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A Guide to PE Injection_molding.pdf

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GUIDE TO PE INJECTION_MOLDING
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OneSource. MoreResourceful. AGuideto PolyolefinInjectionMoldingEquistarisoneofthelargestproducersofethylene,propyleneand polyethyleneintheworldtoday.Oneofthelargest,yetwepayatten- tiontoeventhesmallestneedsofourcustomers. We’realeadingproducerofpolypropylene,oxygenatedchemicals, performancepolymersandresinsandcompoundsforwireandcable. We’reanindustryleaderwithanunwaveringcommitmenttobeing thepremierpetrochemicalsandpolymerscompany. Ourcommitmentstartswitheachofourmorethan5,000employees. ItstretchesoutfromourheadquartersinHoustonacross16manufac- turingfacilitiesalongtheU.S.GulfCoastandintheMidwest.It continuesthroughour1,400-mileethylene/propylenedistribution systemthatspanstheGulfCoast. Wearetheproductofmanymindscomingtogetherwiththesingle focusofprovidingtherightproductforeverycustomer.That’swhat drivesustomaintainanextendedproductline,enhancedoperating efficiencies,greatergeographicdiversityandstrongresearchand developmentcapabilities.That’swhatdrivesustoprovidethe resourcesthathelpusleadtodayandrisetothechallengesofa changingindustrytomorrow. OneSource.MoreResourceful.1 AGuidetoPolyolefinInjectionMolding TableofContents Introduction ..................................................................................................2 Polyolefinsarederivedfrompetrochemicals...................................................2 Molecularstructureandcompositionaffectpropertiesandprocessability.......2 Chainbranching......................................................................................3 Density ..................................................................................................3 Molecularweight ....................................................................................3 Molecularweightdistribution..................................................................4 Copolymers .............................................................................................5 Modifiersandadditives ...........................................................................5 Workingcloselywithmolders ........................................................................5 Howpolyolefinsaremade..............................................................................5 Lowdensitypolyethylene(LDPE)..............................................................6 Highdensitypolyethylene(HDPE) ............................................................6 Linearlowdensitypolyethylene(LLDPE)...................................................7 Polypropylene..........................................................................................7 Shippingandhandlingpolyolefinresins .........................................................7 Materialhandling ....................................................................................8 Howtosolvematerialhandlingproblems................................................9 Othermaterialhandlingpractices..........................................................10 Theinjectionmoldingprocess......................................................................10 Injectionunits........................................................................................10 Plasticatorspecifications ........................................................................13 Screwdesigns........................................................................................13 Nozzles ................................................................................................14 Clampmechanisms ...............................................................................14 Clampspecifications..............................................................................15 Injectionmolds......................................................................................16 Typesofmold........................................................................................16 Spruesandrunners ...............................................................................17 Moldventing.........................................................................................18 Gating ................................................................................................19 Moldcooling.........................................................................................20 Ejectiondevices .....................................................................................20 