Journal of the Brazilian Society of Mechanical Sci.docx

Journal of the Brazilian Society of Mechanical Sci.docx

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JournaloftheBrazilianSocietyofMechanicalSci

JournaloftheBrazilianSocietyofMechanicalSciencesandEngineering

PrintversionISSN1678-5878

J.Braz.Soc.Mech.Sci.&Eng.vol.26no.1RiodeJaneiroJan./Mar.2004

doi:

10.1590/S1678-58782004000100007

TECHNICALPAPERS

Theuseofstereolithographyrapidtoolsinthemanufacturingofmetalpowderinjectionmoldingparts

V.E.BealI;C.H.AhrensI;P.A.WendhausenII

IUniversidadeFederaldeSantaCatarina,DepartamentodeEngenhariaMecânica,LaboratórioCIMJECT,CaixaPostal476,88040-900Florianópolis,SC.Brazil,beal@cimject.ufsc.br,ahrens@cimject.ufsc.br

IIUniversidadeFederaldeSantaCatarina,DepartamentodeEngenhariaMecânica,LaboratórioLABMAT,CaixaPostal476,88040-900Florianópolis,SC.Brazil,paulo@materiais.ufsc.br

ABSTRACT

Theutilizationofstereolithographymoldsinthemanufacturepre-seriesforinjectionmoldedplasticpartsaimstoreducecoststhroughouttheproductlife-time,butmainlyduringdesignandmanufacturingphases.TheuseofthisRapidToolingtechniqueinpowdermetalinjectionmoldingisevaluatedinthiswork.Oneofthegreatestdifferencesbetweentraditionalandstereolithographytoolsisrelatedtotheheatconductivityofthematerialsemployed.Forexample,steelmoldshaveaheatconductivitycoefficient300timeshigherthanmoldsmadewiththephotosensitiveresinusedinthestereolithographyprocess.Thediscrepancyregardingthecoolingrateofthemoldedpartsduringtheinjectioncyclemustbecompensatedwithadjustmentsintheinjectionmoldingparameters,suchastemperature,pressureandspeed.Theoptimizationoftheseparametersmadeitpossibletoejectgreenpartsfromthemoldwithoutcausingdefectswhichwouldbecomeevidentindebindingandsinteringstages.Thedimensionalanalysisperformedattheendofeachcasestudyshowedthattheshrinkingfactorofthecomponentafterthesinteringhadthesamevalueobtainedforcomponentsusingtraditionalmetallicmolds.Moreover,thedimensionalerrorremainsunder2%whichcanbeconsideredlowforapre-seriesofcomponents（orprototypeseries）.

Keywords:

Rapidprototyping,rapidtooling,powdermetallurgy,injectionmolding,stereolithography

Introduction

Inthecompetitivemarket,organizationshavebeenusingtechnologiesthataidtheachievementofrapidnewproductdevelopmentwithdesirablequalityandcosts（Jacobs,1992&1996）.Consideringcomplexgeometricparts,twotechnologiesdeserveattentionfromtheproductionsectors:

RapidPrototyping（RP）andinjectionmolding.Rapidprototypingtechniquesareimportantduetotheirabilitytoreducedevelopmenttime,toidentifyearlyerrorsintheproject,toachieveabettercommunicationwithintheprojectdevelopmentteamandtoevaluatethefunctionalityoftheproductalongwithmanyotherbenefitsusingphysicalprototypes/models.OthersapplicationsandevolutionsfromrapidprototypingareRapidTooling（RT）andRapidManufacturing（RM）.Inthecaseofrapidtooling,itispossibletobuildtools,suchasmolds,toproducepre-seriesofcomponentsbyinjectionmoldinginaperiodshorterthanoneweek.Stereolithography（SL）isoneofthemostversatilerapidprototypingtechniques.Thistechnologyiscapableofdeliveringfastandaccurate3Dobjectswithawiderangeofresinsformanydifferentuses（Wholers,2001）.

PowderInjectionMolding（PIM）competeswellwithtechnologiessuchascasting,machiningandformingwhenthecomponentsproducedaresmall,complexandhavetoleranceswithinanarrowrange.However,PIMhasahighinvestmentcost,becauseitneedsaninfrastructurewithovens,injectionmoldingmachines,arelativelyexpensivefeedstockandahighaccuracymold,thelattertwoalsoinfluencingdirectlythefinalproductioncostofeachcomponent.1

Rapidtoolingmightpossiblybeusedwithpowderinjectionmoldingtoprovidelow-costprototypemolds.However,Hemricketal（2001）andGomide（2000）havedescribedaparticularbehaviorofPIMpartsmoldedinSLtools.Theseauthorsaffirmthatirregularitiesinthemoldsurfacehavecausedastrongattachmentofthemoldedparttothemold.Asaresultthepartbrokeinthemoldopeningandejectingstages.Nevertheless,theinjectionmoldingparametersandmolddesigninfluenceoverthefailuresintheinjectionmoldingofmixturesofpowderandbinderswerenotcompletelydescribed.

Thispaperdescribescasestudiesperformedtoevaluatetheinjectionmoldingofstainlesssteel316Lmetalpowderinrapidmoldsproducedbystereolithography.Toclarify,areviewaboutstereolithographywithitsrapidmoldsandpowderinjectionmoldingispresented.

