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南通大学毕业设计附录(英文资料翻译)-1-SOUDFREEFORMFABRICATIONISFF)ANDRAPIDPROTOTYPING1.1SOUDFREEFORMFABRICATIONISFF)METHODSSeveralmanufacturingprocessesareavailabletomaketheimportanttransitionfromcomputeraideddesign(CAD)toaprototypepart.Severalnewtechnologiesbegantomaketheirappearanceafter1987.Inthatyear,stereolithography(SLA)wasfirstintroducedbv3DSystemsInc.,andoverthenextfiveyearsseveralrivalmethodsalsoappeared.TllScreatedthefamilyofprocessesknownassolidfreeformfabricationfSFFl.Aswithmostnewtechnologiesatthebeginningofthe“marketadoptionS.shapedcurves”inChapter2.theSFFdomainiSaccompaniedbyarela.tiveamountofadvertising“hype.”SFFprocessesaresometimesdescribedas:·Partsondemand·Fromarttopart·Desktopmanufacturing·RapidprototypingAtthetimeofthiswriting,stereolithography(SLA),selectivelasersintering(SLS),fuseddepositionmodeling(FDM),andlayeredobjectmodeling(LOM)arebeingusedonaday-to-daybasisbycommercialprototypingcompanies.Thethree-dimensional(3一D)printingprocessincornstarch,plastic,andceramicsisalsobeingusedcommercially.Themethodsloweronthelistshowpromisebutdonotseemtobeingreatusebythirdpartyprototypinghousestomaketheirdailyincome.Castingisaspecialcase.Itisstillusedtomakeone—of-a—kindprototypes.Furthermore,forbatchrunsinthe10to500categoryitisaverycosteffectivemethodtouseonceanoriginalmoldhasbeenmadebyaprocesssuchasstereolithography.Machiningisalsousedtomakeone…ofakindorseveralprototypes.1.1.1SummaryofSFFandRapidPrototypingProcessesIndailycommercialuse:·Stereolithography(SLA)·Selectivelasersintering(SIS)·Laminatedobjectmodeling(LOM)·Fuseddepositionmodeling(FDM)南通大学毕业设计附录(英文资料翻译)-2-Moreattheresearchanddevelopment(R&D)stage:·3-Dprintingincornstarch,plastic,orceramic·3-Dprintingwithplasticsfollowedbyplanarizationusingmachining·Solidgroundcuring(similartoSLA)·Shapedepositionmodelingracombinationofadditionandsubtraction)Non-SFF(traditional):·Machining·CastingComparisonsdoneintheearly1990sbytheChryslerCorporationrevealedthattheSLAprocesswasaheadofitsrivalnontraditionalprototypingmethodsintermsofcostandaccuracy(thesestudiesexcludedanevaluationofmachiningandcasting).Followingthetechnicaldescriptionsinthischapter.additionalfiguresandtablesarethusincludedtocomparethesecostsandaccuracies.Overthelastdecade,SLAhasfurtheremergedasthemostusedSFFprocess,especiallyforthegenerationofthemasterpatternsforcastingandinjectionmolding.Atthetimeofthiswriting,SLS,FDM.andLOMhavethemostvisibilityafterSLA.1.1.2TheHistoryofSFFMethodsDuringthelate1970s,MeadandConway(1980)createdthegroundworkforthefastprototypingofverylargescaleintegrated(VLSllcircuits.Designerswereencour.agedtothinkintermsoffivetwo-dimensional(2.D)patterns.Thesepatternsdefinedthreestackedinterconnectionlayersonametal.oxide.semiconductorfMOSlwaferandtheirmutualconnectionsthroughholes.Thepatternsdescrybedtheactualgeometrvoftheconnectionrunsandviaholesthatonewouldseewhenlookingdownontothecircuitchip,regardlessoftheexactprocessandnumberofmaskingstepsthatwereusedtoimplementthechip(seeMOSIS,2000).Inspiredbythissuccess,beginninginthe1970s,severalcompaniestriedtocreatelayeredmanufacturingformechanicalparts.AISObythemid.1980s,severaU.S.governmentstudiesanalyzedthepossibilitiesofa“mechanicalMOSIS”(Man.ufacturingStudiesBoard,1990;Bouldin,1994;NSFworkshopI,1994,andII,1995).TheprospectsforamechanicalMOSISwerethusfrequentlylinkedtothefabricationprocessesinthelistsmentioned(Ashley,1991,1998;Heller,1991;Kruth,1991;Woo,1992,1993;AuandWright,1993;Kochan,1993;Kai,1994;UCLA,1994;WleissandPrinz,南通大学毕业设计附录(英文资料翻译)-3-1995;Coheneta1.,1995;Dutta,1995;Jacobs,1992,1996;Beamaneta1.,1997;Kumareta1.,1998;Sachseta1.,2000).TheintroductionofthefirstcommercialSFFtechnology--stereolithography--wasaccompaniedbytheadventoftheSTereoLithography(.STL)representationofaCADobiect.