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1、Lesson1TheInjectionMoldingandMachin1.TheinjectionmoldingInjectionmoldingisprincipallyusedfortheproductionofthethermoplasticparts,althoughsomeprogresshasbeenmadeindevelopingamethodforinjectionmoldingsomethermosettingmaterials.Theproblemofinjectingameltedpla
2、sticintoamoldcavityfromareservoirofmeltedmaterialhasbeenextremelydifficulttosolveforthermosettingplasticswhichcureandhardenundersuchconditionswithinafewminutes①.Theprincipleofinjectionmoldingisquitesimilartothatofdie-casting.Theprocessconsistsoffeedingapla
3、sticcompoundinpowderedorgranularformfromahopperthroughmeteringandmeltingstagesandtheninjectingitintoamold②.Afterabriefcoolingperiod,themoldisopenedandthesolidifiedpartejected.Injection-moldingmachinescanbearrangedformanualoperation,automaticsingle-cycleope
4、ration,andfullautomaticoperation.Theadvantageofinjectionmoldingare:(i)ahighmoldingspeedadaptedformassproductionispossible;(ii)thereisawidechoiceofthermoplasticmaterialsprovidingavarietyofusefulproperties;(iii)itispossibletomoldthreads,undercuts,sideholes,a
5、ndlargethinsections.2.Theinjection-moldingmachineSeveralmethodsareusedtoforceorinjectthemeltedplasticintothemold.Themostcommonlyusedsysteminthelargermachinesisthein-linereciprocatingscrew,asshowninFigure1-1.Thescrewactsasacombinationinjectionandplasticizin
6、gunit.Astheplasticisfedtotherotatingscrew,itpassesthroughthreezonesasshown:feed,compression,andmetering.Afterthefeedzone,thescrew-flightdepthisgraduallyreduced,forcingtheplastictocompress.Theworkisconvertedtoheatbyshearingtheplastic,makingitasemifluidmass.
7、Inthemeteringzone,additionalheatisappliedbyconductionfromthebarrelsurface.Asthechamberinfrontofthescrewbecomesfilled,itforcesthescrewback,trippingalimitswitchthatactivatesahydrauliccylinderthatforcesthescrewforwardandinjectsthefluidplasticintotheclosedmold
8、③.Anantiflowbackvalvepreventsplasticunderpressurefromescapingbackintothescrewflights.Theclampingforcethatamachineiscapableofexertingispartofthesizedesignationandismeasuredintons.Arule-of-thumbcanbeusedtodeter