StereolithographyToolsforInjectionMolding

Stereolithographyisusedtomanufacturethree-dimensionalobjectsbymeansofphotopolymerizationofaresinbytheenergydeliveredthroughanultravioletlaserbeam.AbasicsketchoftheprocessisshowninFig.1.

Basically,everyrapidprototypingprocessstartswithaCAD（ComputerAidedDesign）system.IntheCAD,athree-dimensionalobjectthatrepresentstheproductisdesigned.ThemodelistranslatedintoacommonlanguageknownasSTLwhichisatriangularshellmeshoftheobject.ThisSTLfileisimportedintotheCAM（ComputerAidedManufacturing）ofeachprocessandisslicedintothinlayers.Eachlayerrepresentsonestepoftheprocessintherapidprototypingmachine.TheCAMsystemalsogeneratespathsandmanufacturingparametersaccordingtothematerialandmachinethatisgoingtobeusedtobuildtheprototype.Later,inthestereolithographymachine,datafromtheCAMisloadedtocommencethemanufacturingprocess.Theultravioletlaserbeam,asshownintheFig.1,isdrivenbygalvanometricmirrorsthatscanthevatsurfacewhichcontainsaliquidphotosensitiveresin.Thelaserradiationactivatesapolymerizationprocessandtheresinhardensformingasolidlayerofthethree-dimensionalobject.Afterfinishingonelayertheplatformdivesandtheliquidresinspreadsoverthesolidlayer.Astheresinistooviscous,abladepassesthroughthesurfaceoftheliquidresinleavingagapbetweenthesolidlayerandtheblade.Thescannerstartstosolidifyanewlayerattachingthenewlayertothepreviouslymadelayer.Layer-by-layertheobjectismanufacturedbyscanningselectivelythelaserbeamovertheresinsurface.Thisshortdescriptionistheoriginalconceptionfrom1988developedby3DSystemswhodevelopedthefirstRPcommercialprocess.Furthermore,therearemanyvariationsinotherstereolithographyprocessesthatusethephoto-chemicalprincipletomakeandattachlayers（Jacobs,1996;3DSystems,1998）.

Thistechnologypresentsasanadvantageitsspeedinmanufacturingcomplexpartsdirectlyfromthecomputeranditmaybeusedinmanydifferentapplicationssuchas:

prototypesforfunctionaltesting,bio-modelsforsurgeryplanning,modelsforaerodynamictestsandrapidmoldsforinjectionmolding.Usingrapidmolds,pre-seriesofpartsareobtainedinrecordtimewiththeinjectionmoldingmaterialchosentomanufacturethefinalparts.So,testsmaybeextendedtoawiderangeofapplicationsbeforeexpendingtimeandeffortonadefinitivehardtool.Thistechnologycanbeusedtoevaluatethemoldanalyzingitsinjectiongates,ejectionpins,split-lines,etc.

Ontheotherhand,accordingtoDickens（1999）,SLmoldscanmoldfrom20to500partsofpolypropylenewhichisconsideredverylowcomparedtotheworstmetalmoldsthateasilymoldover100.000shots.Theaveragelifeofthemolddependsbasicallyonthepartcomplexity,injectionmoldingmaterialandprocedurestosetuptheinjectionmoldingparametersintheinjectionmoldingmachine.MostoftheresinsthatareusedtobuildSLmoldshavelowmechanicalpropertiesabove70oC.Alsotheresinshavelowthermalconductivity（forexampletheSLresinDSMSomos7110k=0,2W/m.k）.Thus,theyareweakaftertheyreceivethethermalenergyfromtheinjectionmoldedmaterialthatisinjectedusuallyabove180oC.Whenthemoldisopenandthepartisejectedsomefeaturesofthemoldcanbreakduetotheejectionforcescausedbythecontractionofthepartmaterialoverthemoldcavity.Todecreaseearlyfailuresofthemoldtheinjectionmoldingparametersmustbedifferentfromthoseusedinmetalmolds.Figure2comparesgraphicallythedifferencesintheirmainparameters.NoticethattheinjectionmoldingcyclesarelongerforSLmoldsduetotheirlowthermalconductivity.Asaresulttheinjectedmaterialcanshowdifferentmechanicalpropertiescausedmainlybydifferentdegreesofcrystallizationachieved（Segal&Campbell,2001）.

Eventhoughtheypresentlowmechanicalproperties,stereolithographymoldscanreproduceinjectionmoldingpartswithfinedetailsinlessthan4dayswithoutthehighcostsofadefinitivetool（Gomide,2000）.Consequentlyitmayaidengineeringteamstoevaluatetheirprojects,avoidingerrordetectioninlaterphasesofnewproductdevelopment.

PowderInjectionMolding

PIMtechnologyisacombinationofpowdermetallurgywithinjectionmoldingofthermoplastics.Asaresultitispossibletomanufacturecomplexpartswithmetals,ceramics&composites.Thehardparticlesofceramicsormetalsaremixedwithabindersystemthatcoverstheparticles.Thisbindersystemismadeusuallyofthermoplastic,waxandadditivesthatallowthemixturetobemoltenandinjectedinsideamoldinawaysimilartothatperformedforthermoplasticsalone.AbasicsequenceofthepowderinjectionmoldingprocessispresentedinFig.3.

Theprocessstartsfromchoosingacombinationofpowderandbindersystem.Thepowderwillberesponsibleforgivingthepropermechanical,thermal,electricalandc