“.STU’isamodifiedCADformatthatsuitsasubsequentslicingoper.actionandthe“downstream”laser.scanningpathsonaphysicalSLA.FDM.orSLSmachine.Isasoccerballround?TheanswerdependsonhowcarefullythebalIismeas.ured.Nominally,itisaperfectsphere.However.oncloserinspection.theleatherissewntogetherfromabout20littlehexagonalpatchesandafewpentagonalpatchestocreatethecurvature.Inrealityitisanapproximationtoasphere.Likewise.the“.STL’’formatapproximatestheboundarysurfacesofaCADmodelbybreakingitdownintointerconnectedsmalltriangles--aprocesscalledtes.sellation.Eachtriangleisrepresentedbythex/y/zcoordinatesofeachofitsthreever.tices,enumeratedbytheright.handrule——thatis.Counterclockwise(ccw)orderasviewedfromtheoutsideofthebodvThevectornormaltothesurfaeeofeachtriangleisalsospecified.1histessellatedsurfaceisstoredasan“.STLfile.”thisfile.Perhapscontainingupto200,000triangles,issentovertheInternettoaprototypingshop.AsshowninFigure4.1.thistessellatedCADmodelisthenslicedlikeastackofplayingcards.For3DSystems’machinesthisisknownastheSLIorslicedfile.Otherrapidprototypingmachinesusetheslicingtechniquebuthavetheirownfilecreationdetailsandnames.Eachslicefortheimaginarysoccerballwillthusbeacircle.However.becauseofthetessellationprocedureitwillnotbeaperfectcircle.Theslicingactioncutsthroughthetrianglesontheboundary.Thus,eachcircularslice(ordiscwillactuallybeamultisidedpolygonrunninginsidethe“boundingcircle.”Thenumberofsidesonthisinnerpolygonisofcourserelatedtohowfinelydividedtheoriginaltessellationwasmade.InsidetheSLAmachine.thelaserfirstcreatestheouterboundaryofeachsliceandthen“weaves”acrosseachsliceinahatchingpatterntocreatethelayer.Thenumberofslicesandthestyleoftheweavingpatternarechosenbyeachrapidpro.Totypingshop.EspeciallyforSLAandSLSacertainamountoftrialanderror.Orcraftspersonship,beginstoplayaroleatthisstage.Thisisreviewedinmoredetailoverthenextfewpages.“.STL,’isnowthestandardexchangeformatforSFFprocesses.However.itisinadequate南通大学毕业设计附录(英文资料翻译)-4-formanyreasons.First,thefilesarelargeduetothetessellationmethod.Second.thereareredundanciesinthe“.STL'’format.0neexampleofredundancyisasfollows:thetrianglesarerepresentedbythe“counterclockwiserule”sothatitisclearinwhichdirectiontheouter-surfacenormalacts.However.ithasalsobecomecustomarytospecifythesurfacevectoraswell.Inconsistencycanbeintroducedasaresultofthisredundancy,andnorulesexistforresolvingit.McMains(1996)describeshow“.STL'’doesnotcapturetopologyorconnectivity,makingitdifficulttofixsomeofthecommonerrorsfoundinfiles—suchascracks,penetratingorextraneousfaces,andinconsistentsurfacenormals—withoutresortingtoguessingthedesigner’soriginalintent.MoregeneraldigitalinterchangeformatshavealsobeenusedwithSFlFTheseincludeACIS(1993)andIGES(Heller,1991).However,asdescribedinNSF(1995),problemsarisewiththeseformats,tooOneaspectofongoingresearchisthustoimprovethisrepresentationlanguage(McMainseta1.,1998).1.2CASTINGMETHODSFORRAPIDPROTOTYPING
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