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Production of injection molding tooling with conformal cooling channels using the three dimensional printing process

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Production of injection molding tooling with conformal cooling channels using the three dimensional printing process
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  Production of InjectionMoldingToolingwithConformalCoolingChannelsusingTheThreeDimensionalPrintingProcess EmanuelSachs 1, SamuelAllen2,MichaelCima2,EdwardWylonis 1, andHonglinGu02 1 Department of MechanicalEngineering2Department of MaterialsScienceandEngineeringMassachusettsInstitute of TechnologyCambridge,Massachusetts02139  bstract - ThreeDimensionalPrintingisadesktopmanufacturingprocessinwhichpowderedmaterialsaredepositedinlayersandselectivelyjoinedwithbinderfromanink-jetstyleprinthead.Unboundpowderisremoveduponprocesscompletion,leavingathreedimensionalpart.Stainlesssteelinjectionmoldinginsertshavebeencreatedfrommetalpowderwiththe3DPprocess.Thefreedomtocreateinternalgeometrybytheuse of the3D-Printingprocessallowsforthefabrication of moldswithcomplexinternalcoolingpassages.Toolingwasdevelopedwithcoolingchannelsdesignedtobeconformaltothemoldingcavity.Afinitedifferencesimulationwasconstructedtostudyconformalchanneldesign.Adirectcomparison of themoldsurfacetemperatureduringtheinjectioncycle of a3DPrintedmoldwithconformalchannelsandamoldmachinedwithconventionalstraightchannelswascompleted.Theconformalpassagesproducedwiththe3DPprocessprovidetheabilitytoaccuratelycontrolthetemperature of themoldingcavitythroughouttheprocesscycle.Surfacetemperaturemeasurementsdemonstratedthattheinsertswithconformalcoolingchannelsexhibitedamoreuniformsurfacetemperaturethantheinsertsmachinedwithstraightchannels.Issuessuchaspowderremovalandpostprocessing of greenpartswithsmallcoolingchannelswereinvestigated. I. INTRODUCTION A Motivation Theinjectionmoldingprocessisone of themostwidelyusedmethods of manufacturingpolymerplasticproducts.Manypartsthatareinjectionmoldedtodayhaveveryrigidtolerancerequirements.Properthermalmanagement of metalinjectionmoldingtoolingisnecessarytoincreasepartqualityandproductionrates.Coolingpassagesplacedinthetoolingmanagetheheatflowout of theplastic.Currentfabricationmethodsplaceseverelimitationsontheconfiguration of thecoolingchannelsusedforheatwithdrawal.Thefreedomtocreateinternalgeometrybytheuse of the3D-Printingprocessallowsforthefabrication of moldswithcomplexinternalcoolingpassages.Thesecoolingchannelscanbedesignedtobeconformaltothemoldingcavity.Conformalpassagesproducedwiththe3DPprocessprovidetheabilitytocontrolmoreaccuratelythetemperature of themoldingcavitythroughouttheprocesscycle.Suchtemperaturecontrolhasthepotentialtoproducepartswithlowerresidualstressesandtoshortencycletimes.448  B. CurrentMoldCoolingPractices Coolingincavitygeometriesistypicallyaccomplishedbyroutingstraightcoolingchannelsaroundthepartcavity.Thecoolingcircuitsarepositionedaroundthepartcavityasuniformlyasthepartshapewillallowandasclosetothecavitywallsaspossible.Thenumberandsize of thecoolingcircuitsareoftenlimitedbythepart'sejectorsystem.Often,itisnotpossibletoplacecoolingchannelsintheinserts.Channelsmustbeplacedinthebolstersorsupportblocksinstead.Inthiscasetheinterfacebetweenthebolsterandtheinsertiscritical.Athinlayer of airatthisinterfacecanactasaninsulatorandhinderheatwithdrawal.Shortandindependentroutingsyieldthebesttemperaturecontrolperformance.Parallelcoolingcircuitsasopposedtoseriescoolingcircuitsareconsideredabettercoolingmethod.Shortparallelcircuitsdonotallowthecoolanttoheatupinthemoldandoffermoreconsistentanduniformtemperaturecontrol[Gordon,1993].Cooling of thecoreinsertisthegreatestprobleminmostinjectionmoldingapplications.Often,nocoolingisemployedinthecoreitself.Coolingonlyoccursinthemoldbasethroughthecoremount.Withnocorecooling,eventualheating of thecoreisunavoidable.Cooling of slendercoresisoftenaccomplishedbyusinginsertsmade of materialswithhighthermalconductivity,suchascopper,beryllium-copperorhigh-strengthsinteredcopper-tungstenmaterials.