Microsoft Word 文档Word文档下载推荐.docx
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SubmittedbyK.K.Wang(I),Ithaca,NewYork,USA
1
Abstract
Stereolithographyrapidtoolingmaterials(typicallyepoxy-basedphotopolymers)showgreatpromise
forinjectionmoldingoflimitednumbersofparts,greatlyreducingthetimetoproduct.Yet,theypresentchallengestodesignersbecauseoftheirstrength,thermalcharacteristics,andshorterlifetimesascomparedtoothermoldmaterials.Thispaperpresentsmodelsoftheforcesgeneratedduringtheinjectionmoldingcyclesoastoevaluatethesuitabilityofrapidtools.Themodelscomprisethermalandmechanicalloadingduringinjection,coolingshrinkage,andejection.Theeffectsofprocessconditionsonmaterialpropertiesarestudied.Experimentalresultsarepresented.
Keywords:
rapidprototyping,injectionmold,failure
1INTRODUCTION
RapidToolingshowsgreatpromisefortheinjectionmoldingoflimitednumbersofparts,greatlyreducingthetimetoproduct,e.g.,[I-61.However,stereolithography(SL)rapidtoolingmaterials(typicallyepoxy-basedphotopolymers)presentchallengestodesignersbecausetheirstrengthandthermalcharacteristicsarenotasgoodastraditionalsteelmolds.Inaddition,longercycletimesarerequired.InsomecasesoutsidetheUSA,fillersareaddedtoincreasestrength,butmayinterferewithcure.SLinjectionmoldshavemuchshorterlifetimesthanothermoldmaterials,onereasonbeingthattheyareoperatedattemperaturesclosetotheirglasstransitions(Tg).Also,thestereolithographyprocessoflayeredmanufacturingandthedraftanglerequirementsgeneratestairsteppingonthesurfaceofthepart,whichincreaseejectionforcesandcanproducestressconcentrations.Thispaperpresentsmodelsoftheforcesgeneratedduringthemoldingcyclesoastoevaluatethesuitabilityofrapidtoolsforinjectionmolding.
Thematerialpropertiesoftherapidprototypingmoldmaterialsareevaluatedwithmechanicaltesting(tensileandflexural)anddynamicmechanicalanalysis(DMA)attemperaturessimilartothosepresentinthemoldduringinjectionmolding.MechanicaltestsshowthattheSLmoldingmaterialfailsinabrittlemanner,withlittleifanyfatiguebehavior.Somechangesinmechanicalpropertiesareobservedduetomechanicalandthermalcyclingofthemoldmaterials,eventhoughtheyarefullycuredduringprocessing,asevidencedbydifferentialscanningcalorimetry(DSC).Theseprovidetherequiredpropertiesforthecalculations.
Duringinjection,theflowofthepolymerintothemoldcavitycancausemoldfeaturestodeflectandbreak,particularlyafteranumberofcycleswhenthemoldhasheatedup.Theseforces,coupledwiththetemperaturedependentmoldproperties,aremodeledusingfiniteelementtechniquestodeterminetheextentofthemold
featuredeflectionorpossiblemoldfailure.Experimentsvalidatethesetheoreticalresults.
Duringcooling,aparttypicallywillshrinkontothecoreofamold.TheinherentstairstepgeometryoftheSLmoldismodeledsothatitsgeometrycanbedeterminedtheoretically.Bothofthesecontributionsareincorporatedintoanejectionforcemodel.Finiteelementtechniquesareusedtocalculatetheejectionforces.Experimentaldataforejectionforcesarepresented.
2THEORETICALMODELING
2.1InjectionPhase
Thefirstfeaturestudiedinthispaperisafree-standingribthatismodeledasacantileverbeam(Figure1)withalinearlydecreasingdistributedload(i.e.,higherloadatthefreeend),whichrepresentsloadingbytheincomingpolymerflow.Themaximummoldfeaturedeflection,y-,ispredictedusingafiniteelementmethodandtheelementaryequationforcantileverbeamdeflection(Equation1).Additionally,thestress,o,experiencedinsidethefeatureispredictedusingafiniteelementanalysisandtheelementaryflexureformula(Equation2).
wherePisapointloadattheendofthebeam,Lisitslength,Wisadistributedloadonthebeam,Eisthemodulus,Iisthesecondmomentofarea,Misthebendingmoment,andcisthehalfthicknessofthebeam.TheforcesandmomentactingonthecantileverbeamduetotheincomingpolymerflowarepredictedbyC-Mold,aninjectionmoldingsimulationprogram.ThedetailsofthefiniteelementanalysisarepresentedinSection2.3.Themoldfeatureispredictedtofailifthestressexceedsitsmaterial’smaximumflexuralstress.
2.2CoolingandEjectionPhases
Themodelforejectionforceisdevelopedbycombiningtheeffectsofthemajorcontributingfactors:
thethermalshrinkageandtheinherentstair-stepprofile(Figure2).
