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DOI10.1007/s00170-004-2328-8ORIGINALARTICLEIntJAdvManufTechnol(2006)28:6166Fang-JungShiouChao-ChangA.ChenWen-TuLiAutomatedsurfacefinishingofplasticinjectionmoldsteelwithsphericalgrindingandballburnishingprocessesReceived:30March2004/Accepted:5July2004/Publishedonline:30March2005Springer-VerlagLondonLimited2005AbstractThisstudyinvestigatesthepossibilitiesofautomatedsphericalgrindingandballburnishingsurfacefinishingpro-cessesinafreeformsurfaceplasticinjectionmoldsteelPDS5onaCNCmachiningcenter.Thedesignandmanufactureofagrindingtoolholderhasbeenaccomplishedinthisstudy.TheoptimalsurfacegrindingparametersweredeterminedusingTaguchisorthogonalarraymethodforplasticinjectionmoldingsteelPDS5onamachiningcenter.Theoptimalsurfacegrind-ingparametersfortheplasticinjectionmoldsteelPDS5werethecombinationofanabrasivematerialofPAAl2O3,agrind-ingspeedof18000rpm,agrindingdepthof20m,andafeedof50mm/min.ThesurfaceroughnessRaofthespecimencanbeimprovedfromabout1.60mto0.35mbyusingtheoptimalparametersforsurfacegrinding.SurfaceroughnessRacanbefurtherimprovedfromabout0.343mto0.06mbyusingtheballburnishingprocesswiththeoptimalburnishingparameters.Applyingtheoptimalsurfacegrindingandburnishingparame-terssequentiallytoafine-milledfreeformsurfacemoldinsert,thesurfaceroughnessRaoffreeformsurfaceregiononthetestedpartcanbeimprovedfromabout2.15mto0.07m.KeywordsAutomatedsurfacefinishingBallburnishingprocessGrindingprocessSurfaceroughnessTaguchismethod1IntroductionPlasticsareimportantengineeringmaterialsduetotheirspecificcharacteristics,suchascorrosionresistance,resistancetochemi-cals,lowdensity,andeaseofmanufacture,andhaveincreasinglyF.-J.Shiou(a117)C.-C.A.ChenW.-T.LiDepartmentofMechanicalEngineering,NationalTaiwanUniversityofScienceandTechnology,No.43,Section4,KeelungRoad,106Taipei,TaiwanR.O.C.E-mail:.twTel.:+88-62-2737-6543Fax:+88-62-2737-6460replacedmetalliccomponentsinindustrialapplications.Injec-tionmoldingisoneoftheimportantformingprocessesforplas-ticproducts.Thesurfacefinishqualityoftheplasticinjectionmoldisanessentialrequirementduetoitsdirecteffectsontheappearanceoftheplasticproduct.Finishingprocessessuchasgrinding,polishingandlappingarecommonlyusedtoimprovethesurfacefinish.Themountedgrindingtools(wheels)havebeenwidelyusedinconventionalmoldanddiefinishingindustries.Thegeometricmodelofmountedgrindingtoolsforautomatedsurfacefinish-ingprocesseswasintroducedin1.Afinishingprocessmodelofsphericalgrindingtoolsforautomatedsurfacefinishingsys-temswasdevelopedin2.Grindingspeed,depthofcut,feedrate,andwheelpropertiessuchasabrasivematerialandabrasivegrainsize,arethedominantparametersforthesphericalgrind-ingprocess,asshowninFig.1.Theoptimalsphericalgrindingparametersfortheinjectionmoldsteelhavenotyetbeeninvesti-gatedbasedintheliterature.Inrecentyears,someresearchhasbeencarriedoutinde-terminingtheoptimalparametersoftheballburnishingpro-cess(Fig.2).Forinstance,ithasbeenfoundthatplasticde-formationontheworkpiecesurfacecanbereducedbyusingatungstencarbideballoraroller,thusimprovingthesurfaceroughness,surfacehardness,andfatigueresistance36.Theburnishingprocessisaccomplishedbymachiningcenters3,4andlathes5,6.Themainburnishingparametershavingsignifi-canteffectsonthesurfaceroughnessareballorrollermaterial,burnishingforce,feedrate,burnishingspeed,lubrication,andnumberofburnishingpasses,amongothers3.Theoptimalsur-faceburnishingparametersfortheplasticinjectionmoldsteelPDS5wereacombinationofgreaselubricant,thetungstencar-bideball,aburnishingspeedof200mm/min,aburnishingforceof300N,andafeedof40m7.Thedepthofpenetrationoftheburnishedsurfaceusingtheoptimalballburnishingparameterswasabout2.5microns.Theimprovementofthesurfacerough-nessthroughburnishingprocessgenerallyrangedbetween40%and90%37.Theaimofthisstudywastodevelopsphericalgrindingandballburnishingsurfacefinishprocessesofafreeformsurface62plasticinjectionmoldonamachiningcenter.Theflowchartofautomatedsurfacefinishusingsphericalgrindingandballbur-nishingprocessesisshowninFig.3.Webeganbydesigningandmanufacturingthesphericalgrindingtoolanditsalignmentde-viceforuseonamachiningcenter.TheoptimalsurfacesphericalgrindingparametersweredeterminedbyutilizingaTaguchisorthogonalarraymethod.FourfactorsandthreecorrespondinglevelswerethenchosenfortheTaguchisL18matrixexperiment.Theoptimalmountedsphericalgrindingparametersforsurfacegrindingwerethenappliedtothesurfacefinishofafreeformsurfacecarrier.Toimprovethesurfaceroughness,thegroundsurfacewasfurtherburnished,usingtheoptimalballburnishingparameters.Fig.1.SchematicdiagramofthesphericalgrindingprocessFig.2.Schematicdiagramoftheball-burnishingprocessFig.3.Flowchartofautomatedsurfacefinishusingsphericalgrindingandballburnishingprocesses2DesignofthesphericalgrindingtoolanditsalignmentdeviceTocarryoutthepossiblesphericalgrindingprocessofafreeformsurface,thecenteroftheballgrindershouldcoincidewiththez-axisofthemachiningcenter.Themountedsphericalgrindingtoolanditsadjustmentdevicewasdesigned,asshowninFig.4.Theelectricgrinderwasmountedinatoolholderwithtwoad-justablepivotscrews.Thecenterofthegrinderballwaswellalignedwiththehelpoftheconicgrooveofthealignmentcom-ponents.Havingalignedthegrinderball,twoadjustablepivotscrewsweretightened；afterwhich,thealignmentcomponentscouldberemoved.Thedeviationbetweenthecentercoordi-natesoftheballgrinderandthatoftheshankwasabout5m,whichwasmeasuredbyaCNCcoordinatemeasuringmachine.Theforceinducedbythevibrationofthemachinebedisab-sorbedbyahelicalspring.Themanufacturedsphericalgrind-ingtoolandball-burnishingtoolweremounted,asshowninFig.5.Thespindlewaslockedforboththesphericalgrindingprocessandtheballburnishingprocessbyaspindle-lockingmechanism.63Fig.4.Schematicillustrationofthesphericalgrindingtoolanditsadjust-mentdevice3Planningofthematrixexperiment3.1ConfigurationofTaguchisorthogonalarrayTheeffectsofseveralparameterscanbedeterminedefficientlybyconductingmatrixexperimentsusingTaguchisorthogonalarray8.Tomatchtheaforementionedsphericalgrindingpa-rameters,theabrasivematerialofthegrinderball(withthediam-eterof10mm),thefeedrate,thedepthofgrinding,andtherevolutionoftheelectricgrinderwereselectedasthefourexperi-mentalfactors(parameters)anddesignatedasfactorAtoD(seeTable1)inthisresearch.Threelevels(settings)foreachfactorwereconfiguredtocovertherangeofinterest,andwereidenti-Fig.5.aPhotoofthesphericalgrindingtoolbPhotooftheballburnishingtoolTable1.TheexperimentalfactorsandtheirlevelsFactorLevel123A.AbrasivematerialSiCAl2O3,WAAl2O3,PAB.Feed(mm/