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1.IntroductionInjectionmouldingwithoutrejectsistheidealmoulderstrytoattain.Thisarticledescribestwoauxiliarydevicesthatcouldincreasetherepeatabilityofaninjectionmouldingmachine.Oncetheoptimumparametersareset,thephysicaldimensions,weightandotherphysicalpropertiesofthepartwillstayalmostconstant.Thetwodevicesdoclampingforcemeasurement/controlandcavitypressureswitchovertoholdingpressure.Forinjectionmouldingofhighprecisionopticalparts,orpartswithahighaddedvaluelikeappliancecases,thepayoffofreducedrejectsishigh.Figure1showsthepartweightdistributionofqualitymouldingandsuboptimalmoulding.Thenominalweightis60g,allowabledeviationis+0.1gand-0.05g.Thewhitecubesdenotepartswithinthetoleranceband.Figure1.Qualityinjectionmouldingandotherwise2.Clampingforcemeasurement/controlTheclampingmechanismofinjectionmouldingmachinesfallsmainlyintotwocategories:toggleanddirecthydraulic.Theformerismorewidelyused;thelatterhasthepropertyofautomaticclampingforceregulation.Asaresult,directhydraulicclampmachinedonotneedclampingforcemeasurement/control.2.1ToggleclampA5-pointdoubletoggleclampisshowninFigure2.Afteramplificationbythetogglemechanism,theclampingcylinder,attachedtothetailplaten,extends,pushingthemovingplatentolockthemouldhalvestogether.Wewillinvestigateinvariouswayshowtheclampingforceisgenerated.Figure2.The5-pointdoubletoggles2.2ClampingforceAtitssimplest,theratedclampingforceFoiscalculatedaccordingtothefollowingformula.Fo=Ps*A*M-------------------------------------------(1)where

Ps=systempressure,

A=clampingcylindercrosssectionalarea,

M=mechanicaladvantageofthetoggles.Inmostmachines,Mhasavalueofbetween22and30.Itisafunctionofthetoggledimensionsandthestiffnessofthetogglesandtiebars.2.3RatedclampingforceandadequateclampingforceTheclampingforcefoundinthespecificationtableofaninjectionmouldingmachineistheratedclampingforceFo.Byconsideringthevariousdesignparameters,themachinedesignercalculatesitusingformula(1).Inusinganinjectionmouldingmachine,itisbesttousetheminimalbutadequateclampingforceF.Anadequateclampingforceholdsthemouldhalvestogetheragainstthecavitypressureduringtheinjectionphase.Anexcessiveclampingforcedistortsthemouldandthemouldcavityunnecessarily,affectingtheprecisionofthemouldedpart.Furthermore,ahighclampingforcecompressesthetogglesandthemould,andstretchesthetiebars,reducingthefatiguelivesofthetogglepins,themouldandthetiebars.2.4ClampingforceproblemandsolutionTheproblemwithtoggleclampedinjectionmouldingmachinesiswithonlythehydraulicpressuremeteravailablethereisnowaytosetanaccurateclampingforcewhenthemouldisinstalledandtomaintainitconstantduringinjection.Asthemouldheatsup,itexpands,increasingtheclampingforce.Thesolutionistoattachadevicetomeasuretheclampingforceandtocontroltheclampingforcetowithinatoleranceasmouldtemperaturechanges.Fortheengineers,thefollowingsectionsdetailshowclampingforceisgenerated.Theyalsorelatetotheseconddevice:switchovertoholdingpressurebycavitypressuremeasurement.2.5MouldheightadjustmentmechanismSincenotallmouldshavethesamemouldheight,atoggleclampedinjectionmouldingmachinehasamouldheightadjustmentmechanismforthatpurpose.Basically,thetailplatenismovedforwardorbackwardsothatwiththetogglesalmostfullyextended(>0)themouldhalvesjusttoucheachother.Atthistime,theclampingforceiszero.SeeFigure3a.Togeneratemaximumclampingforce(clampingforce>0)andtoselflock,thetogglesarefullyextended(=0).Thisisdonebyextendingtheclampingstrokefurtherandthroughthetoggles,movesthemovingplatenforwardbyam,whichistheamountbywhichthemouldiscompressed.Atthesametime,thetiebars,attachedbetweenthestationaryandtailplatens,areelongatedbyat.SeeFigure3b.Selflockingmeansevenwhenthehydraulicpressureintheclampingcylinderisrelieved,theclampingforceismaintained.Thiscouldonlybeachievedwhenthetogglesarefullyextended.Figure3.Generationofclampingforce2.6RevisitingclampingforceAssumethemouldandthetiebarsareintheelasticregionattheratedclampingforce.TheirrespectivecompressionandelongationcouldbeanalysedusingFigure4.InFigure4a,attheadequateclampingforceF,themouldiscompressedbyamandthetiebarselongatedbyat.Sincethetiebarsarelongandthin,theyaremoreflexiblethanthemould.Hence,thetiebarslineisshallower.Technicallyspeaking,Kt=tant<Km=tanm.Asanexample,withclampingforcemeasuredintonnesandelongation/compressionmeasuredinmicrons(1micron=0.001mm),aninjectionmouldingmachinewith60mmdiametertiebarshast=9.2o,a300mmthick,170mmsquaresteelmouldhasm=64o.Tofacilitatethefollowinganalysis,themouldcompressionlineismovedrighttointersectthetiebarselongationlineatF.SeeFigure4b.Figure4.ClampingforceanalysisWhenthemeltisinjectedintothemouldcavity,theclampingforceisincreasedtoF1.SeeEquation(2).Inpractice,thisincreaseinclampingforcecouldbeobservedbyaclampingforcemeasuringdevice.SeeFigure5.Figure5.TheeffectofcavitypressureonclampingforceFigure6showsthefree-bodydiagramofthemouldhalveswithcavitypressureintroduced.EachmouldhalfisbalancedbytheforceequilibriumF1=Fc+Fr----------------------------------------------(2)

