TECHNICAL INFORMATION

DuPont™ Delrin® acetal resinMolding Guide

ii

iii

Table of Contents

PagesGeneral Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1SafetyPrecautionstoObserve

WhenMoldingDelrin®AcetalResins . . . . . . . . . . . . . . . 2Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Polymer Structure and Processing Behavior . . . . . . . . . . . . . . 4

GlassTransitionandMelting . . . . . . . . . . . . . . . . . . . . . . . . 4PVTDiagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Heating-CoolingBehavior . . . . . . . . . . . . . . . . . . . . . . . . . . . 6ViscosityandRheologicalBehavior . . . . . . . . . . . . . . . . . . . . 6Injection Molding Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

ScrewDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9ELCeeScrewforOptimumProductivity . . . . . . . . . . . . . . . 10CylinderTemperatureControl . . . . . . . . . . . . . . . . . . . . . . . 10CylinderAdaptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Non-ReturnValve(BackFlowValve—BFV) . . . . . . . . . . . . . 10Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11EvaluationofMeltQuality . . . . . . . . . . . . . . . . . . . . . . . . . . .11Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

AbilitytoFill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14RunnerSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16NozzleandSprue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17HotRunnerMoldforCrystallinePolymers . . . . . . . . . . . . . 18Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20SharpCorners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21RibsDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21WeldLines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21MoldMaintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22MoldCleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

(continued)

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Table of Contents (continued)

PagesMolding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Start-upandShutdownProcedures . . . . . . . . . . . . . . . . . . 23OperatingConditionsforDelrin®

—TemperatureSettings . . . . . . . . . . . . . . . . . . . . . . . . . 24OperatingConditionsforDelrin®

—MoldingCycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26OptimumProductivityMolding . . . . . . . . . . . . . . . . . . . . . . 29StandardMoldingConditions

forISOTensileBars . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30HoldPressureTime

viaIn-cavityPressureMeasurement . . . . . . . . . . . . . . . 30Dimensional Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

FundamentalsofDimensionalControl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

MoldShrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31FactorsAffectingMoldShrinkage . . . . . . . . . . . . . . . . . . . . 31MoldShrinkageofFilledResins . . . . . . . . . . . . . . . . . . . . . 31EffectofPigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Post-MoldingShrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . 34InsertMolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35EnvironmentalChanges . . . . . . . . . . . . . . . . . . . . . . . . . . . 35DimensionalTolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Auxiliary Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

MaterialHandling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37RegroundResin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Coloring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Troubleshooting Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

1

General Information

DescriptionDelrin®acetalresinsaresemi-crystalline,thermoplasticpolymersmadebythepolymerizationofformaldehyde,andarealsocommonlyreferredtoaspolyoxymethylene(POM) .Theyhavegainedwidespreadrecognitionforreliabilityinmanythousandsofengineeringcomponentsallovertheworld .Sincecommercialintroductionin1960,Delrin®hasbeenusedintheautomotive,appliance,construction,hardware,electronics,andconsumergoodsindustries,amongothers .

Delrin®isnotedfor:

• Highmechanicalstrengthandrigidity

• Excellentdimensionalstability

• Naturallubricity

• Fatigueendurance

• Highresistancetorepeatedimpacts

• Excellentresistancetomoisture,gasolines,solventsandmanyotherneutralchemicals

• Toughnessatlowtemperature(downto–50°C[–58°F])

• Wideusefultemperaturerange(inair:–50to90°C[–58to194°F],withintermittentuseupto120°C[248°F]) .

• Goodelectricalinsulatingcharacteristics

• Easeoffabrication

Delrin®acetalresinsareavailableinavarietyofcompositionstomeetdifferentend-useandprocessingrequirements .

CompositionsThemainavailableDelrin®compositionscanbeclassifiedasfollows:

a . Standard

b . Toughened

c . Lowwear/Lowfriction

d . Glassfilled/Reinforced

e . UV-stabilized

Thestandardcompositionscoverabroadrangeofmeltviscosities .Thehighestviscositycomposition,solikeDelrin®100P,areoftenmoldedwhenmaximumtoughnesspropertiesareneeded .TheintermediatemeltviscosityDelrin®500Pisusedforgeneral-purposeinjectionapplications .Theresinshavinglowermeltviscosity,Delrin®900Pisusuallychosenforinjectionmoldingapplicationswithhard-to-fillmolds .

AsummaryofthemaincompositionsisshowninTable 1 .

Safety Precautions to Observe When Molding Delrin® Acetal ResinsDelrin®aswellasmanyotherthermoplasticpolymersdecomposestogaseousproductswhenheatedforaprolongedtime .Thesegasescangeneratehighpressuresifconfined .Ifmaterialisnotfreetoexitfromaninjectioncylinderthroughthenozzle,itmayblowbackthroughthehopper .

InthecaseofDelrin®acetalresin,decompositionisalmostentirelytogaseousproducts,sopressurebuild-upcanberapid .Theproductofdecompositionisformaldehyde .

Aswithanyacetalpolymer,Delrin®,whenoverheated,candiscolorandformgaseousdecompositionproducts,whicharelargelyformaldehyde .Lowlevelsofformaldehydeemissionscanalsooccurattypicalprocessingtemperatures .Repeatedexposuretoformaldehydemayresultinrespiratoryandskinsensitizationinsomeindividuals .Formaldehydeisapotentialcancerhazard .

ProcessingtoughenedgradesofDelrin®canreleaselowlevelsofisocyanates .Repeatedexposuretoisocyanatesmayresultinrespiratoryandskinsensitizationinsomeindividuals .

Lowlevelsofformaldehydemayoccurintheheadspaceofbagsasreceivedorincontainersofformedpartsafterprocessing .Bagsofresinorformedpartsshouldbeopenedinwellventilatedareas .

Useadequatelocalexhaustventilationinprocessareastomaintainexposuresbelowrecommendedcontrollimits .Dropairshotsintowatertoreduceemissions .

WhenmoldingDelrin®,itisimportantthattheoperatorbefamiliarwiththefactorsthatcancausedecomposition,withthedangersignalsthatwarnofthisproblem,andwiththeactionthatshouldbetaken .Thisinformationissummarizedonacardfordisplayatthemoldingmachine .

Theinformationgivenhereisbasedonourexperiencetodate .Itmaynotcoverallpossiblesituationsanditisnotintendedasasubstituteforskillandalertnessoftheoperator .

Follow correct start-up, operating and shut-down procedures as described later in this guide.

Be aware of troublemakers—causes of decomposition:• Hightemperature—stickingtemperaturecontroller,faulty

thermocoupleconnections,incorrectreading,burned-outheaterorheaterwithahotspot,heatsurgesonstart-up .Donotoverheat .Maintainpolymermelttemperaturebelow230°C(446°F) .

• Cycledelay .Avoidprolongedexposureatorabovetherecommendedprocessingtemperature .RecommendedmelttemperaturesaregiveninTable 5 .

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High Viscosity Grades

Delrin® 100/II100 POMhomopolymer .Highviscositymoldingmaterial .Excellenttensilestrengthandresistancetocreepoverawidetemperaturerange,evenunderhumidambientconditions .Highfatigueenduranceandimpactresistance .Applications:moldedpartssuchashighlyloadedgears,plainbearingsandsnap-fits .

Delrin® 100P SamecharacteristicsandapplicationsasDelrin®100,plusimprovedthermalstabilityforlowdepositmoldingindemandingprocessingconditions,e .g .,hotrunnertools .

