IntJAdvManufTechnol(2002)19:253–2592002Springer-VerlagLondonLimitedAnAnalysisofDraw-WallWrinklinginaStampingDieDesignF.-K.ChenandY.-C.LiaoDepartmentofMechanicalEngineering,NationalTaiwanUniversity,Taipei,TaiwanWrinklingthatoccursinthestampingoftaperedsquarecupsandsteppedrectangularcupsisinvestigated.Acommoncharacteristicofthesetwotypesofwrinklingisthatthewrinklesarefoundatthedrawwallthatisrelativelyunsupported.Inthestampingofataperedsquarecup,theeffectofprocessparameters,suchasthediegapandblank-holderforce,ontheoccurrenceofwrinklingisexaminedusingfiniteelementsimulations.Thesimulationresultsshowthatthelargerthediegap,themoresevereisthewrinkling,andsuchwrinklingcannotbesuppressedbyincreasingtheblank-holderforce.Intheanalysisofwrinklingthatoccurredinthestampingofasteppedrectangularcup,anactualproductionpartthathasasimilartypeofgeometrywasexamined.Thewrinklesfoundatthedrawwallareattributedtotheunbalancedstretchingofthesheetmetalbetweenthepunchheadandthestepedge.Anoptimumdiedesignforthepurposeofeliminatingthewrinklesisdeterminedusingfinite-elementanalysis.Thegoodagreementbetweenthesimulationresultsandthoseobservedinthewrinkle-freeproductionpartvalidatestheaccuracyofthefinite-elementanalysis,anddemonstratestheadvantageofusingfinite-elementanalysisforstampingdiedesign.Keywords:Draw-wallwrinkle;Stampingdie;Steppedrectangularcup;Taperedsquarecups1.IntroductionWrinklingisoneofthemajordefectsthatoccurinthesheetmetalformingprocess.Forbothfunctionalandvisualreasons,wrinklesareusuallynotacceptableinafinishedpart.Therearethreetypesofwrinklewhichfrequentlyoccurinthesheetmetalformingprocess:flangewrinkling,wallwrinkling,andelasticbucklingoftheundeformedareaowingtoresidualelasticcompressivestresses.Intheformingoperationofstampingacomplexshape,draw-wallwrinklingmeanstheoccurrenceofwrinklesinthediecavity.Sincethesheetmetalinthewallareaisrelativelyunsupportedbythetool,theeliminationofwallwrinklesismoredifficultthanthesuppressionofflangewrinkles.Itiswellknownthatadditionalstretchingofthematerialintheunsupportedwallareamaypreventwrinkling,andthiscanbeachievedinpracticebyincreasingtheblankholderforce;buttheapplicationofexcessivetensilestressesleadstofailurebytearing.Hence,theblank-holderforcemustliewithinanarrowrange,abovethatnecessarytosuppresswrinklesontheonehand,andbelowthatwhichproducesfractureontheother.Thisnarrowrangeofblank-holderforceisdifficulttodetermine.Forwrinklesoccurringinthecentralareaofastampedpartwithacomplexshape,aworkablerangeofblank-holderforcedoesnotevenexist.Inordertoexaminethemechanicsoftheformationofwrinkles,Yoshidaetal.[1]developedatestinwhichathinplatewasnon-uniformlystretchedalongoneofitsdiagonals.Theyalsoproposedanapproximatetheoreticalmodelinwhichtheonsetofwrinklingisduetoelasticbucklingresultingfromthecompressivelateralstressesdevelopedinthenon-uniformstressfield.Yuetal.[2,3]investigatedthewrinklingproblembothexperimentallyandanalytically.Theyfoundthatwrinklingcouldoccurhavingtwocircumferentialwavesaccordingtotheirtheoreticalanalysis,whereastheexperimentalresultsindicatedfourtosixwrinkles.NarayanasamyandSowerby[4]examinedthewrinklingofsheetmetalwhendrawingitthroughaconicaldieusingflat-bottomedandhemispherical-endedpunches.Theyalsoattemptedtorankthepropertiesthatappearedtosuppresswrinkling.Theseeffortsarefocusedonthewrinklingproblemsassociatedwiththeformingoper...
