Injection molding.docx
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Injection molding.docx
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Injectionmolding
Injectionmolding
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Injectionmolding(BritishEnglish:
moulding)isamanufacturingprocessforproducingpartsfromboththermoplasticandthermosettingplasticmaterials.Materialisfedintoaheatedbarrel,mixed,andforcedintoamoldcavitywhereitcoolsandhardenstotheconfigurationofthemoldcavity.[1]Afteraproductisdesigned,usuallybyanindustrialdesigneroranengineer,moldsaremadebyamoldmaker(ortoolmaker)frommetal,usuallyeithersteeloraluminum,andprecision-machinedtoformthefeaturesofthedesiredpart.Injectionmoldingiswidelyusedformanufacturingavarietyofparts,fromthesmallestcomponenttoentirebodypanelsofcars.
Contents
[hide]
∙1ProcessCharacteristics
∙2History
∙3Applications
∙4ExamplesofPolymersBestSuitedfortheProcess
∙5Equipment
o5.1Mold
o5.2MoldDesign
o5.3Effectsonthematerialproperties
o5.4ToolMaterials
o5.5GeometricalPossibilities
o5.6Machining
o5.7Cost
∙6Injectionprocess
o6.1InjectionMoldingCycle
o6.2TimeFunction
o6.3Differenttypesofinjectionmoldingprocesses
∙7ProcessTroubleshooting
o7.1Moldingtrial
o7.2Moldingdefects
o7.3TolerancesandSurfaces
∙8LubricationandCooling
∙9PowerRequirements
∙10Inserts
∙11Gallery
∙12Seealso
∙13Notes
∙14References
∙15Externallinks
[edit]ProcessCharacteristics
∙Utilizesaramorscrew-typeplungertoforcemoltenplasticmaterialintoamoldcavity
∙Producesasolidoropen-endedshapethathasconformedtothecontourofthemold
∙Usesthermoplasticorthermosetmaterials
∙Producesapartingline,sprue,andgatemarks
∙Ejectorpinmarksareusuallypresent
[2]
[edit]History
Thefirstman-madeplasticwasinventedinBritainin1851byAlexanderParkes.Hepubliclydemonstrateditatthe1862InternationalExhibitioninLondon,callingthematerialheproduced"Parkesine."Derivedfromcellulose,Parkesinecouldbeheated,molded,andretainitsshapewhencooled.Itwas,however,expensivetoproduce,pronetocracking,andhighlyflammable.
In1868,AmericaninventorJohnWesleyHyattdevelopedaplasticmaterialhenamedCelluloid,improvingonParkes'inventionsothatitcouldbeprocessedintofinishedform.TogetherwithhisbrotherIsaiah,Hyattpatentedthefirstinjectionmoldingmachinein1872.[3]Thismachinewasrelativelysimplecomparedtomachinesinusetoday.Itworkedlikealargehypodermicneedle,usingaplungertoinjectplasticthroughaheatedcylinderintoamold.Theindustryprogressedslowlyovertheyears,producingproductssuchascollarstays,buttons,andhaircombs.
Theindustryexpandedrapidlyinthe1940sbecauseWorldWarIIcreatedahugedemandforinexpensive,mass-producedproducts.In1946,AmericaninventorJamesWatsonHendrybuiltthefirstscrewinjectionmachine,whichallowedmuchmoreprecisecontroloverthespeedofinjectionandthequalityofarticlesproduced.Thismachinealsoallowedmaterialtobemixedbeforeinjection,sothatcoloredorrecycledplasticcouldbeaddedtovirginmaterialandmixedthoroughlybeforebeinginjected.Todayscrewinjectionmachinesaccountforthevastmajorityofallinjectionmachines.Inthe1970s,Hendrywentontodevelopthefirstgas-assistedinjectionmoldingprocess,whichpermittedtheproductionofcomplex,hollowarticlesthatcooledquickly.Thisgreatlyimproveddesignflexibilityaswellasthestrengthandfinishofmanufacturedpartswhilereducingproductiontime,cost,weightandwaste.
