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ELSEVIERJournalofMaterialsProcessingTechnology551995408416MaterialsProcessingTechnologyAnalysisofdiedesignsforthestampingofanautomobilerearfloorpanelFuhKuoChen,JiaHongLiuDepartmentofMechanicalEngineering,NationalTaiwanUniversi,Taipei,Taiwan,ROCReceived10October1994IndustrialsummaryThestampingprocessofmanufacturingaonepiecerearfloorpanelofapassengercarhasbeeninvestigatedinthepresentstudy.Ananalysisoftheoriginaldiedesign,inwhichasplitdefectoccurredatthedrawncupwall,wasperformedusingcirclegridanalysisaswellasthe3Dfiniteelementmethod.Thesplitdefectisduetothelargeareaofsheetmetalundertheblankholderthatlimitstheflowofthemetaltowardsthecuparea.Anoptimumdiedesign,whichconsistsofaseparatediefaceandawedgemechanismmountedinthelowerdieframe,wasproposedtoprovideadditionalmetalforthecupareaandtoeliminatethesplitdefectwithoutaddinganextraoperation.Thisoptimumdiedesignwasvalidatedbytheresultsofcirclegridanalysisperformedforthefirstandseconddrawoperationsandalsobythegoodqualitypanelsproduced.KeywordsStampingdiesRearfloorpanelSplitsCirclegridanalysis.1.IntroductionSplitsareamongstthemajordefectsoccurringcommonlyinthestampingprocess.Alotofresearchefforthasbeenmadetoinvestigatethecausesofandsolutionstothesplitproblemduringthelastfewdecades\14\,themethodsusedincludingforminglimitanalysisandthefiniteelementmethod.SinceKeelerandBackofen\5\firstintroducedtheconceptofforminglimitdiagramsFLDsin1963,theyhavebeenusedwidelyintheanalysisofsheetmetalforminginpressshops.TheFLDsindicatethestrainswhichleadtofailureandthusprovideausefultooltodetermineiftheformingprocessislikelytobepronetosplitting,whilstthefiniteelementmethodcancalculatethestraindistributionsinthestampedpartsaccuratelyandthuspredictifthesplitdefectislikelytooccur.Ingeneral,thesolutiontoproblemsofsplittingistoprovidemoremetaltothecriticalareabeforethemajordrawingprocessstarts.Thiscanbeachievedeitherbydecreasingtheblankholderpressureorimprovingthelubricationconditions,butthemoststraightforwardmethodistoaddanextraoperationsolelyforthepurposeoffeedingmoremetalintothecriticalarea.Correspondingauthor.09240136/96/15.00©1996ElsevierScienceS.A.AllrightsreservedSSDI092401369502038NHowever,theextraoperationincreasestheproductioncostbyaddingonemoresetofdiesandadditionalmanpowerandhenceinreality,itshouldbeavoided.Inthepresentstudy,anoptimumdiedesigninwhichaseparatediefaceandawedgemechanismmountedinthelowerdieframeisproposedtoeliminatetheoccurrenceofasplitdefectinthestampingprocessforaonepiecefloorpanelofapassengercar.Thespecialdiefaceandwedgemechanismweredesignedtoprovideadditionalmetalforthecriticalareawherethesplitdefectoccurred,withoutaddinganextraoperation.Bothcirclegridanalysisand3Dfiniteelementsimulationswereperformedtoanalyzethissplitdefect.2.ProblemdescriptionThecommondesignforarearfloorpanelofapassengercarisusuallyatwopiecetype,namely,weldingtwostampedpiecestogether,asshowninFig.1.Thetwopiecetypedesignischosenmainlyduetothedifficultyencounteredinstampingaonepiecerearfloorpanel,inwhichasplittendstotakeplaceatthewallofthedeeplydrawncupusedforstoringthesparetire,asshowninFig.2.Theoccurrenceofthesplitisattributedtothesubstantialdistancebetweenthecupwallandonesideoftheblankholder,asshownbylineABinFig.3,thatF.K.Chen,J.H.Liu/JournalofMaterialsProcessingTechnology551995408416409Fig.1.Atwopiecerearfloorpanel.splitFig.2.Asplitoccurringatthecupwall.Fig.3.Thelocationofthedrawncupinthesheetblank.restrictsthemetalundertheblankholderfromflowingintothecuparea.Whilstinthetwopiecedesignthisdistanceismuchshorterandsufficientmetalcanfloweasilyintothecuptopreventtheedgeofthecupfromsplitting,duetocosteffectiveconsiderations,aonepiecerearfloorpanelisalwaysdesired,sothatthesplitproblemmustthereforebeovercome.Theoriginalprocedureinthepressshopfortheproductionoftheonepiecerearfloorpanelconsistsoffouroperationsfirstdraw,seconddraw,trimming,andflanging.Thefirstdrawoperationwasdesignedonlytoproduceacupshape,asshowninFig.3.Astortheribsaroundthecup,thesewereformedintheseconddrawoperation.Likemoststampingprocesses,themaindeformationoftherearfloorpaneliscompletedinthefirstdrawoperation.Theconventionaldrawprocessallowsthepunchtopullmoremetalintothediecavityfromtheblankholder.Tofacilitatemetalflow,nodrawbeadswereemployedontheblankholdersurface.However,duetothelargedrawdepthandthegeometricdifficultymentionedabove,asplitwasstillfoundatthecupwallnearthebottomafterthefirstdrawoperation,asshowninFig.2.Thelocationofthesplitdefectindicatesthattheconsiderabledistancebetweenonesideofthecupwallandtheblankholderdoespreventthemetalfromflowingtowardsthecup.Someeffortshavebeenmadetohelpthemetaltoflowtowardthecuparea.The,attemptofdecreasingtheblankholderpressureledtomorewrinklesattherootofthecupareabutwithouteliminatingthesplit.Improvementofthesheetmetalqualitywasalsoproventobeinvain.Extremecaretakenwiththelubricationconditionscaneasethesplitproblemhowever,thisisnotcosteffectivefortheprocessofmassproduction.Alsothelargequantityoflubricationoilusedinthestampingprocessmaypollutetheshopfloor.Hence,themostefficientwaylefttosolvethisproblemistoprovidemoremetalforthecupareabeforethepunchstartstoformthecup.Toachievethisend,modificationoftheblankholdersurfacetoformabetterbinderwrapshapewhichprovidesmoremetalaroundthecupareawasconsidered.Thebinderwrapisthedeformedshapeofthesheetblankattheclosureoftheblankholders.However,duetothesamegeometricalreason,i.e.theconsiderabledistancebetweenthecupandonesideoftheblankholder,theoptimumblankholdersurfaceisnoteasytoobtain.Finally,aseparatediefacedesignedforthefirstdrawoperationaidedbyaspecialwedgemechanismmountedinthelowerdieframeprovidedmoremetalforthecupareaandenabledtheproductionofsoundproductswithoutsplitdefects.3.AnalysisoftheoriginaldesignTheproblemofsplittingisusuallyrelatedtothestraindistributioninthecriticalarea.Thestraindistributioninanycrosssectionoftheformedpartisdeterminedbytwofactorsoneistheamountofmetalflowresultingfromdrawinovertheblankholdertheotheristheamountofstretchcreatedbythecontactsbetweenthepunchandthedie\6\.Toquantifytheeffectofthetoolinggeometryonthemetalflow,theoriginaldesignwasanalyzedbycirclegridanalysisCGAandthefiniteelementmethodFEM.410F.K.Chen,J.H.Liu/JournalofMaterialsProcessingTechnology5519954084160.50.40.30.20.1TRUESTRESSGPa00.10.20.3TRUESTRAIN0.40.5Fig.4.Thestressstraincurveforthesheetblank.3.1.CirclegridanalysisCirclegridanalysishasbeenusedwidelyinthepressshoptomeasurethestraindistributionsandthustoenableananalysisoftheformabilityofthesheetmetalbyplottingthemeasuredstrainsontheforminglimitdiagram.Thecirclegridshaveamajoradvantageovertheothertypesofgrid,suchassquaregrids,sincetheydonothaveanypreferredorientations.Thisadvantageliesinthatthedeformationofthecircleswillresultinellipses,thetwoprincipaldirectionsbeingdisplayedclearlybythemajorandminoraxes.Themagnitudesoftheprincipalstrainscanthenbecalculatedfromthemeasurementsofthelengthsofthemajorandminoraxes.Byplottingthesemeasuredmajorandminorstrainsontheforminglimitdiagram,theseverityofareasofaformedpartcanbeevaluated.Inthepresentstudy,aproductionfloorpanelmadeusingtheoriginaldiedesignwasfirstanalyzedwithcirclegridanalysis.Thesteelsheetusedforproductionis0.7mmthickandofDDQquality,withmaterialpropertiesasshowninFig.4.ThecorrespondingforminglimitdiagramforthismaterialprovidedbythesteelsupplierisshownbythesolidcurveinFig.5.Anyareaofaformedpartwhichisdeformedwiththestrainslocatedonorclosetothiscurvewilltendtosplit.Inpractice,theforminglimitcurveshifteddownby10,asshownbythedashedlineinFig.5,isusedasthedesigncurve.Theareaabovetheforminglimitcurveiscalledthefailurezonetheareabetweentheforminglimitcurveandthedesigncurveistermedthemarginalzoneandtheareabelowthedesigncurveisnamedthesafezone.Ingeneral,thestraindistributionsinanyareaofaformedpartshouldfallinthesafezonetomakethestampingprocessstable,astampingprocessbeingsaidtobestableifitislesssensitivetoprocessvariations.Beforebeingstamped,thecriticalareaofthesheetblankwasimprintedwith5mmdiametercircleswith6mmspacingbetweentheircentres.Toimprintthecirclesthecriticalareaofthesheetblankwasfirstcleanedusingaspecialcleaner,thenastencilwiththecorrectgridpatternwasplacedinpositiononthepart.Usingtheelectrolyteasconductor,theareacoveredbythestencilwasmarkedwiththegridpatternbyanetchingprocess.Topreventthemarkedareafromrusting,apieceofclothsaturatedwithcleanerwasusedtowipeoffexcesselectrolyteandresidualoxidesinthemarks.Afterbeingstamped,asplitwasfoundonthecupwallneartothetop,asshowninFig.2.Themajorandminorstrainsofthedeformedcirclesaroundthesplit,asshowninFig.6,weremeasuredandplottedontheforminglimitdiagram,asshowninFig.7.Ascanbeseeninthisfigure,themeasuredstrainsareeitheraboveorclosetotheF.K.Chen,J.H.Liu/JournalofMaterialsProcessingTechnology.551995408416411908580757065555045_4003503r3025\\\Failurezonefezone2015105302520151050510152025303540MinorStrainFig.5.Theforminglimitdiagramfor0.7mmDDQsteelsheet.9085807570656o55r_5O,4540.3530252015105\\\J\302520151050JIIII510152025303540MinorStrainFig.7.Measuredstrainsaroundthesplit.Fig.6.Deformedcirclesonthesheetblank.forminglimitcurve,andthefailureisobviouslyduetostretching,sinceboththemajorandminorstrainsarepositive.Itisalsonotedthatthestrainsareveryclosetotheplanestrainfailuremode,i.e.,closetotheaxisonwhichtheminorstrainiszero.Theresultsofthecirclegridanalysisindicatethattheoriginaldesignisveryunstable.TheFLDalsoindicatesthatthemajorstrainistoolargethisisconsistentwiththepresentauthorsopinionthattheconsiderabledistancebetweenthecupandonesideoftheblankholderlimitstheflowofmetaltowardsthecuparea,resultinginthelargestrains.Asdiscussedintheprevioussections,themosteffectivemethodofdecreasingthemajorstrainistoprovidemoremetalforthecuparea.3.2.FiniteelementanalysisTohelpfurtherunderstandingofthedeformationofthesheetblankduringthestampingprocess,a3Dfiniteelementanalysiswasperformedforthefirstdrawoperationoftheoriginaldesign.TheexplicitfiniteelementcodePAMSTAMP,whichiscapableofhandlinganyarbitrary3Ddieshapes,wasusedtoconductthesimulations.Sincethe3Ddiegeometries,includingthepunch,thediecavity,theblankholderandthedrawbeads,arenotsimplified,thefiniteelementprogramisabletosimulatetheactualproductionprocessesmoreaccurately.Inordertodescribethegeometryofthediecomponents,acommercialCADprogramwasusedtoconstructthesurfacemodelsforthesecomponents.ThemeshsystemsrequiredbyPAMSTAMPastheinputdataforthediegeometrieswerethengeneratedbythesamecommercialCADprogram,asshowninFig.8.Inearlierperiods,itwasverydifficult,ifnotimpossible,togeneratethemeshsystemforacomplicate3Ddieshape,suchasthestampingdies.However,asCADsystemsarebeingusedwithincreasingpopularityinthedieandmouldindustry,theaboveprocedurestogeneratethemeshsystemsforthediegeometriesbecometrivial.SincethePAMSTAMPcodetreatsthediecomponentsasrigidbodies,themeshsystemsareusedonlytodescribethegeometries
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