Delrin® 111PCharacteristics:Delrin®100Pwithenhancedcrystallinity .ResistancetocreepandfatigueenduranceimprovedoverDelrin®100P .Typicalapplications:Highlyloadedgears,bearings,snap-fits .

Medium-High Viscosity Grades

Delrin® 311DPPOMhomopolymer .Optimizedstabilizerformulationwithenhancedcrystallinity;excellentdimensionalstability .Excellentresistancetofuels,lubricants,solventsandmostneutralchemicals .

Medium Viscosity Grades

Delrin® 500PPOMhomopolymer .Generalpurposemoldingresinwithimprovedprocessingstabilityforlowdepositmoldingindemandingprocessingconditions,e .g .,hotrunnertools .

Delrin® 511PCharacteristics:Delrin®500Pwithenhancedcrystallinity,goodresistancetocreepandfatigue .Applications:fuelsystemcomponents,gears,fasteners .

Low Viscosity Grades

Delrin® 900PCharacteristics:Lowviscosity,fastmoldingresinplusimprovedprocessingstabilityforlowdepositmoldingindemandingprocessingconditions,e .g .,hotrunnertools .Applications:Multicavitymoldsandpartswiththinsections,e .g .,consumerelectronicsparts,zippers .

Toughened Grades

Delrin® 100STPOMhomopolymer,SuperTough .Highviscosity,supertoughmaterialforinjectionmoldingandextrusion .Excellentcombinationofsuper-toughness,impactfatigueresistance,solventandstresscrackresistance,aswellashightensileelongationatlowtemperature .Applications:Mainlyusedforpartsrequiringresistancetorepeatedimpactsandloads,suchasautomotivefasteners,helmets,hosesandtubing .

Delrin® 100TPOMhomopolymer,Toughened .Highimpactresistance .

Delrin® 500TMediumviscosity,toughenedresinforinjectionmoldingandextrusion .ExcellentnotchedIzodandtensileimpactstrength .Applications:Mainlyusedforpartssubjectedtorepeatedimpactsandalternatingloads,suchasautomotivefasteners,helmets,hosesandtubing .

Low-Wear/Low-Friction Grades

Delrin® 100ALHighviscositylubricatedgradewithpackageofadvancedlubricants .

Delrin® 100KMDelrin®100PmodifiedwithKevlar®aramidresinforabrasivewearreduction .Applications:Specialtyfrictionandwear .

Delrin® 500AFMediumviscositygradewith20%Teflon®PTFEfibers,outstandingfrictionandwearproperties .Applications:Specialtyfrictionandwear,conveyorsystems .

Delrin® 520MPDelrin®500Pwith20%Teflon®PTFEmicropowder,withlow-wearandlow-frictionproperties .Applications:Specialtyfrictionandwear .

Delrin® 500TLDelrin®500with1 .5%Teflon®powder,withlow-wearandlow-frictionproperties .Applications:Specialtyfrictionandwear,conveyorsystems .

Delrin® 500ALMediumviscosityresinwithadvancedlubricantsystem,verygoodlow-frictionandlow-wearproperties .Applications:Gears,drivetrains,slidingdevices .

Delrin® 500CL ChemicallylubricatedDelrin®500,verygoodlow-frictionandlow-wearproperties .Applications:Gears,drivetrains,slidingdevices .

Delrin® 500MPMediumviscositygradewithTeflon®PTPEmicropowder .

Delrin® 911ALLowviscsoitygradewithadvancedlubricants;excellentdimensionalstability .

Glass-Filled/Glass-Reinforced Grades

Delrin® 570Mediumviscosityresin,with20%glassfiberfiller .Applications:Wherehighstiffnessandcreepresistancearerequired .

Delrin® 510GR10%glass-reinforcedresin .Applications:Partsrequiringhighstiffnessandstrength,andcreepresistance .

Delrin® 525GR25%glass-reinforcedresin .Applications:Partsrequiringveryhighstiffnessandstrength,andcreepresistance .

UV-Stabilized Grades

Delrin® 127UVDelrin®100PwithUVstabilizer .Applications:AutomotiveinteriorpartswithmaximumUVperformancerequirements,skibindings,seatbeltrestraintparts .

Delrin® 327UVMedium-highviscositygradewithUVstabilizerandenhancedcrystallinity .

Delrin® 527UVDelrin®500PwithUVstabilizer .Applications:AutomotiveinteriorpartswithmaximumUVperformancerequirements,interiortrim,seatbeltrestraintparts .

Table 1 Main Compositions of Delrin® Acetal Resins

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• Hold-upareas—incylinder,adapter,nozzle,screwtip,hotrunnerandcheckvalveassembly

• Pluggednozzle—fromscrapmetalorhighermeltingpointresin,orfromclosednozzlevalve

• Foreignmaterials

– Additives,fillersorcolorantsotherthanthosespecificallyrecommendedforuseinDelrin®

– Contaminants(especiallythosecontainingchlorineorgeneratingacidmaterials)suchaspolyvinylchlorideresinorflameretardants

– Copper,brass,bronzeorothercopperalloysincontactwithmoltenDelrin®(notinmoldswheretheresinsolidifiesaftereachcycle)

– Copper-basedlubricantsorgreaseforthreads

– Contaminatedrework—especiallyreworkorreprocessedresinfromoutsideorunknownsources

– DonotmixDelrin®gradeswithotherDelrin®grades,norwithanyotherresins,withoutfirstconsultingDuPont .

– AvoidprocessingDelrin®onequipmentthatisalsousedforincompatibleresins,particularlyhalogenatedpolymers,suchasPVCorhalogenatedthermoplasticelastomers,orUVstabilizedorflameretardantmaterials .

Watch for Danger Signals• Frothynozzledrool

• Spittingnozzle

• Pronouncedodor

• Discoloredresin—brownorblackstreaking

• Badlysplayedparts—whitishdepositonmoldingormold

• Screwpushbackfromgaspressure

Action Required When Any of the Danger Signals Occur• AVOIDPERSONALEXPOSURE—WhenDANGERSIGNALS

arepresent,DONOTlookintohopperorworkaroundnozzleasviolentejectionofmeltispossible .

• MINIMIZEPERSONALEXPOSURETODECOMPOSITIONGASESbyusinggeneralandlocalventilation .Ifnecessary,leaveareaofmachineuntilventilationhasreducedconcentrationofformaldehydetoacceptablelevels .PersonssensitizedtoformaldehydeorhavingexistingpulmonarydisabilitiesshouldnotbeinvolvedinprocessingDelrin® .

• FREENOZZLEPLUGbyheatingwithtorch .Ifthisfails,cooldowncylinder,makesurePRESSUREISRELIEVED,andCAREFULLYREMOVENOZZLEandclean .

• TAKEAIRSHOTStocooltheresin—PURGEWITHCRYSTALPOLYSTYRENE .DROPALLMOLTENDelrin®INTOWATERtoreduceodorlevel .

• Turnoffcylinderheaters .

• Checktemperaturecontrolinstruments .

• Discontinueautomaticmoldingandrunmanuallyuntiljobisrunningsmoothly .

• Provideadequatemeansofventingfeedmechanismincaseofblowback .

• Useexhaustventilationtoreduceformaldehydeodor .

Ifdecompositionoccurs:

1 . Shutoffandpurgemachine .

2 . Minimizepersonalexposuretodecompositiongasesbyusinglocalandgeneralventilation .

3 . Ifnecessary,leaveareaofmachineuntilventilationhasreducedconcentrationofformaldehydetoacceptablelevels .

