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南通职业大学(论文)外文翻译外文翻译原文InjectionMoldingManydifferentprocessesareusedtotransformplasticgranules,powders,andliquidsintoproduct.Theplasticmaterialisinmoldableform,andisadaptabletovariousformingmethods.Inmostcasesthermosettingmaterialsrequireothermethodsofforming.Thisisrecognizedbythefactthatthermoplasticsareusuallyheatedtoasoftstateandthenreshapedbeforecooling.Theromosets,ontheotherhandhavenotyetbeenpolymerizedbeforeprocessing,andthechemicalreactiontakesplaceduringtheprocess,usuallythroughheat,acatalyst,orpressure.Itisimportanttorememberthisconceptwhilestudyingtheplasticsmanufacturingprocessesandpolymersused.Injectionmoldingisbyfarthemostwidelyusedprocessofformingthermoplasticmaterials.Itisalsooneoftheoldest.Currentlyinjectionmoldingaccountsfor30ofallplasticsresinconsumption.Sincerawmaterialcanbeconvertedbyasingleprocedure,injectionmoldingissuitableformassproductionofplasticsarticlesandautomatedonestepproductionofcomplexgeometries.Inmostcases,finishingisnotnecessary.Typicalproductsincludetoys,automotiveparts,householdarticles,andconsumerelectronicsgoods.Sinceinjectionmoldinghasanumberofinterdependentvariables,itisaprocessofconsiderablecomplexity.Thesuccessoftheinjectionmoldingoperationisdependentnotonlyinthepropersetupofthemachinehydraulics,barreltemperaturevariations,andchangesinmaterialviscosity.Increasingshottoshotrepeatabilityofmachinevariableshelpsproducepartswithtightertolerance,lowersthelevelofrejects,andincreasesproductqualityi.e.,appearanceandserviceability.Theprincipalobjectiveofanymoldingoperationisthemanufactureofproductstoaspecificqualitylevel,intheshortesttime,andusingrepeatableandfullyautomatic南通职业大学(论文)外文翻译cycle.Moldersstrivetoreduceoreliminaterejectedpartsinmoldingproduction.Forinjectionmoldingofhighprecisionopticalparts,orpartswithahighaddedvaluesuchasappliancecases,thepayoffofreducedrejectsishigh.Atypicalinjectionmoldingcycleorsequenceconsistsoffivephases1.Injectionormoldfilling2.Packingorcompression3.Holding4.Cooling5.PartejectionPlasticgranulesarefedintothehopperandthroughanintheinjectioncylinderwheretheyarecarriedforwardbytherotatingscrew.Therotationofthescrewforcesthegranulesunderhighpressureagainsttheheatedwallsofthecylindercausingthemtomelt.Asthepressurebuildingup,therotatingscrewisforcedbackwarduntilenoughplastichasaccumulatedtomaketheshot.Theinjectionramorscrewforcesmoltenplasticfromthebarrel,throughthenozzle,sprueandrunnersystem,andfinallyintothemoldcavities.Duringinjection,themoldcavityisfilledvolumetrically.Whentheplasticcontactsthecoldmoldsurfaces,itsolidifiesfreezesrapidlytoproducetheskinlayer.Sincethecoreremainsinthemoltenstate,plasticfollowsthroughthecoretocompletemoldfilling.Typically,thecavityisfilledto9598duringinjection.Thenthemoldingprocessisswitchedovertothepackingphase.Evenasthecavityisfilled,themoltenplasticbeginstocool.Sincethecoolingplasticcontractsorshrinks,itgivesrisetodefectssuchassinkmarks,voids,anddimensionalinstabilities.Tocompensateforshrinkage,additionplasticisforcedintothecavity.Oncethecavityispacked,pressureappliedtothemeltpreventsmoltenplasticinsidethecavityfrombackflowingoutthroughthegate.Thepressuremustbeapplieduntilthegatesolidifies.Theprocesscanbedividedintotwostepspackingandholdingormaybeencompassedinonestepholdingorsecondstage.Duringpacking,meltforcedintothecavitybythepackingpressurecompensatesforshrinkage.Withholding,thepressuremerelypreventsbackflowofthepolymermalt.