Chapter 2Rapid Prototyping review.docx

Chapter 2Rapid Prototyping review.docx

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Chapter2RapidPrototypingreview

2.1RapidPrototypingandApplications

Rapidprototyping（RP）isagroupoftechniquesusedtoquicklyfabricatephysicalmodelsandpartsusingthree-dimensionalcomputeraideddesign（CAD）.Thepartisgeneratedusinglayerbylayerdepositionofmaterials.1RPhasbeendevelopedtoreducetheproductdevelopmenttimeandtoreducethecostofmanufacturing.Asaresult,rapidprototypingoffersthepotentialtorevolutionizetheproductdesignandmanufacturingindustry.

Differenttypesofrapidprototypingmethodshavebeendevelopedsince1980s.RPprocessescanbecategorizedintermsofthestateofmaterialsbeforeformingthepartslikeliquids,powdersorsolidsheetsorthewaybywhichlayersarecreatedsuchaslasers,hotrollersorbinders.

OneoftheearlypatentsinrapidprototypingwasissuedtoCharlesHullin1986forinventingstereolithography（SLA）.SLAisconsideredtobethefirstRPtechniqueworldwide.Itisaprocesstomakingpolymershardenunderultra-violetlightandsettinguptheapparatusformakinganobjectbylayerdeposition.Thefirstobjectthathebuiltwasacupof5cmtallandittookhimmonthstofabricateit.Twoyearslater,Hullco-founded3DSystemsCorporation,oneofthelargestandmostprolificorganizationsoperatinginthe3Dprintingsectortoday.2

Duringthisprocess,curablematerialslikeultraviolet（UV）curablematerials,arefabricatedlayerbylayertoformtheobject.ThecomponentisfabricatedonabuildplatformwhichissubmergedinaUVcurableliquidphotopolymervat.ThebeamofUVlightisdirectedbyxandyscanningmirrorstoshineonthecross-sectionofliquidandtheliquidexposedtoitwillbecuredtocreatealayer.Oncealayeriscomplete,thebuildplatformismoveddownbythethicknessofonelayertomakeroomforcreatingnextlayer.Thisprocessisrepeateduntilthefinalpartisformed.Thecompletedpartiscarefullyremovedfromliquidandseparatedfromtheplatform.TheexcessresinisremovedbychemicalbathandthepartiscuredagainunderUVlight.3

FusedDepositionModelling（FDM）wasdevelopedbyScottCrumpinthelate1980sandthefirstFDMsystemwaslaunchedbyStratasysIncinearly1990s.4Itisalayeradditivemanufacturingprocessthatusethermoplasticmaterialsormetalwirestoproduceparts.Theheatedthermoplasticfilamentsareextrudedfromthenozzlethatmovesinthehorizontalx-yplanewhilethebuildplatformmovesdownintheZaxisafteronelayerisbuilt.5FDMgainsincreasingusebecauseofitssafeandconvenientfabricationprocess,lowcostofmaterialsandtheavailabilityofthermoplastics.

LaminatedObjectManufacturing（LOM）isarapidprototypingprocessthatlayersofheatsensitivematerialslikeadhesivecoatedpaper,plastic,ormetallaminatesaresuccessivelybondedtogethertoformproducts.Inthisprocess,layersofmaterialsaregluedtogetherbyheatedrollerandcuttoshapewithalasercutter.ComparedwithotherRPtechnologies,LOMhassomeadvantagesduetopeculiarityoftheprocess.6LOMcouldproducepartswithlargedimensions.Besides,theLOMpartshavelowinternaltensionswhichpreventdistortion,shrinkinganddeformation.LOMproducewood-likepropertieswhenworkingwithpaper.However,thepartcomplexityislimited.

Selectivelasersintering（SLS）wasinventedinthelate1980sbyTheUniversityofTexasatAustin.7Itisarapidprototypingprocessthatallowstogeneratecomplex3Dpartsbyfusingsuccessivelayersofpowdermaterialsontopofsurfaceusingahighpowerlaser.Thepowderscanbesintereddirectlyorindirectlywiththemixofathermoplasticbinder,dependingonthepropertiesofthepowders.AbeamdeflectionsystemmakesthelaserbeamscaneachlayeraccordingtothecorrespondingcrosssectionofthecomponentascalculatedfromaComputerAidedDesign（CAD）model.8SLScanbeusedtoprocessalmostanytypeofmaterialsincludingmetals,thermoplasticpolymersandelastomers,providedthatthematerialisavailableintheformofpowdersandthatpowderparticlestendtofusewhenheatisprovided.Italsohastheadvantagetoproducepartswithhighdensity.Whenalayeriscompleted,thebedcontainingthepowdersisloweredone-layerthicknesstocreatethenextlayer.Thisprocessrepeateduntilthepartiscreated.Thesinteredpowdersformthepartwhiletheunsinteredpowdersremainsinplaceandprovidestructuresupport.Afterthecomponentisbuilt,theunsinteredpowderswillbecleanedawayandmayberecycled.9

LaserEngineeredNetShaping（LENS）wasdevelopedatSandiaNationalLaboratories,asshowninFig.2.1.TheLENSsystemconsistedofanozzlefeedingpowderstodepositionsurface,whichcreatesaconvergingpowderstream.Meanwhilethelaserbeamisfocusedontheconvergingpointtomeltthetargetedsurface.Thepartiscreatedlayerbylayer.Aftersolidifies,thenewpartiscreatedortheexistingpartsarerepaired.Thedepositscarriedoutintheatmosphereofargon.ThemotioncontrolofthedepositcouldbefromtheCADfileorprogrammedmanually.

