模具专业英语Plastics4

Lesson11PlasticsPlasticmeanspliableorimpressionable.Itisdefinedasthecapabilityofbeingdeformedcontinuouslyindirectionwithoutbreakingapart.Followingthisdefinition,plasticscouldincludeglass,metals,andwax.Thus,itisevidentthatthenameplasticsismisleadingandnotexactlycorrect.Today,however,thenameplasticsisidentifiedwiththeproductswhicharederivedfromsyntheticresins.Thesyntheticresinaremadebyvariouschemicalprocesses.Plasticsmanufacturingisofcomparativelyrecentdate.Thediscoveryofebonite,orhardrubber,byCharlesGoodyearin1839andthedevelopmentofcelluloid(cellulosenitrate)byHyattin1869markedthebeginningofplasticproducts.Itwasnotuntil1909,however,thatoneofthemostimportantmaterials,phenolformaldehyderesin,wasdevelopedbyDr.L.H.Baekelandandhisassociates.Todayplasticsarecommonlyfoundinhomes,automobiles,numerousotherproducts,andmachines.Somemachineswhichutilizeelectricityrequireplasticpartswithelectricalinsulatingproperties.Oneofthelargestplasticproductseverbuiltisa2600-pound28-foot-longboathull.Theunusualpropertiesofsomeplasticmaterialsareusedinmanyinterestingapplications.Advantageshouldbetakenoftheseunusualpropertieswhereverpossible.ThermosettingorThermoplasticPlasticmaterials,commonlycalledplastics,areknowneitherasthermosettingorasthermoplastic.Upontheapplicationofinitialheatthethermosettingplasticssoftenandmelt,andunderpressuretheywillfilleverycreviceofamoldcavity.Uponcontinuedapplicationofheattheypolymerize,orundergoachemicalchange,whichhardensthemintoapermanentlyhard,infusible,andinsolublestate.Afterthistheycannotagainbesoftenedormeltedbyfurtherheating.Thethermoplasticmaterialsarethosewhichsoftenwithheatingandsolidifyandhardenwithcooling.Theycanberemeltedandcooledtimeaftertimewithoutundergoinganyappreciablechemicalchange.FillersPlasticoftencontainotheraddedmaterialscalledfillers.Fillersareemployedtoincreasebulkandtohelpimpartdesiredproperties.Plasticscontainingfillerswillcurefasterandholdclosertoestablishedfinisheddimensions,sincetheplasticsshrinkagewillbereduced.Woodflouristhegeneral-purposeandmostcommonlyusedfillerCottonfrock,producedfromcottonlinters,increasesmechanicalstrength.Forhigherstrengthandresistancetoimpact,cottonclothchoppedintosectionsabout1/2-inchsquarecanbeprocessedwiththeplastic.Asbestosfibermaybeusedasafillerforincreasedheatandfireresistance,andmicaisusedformoldingplasticpartswithsuperiordielectriccharacteristics.Glassfibers,silicon,cellulose,clay,ornutshellflourmayalsobeused.Nutshellflourisusedinsteadofwoodflourwhereabetterfinishisdesired.Plasticpartsusingshortfiberfillerswillresultinlowercosts,whilethosewithlongfibersfillershavinggreaterimpactstrengthsaremoreexpensiveOthermaterials,notdefinedasfillers,suchasdyes,pigments,lubricants,accelerators,andplasticizersmayalsobeadded.Plasticizersareaddedtosoftenandimprovethemoldabilityofplastics.Fillerandmodifyingagentsareaddedandmixedwiththerawplasticbeforeitismoldedorformed.Properties染料颜料润滑剂增塑剂改良剂Numerousplastichavealreadybeendiscoveredanddevelopedbythechemistandchemicalengineer,andresearchonthesyntheticresinsisoneofthemostprominentfieldsinorganicchemistrytoday.Themechanicalpropertiesareconsiderablylowerthanthoseformetalswiththetensilestrengthsusuallyrangingfrom5000to15000psi.chemistchemicalchemistrySomeoftheplasticlisted,suchasphenolformaldehyde,consistofseveraldifferentvarietiesandmixturesincludedundertheonefamily.Numerousotherplasticswithspecialpropertiesareavailable.However,theoneslistedshouldserveasabriefintroduction.