Qualifyingamoldoftencantakeweeksandinvolveseveralcostlyandtime-consumingsteps.Withcomputedtomography,orCTscanningusingx-rays,thetimecanbecuttoafewdaysandcostscanbereduced.Itallstartswhenanewinjectionmoldissetupinthemoldingmachine.Everyonegatherseagerlytoseethefirstpart.Thepartlooksreasonablebutclearlyleavesroomforimprovement.
Themoldingengineerproceedstotweaktheprocess,andafterseveraltrialsafewdecentpartsareobtained.
Thenfollowsthetypicaldiscoveryphase,whichcommonlyinvolvesseveraldifferenttypesofmeasuringequipmentandcantakeuptothreeweeks,dependingonthecomplexityofthepart.SomecompaniestakeastatisticalapproachanduseaDesignofExperiments(DOE)methodologytozeroinonoptimalmachinesettings.AseriesofpartsismoldedwitharangeofmachinesettingsdefinedbytheDOE,sothepartswillshowsomedimensionalvariation.Thequestionis:Howmuchvariationandwhere?
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AlookinsidetheCarlZeissMetrotom1500industrialCTscanner.Inmostcases,thetoolsnecessarytofindthevariationsarefoundinthematerialsandQClabandincludecontactandnon-contactcoordinatemeasuringmachines(CMMs)tocheckthepartcharacteristicsthatcanbeseenandtouched.Torevealhiddeninternaldefects,destructivetestingisperformedbypotting,cutting,andpolishing.Yetacommoncomplaintaboutdestructivetestingisthattheevidenceoneislookingforisoftendestroyedduringtheprocessofpreparingtheparts.Somecompaniessendoutplasticpartsforx-rayinspectiontolookforvoidsandwall-thicknessvariation.Allofthistestingoftenresultsinanexpensivemoldsittingidleor,worse,makingpartsbeforethequalificationresultsarein.
CTMAKESDEFECTSVISIBLE
Afasterandmoreaccuratetechnologyavailabletodayiscomputedtomography,orCTscanning.Itpermitsinternalandexternalanalysisofapartbyprojectingabeamofx-raysthroughitwhiletheobjectisrotated.Itmeasuresinternalcavities,undercuts,anddeeprecessesnotreadilycapturedbyCMMsandlaserscanners,allinlessthananhour,includingdatareconstruction.
CTistheindustrialcounterpartofcomputer-aidedtomography,orCATscanning,whichusesx-raystopenetratethebodyandproducea3Dimageoftheregionofinterest.ThedifferencesbetweenCTandCATscanslieinthex-rayenergy,machineconfiguration,anddatareconstructionalgorithms.TheimagesthatareprovidedbyCTscanningmakevisibleanyvoids,cracks,andinternaldefectsinaplasticpart.Thistechnologyalsodoesnotrequiresophisticatedfixtures,thereforeeliminatingpartdistortionfromclampingforces.
CTscanningisusedinthemedical,electronics,aerospace,andautomotiveindustriesformaterialsevaluation,prototyping,andqualitycontrol.Dependingonphysicalsizeandx-raypenetratingpower,industrialCTsystemscancostfrom$300,000to$5million.Forcompaniesthatdon’tneedtheirownfull-timeCTscanner,scanningservicesareavailabletomakeCTinspectionaffordable.
CarlZeisssuppliesCTscannersandnewlyoffersscanningservicesusingitsMetrotomCTscanner.Thisunitcanscanpartsthatfitintoacylindricalvolumeof350mm(13.65in.)diam.x350mmhighandweighamaximumof51kg(112lb).ZeissalsooffersscanningservicesusingalargerVarianBIR450kVscannerfordefectdetectioninlargecomponentslikeplasticchairs.Itcanhandlepartsthatfitintoacylindricalvolumeof520mm(20.3in.)diam.x1200mm(46.8in.)highandweighupto100kg(220lb).
(Editor’snote:Afewothercompaniesoffersuchaservice—suchasVarianMedicalSystems,PaloAlto,Calif.,andBoltonWorks,E.Hartford,Conn.).
NEWLEVELOFCTSCANNING
TheZeissMetrotom1500computertomographrepresentsanewlevelofindustrialCTscanningmeasurementtechnology.Dubbed“metrotomography,”itwasdevelopedspecificallytoallowcomprehensivequalityassurance.UnlikestandardindustrialCTscanningprocedures,metrotomographygoesbeyondpurevisualinspectionandoffersthepossibilityofalsomeasuringtheinternalandexternalgeometriesofhighlycomplexparts.Ifthepartsaresmallenough,asisthecasewithsomemedicaldevices,itcanscanmultiplepartssimultaneously.
TheMetrotomwasdesignedfromthegroundup,usingtechnologyfromZeiss’shigh-performanceCMMs,suchastheguideways,drivesystems,controller,rotarytable,andCalypsometrologysoftware.Theresultisanon-destructivetestingsystemthatisveryaccurate—downto9micronsfortypicalplasticparts—andextremelyrepeatable,asitwillpassGageRepeatabilityandReproducibility(GR&R)tests.MetrotomalsousessoftwarethatcanevaluateANSI(AmericanNationalStandardsInstitute)Y14.5GD&Tdrawingstandards.
HOWITWORKS
Apartisplacedonarotarytableinthex-rayscanningchamber.Afteronefullrevolution,whichtakes30to60min,thesystemproducesacomplete3Ddatasheet(seeFig.1).IncontrasttotypicalindustrialCTsystems,thereisnoneedfordatamanipulationorscaling,becauseMetrotomproducesadatasetwithanMPEEof+9+L/50micronseverywhereinsideandoutsidethepart.MPEEistheISOgaugingindustryacronymforMaximumPermissibleErrorandisastatementoftraceablesystemmeasurementuncertainty.