Thismethodhastheabilitytoprocessmorecomplexgeometriesduetothe3or5axissystemsflexibleforthenozzlemoves.PartsachievedwithLENSusuallyhaveanetshapegeometrybutwitharoughfinishing.Post-processingboththermallytorelievetheinternalstressandmechanicallytogetthedesiredresolutionareneeded.10

Fig.2.1LENSprocess（CourtesyofWeldingJournal）

Three-Dimensionalprinting（3DP）isapowderbasedrapidprototypetechnologythatdevelopedbyMassachusettsInstituteofTechnologyin1990s.11Itisalsocalledbinderjetting.Inthisprocess,eachlayerstartswithathindistributionofpowderspreadoverthesurfaceofapowderbed.TheprocessissimilartotheprocessofSLS,butinsteadofusingalasertosintermaterials,anink-jetheaddepositsaliquid,whichisabindertobindsthematerials.Abindermaterialselectivelyjoinsparticlestofusethepowderstogetherbyaninject-printinghead.Afteronelayerisformed,thepistonthatsupportsthepowderbedmovesdownintheheightofonelayersothatnextpowderlayercanbespreadandfused.3DPsystemhashighbuildingspeedsandeasytohandle.Formetals,3DPisessentiallyapowdermetallurgyprocess,soporosityismajorproblem.Sometimesinfiltrationisneededtodecreaseporosity.Infiltrationreferstouseasecondmetal,whichisalowermeltingtemperaturealloytoprintedstructuretoachievedensematerials.

RPsystemsareeffectiveinreducingtimeandcostoffabricatingnewproducts;however,thepartaccuracy,surfacefinish,varietyofmaterialsinmostRPsystemsneedtobeimproved.AnewformofhybridRPsystems,combinesmaterialdeposition（RP）andComputerNumericControl（CNC）machininginasinglestationwasputforwardtofindthesolutiontothoseproblems.12

ShapeDepositionManufacturing（SDM）isahybridprocessthatinvolvesdepositingmaterialsinanear-netshapeonasubstrateandthenremovingunwantedmaterialsusingCNCmachining.

Inthisprocess,microcasting,whichisaweld-basedprocesswasusedtodepositthematerials.Thepartisthentransferredtotheshapingstationformachining,wherethedesiredaccuracyandfinishisachieved.Stressreliefcouldbeusedtocontrolresidualstress.Therobotizedpalletsystemisusedtotransferpartsbetweenstations.Thesupportmaterialisembeddedordepositedandshaped.13Byvaryingdifferentmaterialsinthedepositionprocess,thematerialpropertiescanbechanged.TheprocesscycleofSDMisshowninFig.2.1.

Fig.2.2TheprocesscycleofSDMwithadditiveandsubtractiveprocess14

Solventweldingfreeformfabricationtechnique（SWIFT）isahybridRPprocessthatbasedonsolventweldingandCNCcontourmachining.15Thisprocessisapplicableforthermoplasticmaterialssuchaspolystyrene,whichissolventweldableandavailableinsheetform.Foreachlayer,alaserprinterprintedathinfilmhighdensitypolyethylene（HDPE）.HDPEisathermoplasticmaterialwhichnotsolubletoacetoneandthereforeservesasasolventmask.Aftermasking,acetonesolventisappliedtothebottomsideofthesheetandpressedtotheexistingstackofsheetswheresolventweldingtakeplace.AcetonebreaksthevanderWaal’sbondsbetweenpolymerchainsandthusdissolvesthesurfaceofthethermoplasticsheet.Thedissolvedinterfacesofeachsheetblendtogetherandformnewpolymerchainsbetweensheets.

Computer-aidedmanufacture（CAM）oflaminatedengineeringmaterials（LEMS）isahybridfreeformfabricationprocessforfabricatinglaminatedpartsfromsheetmetals.Lasercuttingisappliedtomaterialssheetstocktocutslices.Theresultingpartslicesareextractedfromthestockandtoassemblythecomponentbyachievingintimateinterlayercontactandsubsequentsinteringprocesses.16

Themainpurposeofhybridprocessesistoovercomethelimitationsofrapidprototypingsystems,toimprovethequalityofthefinishesproducts,especiallyinsurfacefinishandroughness.Thehybridprocessesarealsoexpectedtoimprovethecuttingrateandmaterialsremovalrateandthusreducetheprocesscost.14

2.2LaminatedTooling

RapidTooling（RT）referstoaprocessthatusesRPmodeltoquickcreateamoldorusestheRPprocessfabricateatoolforalimitedvolumeofprototypes.RTisanaturalextensionofRapidPrototyping.Comparedwithconventionaltooling,RTprovidesasignificantincreaseinspeedandreductionincost.

RTisbroadlyclassifiedassofttoolingandhardtoolingandalsoasindirecttoolinganddirecttooling.Theshortrunandlowervolumeofpartsisproducedbysofttoolingfromsilicon,epoxyresinsorlowmeltingpointalloys.Thelongrunandhighervolu