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MOLDMATERIALSMAKINGTHEMOSTOFHIGH—PERFORMANCEMOLDMATERIALSUnderstandinghighconductivityalloysandoptimizingtheirusecanhelpyoubuildbettermolds.ByDouglasVeitch,Director,BrushWellmanInjectionmoldersandblowmolderscanbenefitfromhighconductivityalloysbyachievingfastercycletimesandbetterpartquality.Therearecertainpropertiesofthemoldmaterialandpolymerthatenabletheseefficienciestoberealized.Oncethesecharacteristicsareunderstood,moldbuilderscanoptimizetheiruseofhigh-performancematerialstoprovideadurable,fast-cyclingmoldfortheircustomers.CoolingTimeMoldAlloyThermalPropertiesSomecharacteristicsofmoldmaterialsenableustobetterunderstandthethermalprocessthatoccurswhilemolding.Threeimportantpropertiesare:1.ThermalConductivityHigherthermalconductivityequatestothetransferofmorethermalenergyperunitoftimeundersteadystateconditions.2.ThermalDiffusivityHigherthermaldiffusivitymeansthatthermalequilibriumwillbereachedfasterwhenthetemperaturechanges.Agoodthermaldiffuserwillreactmorequicklytoenvironmentaltemperaturechanges.3.ThermalEffusivity(conductivitydividedbythesquarerootofthediffusivity)Higherthermaleffusivityisameasureofthematerial’sefficiencyatinstantlyremovingheatfromanobjectatahighertemperaturewithwhichitsuddenlymakescontact(seeChart1).Thefollowingexplainswhatallofthismeanswhenmoldingplastics.1.Heatmolduptooperatingtemperature(viawaterchannels).Thehigherdiffusivityallowsthecoppermoldalloytoreachequilibriumfaster,sothemoldingoperationcanbeginsooner.2.Injecthotplasticmeltintothemoldandcool.Highereffusivitymeansthemoldwillbegintoinstantlyandefficientlyremoveheatfromtheplastic.Thenthehighdiffusivitytranslatestoreachingsteadystate,uniformtemperaturequickly.Finally,onceatequilibriumtheconductivitydetermineshowfastthethermalenergywillberemovedfromtheplasticuntilthepartreachesthedesiredejectiontemperature.3.Maintainsetpointtemperature(equaltowatertemperature)duringmold-open,ejectionandmold-closeportionsofthecycle.Again,thehighdiffusivityenablesthemoldtomaintainequilibriumatsetpointduringmoldopen,ejectionandmoldclose.Sincetheairisapoorthermalmedium,thecontactbetweenthewaterandcopperistheoverridingfactor.Chart1MoldMaterialThermalConductivity,W/(cm-oK)ThermalDiffusivity,cm2/secThermalEffusivitySteel-P200.290.0811.02CopperBeryllium1.250.4151.94Figure1:IRtempdistribution.ImagescourtesyofBrushWellmanInc.Figure1showspicturesfromathermalFEAillustratingtheuniformtemperatureofacopperberylliummoldcomparedtothatofamoldmadeofP-20steel.PolymerTypesThetwomainpolymerfamilies—semi-crystallineandamorphous—bothbenefitfromhigherconductivitymoldmaterials.Semi-crystallinepolymershaveadenselypacked,uniformmolecularstructureandincludematerialssuchaspolyamide(nylon),polyethylene,polypropyleneandpolyacetal.Thesepolymersbecomeamorphouswhenmeltedduringprocessingandwillbecomesemi-crystallineagainwhencooled.Amorphouspolymershavealooseandrandommolecularstructure,sothatinsomecasesamorphousmaterialsaretransparent.Bothtypesofpolymerscanbenefitfromimprovedheattransferandreducedcoolingtime.Thefollowingaresomedifferencesthatneedtoberealizedtoprovideabetterunderstandingoftheapplication.Crystallinematerialshaveasharpmeltingpoint,andthusalatentheatenergythatmustbeaddedwhenmelting,andremovedwhencooling.Theplasticneedstobesolidifiedandcooledbelowtheheatdeflectiontemperaturebeforeejectionfromthemold.Theheatdeflectiontemperature(HDT)isavailableonmostresindatasheets.Justgettingbelowthemeltingpointisnotenough.Theparthastobecooledtothepointwhereitisstiffenoughtoeject.GlassandmineralfillersincreasethecrystallizationrateandtheHDTsothepartcanbeejectedatahighertemperaturewithoutdeformation.Amorphouspolymersdonothaveameltingpoint,butastheheatinputisincreasedabovetheglasstransitiontemperature(Tg),theviscosityofthepolymerdecreasesuntilitbeginstoflow.Heatisaddeduntiltheplasticcanflowadequatelytofillthemold.ThentheheathastoberemoveduntilthepolymerisbelowtheTg—inmanycasesbeforethepartwillbestiffenoughtobeejected.Ingeneral,crystallinepolymerscontainmoreheatenergyduetothelatentheat.Forexamplepolycarbonate—whichisamorphous—hasaheatcapacityof1.2J/(goK)whilepolypropylene—whichissemi-crystalline—hasaheatcapacityof1.9J/(goK)or58percenthigher.Molderswillexperiencecycletimereductionsandimproveduniformityofcoolingforbothfamiliesofplasticswhenusinghighconductivitymoldalloys.Somesemi-crystallinematerials—suchasnylon—requirerelativelyhighmoldtemperaturestoprovidegoodsurfacefinishandmaximumcrystallinity.Highconductivitymoldalloyscanimprovebothcharacteristics,andreducecycletimeasanaddedbonus.Thiseffectisachievedbysimplyrunningthemoldatthedesiredtemperature—forexample180oF.Thehighconductivityalloywillbeabletoremoveheatfasterthansteel,butattherecommendedtemperature,andtheheatremovalwillbemoreuniform.Theresultisreducedcoolingtimeandmoreuniformcrystallinityinthemoldedpart.Whenmoldingamorphousplastics,uniformcoolingalsoisveryimportant.Forclearpolymers—likepolycarbonate—thepartwillhavebetterclarityandtoughness.WaterCoolingWithsteeltools,moldersoftenrunchillerstoreducecycletimesandtocompensatefortheredu