PackagingDelrin®acetalresinissuppliedassphericalorcylindricalpelletsapproximately3mm(0 .12in)indimensions .Theyarepackagedin1,000kg(2,200lb)netweightbulkcorrugatedboxes,500kg(1,100lb)netweightflexiblecontainer,or25kg(55 .16lb)moistureprotected,tearresistantpolyethylenebags .Thebulkdensityoftheunfilledresingranulesisabout0 .8g/cm3 .

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Polymer Structure and Processing Behavior

Thebehaviorofapolymerduringthemoldingprocessandthebehaviorofamoldedpartduringitswholeend-uselifearehighlydependentonthetypeofstructurethatthepolymertendstoformduringsolidification .

Somepolymersexhibitinthesolidstateroughlythesamemoleculararrangementasinthemelt,i .e .,arandommassofentangledmoleculeswithnoorder .Thisclassisnamed“amorphouspolymers”andincludesforexampleABS,polycarbonateandpolystyrene .

Otherpolymerstendtosolidifyinanorderedmanner:themoleculesarrangingintocrystallineforms(lamellae,spherulites) .Becauseofthelengthofthemacromolecules,partsofthemcannotbelongtocrystals(duetolackofspaceandmobility)andcreateanamorphousinter-crystallinezone .Thesepolymersarethereforepartiallycrystallineorsemi-crystalline;forsimplicity,inthistextwewillrefertothemas“crystalline”(asopposedto“amorphous”) .

TypicalcrystallinematerialsareDelrin®(acetalresins),Zytel® (polyamideresins),Rynite® PETandCrastin® PBT(thermoplasticpolyesterresins),polyethyleneandpolypropylene .

Table 2summarizessomefundamentaldifferencesbetweenamorphousandcrystallinepolymers .Thesepointsaredescribedinmoredetailinthefollowingparagraphs .Thisinformationisessentialtounderstandwhytheoptimizationofthemoldingprocessissubstantiallydifferentforthetwocategoriesofpolymers .

Table 2 Comparison of Amorphous and Crystalline Polymers

Resin type Amorphous Crystalline

Properties Thermal parameters Tg Tg, Tm

Maximum T in use* Below Tg Below Tm

Specific volume vs. T Continuous Discontinuity at Tm

Melt viscosity vs. T High dependence Low dependence

ProcessingSolidification Cooling below Tg Crystallization below Tm

Hold pressure Decreased during cooling Constant during crystallizationFlow through gate Stops after dynamic filling Continues until end of crystallizationDefects if bad process Over-packing, stress-cracking, Voids, deformations, sink marks sink marks

* For typical engineering applications

Glass Transition and Melting Amorphous Polymers

TheoverallbehaviorofamorphouspolymersislargelydeterminedinrelationtotheirglasstransitiontemperatureTg .

Belowthistemperature,themoleculesareessentiallyblockedinthesolidphase .Thematerialisrigidandhasahighcreepresistance,butitalsotendstobebrittleandsensitivetofatigue .

WhenthetemperatureisincreasedabovetheglasstransitiontemperatureTg,themoleculeshavesomefreedomtomovebyrotationaroundchemicalbonds .Therigiditydecreasesgraduallyandthematerialshowselastomericproperties,lendingitselftoprocesseslikethermoforming,blowmoldingand(attemperatures120–150°C[248–302°F]aboveTg)injectionmolding .

AmorphouspolymersusedinengineeringapplicationshaveTgabovetheambienttemperature,andthemaximumtemperatureforend-useshouldbebelowTg;forexamplepolystyrenehasTg=90–100°C(194–212°F),andisinjectionmoldedbetween210and250°C(410and482°F) .

Crystalline Polymers

Incrystallinepolymers,theonsetofmolecularmovementinthematerialalsodefinestheglasstransitiontemperatureTg .

WhenthetemperatureisincreasedaboveTg,thecrystallinepolymersmaintainrigidityappropriateforengineeringapplications(forexamplewithDelrin®apartcaneasilywithstandtemperatureswellabovetheTg) .

UponfurtherheatingthematerialreachesitsmeltingtemperatureTm,wherethecohesionofthecrystallinedomainsisdestroyed .Withinafewdegrees,thereisaconsiderablechangeofmechanicalpropertiesfromsolidtoliquidbehavior .AboveTm,thecrystallinepolymersbehaveashighviscosityliquids,andcangenerallybeprocessedbyinjectionmolding,typicallyattemperatures30–60°C(86–140°F)abovetheirmeltingtemperature .Asaconsequence,thetemperaturedomainfortheuseofcrystallinepolymersisnotlimitedbytheglasstransitiontemperatureTg,butbythemeltingtemperatureTm .ForDelrin®,theeffectoftheTgisnegligibleandverydifficulttomeasure,duetoitsverylowamorphouscontent .TherearetwotransitionsforDelrin®,aweakonearound0–15°C(32–59°F)andastrongeroneat–80°C(–112°F) .Thetransitionjustbelowroomtemperatureissoweakthereisminimaleffectonproperties .ForDelrin®acetalhomopolymrs,Tm=178°C(352°F)andthetypicalprocessingrangeis210–220°C(410–446°F) .

PVT DiagramsThePVTdiagramisacondensedpresentationoftheinterrelationsofthreevariablesthataffecttheprocessingofapolymer:Pressure,VolumeandTemperature .

Theeffectofthetemperature(T)orvolume(V)isillustratedinFigure 1foranamorphousandacrystallinepolymer .Whenthe

5

temperatureofthematerialisincreased,itsspecificvolume(theinverseofdensity)alsoincreasesduetothermalexpansion .Therateofincreasebecomeshigherattheglasstransitiontemperature,becausethemoleculeshavemorefreedomtomoveandtheyoccupymorespace .Thischangeofslopeisobservedwithbothamorphousandcrystallinepolymers .Athighertemperature,themeltingofcrystallinepolymersismarkedbyasuddenincreaseofthespecificvolume,whenthewell-orderedandrigidcrystallinedomainsbecomerandomlyorientedandfreetomove .Thespecificvolumeisthereforeasignatureofthechangesofstructureofthepolymerasafunctionoftemperature .

APVTdiagramissimplythepresentationoftheseriesofcurvesobtainedwhenthemeasurementofspecificvolumeversustemperatureisrepeatedatdifferentpressures .ThePVTdiagramofatypicalamorphouspolymer(polystyrene)isshowninFigure 2,andthePVTdiagramofDelrin®isshowninFigure 3 .

ThemoldingprocesscanbeillustratedbyacycleoftransitionsonthePVTdiagram .Forsimplification,itwillbeassumedinthefollowingdescriptionthatheatingtakesplaceatconstantpressure(“alongisobarlines”)andthatapplicationofpressureisisothermal(verticallines) .

Foranamorphousmaterialthemoldingcycleisasfollows(seeFigure 2):

• Startingfromroomtemperatureand1MPapressure(pointA)thematerialisheatedinthebarrel .Thespecificvolumeincreasesaccordingtotheisobarat1MPatoreachthemoldingtemperature(pointB) .

• Thematerialisinjectedintothecavityandthepressureisapplied .Thisprocessisroughlyisothermal(topointC),andthespecificvolumeisdecreasedtoavalueclosetothatat1MPaandTg .

• Theresiniscooledinthemold,andatthesametimetheholdpressureisdecreased,tofollowahorizontallineinthePVTdiagramandreachpointDwherethepartcanbeejectedwhenitisat1MPapressureandatemperaturebelowTg .Ideally,thereshouldbenoflowofmaterialthroughthegateduringthiscoolingphasetoproduceastress-freepart .