[Menges,1993]Thesemetalinsertsarepress-fittedintotheslendercoreandextendintothemoldbase.Thehighthermallyconductiveinsertusuallyextendsintotheflow of coolantinacoolingchannel.Amajorconcerninutilizingthiscorecoolingmethodismaintaininganextremelytightpress-fit. If thereisapoorfit,theresultingthinlayer of airbetweenthehighthermallyconductiveinsertandtheholeinthecorewillact asan insulatortoheattransfer.Abaffleisacommoncoolingmethodinthosecasesinwhichcoolantisdirectlychanneledthroughthecore.Abaffleusuallyemploysaflatorspiraldividerinaholerunningthroughthecenter of thecore.Theinletandreturnflowisseparated.Thismethodprovidesmaximumcrosssectionsforthecoolanttoflowthrough.Thedividermustbemountedexactlyinthecenter of theholetoensurethatthecoolantdoesnotbypassthehole.Themosteffectivecooling of slendercoresisachievedwithbubblers.Aninlettubedirectsthecoolantintoablindholeinthecore.Thediameters of bothhavetobeadjustedinsuchawaythattheresistancetoflowinbothcrosssectionsisequal.Bubblersarecommerciallyavailableandareusuallyscrewedintothecore.Oneproblemwithbafflesandbubblercoolingsystemsisthatthenecessaryhollowcentercanresultinastructurallyweakcoreinsert.[Menges,1993] C. ThreeDimensionalPrinting - ApplicationtoInjectionMoldingTooling ThreeDimensionalPrinting(3DP)isaprocessfortherapidfabrication of threedimensionalpartsdirectlyfromcomputermodels[Sachset aI, 1990].Asolidobjectiscreatedbyprintingasequence of two-dimensionallayers.Thecreation of eachlayerinvolvesthespreading of athinlayer of powderedmaterialfollowedbytheselectivejoining of powderinthelayerbyink-jetprinting of abindermaterial.Acontinuous-jetprintheadisrasterscannedovereachlayer of powderusingacomputercontrolledx-ytable.Individuallinesarestitchedtogethertoform2Dlayers,andthelayersarestitchedtogethertoforma3Dpart.Unboundpowdertemporarilysupportsunconnectedportions of thecomponent,allowingoverhangs,undercutsandinternal449  volumestobecreated.Theunboundpowderisremoveduponprocesscompletion,leavingthefinishedpart.The3DPrintingprocesssequenceisshowninFig. 1.   I:AitiK Spread Powder   o ·   z  0 Print Layer   t Drop Piston 1.- RepeatCycle -------------- Fig. 1. The3DPrintingsequence.3DPrintingwasinitiallydevelopedfortheproduction of ceramicshellsandcorestobelaterusedforthecasting of metalparts.Inthisembodiment of theprocess,aluminapowderisspreadintothepowderpistonandselectivelyboundusingcolloidalsilica as abinder.Aftercompletion of printing,thepartisfiredinafurnacetofurtherbondthesilicatothealuminaandstrengthenthepartsufficientlysothatitcanbeused as aceramicmold.[Sachs,1992]Animportantaspect of the3DPrintingprocessitsinherentflexibilitywithrespecttomaterialssystems.Althoughdevelopedaroundaluminapowderandsilicabinder,manytypes of powdersandbindersmaybeused.Thisresearcheffortemploysthe3DPrintingprocessforthedirectfabrication of metalinjectionmoldingtoolingwithconformalcoolingchannels.Stainlesssteel(316L)powderisselectivelyboundwithalatexemulsionbinderusingthe3DPprocessresultinginagreenpart.Aseries of post-processingstepssimilartothosefoundinpowdermetallurgyprocessingareusedtoobtainall-metalinjectionmoldingtoolingwithconformalcoolingpassages.Detailedinformationonthespecificexploration of theapplication of 3DPrinting to theproduction of metalpartsandinjectionmoldingtoolingisprovidedbyMichaels.