where

Fc=cavitypressureforce,

Fr=residualclampingforceonthemould.Figure6.FreebodydiagramofmouldhalvesThecavitypressureforceFcoffsetspartoftheclampingforceF1,leavingonlyF1-Fctocompressthemould.Asaresult,themouldcompressionisreducedfromamtoam'.Thedifferenceistakenupbythetiebarselongatingmorefromattoat',increasingtheclampingforcetoF1.SeeFigure7.ThemouldopeningforceFcduetocavitypressureisseenbetweenthetiebarslineandthemouldline.Thisisthegraphicalwayofshowingequation(2).Figure7.AddingcavitypressureFromFigures3and7,onecanseethatthedistancebetweenthemovingplatenandthestationaryplatenisincreased(byam-am')duringinjection.Inpractice,thiscouldbemeasuredbyadialgaugebetweentheplatens.Intheextremecasewhenthecavitypressureissohighthattheresidualclampingforceisreducedtozero,themouldopensandflashingoccurs.Atthispoint,themouldcompressioniszero,andcavitypressureforceFc=F2,theclampingforcewhenflashingoccurs.SeethedashedlineinFigure7.Asanexample,takeF=75tonnes,theratedclampingforceofTatMingME75III.Fora300mmthick,170mmsquaresteelmould,am=0.037mm.Withsuchamouldmounted,theME75IIItoggleclampwillopenatF2=81tonnes,6tonnesaboveitsrating.Everythingelseequal,aninjectionmouldingmachinewith50mmdiametertiebarswillopenatF2=78tonnes,3tonnesaboveitsrating.Asthemouldheatsup,itexpands.Theclampingforceisincreasedasthemechanicalinterferenceisincreasedbytheamountofthemouldexpansion.ThisisshowninFigure8inwhichthemouldlineismovedfurtherrightbytheexpansion,intersectingthetiebarslineatahigherclampingforceF".Inthisdiagram,thetiebarsareelongatedmore(at"-at)andgeneratedtheadditionalclampingforceF"-F.Torestoretheclampingforce,amouldheightadjustmentismadetorestorethemould(nowhot)compressiontoambeforethenextshotisinjected.Suchadjustmentisclampingforcecontrol.Asanexample,a300mmthicksteelmouldheatedupby10oCexpandsby0.045mm.OntheME75IIIinjectionmouldingmachine,theincreaseinclampingforceis7.3tonnes,whichisalmost10%oftheratedclampingforce.Figure8.Mouldexpansionincreasesclampingforce2.7NoproblemswithhydraulicclampForthesakeofcomparison,letusdoasimilaranalysisforahydraulicclampmachine.TheadequateclampingforceisF=P*A--------------------------------------------------(3)