Foracrystallinematerial,thepictureisdifferent(seeFigure 3):

• thematerialisheatedat1MPapressurefromroomtemperature(pointA)uptotheprocessingtemperature(pointB) .Thisresultsinalargechangeofvolume(almost25%forDelrin®);

• theresinisinjectedandcompressedinthecavity .ThespecificvolumeisdecreasedtopointC,whereitsvalueisstillmuchhigherthanat1MPa/23°C(73°F);

• crystallizationtakesplaceinthemoldunderconstantholdpressure .Whenthecrystalsbuildupfromtheliquidphase,alargedifferenceofvolumeoccurs,whichmustbecompensatedbyinjectionofadditionalliquidresinthroughthegate(otherwisevoidsarecreatedwithinthepart);

Temperature, °C

Spec

ific

volu

me,

cm

3 /g

Tg

Temperature, °C

Spec

ific

volu

me,

cm

3 /g

Tm

"Liquid" phase

"Solid" phase

Tg

AMORPHOUS

CRYSTALLINE

Figure 1. Specific Volume as Function of Temperature for Amorphous And Crystalline Polymers

Figure 2. Pressure-Volume-Temperature (PVT) Diagram for Polystyrene. Points A, B, C, and D Refer to Different Steps of the Molding Process (see text).

Temperature, °C

Spec

ific

volu

me,

cm

3 /g

30050

AD

B 1

4060

160

100C

100 150 200 2500

0.95

1.00

1.05

1.10

0.90

20

Polystyrene

P (MPa)

6

• attheendofcrystallization(pointD),thepartissolidanditcanbeejectedimmediately;themoldingshrinkageisthedifferencebetweenthespecificvolumesatthecrystallizationtemperature(pointD)andatroomtemperature(pointA) .

Thisdifferenceinbehaviorhasimportantimplicationsforinjectionmolding .Duringthesolidificationprocess(afterdynamicfilling):

• theholdpressureisdecreasedwithtimeforamorphouspolymers,whereasitismaintainedconstantforcrystallinepolymers;

• theflowthroughthegateisstoppedforamorphouspolymers,whileitcontinuesuntiltheendofthecrystallizationforcrystallinepolymers .Thisimpliesthatforcrystallinematerialsthedesignofparts,gates,runnersandsprueshouldfollowspecialrulesthatwillbedescribedintheMoldssection .

Heating-Cooling BehaviorForanysubstance,theenergyneededtoincreasethetemperatureof1gmaterialby1°C(1 .8°F)isdefinedasitsspecificheat .ThisquantityisgenerallydeterminedbyDifferentialScanningCalorimetry,andtheresultsforDelrin®,polyamide6-6andpolystyreneareshowninFigure 4 .Thetwocrystallinepolymers,Delrin®andpolyamide6-6,showalargepeakthatisduetotheadditionalheatrequiredtomeltthecrystallinephase(latentheatoffusion) .Theamorphouspolymerdoesnotshowsuchapeak,butexhibitsachangeofslopeatTg .

Thetotalenergytobringeachmaterialuptoitsmoldingtemperatureisgivenbytheareaunderthecurve .FromFigure 4itisclearthatthecrystallinepolymersneedmoreenergythantheamorphousones .ThisexplainswhythedesignofascrewforacrystallinepolymerlikeDelrin®shouldbedifferent(andusuallymorecritical)thanforanamorphouspolymer .

Temperature, °C

Spec

ific

volu

me,

cm

3 /g

25050

A

C

D

B

100 150 2000

0.70

0.75

0.80

0.85

0.90

0.65

Delrin® 500 1

40

80120140180

P (MPa)

Figure 3. Pressure-Volume-Temperature (PVT) Diagram For Delrin® 500. Points A, B, C, and D Refer to Different Steps of the Molding Process (see text).

Figure 4. Specific Heat versus Temperature for Delrin® 500, PA66 and Polystyrene

Spec

ific

heat

, kJ

kg–1

K–1

400300

PS

PA66

2001000

0.4

0.8

1.2

1.6

2.0

2.4

2.8

0

Temperature, °C

Delrin® 500

Viscosity and Rheological BehaviorMeltviscositydeterminestoalargeextenttheabilitytofillthemoldcavity .Highviscositymeansdifficultflowthroughthinsectionsandhigherinjectionfillpressure .

Temperatureandshearratearecrucialparameterswhenconsideringtheviscosityofmoltenpolymers,andtheyshouldalwaysbespecifiedtogetherwithavalueformeltviscosity .

ForpolymersconsistingoflinearmoleculeslikeDelrin®,theviscosityisalsoindirectrelationtotheaveragemolecularweight .

Influence of Temperature

Thegeneralrulethatliquidsbecomelessviscouswhenincreasingtemperatureisalsotrueformoltenthermoplastics .Howevercrystallineandamorphouspolymersbehavedifferently,asshowninFigure 5 .ThecurvesforDelrin®andpolystyrenewerebothobtainedbyreducinggraduallythetemperatureofthematerialsfrom230to100°C(446to212°F) .Twodifferencesareworthmentioning .First,attemperaturesabove180°C(356°F),thedependenceofviscosityontemperatureismorepronouncedfortheamorphouspolystyrenethanforDelrin®;therefore,increasingthemelttemperatureofDelrin®doesnotgreatlyimproveitsabilitytoflowthroughathinsection .Second,below170°C(338°F)theviscosityofDelrin®risessharplybecausethematerialcrystallizeswithinafewdegreesofthattemperature .

7

Figure 5. Viscosity/Temperature Curves for Delrin® 500P and for Polystyrene at a Constant Shear Rate of 1000 s–1 (Temperature Reduced from 230 to 100°C [446 to 212°F])

Temperature, °C

App

aren

t vis

cosi

ty, P

a.s

180 220 240200120 140 160100

600

400

200

0

Delrin® 500

Polystyrene

Thermoforming

Extrusion

Injection molding

Y

V (Y)

Figure 6. Approximate Shape of the Velocity Distribution Between Two Parallel Plates. The Shear Rate is the Derivative dv(y)/dy.

Figure 7. Viscosity versus Shear Rate of Delrin® 500P at Three Temperatures

Figure 8. Viscosity versus Shear Rate for Various Grades of Delrin® at a Constant Temperature of 215°C (419°F)

Influence of Shear Rate

Theshearratecharacterizestherateofdeformationofthematerialandisdefinedasthederivativeofthevelocityoverthedirectionperpendiculartoflow(seeFigure 6);inotherwords,theshearrateisproportionaltothevariationofspeedwithinthepartthickness .Soitdependsonthevelocityoftheflowandonthegeometryoftheflowchannels .

Shear rate, s-1

Visc

osity

, Pa·

s-1 200°C

215°C230°C

102 103 104

100

1000

10

Table 3 Viscosity, Flow and Molecular Weight (Mw)

of the Delrin® Grades

Spiral flow length MFR MFR (215°C/100 MPa/2 mm) (190°C/ (190°C/ Ease Mw, 90°C mold Grade 1.06 kg) 2.16 kg) of flow toughness temperature

100 1 2.3 lowest highest 170 mm

300 3.5 7 —

500 7 14 295 mm

900 11 24 350 mm

ForDelrin®,themeltviscositydecreasesconsiderablywhentheshearrateincreases,asshowninFigure 7 .Thiseffectismoreimportantthanthedifferencesresultingfromvariationsofthemelttemperaturewithintheprocessingwindowforinjectionmolding .