南通职业大学(论文)外文翻译Aftertheholdingstageiscompleted,thecoolingphasestarts.During,thepartisheldinthemoldforspecifiedperiod.Thedurationofthecoolingphasedependsprimarilyonthematerialpropertiesandthepartthickness.Typically,theparttemperaturemustcoolbelowthematerialsejectiontemperature.Whilecoolingthepart,themachineplasticatesmeltforthenextcycle.Thepolymerissubjectedtoshearingactionaswellastheconditionoftheenergyfromtheheaterbands.Oncetheshortismade,plasticationceases.Thisshouldoccurimmediatelybeforetheendofthecoolingphase.Thenthemoldopensandthepartisejected.Whenpolymersarefabricatedintousefularticlestheyarereferredtoasplastics,rubbers,andfibers.Somepolymers,forexample,cottonandwool,occurnaturally,butthegreatmajorityofcommercialproductsaresyntheticinorigin.AlistofthenamesofthebetterknownmaterialswouldincludeBakelite,Dacron,Nylon,Celanese,Orlon,andStyron.Previousto1930theuseofsyntheticpolymerswasnotwidespread.However,theyshouldnotbeclassifiedasnewmaterialsformanyofthemwereknowninthelatterhalfofthenineteenthcentury.Thefailuretodevelopthemduringthisperiodwasdue,inpart,toalackofunderstandingoftheirproperties,inparticular,theproblemofthestructureofpolymerswasthesubjectofmuchfruitlesscontroversy.Twoeventsofthetwentiethcenturycatapultedpolymersintoapositionofworldwideimportance.ThefirstofthesewasthesuccessfulcommercialproductionoftheplasticnowknownasBakelite.Itsindustrialusefulnesswasdemonstratedin1912andinthenextsucceedingyears.TodayBakeliteishighonthelistofimportantsyntheticproducts.Before1912materialsmadefromcellulosewereavailable,buttheirmanufactureneverprovidedtheincentivefornewworkinthepolymerfieldsuchasoccurredaftertheadventofBakelite.ThesecondeventwasconcernedwithfundamentalstudiesofthenaturepolymersbyStaudingerinEuropeandbyCarohers,whoworkedwiththeDuPontcompanyinDelaware.Agreaterpartofthestudiesweremadeduringthe1920s.Staudingersworkwasprimarilyfundamental.Carothersachievementsledtothedevelopmentofourpresenthugeplasticsindustrybycausinganawakeningofinterestinpolymerchemistry,aninterestwhichisstillstronglyapparenttoday.南通职业大学(论文)外文翻译TheNatureofThermodynamicsThermodynamicsisoneofthemostimportantareasofengineeringscienceusedtoexplainhowmostthingswork,whysomethingsdonotthewaythattheywereintended,andwhyothersthingsjustcannotpossiblyworkatall.Itisakeypartofthescienceengineersusetodesignautomotiveengines,heatpumps,rocketmotors,powerstations,gasturbines,airconditioners,superconductingtransmissionlines,solarheatingsystems,etc.Thermodynamicscentersaboutthenotionsofenergy,theideathatenergyisconservedisthefirstlowofthermodynamics.Itisstartingpointforthescienceofthermodynamicsisentropyentropyprovidesameansfordeterminingifaprocessispossible.Thisideaisthebasisforthesecondlowofthermodynamics.Italsoprovidesthebasisforanengineeringanalysisinwhichonecalculatesthemaximumamountofusefulthatcanbeobtainedfromagivenenergysource,ortheminimumamountofpowerinputrequiredtodoacertaintask.Aclearunderstandingoftheideasofentropyisessentialforonewhoneedstousethermodynamicsinengineeringanalysis.Scientistsareinterestedinusingthermodynamicstopredictandrelatethepropertiesofmatterengineersareinterestedinusingthisdata,togetherwiththebasicideasofenergyconservationandentropyproduction,toanalyzethebehaviorofcomplextechnologicalsystems.Thereisanexampleofthesortofsystemofinteresttoengineers,alargecentralpowerstations.Inthisparticularplanttheenergysourceispetroleuminoneofseveralforms,orsometimesnaturalgas,andtheplantistoconvertasmuchofthisenergyaspossibletoelectricenergyandtosendthisenergydownthetransmissionline.Simplyexpressed,theplantdoesthisbyboilingwaterandusingthesteamtoturnaturbinewhichturnsanelectricgenerator.Thesimplestsuchpowerplantsareabletoconvertonlyabout25percentofthefuelenergytoelectricenergy.Butthisparticularplantconvertsapproximately40percent
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