Mathematically,
Figure1:
Samplefeatureandcavitygeometry
whereFfric,thermandFdef,dalrdenotetheejectionforcecomponentsduetothermalshrinkageandtostair-stepping,respectively.Forageneralmoldwithacorefeature,themoldedpartcoolsdownandcontractsontothecore,creatingcontactpressureattheinterface.Thecontactpressuregeneratesfrictionduringejection,Ffric,therm,whichmustbeovercometomakeejectionpossible.Foramoldwithacorefeature,Ffric,thermcanbeestimatedusingathick-walledvesselapproximation[I].ThelayerednatureoftheSLprocessleadstothestair-stepphenomenondepictedinFigure2.Thisstair-stepprofilecreatestheundercutoroverlap6thatpreventsthemoldedpartfromejecting.Thestair-stepprofileispreservedthroughmultipleejectioncycles[2].Hence,thepartandmoldmustdeformelasticallytoovercometheoverlap(Fdef,stalr)inordertomakeejectionpossible.
InordertoformulateFdef,stalr,theoverlap6wasquantifiedmathematically.Toachievethisgoal,thestair-stepprofilewasmodeledusingtheequationofthehatchedbulletprofile[3].Thearithmeticaverageroughness(Ra),andconsequently,theoverlap6ofaSLtoolsurfacecanbeestimatedasfollows:
where0isthedraftangle,CL~isthelinewidthcompensationorbeamcompensation,Iisthebuildlayerthickness,andOCistheover-cure.CL~andOCdepend
onresintype,lasertype,andmachineset-up.BycombiningFfrlc,thermandFdef,stalr,theejectionforceequationbecomesthefollowing:
wherePthermisthecontactpressureduetothermalshrinkage,bqisanequivalentfrictioncoefficient,pisthefrictioncoefficientbetweenmoldandpartmaterials,SAisthecontactareaalongtheejectiondirection,ATisthechangeintemperaturefrominjectiontoejection,risthehydraulicradius=(2*area/perimeter),EisYoung'
smodulus,visPoisson'
sratio,andm,paresubscriptsdenotingmoldandpart,respectively.InregardstoATp,theYoung'
smodulusofthepart'
smaterialisonlysignificantattemperaturesbelowitsglasstemperatureTg,p.Therefore,thethermalstrainduetopartcooling
shouldbeincludedonlyafterthepartcoolsbelowTg,p.Inotherwords,ATp=Tg,p-Te,pwhereTe,pdenotestheejectiontemperatureofthepart.
2.3FiniteElementModelingandAnalysis
FEanalyseswereperformedtovalidatetheforceequations(Equations2and5)andtheexperimentalresults,andforuseasindependentmodelstopredictinjectionandejectionforcesforthecaseswheretheforceequationsmightbeinsufficient,e.g.,moldsoflargersizeormorecomplexgeometry.Acoupled-fieldsequentialtechniquewasemployedtoperformthermalanalysisandthesubsequentstructuralanalysisforthemold/partset.Forinjection,C-Moldwasusedtodeterminethethermalandpressurestates.ANSYSthenwasusedtodeterminethestressstate.Forejection,3-Dmodelingofthemold/partsetwasdoneusingProEandANSYS.ThermalanalysisthenwasperformedinANSYStoobtainthetransienttemperaturedistributionwithinthemold/partset.Thethermalresultswereusedasinputtothestructuralanalysistodeterminethestressstateatejection.Thecontactstressthenwasobtained.
3EXPERIMENTALPROCEDURES
3.1MaterialCharacterization
Experimentswereperformedtodeterminetheeffectofprocessconditionsonthemechanicalpropertiesoftwostereolithographyresins:
CibaTool7510andDSMSomos7110.DynamicMechanicalAnalysis(DMA)(singlecantilever35x12~2mm),compactdogbone(ASTMD638-94b:
typeII),andfracturetoughnessbend(ASTME399-90;
thickness(B)of12mm)specimenswerebuiltinthreeorientations:
top,bottom,andside.Thesedesignationsrefertothelocationofthelayering(Figure2)onthepartresultingfromthebuildprocess.AfterbuildandonehourinanUVchamber,partswerepostcured
fortwohoursinathermalovenat8OoC.Samplesweretestedinanotfurtheragedcondition(NA);
athermallyagedcondition(TA)-subjectedtosixmorehoursat8OoC;
andamechanicallyagedcondition(MA)-subjectedto1000bendingcyclesof1mmamplitudeinaDMAat3OoC.Thelatterconditionssimulated,inamanner,theeffectsofprocessing.Amodel2890DMAfromTAInstrumentsandamodel4446universaltestingmachine(UTM)withtemperaturechambermodelA74fromlnstronwereusedforthetests.DMAtestswereperformedwitha3'
C/minsweeprate,20pmdisplacement,and1HzfrequencytodetermineT,,E'
(storagemodulus),andE"
(lossmodulus).FatiguetestingalsowasperformedontheDMA.Itwasperformedfor300bendingcyclesof1mmamplitudeforthe7510resin,anduntilfailureforthe7110resin.TensiletestingwasperformedontheUTMat1.5mm/minwitha25mmgaugelengthandanextensometer.
3.2InjectionMoldingExperiments
Parts(Figures1and3)wereinjectionmoldedonaSumitomoinjectionmoldingmachine(SG75-CI60-Mlll)usingChevronMC3600polystyrene.Themoldswerebui
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