Influence of Molecular Weight

Delrin®isavailableinfourgradesofmolecularweight .Theyarecodedaccordingtotheirabilitytoflow,asmeasuredbyMFRormeltflowrate(seeTable 3) .Highvaluesmeaneasyflowandabilitytofillthinparts,whereaslowvaluesmeanhighviscosity,highmolecularweightandhightoughness(impactresistance,elongationatbreak) .

Shear rate, s-1

Visc

osity

, Pa·

s-1

100 1,000 100,000

100

1,000

10

900500

100

MFRisameasurementperformedatlowshearrate,buttherelativedifferencesbetweenthegradesaremaintainedathighshearrates,asshowninFigure 8 .

Amoredirectcomparisonoftheabilitytofillcanbeobtainedusinganopen-endedsnake-flowmold .ResultsforthedifferentgradesofDelrin®arepresentedintheMoldssection .

8

Hold-up time, min

Mel

t tem

pera

ture

, °C

60 80 10040200

200210220230240250

190

Minimum recommendedmelt temperature

Recommended operatingzone

Figure 9. Effect of Temperature on Hold-up Time of Delrin® 500

Withascrewstrokeof1diameter(asmallshot)andacycletimeof1min(averylongone),theaverageHUTisequalto8min .AccordingtothedegradationcurveshowninFigure 9,Delrin®shouldbestableenoughforinjection-moldingwiththisHUTatamelttemperatureof240°C(464°F) .SomecustomershaveexperiencedmoldingDelrin®successfullyatthattemperature .

Attherecommendedmelttemperatureof215°C(419°F),themaximumHUTisover30minandDelrin®(standardgrades)isthermallystableevenundertheseextremeconditions .

Thereare3mainpotentialcausesofdegradation:

• Material trapped in Hold-up spots .Intheinjectionunit,trappedmoltenmaterialwillstayforverylongtimesinanydeadspotsandwillstarttodegrade .Soalltheinjectionunit(screw,backflowvalve,adaptor,nozzleandhotrunners)shouldbedesignedtoavoidHold-upspots(seefollowingrecommendeddesign) .

• Material sticking to “hot” steel .Duetothehighviscosityofpolymers,thespeednexttothesteeloftheinjectionunit(screw,backflowvalve,adaptor,nozzleandhotrunner)isalmostzeroandtheresidencetimeisalmostinfinite(asevidencedbyhowlongittakestochangecolorsinaninjectionunit) .Whereasinsidethebarrelthemoltenpolymeriscleanedbythescrewandthevalve,insideallotherareasthematerialwillsticktothewalls .Towithstandaverylongresidencetime,thesteelincontactshouldbecontrolledatatemperaturelowerthan190°C(374°F)(seeFigure 9) .

• Chemical degradation .Contamination(e .g .,PVC,flameretardantresins,acidgeneratingresins),incompatiblecoloringsystems(acidorbasicpigments),contactwithcopper(pure,alloys,grease)willacceleratethethermaldegradationofmoltenDelrin®intheinjectionunit .Notethatmoldcomponentsincopperorcopperalloys(suchascopper-beryllium)donotcauseanydegradationandhavebeenusedforyearswithoutproblems .

Injection Molding UnitDelrin®acetalresinsaremoldedthroughouttheworldinawidevarietyoftypesanddesignsofinjectionandextrusionequipment .

Thefirstpurposeoftheinjectionunitformoldingacrystallinematerialistodelivertothemoldthenecessaryamountofahomogeneousmelt(withnounmeltandnodegradedmaterial) .Therulesofconstructionoftheinjectionunitarethendependentonthemoldingmaterialrequirementsintermofthermalbehaviorandheatneeded .Thefirstpointtotakeintoaccountforacrystallinematerialisthethermalstabilityatmelttemperature,toavoiddegradation .Then,screw,nozzle,backflowvalve,adaptor,shouldbedesignedtoprovideefficientmeltingofcrystallinematerialanddeliveryofmoltenpolymertothemold .

Tworoughmethodstoevaluatethepresenceofunmeltandofdegradedmaterialwillbepresentedin“EvaluationofMeltQuality”(seepage11) .

Thermal Stability During Processing

Aspresentedinthepreviouschapter,onedifferencebetweenamorphousandcrystallinematerialisthe“melting”behavior .TheamorphouspolymerstartssofteningjustafterTgandpresentsacontinuouschangeinviscosity .Thisgivesaverylargetemperaturerangetooperate(butalargevariationofviscositywithtemperature) .Incontrast,thecrystallinepolymerstayssoliduptothemeltingpointandsuddenlymeltstotheliquidphaseathightemperature .Thislimitstheprocessingrangeoftemperaturebetweenunmeltandthermaldegradation(specificallyforDelrin®190–250°C[374–482°F]) .

Thesecondfactoristhetimethematerialstaysatthattemperature .Forallpolymers,themoleculescanwithstandacertaintimeatacertaintemperaturebeforedegradationcanstart .Obviouslythisacceptabletimelimitbecomesshorterwhenthetemperatureishigher .ThetypicalbehaviorofDelrin®ispresentedinFigure 9 .DegradationofDelrin®willresultingenerationofgaseswhichcausebubblesinthemelt,splaysonparts,molddeposit,yellowandbrownmarksontheparts .

Theaverageresidencetime(orHold-UpTime,HUT)intheinjectionunitislinkedtotheamountofpolymerinthecylinder,theshotweightandthecycletimeandcanbecalculatedwiththefollowingequation:

AverageHUT= weightofresinincylinder×cycletime

shotweight

Aquickapproximationcanbedoneby:

AverageHUT=maximumscrewstroke×2×cycletime

currentscrewstroke*

*Effectivescrewstroke=distancethescrewtravelsduringrotationonly

9

Screw DesignScrewdesignisakeyparameterforproductivity,becauseforcrystallinematerialsthescrewrotationtimeisaninherentpartofthecycletime .

Asmentionedabove,itshouldtakeinconsiderationthespecificmeltingbehaviorofthecrystallinematerial,i .e .,soliduptothemeltingpoint,highdemandofheatduringmeltingandlowviscosityofthemoltenmaterial .

Althoughgeneral-purposescrewsarewidelyusedformoldingDelrin®,optimumproductivitywillrequireaspecificdesign .Exceedingtheoutputcapabilityofaninadequatelydesignedscrewwillcausewidetemperaturevariationsandunmeltedparticles(sometimesunmeltanddegradedmaterialhavebeenobservedatthesametime) .Theresultislossoftoughness,variabilityinshrinkageanddimensions,warping,surfacedefects,pluggedgates(leadingtoshortshots)orothermoldingproblems .

Duetothespecificsofthemeltingprocessofacrystallinepolymer,ascrewdesignedforDelrin®willhaveshallowflightdepthsinthemeteringsectionandaslightlyhighercompressionthanageneral-purposescrew .SpecificsuggestionsaregivenforvariousscrewdiametersandcompositionofDelrin®acetalresininTable 4 .Compressionratioistheratioofvolumeofoneturninthefeedsectiontothatinthemeteringsection(canbeap-proximatedtotheratioofthedepthofthetwozones) .

