Injection Moulding Techniques english文档格式.docx
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Injection Moulding Techniques english文档格式.docx
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Plasticsprocessingbeganinthemiddleofthe19thcentury.Earlypartgeometrywasquitebasicandthemoldingprocesswasquitesimpleandnotautomated.Mainlyitwasdonewithduroplastandelastomermaterials,thematerialsbeinggrouted,shreddedorsoakedtoworkformoldingpurposes.
Thedevelopmentoftheinjectionmoldingmachinehasmadethemassproductionofplasticpartseasierandtheproductionisdonelargelyautomatically.
Withtheintroductionofelectroniccontrolsystemsandlaterthatofthemicroprocessor,evenhigherlevelsofautomationandcarefullycontrolledmass-productionofplasticarticlesispossibletoday.
AgoodexampleofamoderninjectionmachineistheArburgAllrounder420C.
2.TheinjectionProcess
Therawplasticmoldingbatchisprovidedinsmall,uniformlysizedpellets.Thisispouredintothehopperoftheinjectionmoldingmachinewheregravityfeedsitintotheinjectioncylinder.Theplasticentersthecylinderwhereitismeltedbytheheatingtapesandthefrictionofthescrew,which‘swims’inthecylinder.Theheated,meltingplasticpelletscombineastheymeltandaretheninjectedunderhighpressure(usuallyfrom500to2000bars)intoamold.Themouldhasthenegativegeometryoftheproductcutintoitandtheplasticfillsupthespace,formingthepart.Theplasticbecomessolidasitcoolsdownafterinjectionandthepartshrinksasitfreezes.Tocompensateforthisshrinkingitiskeptunderpressureasitbecomessolid.Thepartiscooledandthende-molded.
Theprocesscycleisthewaytimeand/orpressurecanbecontrolledandmeasured.Thecyclecanbeseparatedintofilling,injection,packingpressure,holdingpressure,cooling,andde-molding
Belowisasamplechartofthemanufacturingprocesscycle(simplified),shownascycle-time-chart.Coolingtimeproceedsatthesametimeasthepackingandholdingpressureandfillingprocesses.
Cyclestart
Time0[sec]
Moldclose
1
Injectionunitforward
0.8
Injection
2
After-pressure
7
Coolingtime
Filling
4
Remainingcoolingtime
9
Screwdischarge
0,5
Injectionunitbackward
0,7
Moldopen
0,8
Ejectorfor-/backward
Cycleend/start
26,8
2.1Filling
Themoldingbatchisinsertedintotheinjectionmachinecylinderthroughthehopper.Thecylinderisheatedbyheatingtapes.Thescrewtransportstheplasticwithacircularmovement,movingthemoldingbatchtowardsthenozzleendofthecylinder.Theplasticpelletsmeltandbecomeplasticized.Ameasuredamountofmeltedplasticmaterialcollectsinthescrewantechamber.Thisamountisequaltothematerialrequiredforoneinjectionmoldingcycle,thatis,theamountofplasticneededtoformthepartandtherunner.Fillingbeginsaftertheplastichasreachedthepropertemperaturetoachieveshrinkagebalancing(afterpressure).
ImportantParameters:
cylindertemperature,thenumberofscrewrevolutions,holdingpressure,packingpressure,backpressure,fillingtimeandthescrewreturncycle(decompression).
-Thecylindertemperatureisdeterminedbythechosenmoldingmaterial.Forexample,PEandPPareheatedtoaround180degrees,PA66toaround270degrees,PMMAtoaround270degrees.Eachplastictypewillhaverecommendedtemperatures.
-Thenumberofscrewrevolutionsisthenumberofrotationsofthescrewperminute.Byincreasingtherotationsperminuteitispossibletodecreasethefillingtime.Ifthespeedistoohigh,however,thefriction(shearofthematerial)canbetoogreatandthiscandamagetheplastic(thisismaterialspecific).
-Backpressureisthehydraulicpressurewhichpowersthefillingprocess.Acorrectbackpressuresettinggivesabettermix,guaranteeingmaterialhomogeneityandevenmaterialdelivery.
-
-Fillingtimeisthetimeofthecompletefillingprocess;
whatisnecessaryforfillingthepartfollowedbydecompression(screwreturn).Asageneralguideline,thefillingtimeshouldbe,ifpossible,shorterthanthecoolingtime.
-Screwreturn(decompression)ismovingbackofthescrewtothestartingposition.Thisoccursafterthefillingprocess,reducingthepressureinthescrew-antechamberandinthehotrunnerandalsotoavoidleakingofthenozzlesafterthemeltedplastichasbeendischargedintothemould.
-Packingpressureisthepressurewhichisneededtocompletelyfillthepartandtobalancetheshrinkingofthecoolingpart.
