary.Typicalproductsincludetoys,automotiveparts,householdarticles,andconsumerelectronicsgoods,Sinceinjectionmoldinghasanumberofinterdependentvariables,itisaprocessofconsiderablecomplexity.Thesuccessoftheinjectionmoldingoperationisdependentnotonlyinthepropersetupofthemachinevariables,butalsooneliminatingshot-to-shotvariationsthatarecausedbythemachinehydraulics,barreltemperaturevariations,andchangesinmaterialviscosity.Increasingshot-to-shotrepeatabilityofmachinevariableshelpsproducepartswithtightertolerance,lowersthelevelofrejects,andincreasesproductquality(i.e.,appearanceandserviceability).Theprincipalobjectiveofanymoldingoperationisthemanufactureofproducts:toaspecificqualitylevel,intheshortesttime,andusingarepeatableandfullyautomaticcycle.Moldersstrivetoreduceoreliminaterejectedparts,orpartswithahighaddedvaluesuchasappliancecases,thepayoffofreducedrejectsishigh.Atypicalinjectionmoldingcycleorsequenceconsistsoffivephases:InjectionormoldfillingPackingorcompressionHoldingCoolingPartejectionInjectionMoldingOverviewProcessInjectionmoldingisacyclicprocessofformingplasticintoadesiredshapebyforcingthematerialunderpressureintoacavity.Theshapingisachievedbycooling(thermoplastics)orbyachemicalreaction(thermosets).Itisoneofthemostcommonandversatileoperationsformassproductionofcomplexplasticspartswithexcellentdimensionaltolerance.Itrequiresminimalornofinishingorassemblyoperations.Inadditiontothermoplasticsandthermosets,theprocessisbeingextendedtosuchmaterialsasfibers,ceramics,andpowderedmetals,withpolymersasbinders.ApplicationsApproximately32percentbyweightofallplasticsprocessedgothroughinjectionmoldingmachines.Historically,themajormilestonesofinjectionmoldingincludetheinventionofthereciprocatingscrewmachineandvariousnewalternativeprocesses,andtheapplicationofcomputersimulationtothedesignandmanufactureofplasticsparts.DevelopmentoftheinjectionmoldingmachineSinceitsintroductionintheearly1870s,theinjectionmoldingmachinehasundergonesignificantmodificationsandimprovements.Inparticular,theinventionofthereciprocatingscrewmachinehasrevolutionizedtheversatilityandproductivityofthethermoplasticinjectionmoldingprocess.BenefitsofthereciprocatingscrewApartfromobviousimprovementsinmachinecontrolandmachinefunctions,themajordevelopmentfortheinjectionmoldingmachineisthechangefromaplungermechanismtoareciprocatingscrew.Althoughtheplunger-typemachineisinherentlysimple,itspopularitywaslimitedduetotheslowheatingratethroughpureconductiononly.Thereciprocatingscrewcanplasticizethematerialmorequicklyanduniformlywithitsrotatingmotion,asshowninFigure1.Inaddition,itisabletoinjectthemoltenpolymerinaforwarddirection,asaplunger.DevelopmentoftheinjectionmoldingprocessTheinjectionmoldingprocesswasfirstusedonlywiththermoplasticpolymers.Advancesintheunderstandingofmaterials,improvementsinmoldingequipment,andtheneedsofspecificindustrysegmentshaveexpandedtheuseoftheprocesstoareasbeyonditsoriginalscope.AlternativeinjectionmoldingprocessesDuringthepasttwodecades,numerousattemptshavebeenmadetodevelopinjectionmoldingprocessestoproducepartswithspecialdesignfeaturesandproperties.Alternativeprocessesderivedfromconventionalinjectionmoldinghavecreatedaneweraforadditionalapplications,moredesignfreedom,andspecialstructuralfeatures.Theseeffortshaveresultedinanumberofprocesses,including:Co-injection(sandwich)moldingFusiblecoreinjectionmolding)Gas-assistedinjectionmoldingInjection-compressionmoldingLamellar(microlayer)injectionmoldinLive-feedinjectionmoldingLow-pressureinjectionmoldingPush-pullinjectionmoldingReactivemoldingStructuralfoaminjectionmoldingThin-wallmoldingComputersimulationofinjectionmoldingprocessesBecauseoftheseextensionsandtheirpromisingfuture,computersimulationoftheprocesshasalsoexpandedbeyondtheearly"lay-flat,"empiricalcavity-fillingestimates.Now,complexprogramssimulatepost-fillingbehavior,reactionkinetics,andtheuseoftwomaterialswithdifferentproperties,ortwodistinctphases,duringtheprocess.TheSimulationsectionprovidesinformationonusingC-MOLDproducts.AmongtheDesigntopicsareseveralexamplesthatillustratehowyoucanuseCAEtoolstoimproveyourpartandmolddesignandoptimizeprocessingconditions.Co-injection(sandwich)moldingOverviewCo-injectionmoldinginvolvessequentialorconcurrentinjectionoftwodifferentbutcompatiblepolymermeltsintoacavity.Thematerialslaminateandsolidify.Thisprocessproducespartsthathavealaminatedstructure,withthecorematerialembeddedbetweenthelayersoftheskinmaterial.Thisinnovativeprocessofferstheinherentflexibilityofusingtheoptimalpropertiesofeachmaterialormodifyingthepropertiesofthemoldedpart.F讪】g】llic舛忙psinvohcdin(liemoldiiigpniucss.FIGURE1.Fourstagesofco-injectionmolding.