min)50100200C.Depthofgrinding(m)205080D.Revolution(rpm)120001800024000fiedbythedigits1,2,and3.Threetypesofabrasivematerials,namelysiliconcarbide(SiC),whitealuminumoxide(Al2O3,WA),andpinkaluminumoxide(Al2O3,PA),wereselectedandstudied.Threenumericalvaluesofeachfactorweredeterminedbasedonthepre-studyresults.TheL18orthogonalarraywasse-lectedtoconductthematrixexperimentforfour3-levelfactorsofthesphericalgrindingprocess.3.2DefinitionofthedataanalysisEngineeringdesignproblemscanbedividedintosmaller-the-bettertypes,nominal-the-besttypes,larger-the-bettertypes,signed-targettypes,amongothers8.Thesignal-to-noise(S/N)ratioisusedastheobjectivefunctionforoptimizingaproductorprocessdesign.Thesurfaceroughnessvalueofthegroundsur-faceviaanadequatecombinationofgrindingparametersshouldbesmallerthanthatoftheoriginalsurface.Consequently,thesphericalgrindingprocessisanexampleofasmaller-the-bettertypeproblem.TheS/Nratio,isdefinedbythefollowingequation8:=10log10(meansquarequalitycharacteristic)=10log10bracketleftBigg1nnsummationdisplayi=1y2ibracketrightBigg.(1)where:yi:observationsofthequalitycharacteristicunderdifferentnoiseconditionsn:numberofexperimentAftertheS/NratiofromtheexperimentaldataofeachL18orthogonalarrayiscalculated,themaineffectofeachfactorwasdeterminedbyusingananalysisofvariance(ANOVA)tech-niqueandanF-ratiotest8.Theoptimizationstrategyofthe64smaller-thebetterproblemistomaximize,asdefinedbyEq.1.Levelsthatmaximizewillbeselectedforthefactorsthathaveasignificanteffecton.Theoptimalconditionsforsphericalgrindingcanthenbedetermined.4ExperimentalworkandresultsThematerialusedinthisstudywasPDS5toolsteel(equiva-lenttoAISIP20)9,whichiscommonlyusedforthemoldsoflargeplasticinjectionproductsinthefieldofautomobilecom-ponentsanddomesticappliances.ThehardnessofthismaterialisaboutHRC33(HS46)9.Onespecificadvantageofthisma-terialisthataftermachining,themoldcanbedirectlyusedforfurtherfinishingprocesseswithoutheattreatmentduetoitsspecialpre-treatment.Thespecimensweredesignedandmanu-facturedsothattheycouldbemountedonadynamometertomeasurethereactionforce.ThePDS5specimenwasroughlyma-chinedandthenmountedonthedynamometertocarryoutthefinemillingonathree-axismachiningcentermadebyYang-IronCompany(typeMV-3A),equippedwithaFUNUCCom-panyNC-controller(type0M)10.Thepre-machinedsurfaceroughnesswasmeasured,usingHommelwerkeT4000equip-ment,tobeabout1.6m.Figure6showstheexperimentalset-upofthesphericalgrindingprocess.AMP10touch-triggerprobemadebytheRenishawCompanywasalsointegratedwiththemachiningcentertoolmagazinetomeasureanddeterminethecoordinatedoriginofthespecimentobeground.TheNCcodesneededfortheball-burnishingpathweregeneratedbyPowerMILLCAMsoftware.ThesecodescanbetransmittedtotheCNCcontrollerofthemachiningcenterviaRS232serialinterface.Table2summarizesthemeasuredgroundsurfaceroughnessvalueRaandthecalculatedS/NratioofeachL18orthogonalar-rayusingEq.1,afterhavingexecutedthe18matrixexperiments.TheaverageS/Nratioforeachlevelofthefourfactorscanbeobtained,aslistedinTable3,bytakingthenumericalvaluespro-videdinTable2.TheaverageS/NratioforeachlevelofthefourfactorsisshowngraphicallyinFig.7.Fig.6.Experimentalset-uptodeterminetheop-timalsphericalgrindingparametersTable2.GroundsurfaceroughnessofPDS5specimenExp.InnerarrayMeasure