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ApplicationNotesPressureSensorinPlasticMachinaryByMichaelSaliPlasticinjectionmoldingmachinePlasticMachineryPlasticInjectionmoldingmachinesareusedtomanufactureplasticparts.Injectionmoldingmachinestypicallyincludeareservoirofmoltenplasticandanozzlethroughwhichthemoltenplasticisforcedathighpressuresintoamold.Theplasticisthereafterpackedandcooledtoformthepart.Inthemanufactureofsuchparts,itisimportanttopreciselydeterminethepressureoftheinjectedmelttomaintainaccuracyofthepartbecausethepartmustbemadeataconstantpressure.Thisissobecauseotherwisetheresultingpartmayincludevoids,ripples,poordimensionalaccuracyorthelike.PlasticMachineryWorkingProcessCyclesCST3Pressuresensormountedonplasticmachine•Inaplasticinjectionmachine,pressuresensorsaremountedintheinjectionmoldingnozzle,hotrunnersystemorcoolrunnersystemandinsidethecavityofthemold.•itcanmeasureplasticpressurefromtheinjectionmoldingnozzletocavityplasticpressuresomewhereinbetweenplasticinjectionmolding,moldfilling,pressuremaintainingandcoolingprocess.•Thosedatacanberecordedbythemonitoringsystem,tobeusedfortruetimecontrolofthemoldingpressure,combineinspectorexcludeproductprocessappearmalfunctionaftermolding.CST4Pressuresensorsonplasticmachine•thereisaneedforpressuretransducerdeviceswhichcanaccuratelymeasurethepressureofmoltenplasticwithininjectionmoldingmachines.•Additionally,suchpressuretransducersarefacedwiththefollowingconstraints:•1.thepressuretransducermusthavelowworkingstressestoavoidmetalfatiguecrackingunderrepetitivecyclicloading;•2.thepressuretransducer,foreconomicpracticality,mustbephysicallyarrangedsuchthatitcaneasilybeintegratedwithininjectionmoldingmachinesshotchamber/nozzlearea;and•3.thepressuretransducermustnotdisrupttheplasticfloworhavepocketswhereplasticwillremainandeventuallydegradeandcausestainingofthepartsasthedegradedplasticmovesintotheflow.CST5Methodsofswitchoverinaplasticmachine•TheavailablemeanstoswitchoverinamodernInjectionMoldingmachine,inincreasingorderofaccuracy,are•1.injectiontime•2.screwposition•3.hydraulicpressure•4.nozzlepressure•5.cavitypressureCST6Injectiontime•Temperatureeffectstheviscosityofthemelt,whichpresentsresistancetotheadvanceofthescrew.•Increasedresistanceslowsdownthescrewandpreventsthecavityfromfillinginthegiveninjectiontime.Ontheotherhand,reducedresistancewouldleadtooverpacking.•InjectiontimeswitchoveristheonlymeansavailableinInjectionMoldingmachineswithoutscrewpositionandpressuresensors.CST7Screwpositionswitchover•Screwpositionswitchoverisnoteffectedbytemperaturenorviscosity.Thisisthepreferredmethodinmachineswithscrewpositionpotentiometer.Likeinjectiontimeswitchover,screwpositionswitchovercouldbeconsideredopen-loopasscrewpositionisnotadirectmeasureofvolumetricfilling.•Aleakynozzlemisleadsthemachinecomputerintoswitchingoverbeforethecavityisfilled.Socouldawornscrewvalveandaworninjectioncylinder.Furthermore,ifthescrewdiameterislargerelativeto(thecuberootof)themoldcavityvolume,variationof0.1mmcouldgiveanoverpackedorunderpackedfill.•Despiteitsdeficiencies,thisisthemostwidelyusedswitchovermethodinamodernInjectionMoldingmachinemostprobablybecauseitisastandard(notoptional)feature.CST8Hydraulicpressureswitchover•Thepackingofthemeltinthemoldcavityhastobebalancedbythehydraulicpressuredrivingthescrewforward.Ariseinthehydraulicpressureduringinjectioncouldbeusedtosignaltheswitchover.Duetoaroughly10:1ratiobetweenthetwininjectioncylindersandthescrewcrosssectionalareas,theinjectioncylinderhydraulicpressureislessthanthescrewtippressurebythesameratio.Thepressuredropattherunnersandspruegateseparatesthecavitypressurefromthescrewtippressure.Thecompressibilityofthemelt(betweenthecavityandthescrewtip)delaysthetimethepressureisfelt.Asaresult,hydraulicpressureisnotanaccuratedetectorofthevolumetricfillingpoint.However,hydraulicpressureswitchoverdoeshavetheadvantageofthesensorworkinginacongenialenvironment(oiltemperaturebelow50oC,oilpressureatsystempressure,(usually140bars)andthesensingisindependentofthemold(notattachedtothemold).HydraulicpressuresensorisusuallyanoptioninamodernInjectionMoldingmachine.CST9Nozzlepressureswitchover•Nozzlepressureisalsocalledinjectionpressure,whichisthepressureofthemeltinthenozzleoratscrewtip.•Nozzlepressureswitchoverisimprovedoverhydraulicpressureasthecompressibilityofthemeltcushionisavoided.•Theenvironmentisharsher(melttemperaturebelow400oC,meltpressureat1400bars,themeltcouldbecorrosive/abrasive),andthesensorfacemustbeflushwiththebarrelinteriorwall.Thisswitchovermethodisnotoftenusedexceptinresearch.CST10Cavitypressureswitchover•Themostaccuratemeasureofvolumetricfillingisviacavitypressure.Twomethodsareincommonuse:directandindirect.Indirectcavitypressuremeasurement,asensorinthemoldsensesthemeltpressureinthecavity.•Directcavitypressuremeasurementisthemoreaccurateofthetwo,butrequiresonetodrillaholeatthemoldforthesensor.Sinceitisinconvenienttoremovethesensor,oneneedstodedicateatleastonesensorpermold.•Inamulti-cavitymold,cavitypressuremeasurementrequiresonesensorpercavity,increasingthesensorinvestmentfurther.CST11•Inindirec