英文原文TheInjectionMolding1.TheinjectionmoldingInjectionmoldingisprincipallyusedfortheproductionofthethermoplasticparts,althoughsomeprogresshasbeenmadeindevelopingamethodforinjectionmoldingsomethermosettingmaterials.Theproblemofinjectionamethodplasticintoamoldcavityfromareservoirinjectionmoldingisquitesimilartothatofdie-casting.Theprocessconsistsoffeedingaplasticcompoundinpoweredorgranularformfromahopperthroughmeteringandmeltingstagesandtheninjectionitintoamold.Afterabriefcoolingperiod,themoldisopenedandthesolidifiedpartejected.Injection-moldingmachineoperation.Theadvantageofinjectionmoldingare:(i)ahighmoldingspeedadapterformassproductionispossible；(ii)thereisawidechoiceofthermoplasticmaterialsprovidingavarietyofusefulproperties；(iii)itispossibletomoldthreads,undercuts,sideholes,andlargethinsection.2.Theinjection-moldingmachineSeveralmethodsareusedtoforceorinjectionthemeltedplasticintothemold.Themostcommonlyusedsysteminthein-linereciprocatingscrew,asshowninFigure2-1.Thescrewactsasacombinationinjectionandplasticizingunit.Astheplasticisfedtorotatingscrew,itpassesthroughthreezonesasshown:feed,compression,andmetering.Afterthefeedzone,thescrew-flightdepthisgraduallyreduced.forcetheplastictocompress.Theworkisconvertedtoheatbyconductionfromthebarrelsurface.Asthechamberinfrontofthescrewbecomesfilled,itforcesthescrewback,trippingalimitswitchthatactivatesahydrauliccylinderthatforcesthescrewforwardandinjectsthefluidplasticintotheclosedmold.Anantiflowbackvavlepresentsplasticunderpressurefromescapingbackintothescrewflight.Theclampingforcethatamachineiscapableofexertingispartofthesizedesignationandismeasuredintons.Arule-ofthumbcanbeusedtodeterminethetonnagerequiredforaparticularjob.Itisbasedontwotonsofclampforcepersquareinchofprojectedarea.Iftheflowpatternisdifficultandthepartsarethin,thismayhavetogotothreeorfourtons.Manyreciprocating-screwmachinesarecapableofhandingthermosettingplasticmaterials.Previouslythesematerialswerehandledbycompressiongortransfermolding.Thermosettingmaterialscureorpolymerizeinthemoldandareejectedhotintherangeof375410.Thermosettingpartsmustbeallowedtocoolinthemoldinorderorremovethemwithoutdistortion.Thusthermosettingcyclescanbefaster.Ofcoursethemoldmustbeheatedratherthanchilled,aswiththermoplastics.3.BasicUnderfeedMouldThedescriptionofthedesignandtheopeningsequencefollows.Themouldconsistsofthreebasicparts,namely:themovinghalf,thefloatingcavityplateandthefeedplaterespectively.ThemovinghalfconsistsofThemovingmouldplateassembly,supportblock,backingplate,ejectorassemblyandthepinejectiongsystem.Thusthemovinghalfinthisdesignisidenticalwiththemovinghalfofbasicmoulds.Thefloatingcavityplate,whichmaybetheintegerorinsert-bolsterdesign,islocatedonsubstantialguidepillars(notshown)fittedinthefeedplate.Theseguidepillarsmustbeofsufficientlengthtosupportthefloatingcavityplateoveritsfullmovementandstillprojecttoperformthefunctionofalignmentbetweenthecavityandcorewhenthemouldisbeingclosed.Guidebushesarefittedintothemovingmouldplateandthefloatingcavityplaterespectively.Themaximummovementofthefloatingcavityplateiscontrolledbystoporsimilardevice.Themovingmouldplateissuitablyboredtoprovidesufficientaclearanceforthestopboltassembly.Thestopboltsmustbelongenoughtoprovidesufficientspacebetweenthefeedplateandthefloatingcavityplateforeasyremovalofthefeedsystem.Theminimumspaceprovideforshouldbe65mmjustsufficientforanoperatortoremovethefeedsystembyhandifnecessary.Thedesireoperatingsequenceisforthefirstdaylighttooccurbetweenthefloatingcavityplate.Thisensuresthesprueispulledfromthespruebushimmediatelythemouldisopened.Toachievethissequence,springsmaybeincorporatedbetweenthefeedplateandthefloatingcavityplate.Thespringsshouldbeenoughtogiveaninitialimpetustothefloatingcavityplatetoensureitmovesawaywiththemovinghalf.Itisnormalpracticetomountthespringsontheguidepillarsandaccommodatetheminsuitablepocketinthecavityplate.Themajorpartofthefeedsystem(runnerandsprue)isaccmmodatedinthefeedplatetofacilitateautomaticoperation,therunnershouldbeofatrapezoidalformsothatonceitispulledfromthefeedplateiscaneasilybeextracted.Notethatifaroundrunnerisused,halftherunnerisformedinthefloatingcavityplate,whereitwouldremain,andbepreventedfromfallingorbeingwipedclearwhenthemouldisopened.Nowthatwehaveconsideredthemouldassemblyinthesomedetail,welookatthecycleofoperationforthistypeofmould.Theimpressionsarefilledviathefeedsystemandafterasuitabledwellperiod,themachineplatenscommencetoopen.Aforceisimmediatelyexertedbythecompressionsprings,whichcausethefloatingcavityplatetomoveawaywiththemovinghalfaspreviouslydiscussed.Thesprueispulledfromthespruebushbythespruepuller.Afterthefloatingcavityplatehasmovedapredetermineddistance,itisarrestedbythestopbolts.Themovinghalfcontinuedtomovebackandthemoldings,havingshrunkontothecores,arewithdrawnfromthecavitities.Thepingatebreaksatitsjunctionwiththerunner.Thespruepuller,beingattachedtothemovinghalf,ispulledthroughthefloatingcavityplateandtherebyreleasethefeedsystemwhichthenfreetofallbetweenthefloatingcavityplateandthemoldingsareejected.Whenthemouldisclosed,therespectiveplatesarereturnedtotheirmoldingpositionandthecycleisrepeated.4.FeedSystemItisnecessarytoprovideaflow-wayintheinjectionmouldtoconnectthenozzle(oftheinjectionmachine)toeachimpression.Thisflow-wayistermedthefeedsystem.Normallythefeedsystemcomprisesasprue,runnerandgate.Thesetermsapplyequallytotheflow-wayitself,andtothemoldedmaterialwhichisremovefromtheflow-wayitselfintheprocessofextractedmolding.Atypicalfeedsystemforafour-impression,twoplate-typemould.Itisseenthatthematerialpassesthroughthesprue,mainrunner,branchrunnerandgatebeforeenteringtheimpression.Asthetemperatureofmoltenplasticisloweredwhichgoingthroughthesprueandrunner,theviscositywillrise；however,theviscosityisloweredbysheargeneratedwhengoingthroughthegatetofillthecavity.Tiisdesirabletokeepthedistancethatthematerialhastotraveldowntoaminimumtoreducepressureandheatlosses.Itisforthereasonthatcarefulconsiderationmustbegiventotheimpressionlayoutgatesdesign.4.1SprueAsprueisachannelthroughwhichtotransfermoltenplasticinjectedfromthenozzleoftheinjectorintothemold.Itisapartofspruebush,whichisaseparatepartfromthemold.4.2RunnerArunnerisachannelthatguidesmoltenplasticintothecavityofamold.4.3GateAgateisanentrancethroughwhichmoltenplasticentersthecavity.Thegatehasthefollowingfunction:restrictstheflowandthedirectionofmoltenplastic；simplifiescuttingofarunnerandmoldingstosimplifyfinishingofparts；quicklycoolsandsolidifiestoavoidbackflowaftermoltenplastichasfilledupinthecavity.4.4ColdslugwellThepurposeofthecoldslugwell,shownoppositethesprue,istheoreticallytoreceivethematerialthathaschilledatthefrontofnozzleduringthecoolingandejectionphase.Perhapsofgreaterimportanceisthefactthatitprovidespositionmeanswherebythespruebushforejectionpurposes.Thesprue,therunnerandthegatewillbediscardedafterapartiscomplete.However,therunnerandthegateareimportantitemsthataffectthequalityorthecostofparts.5.EjectionAmoldingisformedinmouldbyinjectingaplasticmelt,underpressure,intoanimpressionviaafeedsystem.Itmustthereforeberemovedmanually.Furthermore,allthermoplasticmaterialscontractastheysolidify,whichmeansthatthemoldingwillshrinkontothecorewhichformsit.Thisshrinkagemakesthemoldingdifficulttoremove.Facilitiesareprovidedontheinjectionmachineforautomaticactuationofanejectorsystem,andthisissituatedbehindthemovingplaten.Becauseofthis.Themouldsejectorsystemwillbemosteffectivelyoperatedifplacedinthemorninghalfofthemould,i.e.Thehalfattachedtothemovingplaten.Wehavestatedpreviouslythatweneedtoejectthe,molding,fromthescoreanditthereforefollowsthatthecore,too,willmostsatisfactorilybelocatedinthemovinghalf.Theejectorsysteminamouldwillbediscussedunderthreeheadings,namely:(i)theejectorgird；(ii)theejectorplateassembly；and(iii)themethodofejection.5.1EjectorgirdTheejectorgirdisthatpartofthemouldwhichsupportsthemouldplateandprovidesaspaceintowhichtheejectorplateassemblycanbefittedandoperated.Thegirdnormallyconsistsofabackplateontowhichismountedanumberofconvenientlyshaped“supportblocks”.Theejectorplateassemblyisthatpartofthemouldtowhichtheejectorelementisattached.Theassemblyconsistsofanejectorplate,aretainingplateandanejectorrod.Oneendofthislettermembe