where

P=theclampingpressure,

A=theclampingcylindercrosssectionalarea.ClampingforceiseasilysetasitisproportionaltoP,which,nowadays,issetthroughaproportionalpressurevalve.Mouldheightadjustmentisdonebyextendingorretractingtheclampingcylinderrodtoaccommodatedifferentmouldheights.Whenthemouldheatsupandexpands,itsimplepushestheclampingcylinderrodbackintothecylinder,butdoesnotincreaseclampingforce.Inotherwords,theclampingmechanismhastheautomaticregulationproperty.Clampingforcemeasurementandcontrolisnotnecessary.2.8TheclampingforcemeasuringdeviceSincefastresponseisnotneeded,astraingauge-baseddeviceissufficienttomeasureandcontrolclampingforce.Thesimplestmeansisastraingaugeattachedtothetailorstationaryplaten,whichdeflectsundertheclampingforce.Alternatively,astraingaugeisattachedtoatiebarwhichextendsasthemouldislocked.Anassumptionismadethatthetiebarsareevenlystretchedwhichmaynotbetrueifthemouldfacesarenotparallel,themouldcavityisnotsymmetricalorthetiebarsarenotbalancedoutofthefactory.Thestraingaugeoutputisamplifiedanddigitallydisplayed.Thedisplayiscalibratedtoreadintons.Suchadeviceissufficienttohelptheoperatorsetupanadequateclampingforceinitially(duringmouldheightadjustment).Whenforexamplea5%deviationfromtheinitialclampingforceisdetected(afterthemouldisclosedbutbeforeinjection),theoperatorcoulddoamouldheightadjustmenttorestoretheclampingforcetoitsoriginalvalue.Alternatively,thecomputerintheinjectionmouldingmachinecouldsetuptheclampingforceduringtheinitialmouldheightadjustment,andtorestoretheclampingforcebyanothermouldheightadjustmentwhenaprescribeddeviationisdetected.3.InjectionTheinjectionofmeltintothemouldcavityismadeupofthreephases:thefillingphase,thepackingphaseandtheholdingphase.Theinjectionphasescouldbevividlyillustratedusingthecavitypressurecurve.Figure9showstheidealcurvewhichisachievedwhentheswitchovertoholdingpressureisoptimum.Theswitchoverissometimescalledvelocitytopressuretransfer,wherevelocityreferstoinjectionvelocityandpressuretoholdingpressure.Figure9.IdealcavitypressurecurveThefillingphasestartsat1.Inthefillingphase,themeltisinjectedintothecavityatacertainvelocity.At2,themeltreachesthecavitypressuresensor.Duetotheviscosityofthemelt,pressurestartstorise.Thecavityisvolumetricallyfilledat3.Furtherscrewadvancecompressesthemeltupto4whenthemachineswitchesfrominjectionpressuretothemuchlowerholdingpressure.Attheholdingphase,thelowholdingpressureincrementallyfillsthecavityasthepartcoolstocompensatefortheshrinkage.At5,thespruegateisfrozenandtheholdingpressurecouldberemoved(andthemouldcouldbeopened).1-2-3makesupthefillingphase.3-4isthepackingphase.4-5istheholdingphase.Furthercoolingoccursin5-6.3.1OverpackingandunderpackingAnoverpackedcavitypressurecurveisshowninFigure10b.Itischaracterizedbyapressurepeakinthepackingphase.Thepressurepeakiscausedbythedelayinswitchovertoholdingpressure,sothehighinjectionpressureisstillappliedaftervolumetricfilling.Thepressurepeakisrelievedattheswitchovertothelowerholdingpressure.Hereliesanoftenoverlookedcauseofflashingwhichiseasilydetectedifonehascavitypressuresensing.Figure10.UnderpackedandoverpackedcavitycurvesRefertoEquation(2)orFigure7.ThemomentarycavitypressurepeakcouldproduceamomentaryFcbigenoughtoreducetheresidualclampingforceFrtozero,causingthemouldtoopenandtheparttoflash.Toremovetheflashing,thestraightforwardthinkingwouldbetoincreasetheclampingforce.Reachingthelimitofthemachineratedclampingforce,onewouldevenmovethemouldtoabiggermachine.Eveniftheincreasedclampingforceovercomesflashing,overpackingaddsweightandstresstothepartandmakesthepartmoredifficulttodemould.Analternativeistoreducetheinjectionpressure.Toolowaninjectionpressurecausesdefectssuchassinkmarks.Inactuality,theproblemiseasilyandbettersolvedbyswitchingoverearliertogetbacktotheidealcavitypressurecurve.Inprecisioninjectionmoulding,overpackingcreatesareject.AnunderpackedcurveisshowninFigure10a.Itischaracterizedbyapressuredipinthepackingphase.Theswitchoveroccurstooearly,beforethecavityisvolumetricallyfilled.Partofthefillingtakesplaceatthelowerholdingpressure.Subsequently,thescrewadvanceincreasesthepressure.Theparthasreduceddimensions,isunderweight,hassinkmarksandsurfacemarks.Itisagainareject.Adevicethatswitchesoveratvolumetricfillingwouldavoidtheproblemsofoverpackingandunderpackingandproducestheidealcavitypressurecurve.Switchingisinitiatedatpoint3andcompletedinpoint4inFigure9.3.2MethodsofswitchoverTheavailablemeanstoswitchoverinamoderninjectionmouldingmachine,inincreasingorderofaccuracy,are