Thelengthofthescrewwillalsoaffectthemeltquality(aninsulatingmaterialneedssometimetogetthethermalenergytransferredeveniftheshearcontributestotheheatingprocess) .Thepreferredlengthisabout20timesthescrewdiameteror20turnswhenthepitchanddiameterareequal .Thescrewshouldbedividedasfollows:30–40%(6–8turns)feedsection,35–45%(7–9turns)transitionand25%(5turns)metering

Table 4 Screw Design for Delrin® Acetal Resins

Medium and Low Viscosity Grades: Delrin® 500P, 900P, 500T High Viscosity Grades of Delrin®: Delrin® 100P, 100ST

Nominal diameter (D) Depth of feed section (h1) Depth of metering section (h2) Depth of feed section (h1) Depth of metering section (h2) mm mm mm mm mm   30   5.4  2.0   5.2  2.6   45   6.8  2.4   6.5  2.8   60   8.1  2.8   7.5  3.0   90  10.8  3.5   8.7  3.6  120  13.5  4.2 (in) (in) (in) (in) (in)   (1-1⁄2)  (0.240)  (0.087)  (0.230)  (0.105)   (2)   (0.290)  (0.100)  (0.270)  (0.115)   (2-1⁄2)  (0.330)  (0.110)  (0.300)  (0.120)   (3-1⁄2)  (0.420)  (0.140)  (0.340)  (0.140)  (4-1⁄2)  (0.510)  (0.160)

D h1

FEED SECTION

Pitch h2

METERINGSECTION

TRANSITION

(20/1 Length/Diameter Ratio)

section .Screwswith20turnsarecommonlydividedinto7turnsfeed,8turnstransitionand5turnsmetering .Inscrewslessthan16diameterslong,itmaybenecessarytoreducethepitchtogetupto20turns .Definitively,thefeedsectionshouldneverbelessthan6turns .

TherelativelyhighcompressionratioscrewssuggestedforDelrin®aredesignedtoincreasetheheatinputbymechanicalworkingoftheresin .Becausetheenergyforthisincreasecomesfromthescrewmotor,additionalhorsepowermustbeavailableifanincreaseinmeltingcapabilityistoberealized .

Screw Size

Theidealscrewsizeisdeterminedbythevolumeofthecurrentshot .Optimumproductivitywillbeachievedwhentheshotsizerequiresascrewtravelduringplasticizationequaltoorlowerthan50%ofthecapacityoftheinjectionunit .Otherwise,screwrotationspeedwillhavetobedecreasedattheendofthetraveltoguaranteeanhomogeneousmelt,leadingtoalossinproductivity .Practically,optimumproductivityisachievedwithascrewtravelofbetween1and2diametersofthescrew .

Thermalsettingsoftheinjectionunitwillbedependentontheresidencetime(HUT)andhencedependentonthecycletime .Ruleswillbepresentedunder“MoldingProcess .”

Screw Design for the Use of Color Concentrate

Aflowanalysisshowsthatthemajorpartoftheflowinthescrewislaminar,thendividedinthebackflowvalve(duetothechangesinflowdirection),andstilllaminarintheadaptor,nozzle,sprue,etc .Togetoptimummeltquality,todispersepigmentsandcolorconcentrates,itisstronglyrecommendedtoaddamixinghead .Thepurposeofaproperlydesignedmixingheadisnottomixmaterialbyturbulence(turbulentflowisimpossiblewithhighlyviscousmoltenpolymer),butbyforcedchangesinflowdirection .

10

ELCee™ Screw for Optimum Productivity AnothersolutiontoachieveoptimumproductivitywhenmoldinghighlycrystallineresinssuchasDelrin®,istouseaspecialscrewdesignedbyDuPontcalledthe“ELCee™screw”(patentappliedfor) .TheELCee™screwisdesignedtocapitalizeontherheologicalcharacteristicsofresinswithbettercontrolofshearonthemelt,makingthescrewmoreefficient .Thisallowsthemoldertorunthemoldingmachinesatfastercyclesandproducepartsofhigherquality .(ConsultyourlocalDuPontrepresentativeformoreinformation .)

Cylinder Temperature ControlThisisdeterminedbythemachinemanufacturer,buttwocommentsshouldbemade .

• Thetemperaturecontrolshouldprovideatleastthreeindependentzones,withthermocouplesplacednearthecenterofeachzone .Burn-outofoneormoreheaterbandswithinazonemaynotbereadilyapparentfromthetemperaturecontrollers,sosomemoldershaveusedammetersineachzonetodetectheaterbandmalfunctions .

• UsuallyforDelrin®thereisnoneedtocoolthefeedthroat,butincasesuchaneedexists,thewaterflowshouldbekepttoaminimum .Overcoolingthefeedthroathasbeenobservedasamajorreasonforcontaminationbyblackspecks .Thesearegeneratedinthebarrel,betweenthefirstandsecondheatingzones,withthefollowingmechanism(seeFigure 10) .ThethermocoupleTC1isinfluencedbythelowtemperatureduetoexcessivecooling,andthesystemwillrespondbyswitchingONtheheatingbandsHB1andHB2 .ThiscausesnoproblemwithHB1,butresultsinoverheatinganddegradationintheareaunderHB2 .Toreducetheriskofformationofblackspecks,thefollowingrecommendationsshouldbeobserved:

a) thefeedthroatcoolingshouldbelimitedtoaminimumtemperatureof80–90°C(176–194°F);

b) theheaterbandHB2shouldbecontrolledbyTC2,orTC1shouldbeplacedinthemiddleofHB2,orHB2shouldhavehalfthepowerdensityofHB1 .

HB1 HB2

TC1

Cooling channels Contaminationsource

TC2 TC3 TC4

HB3 HB4 HB5 HB6 HB7

HB8

Figure 10. The Risk of Black Specks Contamination That Could Arise From the Presence of a Cooling System of the Feed Throat

Figure 11. Design of Adaptor and Non-Return Valve

Cylinder AdaptorTheadaptorshowninFigure 11isdesignedtoavoidholdupareasandflowrestrictions,thetwomaincausesofdegradationandproblemslinkedtothisarea .NotethattheconceptisthesameforscrewedadaptorsasrepresentedinFigure 11(usedforsmallscrews≤∅40mm)andforboltedadaptors(usedforlargerscrews) .Theadaptorshasshortcylindricalsections(AandB)whereitjoinsboththenozzleandthecylindertomaintainaccuratematchingofthesediameters,evenifitbecomesnecessarytorefacethematingsurfaces .Thematingsurfaces(C)shouldbenarrowenoughtodevelopagoodsealwhenthenozzleoradaptoristightenedandyetwideenoughtoavoiddeformation .Inadditiontoitsmechanicalfunctionofreducingthediameter,theadaptoractstoisolatethenozzlethermallyfromthefrontofthecylinderforbettercontrolofnozzletemperature .Aseparateadaptor,madeofsoftersteelthantheoneusedforthecylinder,iseasierandlessexpensivetorepairandchangethanacylinder .Italsoprotectsthecylinderfromdamageduetofrequentchangingofthenozzle .Withtheboltedadaptor,specialcareshouldbetakenduringassemblytoensureparallelism(don’tovertightenscrewsfromonesideonly) .

Nozzle

Adaptor

A

D HE

B

G FC

Non-Return Valve (Back Flow Valve—BFV) Thenon-returnvalveorcheckringshowninFigure 11preventsmeltfromflowingbackwardduringinjection .Thisunitisfrequentlynotproperlydesignedtoeliminateholdupofresinandflowrestrictions .Malfunctioningthatallowsresinbackflowisalsoacommonexperienceandiscausedbypoordesignormaintenance .Aleakingnon-returnvalvewilladdtoscrewretractiontime,whichcanincreasecycle,anditwillalsocausepoorcontrolofpackinganddimensionaltolerances .