(a)Shortshotofskinpolymermelt(shownindarkgreen)isinjectedintothemold.(b)Injectionofcorepolymermeltuntilcavityisnearlyfilled,asshownin(c).(d)Skinpolymerisinjectedagain,topurgethecorepolymerawayfromthesprue.FusiblecoreinjectionmoldingOverviewThefusible(lost,soluble)coreinjectionmoldingprocessillustratedbelowproducessingle-piece,hollowpartswithcomplexinternalgeometry.Thisprocessmoldsacoreinsidetheplasticpart.Afterthemolding,thecorewillbephysicallymeltedorchemicallydissolved,leavingitsoutergeometryastheinternalshapeoftheplasticpart.FIGURE1.Fusible(lost,soluble)coreinjectionmoldingGas-assistedinjectionmoldingGas-assistedprocessThegas-assistedinjectionmoldingprocessbeginswithapartialorfullinjectionofpolymermeltintothemoldcavity.CompressedgasistheninjectedintothecoreoftheFIGURE1.Gas-assistedinjectionmolding:(a)theelectricalsystem,(b)thehydraulicsystem,(c)thecontrolpanel,and(d)thegascylinder.Injection-compressionmoldingOverviewTheinjection-compressionmoldingprocessisanextensionofconventionalinjectionmolding.Afterapre-setamountofpolymermeltisfedintoanopencavity,itiscompressed,asshownbelow.Thecompressioncanalsotakeplacewhenthepolymeristobeinjected.Theprimaryadvantageofthisprocessistheabilitytoproducedimensionallystable,relativelystress-freeparts,atalowclamptonnage(typically20to50percentlower).Lamellar(microlayer)injectionmoldingOverviewThisprocessusesafeedblockandlayermultiplierstocombinemeltstreamsfromdualinjectioncylinders.Itproducespartsfrommultipleresinsindistinctmicrolayers,asshowninFigure1below.Combiningdifferentresinsinalayeredstructureenhancesanumberofproperties,suchasthegasbarrierproperty,dimensionalstability,heatLive-feedinjectionmoldingOverviewThelive-feedinjectionmoldingprocessappliesoscillatingpressureatmultiplepolymerentrancestocausethemelttooscillate,asshownintheillustrationbelow.Theactionofthepistonskeepsthematerialinthegatesmoltenwhiledifferentlayersofmolecularorfiberorientationarebeingbuiltupinthemoldduetosolidification.Thisprocessprovidesameansofmakingsimpleorcomplexpartsthatarefreefromvoids,cracks,sinkmarks,andweld-linedefects.weldlines,voids,andcracks,andcontrolsthefiberorientation.Low-pressureinjectionmoldingOverviewIQ.yIrijsctinnFreasuriiancCampPECLirsnertHi肿Flo'-vTriir：kftrwaiiM.HtAri.^li—GcubC^tcGiJtjdrid'血Iwm©肌三匚ont-olLow-pressureinjectionmoldingisessentiallyanoptimizedextensionofconventionalinjectionmolding(seeFigure1).Lowpressurecanbeachievedbyproperlyprogrammingthescrewrevolutionsperminute,hydraulicbackpressure,andscrewspeedtocontrolthemelttemperatureandtheinjectionspeed.Italsomakesuseofagenerousgatesizeoranreduceumberofvalvegatesthatopenandclosesequentiallytoreducetheflowlength.Thepackingstageiseliminatedwithagenerallyslowandcontrolledinjectionspeed.Thebenefitsoflow-pressureinjectionmoldingincludeareductionoftheclampforcetonnagerequirement,lesscostlymoldsandpresses,andlowerstressinthemoldedparts.PrugraiTEdRamSpeed,,ScrevRPhtardSack3reEsureFIGURE1Low-pre^ureinjectionmoldingPush-pullinjectionmoldingOverviewThepush-pullinjectionmoldingprocessusesaconventionaltwin-componentinjectionsystemandatwo-gatemoldtoforcematerialtoflowbackandforthbetweenamasterinjectionunitandasecondaryinjectionunit,asshownbelow.ThisprocesseliminatesReactivemoldingProcessingMajorreactivemoldingprocessesincludereactiveinjectionmolding(RIM),andcompositesprocessing,suchasresintransfermolding(RTM)andstructuralreactiveinjectionmolding(SRIM).Thetypicallylowviscosityofthereactivematerialspermitslargeandcomplexpartstobemoldedwithrelativelylowerpressureandclamptonnagethanrequiredforthermoplasticsmolding.relativelyForexample,tomakehigh-strengthandlow-volumelargeparts,RTMandSRIMcanbeusedtoincludeapreformmadeoflongfibers.AnotherareathatisreceivingmoreattentionthaneverbeforeistheencapsulationofmicroelectronicICchips.Theadaptationofinjectionmoldingtothesematerialsincludesonlyasmallincreaseintemperatureinthefeedmechanism(barrel)toavoidpre-curing.Thecavity,however,isusuallyhotenoughtoinitiatechemicalcross-linking.Asthewarmpre-polymerisforcedintothecavity,heatisaddedfromthecavitywall,fromviscous(frictional)heatingoftheflow,andfromtheheatreleasedbythereactingcomponents.Thetemperatureofthepartoftenexceedsthetemperatureofthemold.Whenthereactionissufficientlyadvancedfortheparttoberigid(evenatahightemperature)thecycleiscompleteandthepartisejected.DesignconsiderationsThemoldandprocessdesignforinjectionmoldingofreactivematerialsismuchmorecomplexbecauseofthechemicalreactionthattakesplaceduringthefillingandpost-fillingstages.Forinstance,slowfillingoftencausesprematuregellingandaresultantshortshot,whilefastfillingcouldinduce