1.injectiontime,

2.screwposition,

3.hydraulicpressure,

4.nozzlepressure,

5.cavitypressure.3.2.1InjectiontimeswitchoverTemperatureaffectstheviscosityofthemelt,whichpresentsresistancetotheadvanceofthescrew.Increasedresistanceslowsdownthescrewandpreventsthecavityfromfillinginthegiveninjectiontime.Ontheotherhand,reducedresistancewouldleadtooverpacking.Injectiontimeswitchoveristheonlymeansavailableininjectionmouldingmachineswithoutscrewpositionandpressuresensors.3.2.2ScrewpositionswitchoverScrewpositionswitchoverisnotaffectedbytemperaturenorviscosity.Thisisthepreferredmethodinmachineswithscrewpositionpotentiometer.Likeinjectiontimeswitchover,screwpositionswitchovercouldbeconsideredopen-loopasscrewpositionisnotadirectmeasureofvolumetricfilling.Aleakynozzlemisleadsthemachinecomputerintoswitchingoverbeforethecavityisfilled.Socouldawornscrewvalveandaworninjectioncylinder.Furthermore,ifthescrewdiameterislargerelativeto(thecuberootof)themouldcavityvolume,variationof0.1mmcouldgiveanoverpackedorunderpackedfill.Despiteitsdeficiencies,thisisthemostwidelyusedswitchovermethodinamoderninjectionmouldingmachinemostprobablybecauseitisastandard(notoptional)feature.3.2.3HydraulicpressureswitchoverThepackingofthemeltinthemouldcavityhastobebalancedbythehydraulicpressuredrivingthescrewforward.Ariseinthehydraulicpressureduringinjectioncouldbeusedtosignaltheswitchover.Duetoaroughly10:1ratiobetweenthetwininjectioncylindersandthescrewcrosssectionalareas,theinjectioncylinderhydraulicpressureislessthanthescrewtippressurebythesameratio.Thepressuredropattherunnersandspruegateseparatesthecavitypressurefromthescrewtippressure.Thecompressibilityofthemelt(betweenthecavityandthescrewtip)delaysthetimethepressureisfelt.Asaresult,hydraulicpressureisnotanaccuratedetectorofthevolumetricfillingpoint.However,hydraulicpressureswitchoverdoeshavetheadvantageofthesensorworkinginacongenialenvironment(oiltemperaturebelow50oC,oilpressureatsystempressure(usually140bars))andthesensingisindependentofthemould(notattachedtothemould).Hydraulicpressuresensorisusuallyanoptioninanmoderninjectionmouldingmachine.Hydraulicpressure,nozzlepressureandcavitypressuresensinglocationsareshowninFigure11.Figure11.Hydraulicpressuresensorintheinjectioncylinder3.2.4NozzlepressureswitchoverNozzlepressureisalsocalledinjectionpressure,whichisthepressureofthemeltinthenozzleoratscrewtip.Nozzlepressureswitchoverisimprovedoverhydraulicpressureasthecompressibilityofthemeltcushionisavoided.Theenvironmentisharsher(melttemperaturebelow400oC,meltpressureat1400bars,themeltcouldbecorrosive/abrasive),andthesensorfacemustbeflushwiththebarrelinteriorwall.Thisswitchovermethodisnotoftenusedexceptinresearch.Figure12.Nozzlepressuresensor3.2.5CavitypressureswitchoverThemostaccuratemeasureofvolumetricfillingisviacavitypressure.Twomethodsareincommonuse:directandindirect.Indirectcavitypressuremeasurement,asensorinthemouldsensesthemeltpressureinthecavity.Directcavitypressuremeasurementisthemoreaccurateofthetwo,butrequiresonetodrillaholeatthemouldforthesensor.Sinceitisinconvenienttoremovethesensor,oneneedstodedicateatleastonesensorpermould.Inamulticavitymould,cavitypressuremeasurementrequiresonesensorpercavity,increasingthesensorinvestmentfurther.Figure13.DirectcavitypressuresensorInindirectpressuremeasurement,aforcesensorisplacedbehindanejectorpintheotherendofwhichisincontactwiththemelt.Cavitypressurecouldbecalculatedfromforce/ejectorpincrosssectionalarea.Thetemperatureatthesensorismuchlessthanthatofthemelt.Withindirectcavitypressureswitchover,thesensorisnotdedicatedtothemould(mouldindependent),whichcomesinhandywhenmouldchangingisoften.Italsoreducesthesensorinvestment.Duetothefrictionattheejectorpin,indirectcavitypressuresensingislessaccuratethanitsdirectcousin.Figure14.IndirectcavitypressuresensorWheretherequiredqualityonthesurfaceofthemouldedpartsdoesnotallowmarkseitherbythesensorortheejectorpin,astrainsensorthatmeasuresmoulddeformationcouldbeused.Aftercalibrationinatestmould(whichhasacavitypressuresensor),itmaybeusedforcavitypressuremeasurementintheproductionmould(whichdoesnothaveacavitypressuresensorbuthasthecalibratedstrainsensor).Adevicebasedoncavitypressuresensingcoulddetectthevolumetricfillingpointaccurately.Switchovercouldbeinitiatedbycomparingtheactualpressurewithasetvalueequalstothecavitypressureatpoint3inFigure9.Alternatively,KistlerhasdevelopedSmartAmpwhichdetectsthevolumetricfillingpoint.SmartAmpcontainsachargeamplifierforthequartztypecavitypressuresensorandachipwhichusestheprinciplesofartificialintelligencetodetectthekinkinthepressurecurveatvolumetricfilling.Usually,thelearningtakesthefirstfewshots.3.3MonitoringthecavitypressurecurveThecavitypressurecurveprovidesmoreinformationoftheprocessinthecavitythancanthenozzlepressureorhydraulicpressurecurves.Thenozzlepressuresensorisalwayssurroundedbythemeltandcannotmeasuretheprocesspressureduringthecoolingperiod.Neith