-Holdingpressureisthepressurewhichisappliedtothematerialwithoutthemovementofthescrew.Thescrewstopsmovingandholdsthepressure.Thispressurehastobemaintaineduntilthegatefreezehasoccurred.
2.2Injection
Theinjectionprocessisdefinedasthefillingofthemoldcavitywithplasticizedplastic.Themeltedplasticisinjectedintothemouldunderhighpressurebyaforwardmovementofthescrew.
injectionspeed,injectionpressure,injectiontime.
-Injection-speedistheforwardmovementspeedofscrewinmmpersecond.Throughthisthesurfacefinish,thefilling,theorientationandtheshrinkingoftheplasticproductcanbecontrolled.Thespeedsettingisdependentonthechosenplasticmaterialandthemouldstructure.Materialrequirementsaredeterminedbyviscosity,useofreinforcedmaterial(glassfiber,etc.)andothermaterialadditionssuchastalcorcoloring.Carefulattentionmustbepaidwhenusinghighinjectionpressurewheretherearesmallpartcross-sectionsinthemould.Veryhighshearspeedcanoverheatanddamagetheplastic.Theadjustedinjectionspeedisonlythescrewspeedandnottheeffectivemoldingmaterialspeed.Materialspeedcanbemanytimeshigherinsidethemould(thesmallerthecross-section,thehigherthespeed).
Injectionpressureistheeffectivepressureofinjection.Onoldermachinesthehydraulicpressurecanberegulated.Onmodernmachinestheinjectionspeedregulatestheinjectionpressure,becausetheelectroniccontrolschoosethenecessaryhydraulicpressuredependingontheinjectionspeed.
-Injectiontimeisthetimeneededforthecompleteinjectionprocess.Thiscanbeusedformanuallyswitchingtoafter-pressure(unfavorablebecausenotconstant)asageneralguideline.
2.3Packingpressure
Theafterpressureservestobalancetheshrinkingofthecoolingpart.Thepackingpressureisimmediatelyswitchedonaftertheinjection-process.
packingpressureelevation,packingpressuretime
-Packingpressureelevationisthepressureincreasethatallowstheafterpressurecycletobeeffective(usuallyabout30to50%oftheinjectionpressure,semi-crystallineplasticshavehighershrinkageratesthanamorphousplastics).
-Packingpressuretimeisthelengthoftimepackingpressureisapplied.
2.4Holdingpressure
Holdingpressureismaintaineduntilthegatefreezeofthecoolingparthasoccurred.Holdingpressureisappliedimmediatelyafterthepackingpressurephase.
holdingpressureelevation,holdingpressuretime
-Holdingpressureelevationistheamountofpressurewhichisappliedafterthepackingpressurephase.Forthispressurethescrewstopsandmaintainsthepressureofpacking.
-Holdingpressuretimeisthelengthoftimewhichisnecessarytoreachthegatefreeze.Ifthescrewmovesbackbeforegatefreezeoccurs,thelossofpressurewillcausethestillmeltedplastictoflowoutofthemouldandbackintotheinjectionchamber.Thepartwillnotbefilledproperly.
2.5Freezing–TheCoolingProcess
Themeltedplasticbeginstocoolinthemoldonceinjected.Themoldisinmostcasesdesignedwithcoolingchannelsaroundthecavityandcore,throughwhichcoolantliquidiscirculatedtoremoveheatfromaroundthepart.Theneededcoolingtimeisprimarilydependentonthewallthicknessofthepartandthemoldcoolingdesign.Thisoftendeterminesthewholecycletime.Averysimplifiedcoolingcalculationis
Tk=2xs2
Tk=coolingtimes=biggestwallthickness
Formoreprecisecoolinganalysisadvancedcalculationorcomputersimulation(MoldFlow,CadMould)canbeused.
Duringthecoolingtimetheplasticpartmustreachthenecessarystrength,shapeconsistencyandamountholdingbeforede-moldingispossible.Duringthistimethefilling-unit(aggregate)readiesthemoldingbatchforthenextcycle.
Thecoolingtimealsodependsonthetimeneededtoachievethegatefreeze.Youcanonlyguaranteegoodshapeconsistencyofthepartafterreachingthegatefreezepointduetolossofbalanceinthepartshrinkage.
It’sonlyonepartofcoolingtime,asfurthercoolingcouldbenecessaryafterthegatefreezes.
2.6Demolding
Theproperlyfrozenproductgetsde-molded.Toachievethis,themoldopensandthepartispushedoutbythemechanicalejectorsystem,compressedairejectionand/ormaybetakenoutofthecavitybyhand.Theinjectioncycleisnowcompletedandanewonebegins.
3.Assemblyofthemachine
Aninjectionmolding-machinecanvaryfromtablesizedunitstoheavyweapontransportersizedmachines.Theinvestmentcostalsocanrangefromthesamepriceasasm
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