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外文翻译--热塑性塑料注射模中焊缝形成的流体分析 英文版.pdf

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外文翻译--热塑性塑料注射模中焊缝形成的流体分析 英文版.pdf

IntroductionWeldlinesareformedduringmoldfillingwhenevertwoseparatedmeltstreamsrecombine.Thisoccurseitherduetoinjectionthroughmultiplegatesorasaconsequenceofflowaroundanobstacle.Twomaintypesofweldlinesareusuallydistinguished.Coldorstagnatingweldlineisformedbyaheadonimpingementoftwomeltfrontswithoutadditionalflowafterthat.Hotorflowingweldlinesoccurwhentwomeltstreamscontinuetoflowaftertheirlateralmeeting.Sinceweldlinesoftenresultinreducedmechanicalstrengthsand/orpooropticalsurfaceappearanceofinjectionmoldedpartstherehavebeenagreatnumberofinvestigationsabouttheeffectofprocessingconditionsontheweldlines.MalguarneraandManisali1981measuredtheweldlinestrengthforseveraltypesofpolymersandfoundthatmeltandmoldtemperaturehadaremarkableinfluenceontheweldlinestrength.CriensandMosle´1983investigatedtheinfluenceofdesignandprocessingparametersonthemechanicalpropertiesofaplatewithhole.Theyrecognizedthattheeffectofmelttemperaturechangesfrompolymertopolymer.KimandSuh1986haveshownthatincreasingmelttemperaturecanleadtoadeteriorationofweldlinestrengthjustbelowthedegradationtemperature.Injectionpressure,injectionspeed,holdingtimeandholdingpressurehavealsobeeninvestigatedandonlylittleeffecthasbeenobservedPiccaroloandSaiu1988.Recently,Liuetal.2000designedtheirexperimentsaccordingtotheTaguchiC213smethodandshowedagainthatthemeltandmoldtemperaturearetheprinciplefactorsaffectingweldlinepropertiesofinjectionmoldedthermoplastics.Itshouldbenotedthatthesensibilityofweldlinesdependsnotonlyonthematerialpropertiesandtheprocessingconditions,butalsoonthetestingmethodsappliedSelde´n1997.Althoughintheliteraturemechanicalweaknessofweldlinesisusuallyexplainedby1lackofdiffusionThamNguyenChungFlowanalysisoftheweldlineformationduringinjectionmoldfillingofthermoplasticsReceived10February2003Accepted22October2003Publishedonline19December2003C211SpringerVerlag2003AbstractTostudytheweldlineformationofcollidingflowfrontsthefillingofamoldcavitywassimulated.Thethermorheologicalfindingswereusedtoinvestigatethesourcesofweldlineweakness.Inthiswaycriticalareasoftheinterfaceinregardtothelackofinterdiffusionandtheinappropriatemolecularorientationwerefoundtobeplacednearthesurfaceofthefinishedparts.ThemainsourcefortheweldlineweaknessseemstobetheVnotchthatarisesduetothepoorlybondedregionnearthesurfaceincombinationwiththelargeshrinkageasaresultofextremelyhighmolecularorientationsinducedattheendofthefilling.Furthermore,theempiricalknowledgewasconfirmedthatweldlinesarerathermoresensitivetothelocalflowsituationthantheglobalprocessingconditions.Meltandmoldtemperaturescanbeconsideredtobethemostimportantfactorswhichinfluencetheweldlinestrength.KeywordsPolymerÆInjectionmoldingÆThermoplasticsÆWeldlineÆSimulationRheolActa200443240–245DOI10.1007/s0039700303392ORIGINALCONTRIBUTIONInpartpresentedatthe6thEuropeanConferenceonRheology,Erlangen,2002T.NguyenChungInstitutfu¨rAllgemeinenMaschinenbauundKunststofftechnik,ChemnitzUniversityofTechnology,09107Chemnitz,GermanyEmailtham.nguyen.chungmb.tuchemnitz.deofpolymermolecules,2unfavorablemolecularorientationattheinterface,and3formationofaVnotchatthesurfaceofinjectionmoldedpartsKimandSuh1986Fellahietal.1995,littlewasknownabouttheinterrelationshipbetweenthesefactors.KimandSuh1986analyzedthefirstandsecondfactorsseparatelyandthenintegratedthemtopredictthestrengthofweldlines.Intheirtheoreticalapproachforthediffusionprocessthetemperaturegradientacrossthepartthicknesswasneglected.Tomarietal.1990clarifiedtheVnotchstructureanditseffectonthestrengthofgeneralpurposepolystyreneinjectionmoldings.Theymeasuredtheweldstrengthofdogbonetypetensilespecimensthesurfaceofwhichwaspartiallyeliminatedbymilling.TheirresultssuggestedthattheVnotcheffectiscausedratherbyapoorlybondedlayernearthesurfacethanthefinegrooveonthesurface.ItisalsoworthnotingthattheVnotchmaybealsoattributedtotheairentrappedattheinterfacebetweentheflowfrontsHagerman1973orvolumetricshrinkageduringcoolingPiccaroloandSaiu1988.Todate,modelingoftheweldlinemainlyfocusesonpredictingtheweldlinepositionandinvestigatingtheinfluenceofthethermorheologicalsituationonthemeasuredweldlinestrengths.However,mostofthesimulationisbasedonthepressuredropformulation,whichdoesnotgivedetailedinformationabouttheflowsituationattheadvancingfront.Therehavebeenonlyafewpapersonsimulationoftheweldlineformationconsideringthefullflowhistory.Weietal.1987calculatedthestresswhichaviscoelasticmeltexhibitsinaflowpastobstaclesbyassumingthatthekinematicsareclosetothoseofashearthinningfluidsuchastheCarreaumodel.Thecalculatedvaluesofmolecularorientationshowedahighlyorientedregionsurroundingtheweldinterfacejustdownstreamoftheobstacle,whichwasverifiedbyexperimentsusingtherheoopticalmethod.Mavridisetal.1988simulatedthesituationofcollidingflowfrontsforaNewtonianfluidandshowedthattheorientationofpolymermoleculesatastagnatingweldlineismainlydeterminedbythefountainflowbeforethecollisionoccurs.Recently,NguyenChungetal.1998investigatedtheflowmechanismsbehindanobstacleclarifyingtheinfluenceofthethermorheologicalhistoryofthemeltontheperformanceoftheweldline.Thepresentedpaperrepresentsanonisothermalsimulationoftheweldlineformationduetocollisionoftwoflowfronts.Thiswaytheaforementionedsourcesoftheweldlineweaknessandtheirinterrelationshipcanbeinvestigatedwithregardtotheflowhistoryandthethermorheologicalsituation,whichasawholeenablesabetterunderstandingofthemechanismsoftheweldlineformation.SimulationSimulationhasbeencarriedoutofaviscousfluidfillingarectangularcavityfrombothendsFig.1.Byconsideringthesymmetryaquarterofthecavitywasmodeledastwodimensionalgeometry.Neglectinggravityandsurfacetensionmeansthatthefreesurfacescanbeassumedtobeinitiallyflat,thefluidbeingatrest.Themass,momentumandenergyconservationequationsforanincompressiblefluidcanbewrittenasfollowsrC1t¼0ð1ÞqttþtC1rtC18C19¼C0rpþrC1C0sð2ÞqcpTtþtC1rTC18C19¼rC1krTðÞþC0s_C0cð3Þwheret,t,T,p,C0s,_C0c,q,cpandkdenotetime,velocityvector,temperature,hydrostaticpressure,deviatoricstresstensor,rateofdeformationtensor,density,specificheatandheatconductivityrespectively.TheconstitutiveequationforageneralizedNewtonianfluidwasusedC0s¼2gT_cðÞ_C0c_C0c¼12rtþrtTðÞð4ÞwiththeviscositygivenbytheBirdCarreaumodelBirdetal.1977g¼g01þkc_cðÞ2hinC012_c¼ffiffiffiffiffiffiffiffiffiffiffiffi2_C0c_C0cpð5ÞFortemperaturedependencetheArrheniusmodelwasappliedontheviscosityatareferencetemperatureT0g_cTðÞ¼aTgaT_cT0ðÞaT¼expa1TC01T0C18C19C20C21ð6ÞFig.1Initialstatetopandboundaryconditionsbottomforafillingsimulationofarectangularcavity241ThefollowingboundaryconditionscompletethestatementoftheproblemattheinletaconstantvelocityandaconstanttemperatureofthemeltareassumednoslipconditionandaconstantmoldtemperatureareimposedonthewallTable1atthesymmetrylinessymmetryconditionsareappliedattheflowfrontzerosurfacetractionisappliedandheattransportthroughthissurfaceisneglected.WiththecommercialcodeFIDAPFluent1998,theGalerkinfiniteelementmethodwasusedtosolvethecontinuity,momentum,andenergyequationswhicharediscretizedbystandardprocedures,usingamixedformulationinwhichpressureisinterpolatedoneorderlowerthanvelocityandtemperature.ThefreesurfacesaretrackedbyusingtheVOFmethodappliedonafixedmeshHirtandNichols1981.AnadditionalequationistobesolvedtogetherwiththegoverningflowequationsFtþtC1rF¼0ð7ÞwherebyFisdefinedasamaterialdensityfunction.Ithasavalueofunityinafilledsectionoftheflowdomainandiszerooutsideofthefluid.Atthefreesurfaceitselfthisfunctionhasavaluebetween0and1.Asmaterial,polystyrene165HsuppliedbyBASF,Ludwigshafen,Germanyhasbeenused.ThethermorheologicalpropertiesandthecoefficientsoftheviscositymodelareshowninTable2.ResultsResultswillbeshownusingthenondimensionalizedvariablesasfollowsvC3¼vv0tC3¼tt0tC3¼tt0v0_cC3¼_cv0t0pC3¼pv0g0T0ðÞt0ð8Þwithacharacteristiclengthv00.004m,acharacteristicvelocitym00.1m/s,andazeroshearrateviscosityg0T03760Pas.TheflowfrontsatdifferenttimesFig.2showthattheweldlineisformedasexpectedfromthemiddleofthecavitytowardsthewall.InFig.3thepathlinesofselectedmaterialelementscanbeobservedwhichareoriginallypositionedontheflatflowfront.Thedistancebetweentheoriginallyflatflowfrontandtheweldlinepositionislargeenoughsothattheflowfronthasbeenfullydevelopedbeforetheweldlineisformed.ItcanbeseenthattheweldlineconsistsofthematerialelementscomingfromthecoreregionofthecavitywheretheygenerallydidnothaveexperiencedlargedeformationsNguyenChungandMennig2001.Onlyduringthetransitionfromtheflattothefullydevelopedflowfrontmaydeformationsoccur,butthatisratheranexceptionduetothespecificproblemdefinition.Onthewhole,deformationsattheweldlinemustbemostlysubjectedtothelocalflowsituation.Attheinterfacethematerialelementschangetheirflowdirectionandcontinuetomovealongthethicknessdirection.Duringthesetimesinterdiffusionmayoccur.Inthecorethecontacttimeabovethesolidificationtemperatureislongerthanintheouterregionsothatastrongerdegreeofinterdiffusioncanbeexpectedthere.Bycontrast,inthelayernearthewall,thecontacttimeisveryshortsincethematerialelementsarrivingtherewillbefrozeninimmediatelybeforecontactwiththeircounterpartscanbeestablishedforexamplethematerialelementnumbered8.ThisresultsinalayerwithpoorbondingasfoundbyTomarietal.1990.ThemolecularorientationhasbeenfrequentlyinvestigatedbytracingagreatnumberofmaterialelementswhicharefirstlyplacedonastraightlineCoyleetal.1987.However,inthatwayitisnotpossibletodistinguishbetweentheabsolutedeformationsthatstronglydependontheobservationtimeandtherelativedeformationsthatareameasurementformolecularorientation.Inthiswork,severalgroupsofmaterialTable1ProcessingparametersParametersValuesMelttemperatureTF503C176KMouldtemperatureTW333C176KInletvelocityv00.1m/sTable2Materialpropertiesofpolystyrene165HPropertiesValuesMeltdensityq892kg/m3Specificheatcp1968J/kgKThermalconductivityk0.14W/mKReferencetemperatureT0503C176KZeroshearrateviscosityg0T03760PasTimeconstantk0.15sPowerlawindexn0.23Arrheniuscoefficienta10,842Fig.2Developmentoftheflowfronts242elementsweretraced.EachgroupformsacircleandlocatesoriginallyonthestraightflowfrontFig.4.Bycomparingthedeformationsofthecirclesatdifferenttimestherelativedeformationsofthemeltandsothedevelopmentoftheflowinducedmolecularorientationcanbevisualized.Itshowsagainthatthehighorientationattheweldlineisaresultofratherthelocaldeformationsalongtheinterfacethanofthegeneraldeformationattheflowfront.Inthepast,Mavridisetal.1988alsorecognizedthattherewassignificantextensionaldeformationatthesurfaceoftheadvancingflowfrontwhichwouldleadtoaperpendicularorientationtothewall.TheauthorspointedouttheanalogyofthecollidingflowfrontstotheplanarstagnationflowwhichwasoriginallyusedbyTadmor1974asamodeltodescribethefountainflow.However,inthesamepaperMavridisetal.1988comparedthestretchingofmaterialbandsattheflowfrontwiththoseattheweldlineandrecognizedthatthestretchingduetofountainflowismuchlargerleadingtotheassumptionthatthefountainflowmaybemostlyresponsiblefortheanisotropyatweldlinesofinjectionmoldedparts.Thisassumptionisnotquitecorrectduetothefactthatdifferenttrackingtimeswerecomparedtoeachother,i.e.,thematerialbandsatthefountainflowweretracedlongerthanthoseatthecollidingflowfronts.Actually,Fig.4showsthatthefountainflow,whichleadstohighdeformationsatthemoldwallNguyenChungandMennig2001producingpronouncedmolecularorientationparalleltothewall,affectsonlytheregionsfarawayfromtheweldline.Asthetwoflowfrontsmeet,theextensionalongtheweldlinecanbeconsideredtobethemainsourceforthemolecularorientationperpendiculartothewall.Furthermore,thelargestextensionratewasfoundtooccurnearthecavitysurfacejustbeforethecavityhasbeenfullyfilledFig.5.Inthecoretheextensionratesareatalowerlevelallthetimelikeincaseofasteadystateplanarextensionalflow.Fig.3PathlinesofmaterialelementsoriginallypositionedonthestraightflowfrontFig.4DeformationsofcircularvolumeelementsFig.5Extensionratesalongtheweldlinejustbeforetheendoffilling243

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