mold shrinkage

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1、tech.topicEquistarTechnicalTipMoldShrinkagenINTRODUCTIONMostmoldmakersandmoldersfacetheproblemofmoldshrinkageofpolypropylene(PP)andpolyethylene(PE).Formany,itisdifficulttounderstandwhy,withalltheresourcesandtechnologyavailable,onecan'tgetmoreaccuracyinmoldshrinkagedatathana“typical”.ThisTechnicalT

2、ipbrieflyexplainswhatmoldshrinkageisandwhyit'simpossibletoestablishaconstantvalueforPPandPE.nWHATISMOLDSHRINKAGE?Insomerespects,moldshrinkagecanbecomparedtolinearthermalcontractionorexpansion.Amassofmoltenpolymercoolinginamoldcontractsasthetemperaturedrops.Holdingpressureisusedtominimizeshrinkageb

3、utisonlyeffectiveaslongasthegate(s)remainsopen.Ifthepolymerishomogeneous,allpartsshouldshrinkessentiallythesameamountevenafterthepressureisremovedorthegatesfreezeoff.Thisisgenerallythecasewithamorphouspolymerssuchaspolystyrene,polycarbonate,ABS,etc.Publishedvaluesformoldshrinkageofthesematerialsar

4、everylowanddonotexhibitabroadrange,generallyintheorderoflessthanin./in.COMPARATAIVEMOLDSHRINKAGEVALUESAstheycool,crystalsformwhichshrinkmuchmorethantheamorphousregionsandshrinkatdifferentrates.Thisimbalanceresultsinanetincreaseinshrinkageandintroducesasensitivitytoothermoldingparameterswhichhavead

5、ditionaleffectsontheshrinkage.Anotherfactorinfluencingshrinkageisthevisco-elasticcharacteristicofhigh-molecular-weight-polymermelts.Thelong,molecularweightchainsareliterallystretchedandplacedunderstressastheyfillthemold.Asthestressesarerelievedduringcooling,thechainswanttorelax,analogoustostretchi

6、ngarubberbandandslowlylettingitreturntoitsoriginalsize.Thisrelaxationalsoinfluencestheshrinkage,especiallyindifferentflowdirections.Boththeaveragemolecularweightandthemolecularweightdistributionarekeymaterialfactorsthatcontrolthisfacetofmoldshrinkage.nWHATPROCESSANDDESIGNPARAMETERSINFLUENCEMOLDSHR

7、INKAGE?Therelativeproportionofcrystallinetoamorphouscomponentchangestheshrinkage.Thisisaverycriticalvariablewithpolyethylenebutisnotassignificantwithpolypropylene,asevidencedbythemuchnarrowerrangeofspecificgravit