Spiralflowmeasurement.......................................................................20 Generalinjectionmoldingoperatingprocedures ..........................................21 Generalsafety .......................................................................................21 Heat ................................................................................................22 Electricity...............................................................................................22 Machinerymotion .................................................................................22 Theinjectionmoldingprocessanditseffectonpartperformance................22 Themoldingcycle .................................................................................22 Shrinkage ..............................................................................................27 Warpage ...............................................................................................28 Colordispersionandairentrapment .....................................................29 Partejectionandmoldrelease...............................................................29 Clarity ................................................................................................30 Gloss ................................................................................................30 Polypropyleneintegralhinges ................................................................30 Appendices 1:InjectionMoldingTerms ....................................................................31 2:MetricConversionGuide...................................................................35 3:Abbreviations ....................................................................................37 4:ASTMtestmethodsapplicabletopolyolefins ....................................38 5:Injectionmoldingproblems,causesandsolutions..............................39 6:ASTMandISOsamplepreparationandtestprocedures ....................43 7:CompressionandinjectionmoldedsamplepreparationforHDPE......44Introduction Polyolefinsarethemostwidely usedplasticsforinjectionmolding. Thismanual,AGuidetoPolyolefin InjectionMolding,containsgeneral informationconcerningmaterials, methodsandequipmentfor producinghighquality,injection molded,polyolefinproductsat optimumproductionrates. Polyolefinsthatcanbeinjection moldedinclude: ? Lowdensitypolyethylene(LDPE) ? Linearlowdensitypolyethylene (LLDPE) ? Highdensitypolyethylene(HDPE) ? Ethylenecopolymers,suchas ethylenevinylacetate(EVA) ? Polypropyleneandpropylene copolymers(PP) ? Thermoplasticolefins(TPO) Ingeneral,theadvantagesof injectionmoldedpolyolefinscom- paredwithotherplasticsare: ? Lightweight ? Outstandingchemicalresistance ? Goodtoughnessatlower temperatures ? Excellentdielectricproperties ?Non hygroscopic Thebasicpropertiesofpolyolefins canbemodifiedwithabroad rangeoffillers,reinforcementsand chemicalmodifiers.Furthermore, polyolefinsareconsideredtobe relativelyeasytoinjectionmold. Majorapplicationareasforpoly- olefininjectionmoldingare: ? Appliances ?Automotive products ?Consumer products ?Furniture ?Housewares ?Industrial containers ? Materialshandlingequipment ?Packaging ?Sporting goods ? Toysandnovelties Thismanualcontainsextensive informationontheinjectionmold- ingofpolyolefins;however,it makesnospecificrecommendations fortheprocessingofEquistarresins forspecificapplications.Formore detailedinformationpleasecontact yourEquistarpolyolefinssalesor technicalservicerepresentative. Polyolefinsare derivedfrom petrochemicals Polyolefinsareplasticresinspoly- merizedfrompetroleum-based gases.Thetwoprincipalgasesare ethyleneandpropylene.Ethyleneis theprincipalrawmaterialformak- ingpolyethylene(PE)andethylene copolymerresins;propyleneisthe mainingredientformaking polypropylene(PP)andpropylene copolymerresins. Polyolefinresinsareclassifiedas thermoplastics,whichmeansthat theycanbemelted,solidifiedand meltedagain.Thiscontrastswith thermosetresins,suchasphenolics, which,oncesolidified,cannotbe reprocessed. Mostpolyolefinresinsforinjection moldingareusedinpelletform. Thepelletsareabout1/8inchlong and1/8inchindiameterandusual- lysomewhattranslucenttowhitein color.Manypolyolefinresinscon- tainadditives,suchasthermalstabi- lizers.Theyalsocanbecompound- edwithcolorants,flameretardants, blowingagents,fillers,reinforce- ments,andotherfunctionaladdi- tivessuchasantistaticagentsand lubricants. Molecular structureand composition affectproperties andprocessability Fourbasicmolecularproperties affectmostoftheresincharacteris- ticsessentialtoinjectionmolding highqualitypolyolefinparts.These molecularpropertiesare: ? Chainbranching ? Crystallinityordensity ? Averagemolecularweight ? Molecularweightdistribution Thematerialsandprocessesusedto producethepolyolefinsdetermine thesemolecularproperties. Thebasicbuildingblocksforthe gasesfromwhichpolyolefinsare derivedarehydrogenandcarbon atoms.Forpolyethylene,these atomsarecombinedtoformthe ethylenemonomer,C 2 H 4 . HH || C=C || HH Inthepolymerizationprocess,the doublebondconnectingthecarbon atomsisbroken.Undertheright conditions,thesebondsreformwith otherethylenemoleculestoform longmolecularchains. HHHHHHHHHH | | |||||||| –C–C–C–C–C–C–C–C–C–C– | | |||||||| HHHHHHHHHH Theresultingproductispolyethyl- eneresin. 