Theplasticinjectionmoldingindustryhasevolvedovertheyearsfromproducingcombsandbuttonstoproducingavastarrayofproductsformanyindustriesincludingautomotive,medical,aerospace,consumerproducts,toys,plumbing,packaging,andconstruction.[4]
[edit]Applications
Injectionmoldingisusedtocreatemanythingssuchaswirespools,packaging,bottlecaps,automotivedashboards,pocketcombs,andmostotherplasticproductsavailabletoday.Injectionmoldingisthemostcommonmethodofpartmanufacturing.Itisidealforproducinghighvolumesofthesameobject.[5]Someadvantagesofinjectionmoldingarehighproductionrates,repeatablehightolerances,theabilitytouseawiderangeofmaterials,lowlaborcost,minimalscraplosses,andlittleneedtofinishpartsaftermolding.Somedisadvantagesofthisprocessareexpensiveequipmentinvestment,potentiallyhighrunningcosts,andtheneedtodesignmoldableparts.[6]
[edit]ExamplesofPolymersBestSuitedfortheProcess
Mostpolymersmaybeused,includingallthermoplastics,somethermosets,andsomeelastomers.[7]In1995therewereapproximately18,000differentmaterialsavailableforinjectionmoldingandthatnumberwasincreasingatanaveragerateof750peryear.Theavailablematerialsarealloysorblendsofpreviouslydevelopedmaterialsmeaningthatproductdesignerscanchoosefromavastselectionofmaterials,onethathasexactlytherightproperties.Materialsarechosenbasedonthestrengthandfunctionrequiredforthefinalpart,butalsoeachmaterialhasdifferentparametersformoldingthatmustbetakenintoaccount.[8]CommonpolymerslikeEpoxyandphenolicareexamplesofthermosettingplasticswhilenylon,polyethylene,andpolystyrenearethermoplastic.[9]
[edit]Equipment
Paperclipmoldopenedinmoldingmachine;thenozzleisvisibleatright
Mainarticle:
Injectionmoldingmachine
Injectionmoldingmachinesconsistofamaterialhopper,aninjectionramorscrew-typeplunger,andaheatingunit.[2]Theyarealsoknownaspresses,theyholdthemoldsinwhichthecomponentsareshaped.Pressesareratedbytonnage,whichexpressestheamountofclampingforcethatthemachinecanexert.Thisforcekeepsthemoldclosedduringtheinjectionprocess.Tonnagecanvaryfromlessthan5tonsto6000tons,withthehigherfiguresusedincomparativelyfewmanufacturingoperations.Thetotalclampforceneededisdeterminedbytheprojectedareaofthepartbeingmolded.Thisprojectedareaismultipliedbyaclampforceoffrom2to8tonsforeachsquareinchoftheprojectedareas.Asaruleofthumb,4or5tons/in2canbeusedformostproducts.Iftheplasticmaterialisverystiff,itwillrequiremoreinjectionpressuretofillthemold,thusmoreclamptonnagetoholdthemoldclosed.[10]Therequiredforcecanalsobedeterminedbythematerialusedandthesizeofthepart,largerpartsrequirehigherclampingforce.[11]
[edit]Mold
Moldordiearethecommontermsusedtodescribethetoolingusedtoproduceplasticpartsinmolding.
Sincemoldshavebeenexpensivetomanufacture,theywereusuallyonlyusedinmassproductionwherethousandsofpartswerebeingproduced.Typicalmoldsareconstructedfromhardenedsteel,pre-hardenedsteel,aluminum,and/orberyllium-copperalloy.Thechoiceofmaterialtobuildamoldfromisprimarilyoneofeconomics;ingeneral,steelmoldscostmoretoconstruct,buttheirlongerlifespanwilloffsetthehigherinitialcostoverahighernumberofpartsmadebeforewearingout.Pre-hardenedsteelmoldsarelesswear-resistantandareusedforlowervolumerequirementsorlargercomponents.Thetypicalsteelhardnessis38-45ontheRockwell-Cscale.Hardenedsteelmoldsareheattreatedaftermachining.Thesearebyfarthesuperiorintermsofwearresistanceandlifespan.Typicalhardnessrangesbetween50and60Rockwell-C(HRC).Aluminummoldscancostsubstantiallyless,and,whendesignedandmachinedwithmoderncomputerizedequipment,canbeeconomicalformoldingtensorevenhundredsofthousandsofparts.Berylliumcopperisusedinareasofthemoldthatrequirefastheatremovalorareasthatseethemostshearheatgenerated.[12]ThemoldscanbemanufacturedeitherbyCNCmachiningorbyusingElectricalDischargeMachiningprocesses
Injectionmoldingdiewithsidepulls
"A"sideofdiefor25%glass-filledacetalwith2sidepulls.
Closeupofremovableinsertin"A"side.
"B"sideofdiewithsidepullactuators.
Insertremovedfromdie.
[edit]MoldDesign
Standardtwoplatestooling–coreandcavityareinsertsinamoldbase–"Familymold"of5differentparts
Themoldconsistsoftwoprimarycomponents,theinjectionmold(Aplate)andtheejectormold(Bplate).Plasticresinentersthemoldthroughasprueintheinjectionmold,thespruebushingistosealtightlyagainstthenozzleoftheinjectionbarrelofthemoldingmachineandtoallowmoltenplastictoflowfromthebarrelintothemold,alsoknownascavity[13]ThespruebushingdirectsthemoltenplastictothecavityimagesthroughchannelsthataremachinedintothefacesoftheAandBplates.Thesechannelsallowplastictorunalongthem,sotheyarereferredtoasrunners.[14]Themoltenplasticflowsthroughtherunnerandentersoneormorespecializedgatesandintothecavity[15]geometrytoformthedesiredpart.
Theamountofresinrequiredtofillthesprue,runnerandcavitiesofamoldisashot.Trappedairinthemoldcanescapethroughairventsthataregroundintothepartinglineofthemold.Ifthetrappedairisnotallowedtoescape,itiscompressedbythepressureoftheincomingmaterialandissqueezedintothecornersofthecavity,whereitpreventsfillingandcausesotherdefectsaswell.Theaircanbecomesocompressedthatitignitesandburnsthesurroundingplasticmaterial.[16]Toallowforremovalofthemoldedpartfromthemold,themoldfeaturesmustnotoverhangoneanotherinthedirectionthatthemoldopens,unlesspartsofthemoldaredesignedtomovefrombetweensuchoverhangswhenthemoldopens(utilizingcomponentscalledLifters)
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