Thenon-returnvalvemustmeetthefollowingrequirements:

• Noholdupspots

• Noflowrestrictions

• Goodseal

• Controlofwear

11

Figure 12. Reverse Taper Nozzle

Heater Band

Thermocouple well

A

B

Thermocouple wellHeater BandA B

Figure 13. Straight Bore Nozzle, Only for Machines Without Screw Decompression

Theserequirementsareprovidedforinthenon-returnvalveshowninFigure 11 .

Theslotsorflutes(D)inthescrewtiparegenerouslyproportioned,andthespace(E)betweenthecheckringandtipissufficientforresinflow .

Theseatingofthefixedringiscylindricalwhereitjoinsboththeendofthescrew(F)andthescrewtip(G)topermitaccuratematchingofthesediametersandavoidholdup .

Thescrewtipthreadhasacylindricalsection(H)aheadofthethreadsthatfitscloselyinamatchingcounterboreforsupportandalignmentofthescrewtipandseatring .

Thescrewtipandcheckringseatshouldbeharder(aboutRc52)thanthefloatingring(Rc44),becauseitislessexpensivetoreplacethefloatingringwhenwearoccurs .

Corrosionresistantsteelissuggestedforthetip .Goodmatchingofcylindricaldiametersisessentialtoavoidholdupspots .

NozzleAswithothersemi-crystallinepolymers,Delrin®maydroolfromthenozzlebetweenshotsifthenozzleistoohot,oritmayfreezeiftoomuchheatislosttothespruebushing .

ThenozzledesignshowninFigure 12cansolvetheseproblems .Thefollowingshouldbeconsidered:

1 . Theheaterband(A)shouldextendasclosetothenozzletipaspossibleandcoverasmuchoftheexposedsurfaceaspractical .Thiscounteractsanyheatloss,especiallyheatlosstothespruebushing .

2 . Thethermocouplelocationisimportant .Anappropriatelocation(B)isshowninthesamepicture .

3 . Adequatetemperatureuniformityisrequiredsothatlocaloverheatingorprematurefreezingisavoided .

4 . Topreventpolymerdegradationthesteeltemperatureshouldnotexceed190°C(374°F) .

5 . Thenozzleheatershouldhaveitsownindependenttemperaturecontroller .

Screwdecompressionor“suckback”isfrequentlyusedtomakecontrolofdrooleasierwiththeseopennozzles .Thisfeatureisavailableinmostmachines .

Whennotavailable,adesignsuchastheoneillustratedinFigure 13shouldbeused .

AlthoughshutoffnozzleshaveoccasionallybeenusedsuccessfullywithDelrin®,theytendtocauseholdupofresinthatresultsinbrownstreaksorgassing,especiallyaftersomewearhasoccurredinthemovingpartsofthenozzle .ThesenozzlesarenotgenerallyrecommendedforDelrin®onsafetygroundsalone .

Note:Withalongnozzle,thethermocouplewellBshouldbepositionedinthemiddleofthenozzleandnotatthebackofthenozzle .

Evaluation of Melt QualityBelowarepresentedtwoquickandeasyteststoevaluatethemeltqualitydeliveredbytheinjectionunit .Althoughtheresultislinkedwiththetemperaturesettingoftheinjectionunit,itisalsohighlydependentonthedesignoftheinjectionunit .

Foaming Test

Thefoamingtestisrecommendedtodeterminethequalityoftheresinaftermeltingintheinjectionunit,i .e .,thequalityoftheresinANDthequalityoftheinjectionunit .

Procedure:

1 . Whenthemachineisrunningincycle,stopthemachineafterscrewretractionfor3minforpigmentedDelrin®(10minfornaturalmaterial) .

2 . Purgeatlowspeed(toavoidhotsplashes)intoacupandobservethemoltenmaterialfor1or2min .Thenputthemoltenmaterialinabucketofwater .

3 . Thenrechargethescrewandwait2moreminutes(10moreminutesfornaturalmaterial) .

4 . Repeatoperation2 .

12

Anunstablemeltwillgrow(foam)duringtheobservationandfloatinthebucket .Astablemeltwillstayshinywithatendencytoshrinkduringtheobservation,andwillsinkinthebucket .

Foamingresinwillquicklycausemolddepositandwillacceleratescrewdeposit,whichmayleadtoblackspeckcontamination .

Thistechniqueisusefultoevaluatenon-DuPontcolorsystems(colormasterbatches,liquidcoloring) .

Thefoamingtestcanalsobeusedtodetectinadequatequalityoftheinjectionunit(e .g .,problemsofthroatcoolingandconsequentoverheating,excessivenozzletemperature,hold-upspots,etc) .

Unmelt Test

Theunmelttestisrecommendedtoevaluatemelthomogeneity:

• Whenthepressisrunningoncycle,stopattheendofacycleandpurgeoneshot;

• chargethescrewimmediatelywiththeshotvolumeusedandpurgeagain;

• repeattheoperationuntildetectionoflumps/irregularitiesinthepurgecomingoutofthenozzle .

Ifsuchlumps/irregularitiesappearafterlessthan3purges,theriskofunmeltisveryhighandshouldbedealtwithbyincreasingcylindertemperature,byloweringscrewRPMandbyincreasingbackpressure .Ifsuchchangeslengthenthecycletimetoomuch,amoreappropriatescrewdesignshouldbeused(seeTable 4) .Iflumps/irregularitiesappearafter3purgesbutbefore6,thesituationisacceptable,butthereisnotmuchsafetymargin .Iftheyappearafter6purges,thereisaverylowriskofunmelt .

13

Figure 14. Exploded View of Mold

LocatingRing

SprueBushing

FrontClampingPlate

FrontCavityPlate(“A”Plate)

LeaderPins

LeaderPinBushingsRearCavityPlate(“B”Plate)

SupportPlate

Cavity

SpacerBlock

EjectorRetainerPlate

ReturnPinEjectorPlateKnockOutPins

RearClampingPlateSpruePullerSupportingPullerStopPin

Ability to FillMeltviscositylargelygovernstheabilityofaresintofillamold .Delrin®acetalresinsrangeinmeltviscosityfromDelrin®900P,thelowestinviscosityormostfluid,toDelrin®100P,thehighest .TheviscosityofDelrin®doesnotdecreaserapidlyasmelttemperatureincreases,incontrasttoamorphousthermoplasticresins .IncreasingmelttemperaturewillnotgreatlyimprovetheabilityofDelrin®tofillathinsection .

Inadditiontothepropertiesoftheresin,themoldingconditionsandcavitythicknessdeterminethedistanceofflow .Figure 15showsthemaximumflowdistancesthatcanbeexpectedattwocavitythicknessesforDelrin®acetalresinsasafunctionofinjectionfillpressure .Thesecomparisonsweremadeinanopen-endedsnakeflowmoldwithnogaterestriction .Obstructionsintheflowpath,suchassuddenchangesinflowdirectionorcorepins,cansignificantlyreducetheflowdistance .

Molds

Delrin®acetalresinshavebeenusedinmanytypesofmolds,andmoldershaveawealthofknowledgeconcerningmolddesignforDelrin® .MoldsforDelrin®arebasicallythesameasmoldsforotherthermoplastics .ThepartsofatypicalmoldareidentifiedinFigure 14 .

ThissectionwillfocusontheelementsofmolddesignthatdeservespecialconsiderationforprocessingDelrin®andcanleadtohigherproductivityandlowercostforthemolder .Thesetopicsare:

• Abilitytofill • Undercuts

• Gates • Runnerlessmolds

• Runners • Moldmaintenance

• Vents

Moldshrinkageandotheraspectsofmoldsizingarediscussedin“DimensionalConsiderations”(seepage29) .

