Design guides (performance and value with engineer

DesignGuidePerformanceAndValueWithEngineeringPlasticsOverviewofDesignPrinciples2DimensionalStability4Shrinkage6DesignGuidelines8WallThickness8Warpage10RibsandProfiledStructures12GussetsorSupportRibs15Bosses15Holes17Radii&Corners19Tolerances20Coring20Undercuts21DraftAngle22MoldDesignGuidelines23MoldMachine23MoldConstruction24Multi-CavityMolds25ColdRunnerSystems26HotManifold/RunnerlessMolds29GateDesign32MoldCooling40EjectionSystem45TypeofToolSteel46SurfaceFinish47Venting48TableofContents12OverviewofDesignPrinciplesFigure1Molecularstructureofthermoplastics.Theprocessofdevelopingthermoplasticpartsrequiresafullunderstandingoftypicalmaterialpropertiesundervariousconditions.Thermoplasticscanbecategorizedbytheirmolecularstructureaseitheramorphous,semi-crystallineplastics,orliquidcrystalpoly-mers(LCPs).ThemicrostructuresoftheseplasticsandtheeffectsofheatingandcoolingonthemicrostructuresareshowninFigure1.Amorphousthermoplastics.Amorphouspolymershaveastructurethatshowsnoregularity.Inanunstressedmoltenstatepolymermoleculesarerandomlyorientedandentangledwithothermolecules.Amorphousmaterialsretainthistypeofentangledanddis-orderedmolecularconfigurationregardlessoftheirstates.Onlyafterheattreatmentsomesmalldegreeoforientationcanbeobserved(physicalaging).Whenthetemperatureofthemeltdecreases,amorphouspolymersstartbecomingrubbery.Whenthetemperatureisfurtherreducedtobelowtheglasstransitiontemperature,theamorphouspolymersturnintoglassymateri-als.Amorphouspolymerspossessawidesofteningrange(withnodistinctmeltingtem-perature),moderateheatresistance,goodimpactresistance,andlowshrinkage.Figure2Amorphousversussemi-crystallinethermoplastics.3Semi-crystallinethermoplastics.Semi-crys-tallineplastics,intheirsolidstate,showlocalregularcrystallinestructuresdispersedinanamorphousphase.Thesecrystallinestruc-turesareformedwhensemi-crystallineplas-ticscooldownfrommelttosolidstate.Thepolymerchainsarepartlyabletocreateacompactedstructurewitharelativelyhighdensity.Thedegreeofcrystallizationdependsonthelengthandthemobilityofthepolymersegments,theuseofnucleants,themelt,andthemoldtemperatures.Liquidcrystalpolymers.Liquidcrystalpolymers(LCPs)exhibitorderedmoleculararrangementsinboththemeltandsolidstates.Theirstiff,rod-likemoleculesthatformtheparallelarraysordomainscharacterizethesematerials.Thedifferenceinmolecularstructuremaycauseremarkabledifferencesinproperties.Variouspropertiesaretimeortemperaturedependent.Theshearmodulus,forinstance,decreasesatelevatedtemperatures.Theshearmoduluscurveillustratesthetempera-turelimitsofathermoplastic.Theshapeofthecurveisdifferentforamorphousandsemi-crystallinethermoplastics(seeFigure3)Figure3Glasstransitiontemperature(Tg)andmelttemperature(Tm).Figure4Thefollowinggraphdemonstratestimedependentcreepmoduli.Ingeneralsemi-crystallinematerialshavelowercreepratesthanamorphousmaterials.Glassreinforcementgenerallyimprovesthecreepresistanceofathermoplasticmaterial.DimensionalStabilityThefollowinginformationonmoldshrinkage,thermalexpansion,andwaterabsorptionrelatestotheprecisionofacomponentbothduringmoldingandassecondaryeffectsaftermolding.Moldshrinkage.Shrinkageormoldshrink-ageisthedifferencebetweenthemoldcavitydimensionsandthecorrespondingcompo-nentdimensions.Itisnotpossibletopredictexactshrinkagevaluesforaspecificpolymergrade.Therefore,themaximumandminimumvaluesforthevariousDSMthermoplasticsareprovidedinFigure5.Duringinjectionmoldingthepolymermeltisinjectedintothemold.Oncethemoldiscom-pletelyfilledthedimensionsofthemoldingarethesameasthedimensionsofthemoldcavityatitsservicetemperature(seeBinFigure6).Whilecoolingdown,thepolymerstartstoshrink(seeCinFigure6).Duringtheholdingstageoftheinjectionmoldingcycle,shrinkageiscompensatedbypost-filling/packing.Boththedesignofthepartaswellastherunner/gateshouldallowforsuffi-cientfillingandpacking.Theprocessofshrinkagecontinuesevenaftertheparthasbeenejected.Shrinkageshouldbemeasuredlongenoughafterinjec-tionmoldingtotakeintoaccountpost-shrink-age(seeDinFigure6).Secondaryeffects.Ifcomponentsareheatedaftermolding,forexampleduringpaintcuringoperation,thiscancausetemporaryorevenpermanentdimensionalchanges.Theoperatingenvironmentwillalsohaveconsequencesforthedimensionalstabilityofthecomponent.Thermalexpansion.Animportantconditionforthedimensionsofapartistheusetem-perature.Thermoplasticsshowarelativelyhighthermalexpansion(10-4/°C)comparedtometals(10-5/°C).Thermalexpansioncannotbeignoredforlargepartsthatareusedatelevatedtemperatures(seeFinFigure6).4Figure5ShrinkageindicationofDSMpolymersin%.Figure6Dimensionalstabilitythroughtime%.5Moistureabsorption.AkulonandStanylparts,likeallpolyamidemoldings,showdimensionalchangesincreaseaftermoldingduetomoistureabsorption(seeFigure7).Moistureabsorptionisatimedependent,reversibleprocessthatcontinuesuntilequilib-riumisreached.Thisequilibriumdependsontemperature,relativehumidityoftheenviron-mentandthewallthicknessofthemolding.Achangeinmoisturecontentwillresultindifferentproductdimensions.Thedesignershouldanticipatevaryinghumidityconditionsduringuseoftheproduct(seeEin