Antec-machine-audit[1]

THEMACHINEAUDIT:ASYSTEMATICEVALUATIONOFINJECTIONMOLDINGMACHINES:HOWTOTELLAGOODMACHINE-OLDORNEW(2ndedition)JohnBozzelliInjectionMoldingSolutionsRodneyJ.GroleauandNormWardRJGAssociates,Inc.Injectionmoldingcontinuestodominateastheprocessofchoicefortheproductionofhighvolumeprecisionplasticparts.However,theproductiongoalofidenticalpartsisnotoftenachieved.Mostmoldershavecommonpartproblemssuchasshorts,flash,warp,knitlineproblems,dimensionalvariations,etc.Causesaresubjecttodebateandoftenvarywithindividualsituations.Thesituationisfurthercomplicatedwhencausescannotbeclearlydefinedasaproblemwiththemachine,mold,methodofprocessing,orresin.Thispaperwillfocusonhowtoquantifyamoldingmachine'sperformancesothatwhenaproblemdoesarise,acleardecisioncanbemadeastowhetherornottheproblemiscausedbythemachine.Thepurposeistodefineasystematicevaluationofinjectionmoldingmachinesforprocesscapabilityindependentofthemoldandauxiliaries.Theintentistoestablishwhatisrequiredofamoldingmachinetobeprocesscapable.Theindustrydoesnothaveworldclassstandardsfortheevaluationofinjectionmoldingmachines.Whatisrequiredofaninjectionmoldingmachinetoprovideamolderworldclassprocesscapability?Theanswercanbefoundbytakingtheplastic'spointofview.Emphasisonfourplasticvariablesisthekeytoprocesscontrolandcapability:PlasticflowratePlastictemperaturePlasticpressureCoolingrateandtimeThemachinemustaccuratelycontrolthesevariableswhenitismeltingandinjectingplastic.Takingeachoftheseplasticvariablesandtranslatingthemintomachinerequirementsandlimitsofacceptablefunctionprovidesthefollowingtests.PLASTICFLOWRATE:Precisioncontrolofflowrateasplasticisbeingpushedintothecavitiesiscriticaltoprocesscontrolbecauseplasticchangesviscosityasinjectionrate(filltime)changes.ThisissometimesmisunderstoodbecauseMeltFlowRateisusedasanindustrystandardtodescribeplasticflow.TheMeltFlowtestonlymeasuresonepointofaviscositycurve.Figure1depictsaviscositycurveforatypicalplastic.Itismorecorrecttoplotthisonlogaxis,butisplottedhereonlinearaxistoaccentuatetheviscositychange.Ifthemachinecannotcontrolfillrate(filltime),thenviscositychanges.Ifviscosityvariesshottoshotorruntorunduetoinjectionspeedchanges,partswillnotbeidenticalbecausetheassociatedviscositychangescausedifferentpressuregradientsacrossthepart.1,2Also,varyingdegreesofmolecularorientationmaydevelop.Thus,themachinemusthavecruisecontroloninjectionrate.Teststocheckamachine'sabilitytoreproduciblycontrolfilltimeare:1)loadsensitivity,2)filltimeconsistency,3)positioncutoffconsistency,and4)shotsizeconsistency.Aswithallthefollowingtests,itisrecommendedtouseacalibratedandzeroedhydraulicpressuretransducercombinedwitharecordingmethodthatsamplesthesignalatleast100samplespersecond.Timemeasurementsneedtobeatthe0.01secondresolution.Datashouldbecollectedusingapredeterminedsamplesizeandfrequency;forexample,fiveconsecutiveshotsevery30minutes.Keepaneventlogincasespecialeventsoccurwhenlongtestperiodsarecalledfor.TabulatethedataanddisplayinXbar/Rformat.Ifthedataisincontrolandspecificationshavebeenestablished,acapabilityindexcanbecalculatedasshowninFigure2.Thetransducershouldbeattacheddirectlytothehydrauliccylinderaftertheflowcontrolvalve.SeeFigure3.Relyingonthemachine'scontrollerandplottingcapabilitiesisnotrecommended.Itiswisetolookovertheshoulderofthemachine'scontrollertoassurepropermachineperformanceandpropersamplingrate.ThesetestsarebestperformedunderDECOUPLEDMOLDINGSMconditions.Thatis,thehydraulicpressureintheinjectioncylindershouldnotreachthesetpointoffirststageorboostpressureandthefillingorvelocityphaseofthecycleshouldendjustpriortothecavitybeingfullofplastic.Forexample,ifthepresswasstoppedattheendoffirststage,thepartwouldbe99%filledorslightlyshort.Equipmentneeded:Injectionmoldingmachineequippedwithposition(stroke)transfercapability.Themachinemustbecapableofrunningonsemiautomaticcyclewiththenozzleretracted.Hydraulicpressuresensorasdescribedabove.Positiontransducerwitharesolutionequalto0.1%offullstroke.Stripchartrecorderorotherrecordingmeanswith100hertzresolutionandtimemeasurementtothe0.01second.Afilltimeclockispreferredbutnotrequiredfortimemeasurements.AmoldPlasticresinLoadSensitivityTest:Theloadsensitivitytestchecksamachine'sabilitytodeliverplasticataconstantanduniformvelocityasmaterial,temperature,andotherprocessvariationscauseviscositychangestooccur.Theconceptistovarytheloadrequirementoftheinjectionunitandnotethechangeinfilltime.Procedure:Startupthemachinefollowingyournormalsafetyproceduresmakingsureyouaredecoupledandareconvertingfromfirststagetosecondstageviaposition(stroke).Toensureadecoupledsetup,reduceholdpressuretozero.Ifholdpressurewillnotgobelow0.5MPa(70psi),takeholdtimetozeroandaddthedeletedholdtimetothecuretimertomaintainaconstantcycle.Adjustthepositivecutoffsettingtoobtainshortshotswiththecavityabout95-99%fullmakingsurethatfirststagesetpressureisneverreached.Observethefilltimetothenearest.01secondandpeakhydraulicpressureforthreeshots.Filltimeisdefinedasthetimefromthestartofinjectiontowhenthescrewreachesthepositioncutoff.Filltimeisameasuredtime,notthefir