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Fig.1—Thisshowsthedatasetproducedafterx-rayscanningamedicalfluid-deliveryY-connector.Immediatevisualinformationisavailable,likehowmuchtherubbergrommet(brightwhite)isdeformedbytighteningtheplasticthreadedcapatthetopofthepart.Inthenextstep,theZeissCalypsomeasurementsoftwarealignsthepartdatasettotheCADmodelofthepartusingthedatumreferenceframe(DRF)specifiedinthepartdrawing(Fig.2).Oncealigned,theonlyvisualindicationthataCADmodelisoverlayingtheCTdatasetistheCADsurfaceedgesthatrevealtheunderlyinggeometry.Thissoftwaretoolcanhelptoquicklyidentifysmallfeaturesintheplasticthatwerenotproperlyfilledout(shortshot)andmayoftengounnoticed,duetotheirsizeorhiddenlocation.CalypsosoftwaretakespartmeasurementsdirectlyfromtheCTgrayscaleimagemodelsratherthantheSTLdataformatusedbypopularstereolithographyorotherrapid-prototypingsystems.STLformatwasdevelopedforvisualizationpurposesbutwasneverintendedtorepresentanexactdescriptionofthepartgeometry.
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ABOVEFig.2—Zeiss’sCalypsomeasurementsoftwarealignsthemeasuredpartdatatotheoriginalCADmodel.CADedgegeometryismadevisibletohelpshowmissingormisalignedgeometry.WiththedatasetalignedwiththeCADmodel,theanalysisbegins.Averypracticalandfastfirststepistoshowgraphically,viacolorvariation,wherealltheinternalandexternalsurfacesdifferfromtheCADmodel(Fig.3).Tolookevendeeperinsidethepart,softwaretoolsareavailabletoprovideacross-sectionfromanyorientation.Thiscross-sectioningtoolcanalsobeusedtorevealvoidsinsidethematerial.NotonlyiscolorusedtoshowadeviationfromtheCADmodel,butitcaneasilybeusedasago/no-gogauge.
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Fig.3—Theactualpartgeometry(insideandoutside)ofanautomotive“PRNDL”shiftselectorcoverafterbeingalignedtotheCADmodel.Colorcodingisusedtoshowvariationbetweenthetwo.Greenareasareclosetoperfect,yellow-redareasshowvariationonthepositivesideofthematerial(+)andblueshowsvariationonthenegativesideofthematerial(-).Thecolorspectrumisuser-definedandvaluescanberepresentedinanyunits.(Note:Partdeviationsareexaggeratedhereforillustrativepurposes.)Thescanningandanalysisdescribedthusfartakeplaceinafewhours.Thecolormapsareusefultohelpedtozeroinontheinjectionmoldingissues.Butwhatyoureallyneedisafulllayoutreport.WithCalypsosoftware,toolsareavailabletodevelopameasurementplanforcomplexplasticpartssuchasthefluid-deliveryvalveinFig.1.
WithitsGeometricDimensioningandTolerancing(GD&T)mathengine,CalypsosoftwarecanhandleyourGD&Tcall-outsandreportthedatainawidevarietyofformats.AMicrosoftExceloutputreportand“balloondiagram”withleaderspointingtothecharacteristicsinquestionareillustratedinFigs.4and5,respectively.
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Fig.4—CTmeasurementresultscanbereportedandinawidevarietyofformats,suchasthisExcelspreadsheet.
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Fig.5—AnothersampleofaCTmeasurementreport.Here,thepartisslicedopenforvisualreferenceandtheGD&Tmeasurementresultsareembeddedin“balloons”thathaveleaderlinespointingtothecharacteristicsbeingevaluated.Notethatballooncolorsareusedtoshowpartvariationsfromtolerance.Greenballoonsshowcharacteristicsthatarewithintolerancelimitsandredballoonsshowcharacteristicsthatareoutsidetolerancelimits.THINGSTOCONSIDER
WhetheryouarelookingtobuyaCTscannerorexploringtheadvantagesofascanningservice,considerthefollowing:
Accuracy:Resolutionisnotthesameasaccuracy.Highresolutioncannevercompensateforpoorelectromechanicaldesignofameasuringinstrument.Systemsintendedtobeusedforprecisemeasurementmustbequalifiedagainstknownreferencestandards.Askthemanufacturerand/orserviceprovidertoshowyouhowtheyqualifytheaccuracyoftheirCTsystems.
Repeatability:Whenevaluatinganewgaugingsystem,takethetimetoconfirmthatthesystemiscapable—i.e.,repeatableandfreefromoperatorinfluence.Oneofthe“goldstandards”forjudgingsystemcapabilityisaGR&R.Agoodruleofthumbisthatthegaugemustrepeatto3%ofthecharacteristictolerancetoachievea105GR&R.
Speed:Thegoalistoqualifynewmoldsinaday—notaweekormore.
Software:Evenafast,accurate,andrepeatablesystemisuselesswithoutsoftwaretoolsthatcanprovidemeaningfulresults.MetrologysoftwaremustbeabletoimportanyCADformat,haveastrongGeometricDimensioning&Tolerancing(GD&T)mathengine,offerafastwaytocreateandeditmeasurementplans,and—mostimportant—beabletomeasurefromanativeCTgrayscaledataset.
abouttheauthor
KevinLegacyismanagerofcomputedtomographyandengineeringatCarlZeissIndustrialMeasurementTechnology(IMT)Corp.,MapleGrove,Minn.Hehasbeenwiththecompanyfor11yearsandheldvarioussalesandengineeringmanagementpositions.Hecanbereachedatklegacy@zeiss.comor(248)867-3699.FormoreinformationonZeiss,visitwww.zeiss.com/imt.