专业模具英语.docx
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专业模具英语.docx
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专业模具英语
专业模具英语
1.Theinjectionmoldingandmachine
1.1Theinjectionmolding
Injectionmoldingisprincipallyusedfortheproductionofthethermoplasticparts,althoughsomeprogresshasbeenmadeindevelopingamethodforinjectionmoldingsomethermosettingmaterials.Theproblemofinjectingameltedplasticintoamoldcavityfromareservoirofmeltedmaterialhasbeenextremelydifficulttosolveforthermosettingplasticswhichcureandhardenundersuchconditionswithinafewminutes.Theprincipleofinjectionmoldingisquitesimilartothatofdie-casting.Theprocessconsistsoffeedingaplasticcompoundinpowderedorgranularformfromahopperthroughmeteringandmeltingstagesandtheninjectingitintoamold.Afterabriefcoolingperiod,themoldisopenedandthesolidifiedpartejected.Injection-moldingmachinescanbearrangedformanualoperation,automaticsingle-cycleoperation,andfullautomaticoperation.Theadvantageofinjectionmoldingare:
(i)ahighmoldingspeedadaptedformassproductionispossible;(ii)thereisawidechoiceofmaterialsprovidingavarietyofusefulproperties;(iii)itispossibletomoldthreads,undercuts,sideholes,andlargethinsections.
1.2Theinjection-moldingmachine
Severalmethodsareusedtoforceorinjectthemeltedplasticintothemold.Themostcommonlyusedsysteminthelargermachinesisthein-linereciprocatingscrew.Thescrewactsasacombinationinjectionandplasticizingunit.Astheplasticisfedtotherotatingscrew,itpassesthroughthreezones:
feed,compression,andmetering.Afterthefeedzone,thescrew-flightdepthisgraduallyreduced,forcingtheplastictocompress.Theworkisconvertedtoheatbyshearingtheplastic,makingitasemifluidmass.Inthemeteringzone,additionalheatisappliedbyconductionfromthebarrelsurface.Asthechamberinfrontofthescrewbecomesfilled,itforcesthescrewback,trippingalimitswitchthatactivatesahydrauliccylinderthatforcesthescrewforwardandinjectsthefluidplasticintotheclosedmold.Anantiflowbackvalvepreventsplasticunderpressurefromescapingbackintothescrewflights.
Theclampingforcethatamachineiscapableofexertingispartofthesizedesignation.Theinjection-moldingmachine,thereciprocating-screwinjectionsystemandismeasuredintons.Arule-of-thumbcanbeusedtodeterminethetonnagerequiredforaparticularjob.Itisbasedontwotonsofclampforcepersquareinchofprojectedarea.Iftheflowpatternisdifficultandthepartsarethin,thismayhavetogotothreeorfourtons.
Manyreciprocating-screwmachinesarecapableofhandingthermosettingplasticmaterials.Previouslythesematerialswerehandledbycompressionortransfermolding.Thermosettingmaterialscureorpolymerizeinthemoldandareejectedhotintherangeof375~410℃.Thermoplasticpartsmustbeallowedtocoolinthemoldinordertoremovethemwithoutdistortion.Thusthermosettingcyclescanbefaster.Ofcoursethemoldmustbeheatedratherthanchilled,aswiththermoplastics.
Machinesareavailableformoldingsandwichparts.Onecylinderandplungerinjectsmeasuredamountofskinmaterialintothedie,andthenasecondcylindersquirtsthefillerinsidethemass.Finally,afinalspurtfromthefirstcylinderclearsthecorematerialfromthesprue.Theaimistoproducecompositeswithoptimumproperties.Eithercaseorcoremaybefoamed.
2.Feedsystem
Itisnecessarytoprovideaflow-wayintheinjectionmouldtoconnectthenozzle(oftheinjectionmachine)toeachimpression.Thisflow-wayistermedthefeedsystem.Normallythefeedsystemcomprisesasprue,runnerandgate.Thesetermsapplyequallytotheflow-wayitself,andtothemoldedmaterialwhichisremovedfromtheflow-wayitselfintheprocessofextractingthemolding.
Atypicalfeedsystemforafour-impression,twoplate-typemould.Itisseenthatthematerialpassesthroughthesprue,mainrunner,branchrunnersandgatebeforeenteringtheimpression.Asthetemperatureofmoltenplasticisloweredwhilegoingthroughthesprueandrunner,theviscositywillrise;therefore,theviscosityisloweredbyshearheatgeneratedwhengoingthroughthegatetofillthecavity.Itisdesirabletokeepthedistancethatthematerialhastotraveldowntoaminimumtoreducepressureandheatlosses.Itisforthisreasonthatcarefulconsiderationmustbegiventotheimpressionlayoutandgate’sdesign.
Sprue
Asprueisachannelthroughwhichtotransfermoltenplasticinjectedfromthenozzleoftheinjectorintothemold.Itisapartofspruebush,whichisaseparatepartfromthemold.
Runner
Arunnerisachannelthatguidesmoltenplasticintothecavityofamold.
