外文翻译--塑料注射成型-热力学性质.doc

外文翻译原文：InjectionMoldingManydifferentprocessesareusedtotransformplasticgranules,powders,andliquidsintoproduct.Theplasticmaterialisinmoldableform,andisadaptabletovariousformingmethods.Inmostcasesthermosettingmaterialsrequireothermethodsofforming.Thisisrecognizedbythefactthatthermoplasticsareusuallyheatedtoasoftstateandthenreshapedbeforecooling.Theromosets,ontheotherhandhavenotyetbeenpolymerizedbeforeprocessing,andthechemicalreactiontakesplaceduringtheprocess,usuallythroughheat,acatalyst,orpressure.Itisimportanttorememberthisconceptwhilestudyingtheplasticsmanufacturingprocessesandpolymersused.Injectionmoldingisbyfarthemostwidelyusedprocessofformingthermoplasticmaterials.Itisalsooneoftheoldest.Currentlyinjectionmoldingaccountsfor30%ofallplasticsresinconsumption.Sincerawmaterialcanbeconvertedbyasingleprocedure,injectionmoldingissuitableformassproductionofplasticsarticlesandautomatedone-stepproductionofcomplexgeometries.Inmostcases,finishingisnotnecessary.Typicalproductsincludetoys,automotiveparts,householdarticles,andconsumerelectronicsgoods.Sinceinjectionmoldinghasanumberofinterdependentvariables,itisaprocessofconsiderablecomplexity.Thesuccessoftheinjectionmoldingoperationisdependentnotonlyinthepropersetupofthemachinehydraulics,barreltemperaturevariations,andchangesinmaterialviscosity.Increasingshot-to-shotrepeatabilityofmachinevariableshelpsproducepartswithtightertolerance,lowersthelevelofrejects,andincreasesproductquality(i.e.,appearanceandserviceability).Theprincipalobjectiveofanymoldingoperationisthemanufactureofproducts:toaspecificqualitylevel,intheshortesttime,andusingrepeatableandfullyautomaticcycle.Moldersstrivetoreduceoreliminaterejectedpartsinmoldingproduction.Forinjectionmoldingofhighprecisionopticalparts,orpartswithahighaddedvaluesuchasappliancecases,thepayoffofreducedrejectsishigh.Atypicalinjectionmoldingcycleorsequenceconsistsoffivephases；1.Injectionormoldfilling2.Packingorcompression3.Holding4.Cooling5.PartejectionPlasticgranulesarefedintothehopperandthroughanintheinjectioncylinderwheretheyarecarriedforwardbytherotatingscrew.Therotationofthescrewforcesthegranulesunderhighpressureagainsttheheatedwallsofthecylindercausingthemtomelt.Asthepressurebuildingup,therotatingscrewisforcedbackwarduntilenoughplastichasaccumulatedtomaketheshot.Theinjectionram(orscrew)forcesmoltenplasticfromthebarrel,throughthenozzle,sprueandrunnersystem,andfinallyintothemoldcavities.Duringinjection,themoldcavityisfilledvolumetrically.Whentheplasticcontactsthecoldmoldsurfaces,itsolidifies(freezes)rapidlytoproducetheskinlayer.Sincethecoreremainsinthemoltenstate,plasticfollowsthroughthecoretocompletemoldfilling.Typically,thecavityisfilledto95%98%duringinjection.Thenthemoldingprocessisswitchedovertothepackingphase.Evenasthecavityisfilled,themoltenplasticbeginstocool.Sincethecoolingplasticcontractsorshrinks,itgivesrisetodefectssuchassinkmarks,voids,anddimensionalinstabilities.Tocompensateforshrinkage,additionplasticisforcedintothecavity.Oncethecavityispacked,pressureappliedtothemeltpreventsmoltenplasticinsidethecavityfrombackflowingoutthroughthegate.Thepressuremustbeapplieduntilthegatesolidifies.Theprocesscanbedividedintotwosteps(packingandholding)ormaybeencompassedinonestep(holdingorsecondstage).Duringpacking,meltforcedintothecavitybythepackingpressurecompensatesforshrinkage.Withholding,thepressuremerelypreventsbackflowofthepolymermalt.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.TheNatureofThermodynamicsThermodynamicsisoneofthemostimportantareasofengineeringscienceusedtoexplainhowmostthingswork,whysomethingsdonotthewaythattheywereintended,andwhyothersthingsjustcannotpossiblyworkatall.Itisakeypartofthescienceengineersusetodesignautomotiveengines,heatpumps,rocketmotors,powerstations,gasturbines,airconditioners,super-conductingtransmissionlines,solarheatingsystems,etc.Thermodynamicscentersaboutthenotionsofenergy,theideathatenergyisconservedisthefirstlowofthermodynamics.Itisstartingpointforthescienceofthermodynamicsisentropy；entropyprovidesameansfordeterminingifaprocessispossible.Thisideaisthebasisforthesecondlowofthermodynamics.Italsoprovidesthebasisforanengineeringanalysisinwhichonecalculatesthemaximumamountofusefulthatcanbeobtainedfromagivenenergysource,ortheminimumamountofpowerinputrequiredtodoacertaintask.Aclearunderstandingoftheideasofentropyisessentialforonewhoneedstousethermodynamicsinengineeringanalysis.Scientistsareinterestedinusingthermodynamicstopredictandr