2 AGuidetoPolyolefinInjectionMoldingForpolypropylene,thehydrogen andcarbonatomsarecombinedto formthepropylenemonomer, CH 3 CH:CH 2 . HH || H–C–C=C ||| HHH Thethirdcarbonatomformsaside branchwhichcausesthebackbone chaintotakeonaspiralshape. HHHHHH |||||| –C–C–C–C–C–C– |||||| H H–C–H H H–C–H H H–C–H ||| HHH Ethylenecopolymers,suchasethyl- enevinylacetate(EVA),aremade bythepolymerizationofethylene unitswithrandomlydistributed comonomergroups,suchasvinyl acetate(VA). Chainbranching Polymerchainsmaybefairlylinear, asinhighdensitypolyethylene,or highlybranchedasinlowdensity polyethylene.Forevery100-ethylene unitsinthepolyethylenemolecular chain,therecanbeonetotenshort orlongbranchesthatradiatethree- dimensionally(Figure1).Thedegree andtypeofbranchingarecon- trolledbytheprocess(reactor),cat- alyst,and/oranycomonomersused. Chainbranchingaffectsmanyof thepropertiesofpolyethylenes includingdensity,hardness,flexibili- tyandtransparency,tonameafew. Chainbranchesalsobecomepoints inthemolecularstructurewhere oxidationmayoccur.Ifexcessively hightemperaturesarereached duringprocessing,oxidationcan occurwhichmayadverselyaffect thepolymer’sproperties.Thisoxida- tionordegradationmaycause cross-linkinginpolyethylenesand chainscissioninpolypropylenes. Polypropylene,ontheotherhand, canbedescribedasbeinglinear (nobranching)orveryhighly branched.Althoughthesuspended carbonformsashortbranchon everyrepeatunit,itisalsoresponsi- blefortheuniquespiralandlinear configurationofthepolypropylene molecule. Density Polyolefinsaresemi-crystallinepoly- merswhichmeanstheyarecom- posedofmoleculeswhichare arrangedinaveryorderly(crystalline) structureandmoleculeswhichare randomlyoriented(amorphous).This mixtureofcrystallineandamorphous regions(Figure2)isessentialin providingthedesiredpropertiesto injectionmoldedparts.Atotally amorphouspolyolefinwouldbe grease-likeandhavepoorphysical properties.Atotallycrystallinepoly- olefinwouldbeveryhardandbrittle. HDPEresinshavelinearmolecular chainswithcomparativelyfewside chainbranches.Therefore,the chainsarepackedmoreclosely together(Figure3).Theresultis crystallinityupto95percent.LDPE resinsgenerallyhavecrystallinity from60percentto75percent. LLDPEresinshavecrystallinityfrom 60percentto85percent.PPresins arehighlycrystalline,buttheyare notverydense.PPresinshavea nominalspecificgravityrangeof 0.895to0.905g/cm 3 ,whichisthe lowestforacommoditythermo- plasticanddoesnotvaryappreciably frommanufacturertomanufacturer. Forpolyethylene,thedensityand crystallinityaredirectlyrelated,the higherthedegreeofcrystallinity, thehighertheresindensity.Higher density,inturn,influencesnumer- ousproperties.Asdensityincreases, heatsofteningpoint,resistanceto gasandmoisturevaporpermeation andstiffnessincrease.However, increaseddensitygenerallyresults inareductionofstresscracking resistanceandlowtemperature toughness. ? LDPEresinshavedensitiesrang- ingfrom0.910to0.930grams percubiccentimeter(g/cm 3 ) ? LLDPEresinsrangefrom0.915to 0.940g/cm 3 ? HDPEresinsrangefrom>0.940 to>0.960g/cm 3 Ascanbeseen,allnaturalpoly- olefinresins,i.e,thosewithoutany fillersorreinforcements,have densitieslessthan1.00g/cm 3 .This lightweightisoneofthekey advantagesforpartsinjectionmold- edfrompolyolefins.Ageneral guidetotheeffectsofdensityon thepropertiesforvarioustypesof polyethyleneresinsisshownin Table1. Molecularweight Atomsofdifferentelements,suchas carbon,hydrogen,etc.,havediffer- entatomicweights.Forcarbon,the atomicweightis12andforhydro- genitisone.Thus,themolecular weightoftheethyleneunitisthe sumoftheweightofitssixatoms (twocarbonatomsx12+four hydrogenx1)or28. 3 Figure1.Polyethylenechainwith longsidebranches C–C–C–C–C–C–C–C–C–C–C–C–C–C–C–C–C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Figure2.Crystalline(A)andamor- phous(B)regionsinpolyolefin Figure3.Linearpolyethylene chainwithshortsidebranches C–C–C–C–C–C–C–C–C–C–C–C–C–C–C–C–C C C C CUnlikesimplecompounds,like ethyleneorp
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