Gate
Agateisanentrancethroughwhichmoltenplasticentersthecavity.Thegatehasthefollowingfunctions:
restrictstheflowandthedirectionofmoltenplastic;simplifiescuttingofarunnerandmoldingstosimplifyfinishingofparts;quicklycoolsandsolidifiestoavoidbackflowaftermoltenplastichasfilledupinthecavity.
Coldslugwell
Thepurposeofthecoldslugwell,shownoppositethesprue,istheoreticallytoreceivethematerialthathaschilledatthefrontofthenozzleduringthecoolingandejectionphase.Perhapsofgreaterimportanceisthefactthatitprovidespositivemeanswherebythespruecanbepulledfromthespruebushforejectionpurposes.
Thesprue,therunner,andthegatewillbediscardedafterapartiscomplete.However,therunnerandthegateareimportantitemsthataffectthequalityorthecostofparts.
3.RUNNER
Therunnerisachannelmachinedintothemouldplatetoconnectthespruewiththeentrancetotheimpression.Thewalloftherunnerchannelmustbesmoothtopreventanyrestrictiontoflow.Also,astherunnerhastoberemovedwiththemolding,theremustbenomachinemarksleftwhichwouldtendtoretaintherunnerinthemouldplate.
Therearesomeotherconsiderationsforthedesignertobearinmind:
(i)theshapeofthecrosssectionoftherunner,(ii)thesizeoftherunner;(iii)therunnerlayout.
Runnercross-sectionshape
Thecross-sectionalshapeoftherunnerusedinamouldisusuallyoneoffourforms:
fullyround,trapezoidal,modifiedtrapezoidalandhexagonal.
Thecriterionofefficientrunnerdesignisthattherunnershouldprovideamaximumcross-sectionareafromthestandpointofpressuretransferandaminimumcontactontheperipheryfromthestandpointofheattransfer.Theratioofcross-sectionalareatoperipherywill,therefore,giveadirectindicationoftheefficiencyoftherunnerdesign;thehigherthevaluethegreatertheefficiency.
Runnerlayout
Thelayoutoftherunnersystemwilldependuponthefollowingfactors:
(i)therunnerofimpression,(ii)theshapeofthecomponents,(iii)thetypeofmould,(iv)thetypeofgate.
Whendesigningarunnersystemtherearetwomainconsiderations.Therunnerlengthshouldalwaysbekepttoaminimumtoreducepressurelosses,andtherunnersystemshouldbebalanced.
Runnerbalancingmeansthatthedistancetheplasticmaterialfromthespruetothegateshouldbethesameforeachmolding.Thissystemensuresthatalltheimpressionswillfilluniformlyandwithoutinterruptionprovidingthegatelandsandthegateareasareidentical.
Runnerlayout
Itisnotalwayspracticable,however,tohaveabalancedrunnersystemandthisparticularlyappliestomouldswhichincorporatealargenumberofdifferentlyshapedimpressions.Inthesecases(asshowsexamples)balancedfillingoftheimpressioncanbeachievedbyvaryingthegatedimensionsthatisbybalancedgating.
4.GATES
Thegateisachannelororificeconnectingtherunnerwiththeimpression.Ithasasmallcross-sectionalareawhencomparedwiththerestofthefeedsystem.Thissmallcross-sectionalareaisnecessarysothat:
i)Thegatefreezessoonaftertheimpressionisfilledsothattheinjectionplungercanbewithdrawnwithouttheprobabilityofvoidbeingcreatedinthemoldingbysuck-back.ii)Itallowsforsimpledegatingandinsomemouldthisdegatingcanbeautomatic.ⅲ)Afterdegatingonlyasmallwitnessmarkremains.ⅳ)Bettercontrolofthefillingofmulti-impressionscanbeachieved.ⅴ)Packingtheimpressionwithmaterialinexcessofthatrequiredtocompensateforshrinkageisminimized.
Thesizeofthegatecanbeconsideredintermsofthegatecross-sectionalareaandthegatelength,thelatterbeingknownasgateland.Theoptimumsizeforagatewilldependonanumberoffactorsincluding(i)theflowcharacteristicsofthematerialtobemolded(ii)thewallsectionofthemolding,(iii)thevolumeofmaterialtobeinjectedintotheimpression,(iv)thetemperatureofthemelt(v)thetemperatureofthemould.
Notheoreticalsizeexistsfortheidealgate.Thegatesizechoseninpracticeforaparticularcomponentisnormallybasedonpastexperience.However,thereadermaynothavethisexperienceuponwhichtobaseadecisionand,therefore,aguidetothedimensionsforeachgatetypeisgiven.Tothegeneralcaseofamoldingwithawallsectionbetween0.75mmand4mm.
Figure4-1Typesofgate
Typeofgate
Toobtaintheoptimumfillingconditionsthetypeofgatemustbecarefullychosen.Onmostoccasions,however,thechoicewillbeobviousasonlyonetypeofgatewillmeettheparticularrequirementsforthemoldingonhand.Thetypesofgate(seeFigure4-1)commonlyusedare:
spruegate(f),edgegate(n),overlapgate(a),fangate(b),diaphragmgate(g),ringgate(h),filmgate(c),pingate(d),submarinegate(m)andtabgate(e).
5.Partingsurface
Thepartingsurfacesofamouldarethoseportionofbothmouldplates,adjacenttotheimpressions,whichbutt
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