66Moldflow Design Guide A Resource for Plastics En

10ShrinkageandWarpage•Injectionmoldingandshrinkage•Basiccausesofshrinkageandwarpage•Designingaccuratepartsconsideringwarpage10.1InjectionMoldingandShrinkageInthissectiontherelationshipbetweenprocessingandshrinkageisconsidered.Inparticular,theeffectofpackingpressureonshrinkageisdescribed.10.1.1WhatAreShrinkageandWarpage?Partshrinkagemaybethoughtofasageometricreductioninthesizeofthepart.Iftheshrinkageisuniform,thepartdoesnotdeformandchangeitsshape,itsimplybecomessmaller.Warpageresultswhenshrinkageisnotuniform.Ifregionsofthepartshrinkunequally,stressesarecreatedwithinthepartwhich,dependingonpartstiffness,maycausetheparttodeformorchangeshape.Inthelongtermpartscanevencrack.10.1.2ShrinkageandMachineSettingsAllmoldersknowthatshrinkageandconsequentlywarpageisaffectedbyprocessingconditions.Figure10.1showssomeoftheclassicrelationshipsbetweenmachinesettingsandshrinkage,alsoshownistheeffectofwallthickness.Thesecurvesapplyonlytoaparticularmoldandmaterialcombination.ItisclearfromFigure10.1thatthefinalshrinkageofacomponentisacomplexfunctionofmachinesettings.Nevertheless,amajorfactoristhepressureandtimehistoryofthematerialasitfills,packs,andcoolsinthemold.174ShrinkageandWarpageFigure10.1Effectofmachinesettingsonshrinkage10.1.3MoldFillingandPackingPlasticmeltsareverycompressibleatthepressuresusedininjectionmolding.Astherammovesforward,thematerialinthebarreliscompressedsothattheflowrateinthecavityislessthanindicatedbytherammovement.Astheramslowsdown,theplasticexpandsunderpressure.Meltcompressibilitycausesasmoothtransitionfrommoldfillingtopacking.Themoldingprocessisfrequentlydividedintotwophases.Injectionmolderswillcommonlytalkaboutthefillingandpackingstagesbecausethiscorrespondstomachinesettings.Experimentsonaninstrumentedmoldshowthisconceptisfarfromthetruth.Figure10.2illustratesasimplemoldwithpressuretransducersPT1,PT2,andPT3positionedasshown.ThelineslabeledPT1,PT2,andPT3showthepressuresrecordedbythesetransducersduringfillingofthemold.Becauseofthecompressibilityofplastic,thereisatimedelaybetweenramdisplacementandplasticmovement.Thisactualswitchfromfillingtopackingonthemachineusuallyoccursbeforethecavityisfilled(seeFigure10.2)andthefinalstagesoffillingoccurbyexpansionofthepressurizedmaterial.ShrinkageShrinkageShrinkageShrinkageShrinkageShrinkageMeltTemperatureMoldTemperatureInjectionRatePackingPressurePackingTimePartThicknessInjectionMoldingandShrinkage175Figure10.2Pressuretracesforasimplemolding10.1.4HowPressureandTimeAffectShrinkageThemagnitudeofpressureandthetimeforwhichpressureisappliedgreatlyaffecttheshrinkageofmaterialinthecavity.Theactualpressuretowhichthematerialissubjectedisdeterminednotonlybymachinesettings,butalsobytheviscosityofthematerialandthegeometryofthecavity.Althoughacomplicatedmatter,itispossibletorestrictattentiontotwoimportantregions:closetothegateandattheendofflow.10.1.4.1ShrinkageneartheGateAreasnearthegateareeasiertopressurize(anddepressurize)thanareasattheendofflowandgenerallytherelationshipbetweenpressure,timeandshrinkageissimple.Highpackingpressuregiveslowershrinkagesaslongasthepressureiskeptonuntilthegatehasfrozen.Inthiscasetheshrinkagearoundthegatewillgenerallybelowerthanthatattheendofflow.Ifthepackingpressureisnotheldonuntilthegateorrunnersystemhasfrozen,thenthepressureinthecavitywillcauseplastictoreverseflowbackintotherunnersystem.Thiscanresultinahighershrinkagearoundthegateareathanintherestofthecavity.20100PT1PT2PT3Pressure(MPa)RampositionPT1presstracePT2presstracePT3presstraceTimeatwhichtheramstopsTimeatwhichtheflowfrontreachesPT3176ShrinkageandWarpage10.1.4.2ShrinkageattheEndofFlowPressurehastobetransmittedthroughtheplastictoreachtheextremitiesofthecavity.Cavitygeometry,viscosity,andthetimethemeltchannelinboththefeedsystemandcavityremainopendeterminehowwellpressureistransmitted.Ahighpackingpressureresultsinahighinitialflowasthepressureisquicklydistributedthroughoutthecavity.Oncethecavityispressurized,theflowintothecavitywillresultfromthecontractionofthematerialandmaybeveryslowincomparisonwiththeinitialflow.Inotherwordstherewillbeahighinitialflowfollowedbyaveryslowflow.Alowpackingpressuremaygivetheoppositeeffect.Initiallytheflowratewillbemuchsmallerthanwiththehighpressuresothefrozenlayerwillgrowquickly.Howeverasthematerialcoolsthevolumetricchange(fromhightolowtemperature)ismuchgreateratlowpressuressotheflowrateduetocompensationwillbegreaterthanforthehigherpressure.Highpackingpressuresdonotautomaticallymeanthattherewillbelessshrinkageattheendofflow.Thisisbecausetheplasticwillfreezeoffintheupstreamsectionearlierinthecycle,thuspreventingthepressurepackingouttheareaattheendofflow.10.1.5ThermallyUnstableFlowPlasticflowisself-reinforcing,thatis,flowwillcarryheatintoanareatherebymaintainingflow.ThiswasillustratedinChapter1.Adiskwithathickouterrimwaspackedouttogiveahighcompensatingflowtothethickouterrim.Theplasticdoesnotflowasathindiskbutformsaseriesofflowchannelsthatareself-reinforcing,maintainingplastictemperatureandheatingthemold,whileotherareaswithlowflowsfreezeoffearlyinthepackingphase.Theflowch