外文翻译--对通过塑料注射成型零件的选择性激光融化生产得到的功能梯度材料插入棒的评估 英文版.pdf

EvaluatingtheuseoffunctionallygradedmaterialsinsertsproducedbyselectivelasermeltingontheinjectionmouldingofplasticspartsVEBeal1*,PErasenthiran2,CHAhrens1,andPDickens21UniversidadeFederaldeSantaCaterina,Floriano´polis,Brazil2WolfsonSchoolofMechanicalEngineering,LoughboroughUniversity,Loughborough,UKThemanuscriptwasreceivedon19October2006andwasacceptedafterrevisionforpublicationon9March2007.DOI:10.1243/09544054JEM764Abstract:Thedemandforproductivityandshapecomplexityontheinjectionmouldingindustrynecessitatesnewresearchtoimprovetooldesign,material,andmanufacturing.Aresearchfieldisthedevelopmentoffunctionallygradedmaterials(FGMs)tobuildinjectionmoulds.Forexample,mouldsbuiltwiththeFGMstechniquecanhavedistinctiveregionswithhigherheatconduction.Higherratesofheattransfersfromthickerregionsoftheinjectedpartcanbeusefultoproducebetterandcheaperinjectionmouldedpolymerparts.Itispossibletoobtainmouldswithdifferentialconductivitybyaddinglocally,duringthefabricationofthemould,coppertothemouldbasematerialsuchastoolsteel.Inthiswork,aninvestigationintotheeffectofFGMcopper(Cu)-toolsteelmouldinsertoverpolymerinjectedpartsispresented.Theworkisdividedintwoparts:anumericalthermalanalysiscomparisonbetweenCu-toolsteelgradedandtoolsteelinsertsandaninjectionmouldingexperimentwithcomparisonsbetweenmouldsurfacetemperatureanddegreeofcrystallinityofpolypropyleneparts.Thenumericalmodelwasusedtocomparedifferentbehaviourofthemouldheattransferaccordingtothemouldinsertmaterial.Thereafter,abolsterwasbuilttoholdFGMsandtoolsteelinsertsobtainedbyaselectivelaserfusionprocess.Polypropylenewasinjectedovertheinsertstocomparewiththenumericresults.Toobservetheeffectofthecoolingrateinthepolypropylenepartsusingthegradedinserts,thedegreeofcrystallinityofthepartswasmeasuredbydifferentialscanningcalorimetry(DSC)test.Thetemperatureofthemouldwasalsoevaluatedduringtheinjectioncycles.TheresultsshowedthatthegradedCu-toolsteelinsertstestedhadlowercapacitytostoreheatenergy.AsCuwasaddedtothetoolsteel,themixtureprovedtotransferheatmoreefficientlybutithadlesscapacitytoabsorbheat.Keywords:functionallygradedmaterials,injectionmoulding,rapidmanufacturing,polypropylenecrystallinity1INTRODUCTIONThebenefitofinjectionmouldedpartsdependsonthreegeneralaspects:toolcost,injectionmould-ingrawmaterial,andproductivityofthetool.Thistriomakesitdifficulttochangepart/moulddesignwithoutaffectingproductivityandmaterial.Hence,durabilityofthetool,productivity,andcostsmustbeachievedbytheoptimalmaterialrawselectionandpartandmoulddesigns1.Unfortunately,therearerestraintsthatmakeitdifficulttofindthebestcom-promisingsolution.Asthecomplexityofamoderninjectionmouldishigh,themoulddesignersconcernishowtosolidifythepartwithoutcausingdistortionsandkeepingthemouldwithhighratesofpartspro-ducedperhour.Acomplexchannelsnetworkisdesignedtoenablecoolingliquidtoextracttheheatfromthemould.Thedesignofthechannelsisdifficultasitisnecessarytokeeptheejectionsysteminplace.