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MoldConstruction1.Theinjectionmoldingandmachine1.1TheinjectionmoldingInjectionmoldingisprincipallyusedfortheproductionofthethermoplasticparts,althoughsomeprogresshasbeenmadeindevelopingamethodforinjectionmoldingsomethermosettingmaterials.Theproblemofinjectingameltedplasticintoamoldcavityfromareservoirofmeltedmaterialhasbeenextremelydifficulttosolveforthermosettingplasticswhichcureandhardenundersuchconditionswithinafewminutes.Theprincipleofinjectionmoldingisquitesimilartothatofdie-casting.Theprocessconsistsoffeedingaplasticcompoundinpowderedorgranularformfromahopperthroughmeteringandmeltingstagesandtheninjectingitintoamold.Afterabriefcoolingperiod,themoldisopenedandthesolidifiedpartejected.Injection-moldingmachinescanbearrangedformanualoperation,automaticsingle-cycleoperation,andfullautomaticoperation.Theadvantageofinjectionmoldingare:(i)ahighmoldingspeedadaptedformassproductionispossible;(ii)thereisawidechoiceofmaterialsprovidingavarietyofusefulproperties;(iii)itispossibletomoldthreads,undercuts,sideholes,andlargethinsections.1.2Theinjection-moldingmachineSeveralmethodsareusedtoforceorinjectthemeltedplasticintothemold.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.FeedsystemItisnecessarytoprovideaflow-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.SprueAsprueisachannelthroughwhichtotransfermoltenplasticinjectedfromthenozzleoftheinjectorintothemold.Itisapartofspruebush,whichisaseparatepartfromthemold.RunnerArunnerisachannelthatguidesmoltenplasticintothecavityofamold.GateAgateisanentrancethroughwhichmoltenplasticentersthecavity.Thegatehasthefollowingfunctions:restrictstheflowandthedirectionofmoltenplastic;simplifiescuttingofarunnerandmoldingstosimplifyfinishingofparts;quicklycoolsandsolidifiestoavoidbackflowaftermoltenplastichasfilledupinthecavity.ColdslugwellThepurposeofthecoldslugwell,shownoppositethesprue,istheoreticallytoreceivethematerialthathaschilledatthefrontofthenozzleduringthecoolingandejectionphase.Perhapsofgreaterimportanceisthefactthatitprovidespositivemeanswherebythespruecanbepulledfromthespruebushforejectionpurposes.Thesprue,therunner,andthegatewillbediscardedafterapartiscomplete.However,therunnerandthegateareimportantitemsthataffectthequalityorthecostofparts.3.RUNNERTherunnerisachannelmachinedintothemouldplatetoconnectthespruewiththeentrancetotheimpression.Thewalloftherunnerchannelmustbesmoothtopreventanyrestrictiontoflow.Also,astherunnerhastoberemovedwiththemolding,theremustbenomachinemarksle