[Michaels,1995]II.FABRICATION A Introduction Theoverallprocess of creatingametalpartmaybedividedintoseveralsteps.First,thegreenpartisprintedusingthe3DPsystembyusingatemporaryorganicbinder.Powdermustthenberemovedfromtheinsertcavitiesandcoolingchannels.Thegreenpartisthensubjectedtoaseries of postprocessingsteps. 450  Thefirststepinpost-processingthegreenpartistodebindandsinterthepart.Indebinding,theorganicbinderusedinthe3DPrintingprocessthermallydecomposesinaninertgasfurnace.Metalskeletonshavesufficientstrengthtobehandledafterdebinding,however,itisadvantageoustosinterthepartinthesamefiringscheduleatahighertemperaturetoincreaseitsstrength.Finally,thepartisinfiltratedwithalowermeltingpointalloyto100 density. B. PrintingGreenToolingInserts Thepowderusedincreatinggreen3DPrintedtoolingwasa316Lsphericalstainlesssteelpowderwithasizerange of 60flmfromAnvalCorp. of Rutherford,NJ.Anaqueousacryliccopolymeremulsion(Acrysol)fromRohmandHaas of Philadelphia,PAwasusedasthebinder.Typicalpackingdensity of theprintedpartisapproximately60 .Atypical3DPgreenpartis 10 byvolumebinder,leavingapproximately30 openporosity.Aftercompletion of theprintingprocesstheentirepowderbedisallowedtoairdryforaperiod of 24hours.Theinsertsarethenplacedinanovenat100°Cforonetotwohourstocompletelycuretheacrylicbinder.Thegreenpartisthenremovedfromthepowderbed.Theremainingattachedpowderontheexterior of theinsertiseasilyblownoffwithcompressedair.Fig.2 is aphotograph of alargetoolinginsertinthegreenstateandfinalinfiltratedstate.Fig. 2. Alarge3DPrintedtoolinginsertinthegreen(left)andfinal-infiltrated(right)states. 45]  C. PowderRemovalfromConformalCoolingChannels Afterprinting,complexcavitiesandcoolingchannels of thegreenmetalpartmustbecleared of powder. In ordertotesttheabilitytoremovepowderfromcomplexpassagesapowderremovaltestshellwasdesigned.Thisexperimentwasdevisedtoevaluatetheminimumsizecoolingchannelthatcouldbecleared of powder.Athinboxshapedshellwasdesignedsuchthatitcouldbeeasilysplitopenandthecontents of thechannelscouldbeviewed.One of themiddlelayers of thepartcontainsonlythinconnectionlinesthatwereprintedbetweenthechannelsandaroundtheperimeter of theshell.Thelinesprintedbetweenthechannelspreventpowderfromjumpingfromonechanneltothenextduringpowderremoval.Thelineprintedaroundtheperimeter of thepartaidsinsecuringthetop of theshelltothebottom.Becauseonlythesefewlinesfastenthetop of theshelltothebottom,thisweaklayeractsasadelaminationlayerprovidingeasyseparation of thetop of theshellafterthepowderremovalexperimenthasbeenconducted.Fig.3isaschematic of thefivecentimeterlongtestshellcontainingthreestraightchannelsandonezig-zagchannel.smallestchannelintheshellhasanominalchannelwidth of 525flm.Themiddlechannelis875flmwide,andthelargestchannelandthezig-zagchannelare1.225 mm wide.Allchannelsare2 mmdeep. 25 mm   5 mm 1.25mmx2 mm 0.89 mm x 2 mm 0.53 mm x2 mm Fig. 3. Schematic of openedpowderremovaltestshell.Themethodusedtoclearthetestshellinvolvesplacingthegreenpartinabath of water.Thispowderremovaltechniqueaddsarequirementthatthepolymericbinderinthegreenpartbeabletowithstandwaterimmersionforaperiod of timewithoutresultingindistortion of thegreenpart.Fig.4showsapicture of apowderremovalshellclearedusingthewaterbathprocedure.channelswereclearedusingthistechniqueandthegreenpartsufferednegligibledistortion.452
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