*Correspondingauthor:EngenhariaMecanicaCIMJECT,UniversidadeFederaldeSantaCatarina,CaixaPostal476,CampusUniversita´rioTrindade,Floriano´polis,SantaCatarina88040-900,Brazil.email:valterbealgmail.comJEM764C211IMechE2007Proc.IMechEVol.221PartB:J.EngineeringManufacture945Ejectorpins,slides,andairstreamgatesareusedtoejectthepartfromthemouldcavityavoidingmarksintheaestheticsideofthepart.Dependingonthecomplexityandshapeofthepart,thespaceleftbythecoolingsystemissmallanditisnotfeasibletomanufacturewithoutleavingmarksinthemouldimp-ression.Inmanycases,whentheheatextractionequi-libriumforhomogeneouslyextractingtheheatfromcavityandproductivityarenotachieved,itmightbenecessarytoredesignthepartgeometrytofitmouldlimitations.Analternativetosolvecomplexthermalissuesistheuseofcopperberyllium(CuBe)inserts1.AsCuBeinsertshavehigherthermalconductivitythantheusualsteelalloys,theyareusedtoextractheatfromregionswherethecoolingchannelsdonothaveaneffectduringtheinjectionmouldingcycles.Nevertheless,theyarenotenvironmentallyfriendly,asberylliumiscitedasahighlycarcinogenicelement2.Anotherlimitationisthatinsertsinthemouldimpressionmightleavemarksinthepartasthemouldsurfacehasavisibleinterfacebetweenthebasematerialandtheinsert.Inaddition,theinsertfeaturesthatareneededtoattachittothebasecontributetoreducingthespaceleftforthecoolingchannels.Inthemid1980s,newmanufacturingtechnologiesknownasSFF(solidfree-formfabrication)emerged3.Themaindifferenceofthesetechnologies,asoppo-sedtothetraditionalones,wasthattheywerebasedonthelayeradditiveprinciple.Alsoknownasrapidprototyping(RP),thesetechnologiescanproducepartsinlow-volumeproductioninvirtuallyanyformormaterial.Thevarietyofavailablematerialsislimited；however,RPprocessescanbuildpartsinmetals,ceramics,andpolymers3.RPtechnologiesarehighlyautomatedandtheyarealsocalledthree-dimensional(3D)printersasthemachinesalmostprintsolidpartsfromdatageneratedfromcompu-tersoftware.Designersandengineerscanbuildandverifydesignedpartswithoutmisunderstandings,inaccuracies,anddelays.ThebasicprincipleofRPtechnologiesistobuild,layer-by-layer,materialcor-respondingtothedataofthedesignedpart.Rawmaterialscanbeliquidresins,wires,pastes,powders,andsheets.Thewaytoshapethesematerialsandbondlayerscanbediverseincludingultravioletlasers,lamps,powerlasers,sprayofglue,depositionoffusedmaterial,andothers.Theseadditive-layeredmanufacturingtechnologies(LMTs)havealsobeenusedtoproducetoolsforinjectionmoulding.Depend-ingonthetechnologyandmaterialusedincons-truction,thecomplexityofthemouldimpression(injectedpart),andtheinjectedmaterial,thesemouldscanbecompetitivetotraditionalcast/milledmoulds.Itispossibletobuildmouldsfrom12to10000partsaccordingtothetechnology,material,andapplication4,5.OneinterestingtechniqueusedwithSFFtobuildinjectionmouldsisconformalcoolingchannels.Thechannelsaredesignedinthemouldimpressionwithouttheconcernsofthelim-itationsfromthetraditionalmanufacturingmethod.Theconformalcoolingchannelsmightfollowthemouldimpressionsurface,passingbytheejectorsys-temwithfewerlimitationsthantheusualmoulds.Unfortunately,theyarestilllimitedbytheejectionsystemandsomepartfeaturessuchasdeepgrovesmightnotbeaffectedbythecoolingcapabilitiesofthechannels.Toovercomesomeoftheseconstraints,itispossibletousefunctionallygradedmaterials(FGMs)tobuildinjectionmouldsbySFFtechnologies.FGMshavebeenthesubjectofresearchforthelast25years6.Mostofthenaturalmaterialssuchasmineralsandtissueshaveagradualchangefromonefunctionalregiontoanother.Thisexampleofnatureinspiresintegratedformandfunctiondesignallinthesamecomponent/unit.FGMisnotcomple-telynewtothemanufacturingprocesses,butitwasonlyafterthe1980sthatitstartedtoreceivemoreattentionandtobeclassifiedasaspecificresearchsubject.ThebasicideaofFGMistoimprovethepropertiesofthepartbyvaryingthequantityofaningredientinspecificregionsinordertoachievedif-ferentialproperties.Aningredientcouldbeabasicelementsuchascarbonbeingusedtoincreasethehardnessofasteelpartonlyatthesurface.Anotherexampleistheporosityvariationfromtheoutsidetotheinsideofthemammalsbones.Thelowporosityfromtheoutsideincreasesthestiffnessofthebonebutprovidesinterconnectivitytotheinside.Thecoreoftheboneisporous,therebyallowingweightefficiency.Byusingthisvariationfromonematerialtoanother,optimizedcomponentscanbeobtained.Reducednumberofjointsandfasteners,weightreduction,structuralenhancement,differentialheatextraction,thermalbarriers,embeddedsensors,andbiocompatibleimplantsaresomeofthepotentialadvantagesofusingFGM68.FGMsalsocouldgraduallyjoindissimilarmaterialswithdifferentpropertiesinthesamecomponent.Theprincipleissimilartocompositematerials.Thedifferenceisthatcompositeshavedistinctivephasesanddonotvarytheircompositioninthevolumeofthecompo-nent.DespitetheideaofFGMsbeingverysimple,mostofthepotentialFGMapplicationsarerestric-tedtotechnologicallimitationsandhighcost.Diffi-cultiesincontrollinganddepositingthegradientcompositionandproducingcomplexshapeswithcomputer-aideddesign(CAD),computer-aidedman-ufacturing(CAM),andfiniteelementanalysis(FEA)integrationaresomeofthecausesforrestrictionsofuse.TheuseofRPtechnologiestoproduceFGMpartshasbeeninvestigatedbymanyresearchers9.SinceRPtechnologiescanproducefree-formpartsandcanhandledifferentmaterials,itispossibletousethem946VEBeal,PErasenthiran,CHAhrens,andPDickensProc.IMechEVol.221PartB:J.EngineeringManufactureJEM764C211IMechE2007toproduceFGMcomponents.Mostoftheresearch-ersinvestigatingthefabricationofFGMbyLMTspro-cessthematerialswiththeheatsourcedeliveredbyalaserbeam.Aslaserscanbeeasilyautomatedandcandeliverhigh-energydensitieswithprecisionandspeed,theycanprocessalmostanymaterial10.AnotheraspectofFGMandRPisthefrequentuseofmaterialsintheformofpowdertobefusedorpre-sinteredunderalaserspot.Themainissueforusingrapidprocessingandmanufacturingtechnolo-giesforproducingFGMpartsisthelocalcompositioncontrol(LCC).Thisregardstheprincipleforaddingandjoiningthematerialsbycontrollingtheirpercen-tagesoneachregionofthepartorlayers.Someresearchers11,12usedminiaturehopper-nozzlesandcapillarytubestocontrolthedepositionofpow-dersinthelayer.Enszetal.13studiedtheoptimiza-tionoftwopowderflowsinthelaserengineeringnet-shaping(LENSC212)processtobuildgradientsfromH13toM300steelalloys.Inaddition,computationalmethodstorepresentthegradedgeometryhavebeenthesubjectofstudy.Choetal.14investigatedtheLLCforthe3Dprintingprocessafterobtaininggeometryandmaterialdatafromfiniteelementandvoxelspacegeometries.Bythismethod,itwaspossi-bledigitallytorepresentthe3Dpartwithdifferentvolumetricgradients.TheideaofaddinganextrafunctionalmaterialtoabasematerialtoproduceaFGMinjectionmouldhasbeenresearchedinpreviouswork15.Amulti-compartmenthopperwasusedtoproducegradedstructuresofH13toolsteelandCu.TheH13iscommonlyusedasmaterialforinjectionmouldsasithasdimensionalstability,toughness,andwearresistanceathightemperature.NeverthelesstheheatconductionofthismaterialislowcomparedwithCu(kH13:24.3W/mK；kCu:385W/mK16,17).ElementalCupowderwasmixedwithH13inpropor-tionsof12.5,25,37.5,and50%wttoproduceFGMbars.Themethodforproducingthesebarswastheselectivelaserfusion/melting(SLForSLM)usingahigh-powderNd:YAGpulsedlaser.Thelaserpro-cessedthemulti-compositionpowderbedthatwaspreviouslyloadedwithpowdersfromthemulti-compartmenthopper.Asthelaserscannedthepowderbed,thepowderwasfusedandbondedtothepreviousaddedlayers.Afterprocessingalayer,thepowderbedwasloweredandthepowderswerespreadoverthepreviouslayerandthelaserwassettofusethepowdertoformanewlayer.Thisprocesswasnumericallycontrolledandcontinueduntilthecompletionofthepart.Thefusionprocess,usingthislaser,leftaroughsuperficialaspectandrequiredsomepostprocessingincludingtheremovalofthesubstratethatwasusedtobondthefirstlayersoftheparttothepowderbedplatform.Attheendoftheprocess,gradedpartsofH13andCucouldbeobtained.Therefore,FGMscouldbeusedoninjec-tionmouldstocreatehighheatconductivityregionstoimproveheatextraction.Asthecooling/heatingchannelscanbelimitedbymanufacturabilityandtheejectorsystem,someregionsoftheimpressioncouldbeoverheated.Thisdifferentialheatextractionfromthepartmightcausewarpage,sinkandcoldweldingmarks,andpoorsurfacequality,andcouldreducetheproductionrateofthepart.AnotherapplicationofFGMonmouldsistobuildthecavityedgeswithgradientsoftoolsteelandtungstencar-bide.Thiscouldimprovethepartqualitybyred-ucingdefectssuchasflashingcausedbywearinthemouldedges1.TheuseofFGMtoobtainperformanceinjectionmouldswasoneofthestimuliforthisresearch.Despitethelimitationsofthelaserandlayerdeposi-tionsystemsusedinthiswork,theseexperimentswereplannedtoevaluatetheinfluenceoftheCuadditiontotheH13matrix.TheeffectoftheadditionofCuonthemouldtemperatureandontheinjectedpolymerpartcrystallinitydegree,comparedwiththeH13basematerial,wasanalysed.Intheory,theaddi-tionofCuwouldincreasethethermalconduct-ivityofthemould.Theworkwasdividedintwoparts:numericalmodellingoftheheattransferandexperimentalinjectionmoulding.Thefirstpartpre-sentsthenumericalmodeloftheheattransferfromtheinjectedparttothemouldandthemetallicinserts.Themodelevaluatedthetemperaturetimestamp,simulatingmouldinsertsindifferentmateri-als:H13,Cu,andH13þ50%Cu.Intheinjectionmouldingexperiment,FGMbars(mouldinserts)weremanufacturedbylaserfusionandplacedinastereolithography(SL)mould.Polypropylene(PP)partswereproducedbyinjectingthepolymeroverthesemetallicinserts.Twooutputswereanalysedfromthisexperiment:temperatureofthemouldsur-faceandcrystallinitydegreeofthePPparts.Thetemperatureofthemouldwasmeasuredbythermo-couplesintheexactsamepositiontakeninthenumericalmodel.Thedegreeofcrystallinityoftheparts,mouldedwithdifferentinserts,wasanalysedbydifferentialscanningcalorimetry(DSC).TheDSCtestwasperformedtoidentifyifthepartsmouldedoverdifferentinsertshaddifferentcoolingrates.Asaconsequence,thedegreeofcrystallinityofthepartscouldbedifferenttoo.Thelowerthecoolingrate,thegreateristheorganizationofthepolymerchainsreflectinginthecrystallinitydegreeofthePP.Arapidcoolingratehelpsthepolymertoholdanamorphousstructure.Whenheatingaplastic,moreheatwillbenecessarytodissolvethecrystals(morestableandlowerenergystate)untiltheplasticiscompletelymelted.ThisphenomenoncanbeseenintheDSCcurvesmeasuringtheenergyabsorbedbythesamplebeforemelting18.Evaluatingtheuseoffunctionallygradedmaterialsinserts947JEM764C211IMechE2007Proc.IMechEVol.221PartB:J.EngineeringManufacture2METHODOLOGY2.1NumericalmodelTheinjectionmouldingcycleisatransientphenom-enaandthermalconductivityisnottheonlymaterialpropertythatcountswhenanalysingtheheattrans-fer.Densityandspecificheatcapacityalsodeterminethecapabilityofthematerialtostoreortotransportenergy19.Consideringvolumecontrol,theenergystateisobtainedbythebalanceoftheenergythatisabsorbed,generated,andlost.Thisvariationoftheenergyaccumulatedbythemassinsidethevolumecanbemodelledbyequation(1).Theenergythatenters(_Ein)plustheenergygenerated(_Egen)insidethevolumeminustheenergylost(_Eout)tothesur-roundingsisequaltothevariationofenergy(E)ofthemassinsidethevolumewithrespecttotime(t)._EinC0_Eoutþ_Egen¼dEdtC12C12C12C12vcð1ÞInthecaseofamould,inthemomentsafterthemeltedmaterialfillsthemouldimpression,thereisnoheatgeneratedindefinedvolumecontrol.Consid-eringtheheattransferinonedirection,equation(1)becomesfurthersimplifiedfortheheatfluxthroughanareaA,generatingequation(2).Simplifyingthearea,equation(3)isgenerated.Intheseequations,q00representstheheatfluxes,risthematerialdensity,cpsymbolizesthespecificheat,Tisthetemperature,andxistheaxisofthedirectionoftheheatflux.qin00AC0qout00Aþ0¼ZxrcpTtAdxð2Þqin00C0qout00¼ZxrcpTtdxð3ÞThetemperatureinthemould,awayfromtheimpression,couldbeconsideredconstant.Takingthisintoconsideration,inaveryshortperiodtheheatfluxescanbeconsideredconstantandcanbedescribedasinequation(4),wherekistheheatconductioncoefficient.q00¼kTxC12C12C12C12xð4ÞThereisnoeasysolutionforsolvingequations(3)and(4)andanumericalmodelisusuallynecessarytosolvethemforcomplexshapes.Atwo-dimensional(2D)modelofaninjectionmouldedpartinsideaSLmouldincontactwithametallicinsertisshowninFig.1.Thismodelconsiderednocontactresistancebetweentheparts,moulds,andinsertsurfaces.TheinitialconditionswerethatthetemperatureinthenodesinsidetheareathatrepresentsthehotPPinjectedpartwas195Candthetemperatureforallothernodes,includingtheconnectednodesofthepartwithotherareas,was20C.Temperaturewascal-culatedbyemployingaquadrangularmeshformedbyplanarfour-nodeelements.ThenodeschosentobeanalysedareindicatedinFig.1.Thermocoupletemperature,Ttc,matchedthesamepositionintheexperimentalworkandinsertsurfacetemperature,Tis,matchedtheregionfromwhereDSCsamplesweretakeninthePPpart.ThemodelandanalysiswereperformedusingAnsyssoftware.Forinputtingthematerialproperties(density,specificheat,andthermalconductivity)inthenumer-icalmodel,tabledvalueswereusedfortheH1316.However,theH13þ50percentmaterialpropertiesvalueswereestimatedbasedontheVoightandReussrulesofmixtures5,6.Thebasicruleofmixtures(Voight)ispresentedinequation(5).Anequivalentproperty(«)ofthemixtureformedbyaandbphasesiscalculatedbythesummationofthepropertyofeachphaseandthevolumefraction(V)ofthephasesinthemixture,resultinginalinearvariationbetweeneachphasevalueproperty.Thesecondruleexpressedinequation(6)ismoreelaborate,butneitherrulecountsthephaseinteraction,phasegeometry,spacedistribution,andotherfactorsthataffectthefinalpropertyofthemixture.Nevertheless,thesecondruleismoreconservativethanthefirstone.Themate-rialpropertiesusedinthenumericalmodelarepresentedinTable1."Voight¼Va"aþVb"bð5Þ"Reuss¼"b"aVa"bþVb"að6ÞIntotal,sixsimulationsofthetimeversustemperatureinthenodesTtcandTis(refertoFig.1)weremadeusingdifferentinsertmaterialsspecifiedinTable1.ThefirstfoursimulationswereperformedFig.12DmodelfortheheattransferofaninjectedpartincontactwithametallicinsertinaSLmould(initialtemperatureindicatedforeachareaofthemodel)948VEBeal,PErasenthiran,CHAhrens,andPDickensProc.IMechEVol.221PartB:J.EngineeringManufactureJEM764C211IMechE2007