14

GatesThegatesofamoldplayamajorroleinthesuccessorfailureofamoldingjob .Thelocation,design,andsizeofagatearekeyfactorstoallowoptimumpacking .Obviously,thedesignwillbedifferentthantheoneusedformoldingamorphousmaterial .Inthatcasetheflowshouldstopassoonaspossibleafterfillingthecavitytoavoidoverpacking(flowin)andsinkmarksatgate(flowback) .Withcrystallinematerial,thelocation,designandsizeofthegateshouldbesuchthatitwillallowacontinuousflowduringALLthepackingphase(Holdpressuretime—seepage27) .

Gate Location

Asakeyrule,whenapartisnotuniforminwallthickness,thegatemustbelocatedinthethickestsection .Therespectofthisbasicprincipleplaysanessentialroleinobtainingoptimumpackingandconsequentlythebestmechanicalproperties,dimensionalstabilityandsurfaceaspect .Ofcourseeverybottleneck(reducedsectionalongtheflowofthemelt)shouldbeavoidedbetweenthegateandallareasofthepart .

Anareawhereimpactorbendingwilloccurshouldnotbechosenasthegatelocation,becausethegateareamayhaveresidualstressandbeweakenedsinceitworksasanotch .Similarly,thegateshouldnotcauseaweldlinetooccurinacriticalarea .

Thegateshouldbepositionedsothattheairwillbeswepttowardapartinglineorejectorpin—whereconventionalventscanbelocated .Forexample,aclosed-endtubesuchasapencapshouldbegatedatthecenteroftheclosedend,soairwillbeventedatthepartingline .Anedgegatewillcauseairtrappingattheoppositesideneartheclosedend .Whenweldlinesareun-avoidable,forexamplearoundcores,anescapeforgasesmustbeprovidedtoavoidseriousweaknessandvisualflaws .Specificrecommendationsforventingaregivenlaterinthissection .

AnotherconsiderationinchoosingagatelocationforDelrin®issurfaceappearance .Gatesmearorblush,aswellasjetting,areminimizedbylocatingthegatesothatthemeltenteringthecavityimpingesagainstawallorcorepin .

Acentralgatelocationisoftennecessarytocontrolroundnessofgearsandothercriticalcircularparts .Multiplegates,usuallytwotofour,arecommonlyusedwhenthereisacentralholetoavoidadifficult-to-removediaphragmgate .

Gate Design

Asmentionedabove,forcrystallinematerialslikeDelrin®thethicknessofthegateoritsdiameter(forapin-pointgateortunnelgate)determinesthefreeze-offtime,andthereforealsodetermineswhetheritispossibletopackthepart(tocompensatethevolumereductionduetocrystallization)andmaintainthepressureduringsolidification .Thegateshouldremainopenuntilthepartdensityismaximumforaspecificmaterial .Thethickness(ordiameter)ofthegateshouldamountto50–60%ofthewallthicknessatthegate .Thewidthofthegateshouldalwaysbeequalorgreaterthanthegatethickness .Thelengthofthegateshouldbeasshortaspossibleandneverexceed0 .8mm(0 .03in) .Thegateareaofthepartshouldnotbesubjectedtobendingstressesduringactualservice .Impactstressesareparticularlyliabletocausefailureinthegatearea .

ThemostcommontypesofgatesaresummarizedinFigure 16 .

• DIAPHRAGMGATE:Circulargateusedtofillasinglesymmetricalcavity .Theadvantagesareareductionofweldlineformationandimprovementoffillingrates .Howevertheparthastobemachinedtoremovethegate .

• DIRECTGATE:Thespruefeedsdirectlyintothemoldcavitywithoutrunners .Thisdesignmayoftenleadtosurfacedefectscomingfromthenozzle(e .g .,coldslug,coldskin,entrappedair .)

• EDGEGATE:Usualtypeofgatewithtwoplatemolds .Itisnotselfdegating .

• FANGATE:Thisgateisusedtoenlargetheflowfront .Usuallyitleadstoareductionofstressconcentrationsinthegatearea .Lesswarpageofpartscanusuallybeexpectedbytheuseofthisgatetype .

• PINPOINTGATE:Thisgateisusedwiththreeplatemolds .Itisselfdegating .

Injection pressure, MPa

Flow

dis

tanc

e, m

m

Flow

dis

tanc

e, in

12080 100

Injection pressure, psi17.40011.600 15.000

500 –20

–15

–10

–5

600

400

300

200

100

0

100

100ST

100

500100ST

900

500

900

2.5 mm (0.100 in)1 mm (0.04 in)

Figure 15. Maximum Flow Distance of Delrin® Acetal Resins

15

Diaphragm gate

Submarine gate(bucket type)

Direct gate

Edge gate

Fan gate

Fan gate

Pin point gate

Ring gate

Figure 16. Schematic View of the Most Common Types of Gates

Figure 17. Details of a Typical Edge Gate Suitable for Delrin®

Runner

Side View

z = Max. 0.8 mm

T = Part ThicknessT

x = 0.5T

T+1.0

D1

30°

D

D1

d

T

Figure 18. Details of a Submarine Gate (Tunnel Gate) Adequate for Delrin® (left side). The One on the Right is Not Adequate for Crystalline Polymers and Would Give Problems with Delrin®.

• RINGGATE:SeeDIAPHRAGMGATE .

• SPRUEGATE:SeeDIRECTGATE .

• SUBMARINEGATE:Atypeofedgegatewheretheopeningfromtherunnerintothemoldisnotlocatedonthemoldpartingline .Itisusedtoseparatethegatefromthepartwithatwoplatemold(self-degating) .

• TUNNELGATE:SeeSUBMARINEGATE .

DetailsofatypicaledgegatesuitableforDelrin®areshowninFigure 17 .

Figure 18showsdetailsofasubmarinegateadequateforDelrin®(left),comparedtoasimilartypeofgatenotrecommendedforcrystallinematerials(right) .

Design criteria:

• alwaysgateinthickestareaofthepart;

• diameterofthegate“d”mustbeatleasthalfthepartthickness .Thelengthmustbeshorterthan0 .8mm(0 .03in)topreventprematuregatefreezingduringpacking;

• theinscribeddiameter“D”ofthetunnelnexttothegatemustbeatleast1 .2×thepartthickness“T .”

ThegateshownontherightsideofFigure 18isnotrecommendedforcrystallinematerialslikeDelrin®,becausesuchconicalgatesectionscrystallizebeforetheendofcompletepartpackout .Thisresultsinlowmechanicalperformanceanduncontrolledshrinkage .

Figure 19showsdetailsofathreeplategatedesignadequateforDelrin®(left),comparedtoasimilartypeofgatenotrecommendedforcrystallinematerials .Thedesigncriteriaillustratedabovearealsoapplicabletothiskindofgate .

Note:Restrictionsaroundthespruepullerwillleadtoincompletepartpackout .So,thediameterD1inFigure 19shouldbeatleastequaltodiameterD .

16

Runner SystemGuidelines

Keyguidelinestofollowwhendesigningarunnersysteminclude:

a . runnersshouldstayopenuntilallcavitiesareproperlyfilledandpacked;

b . runnersshouldbelargeenoughforadequateflow,minimumpressurelossandnooverheating;

c . runnersizeandlengthshouldbekepttotheminimumconsistentwithpreviousguidelines .

Eachofthesefactorscanaffectqualityandcostofmoldedparts .Factor(a)shouldberegardedasthemostcritical .

Thecrosssectionoftherunnersismostoftentrapezoidal,whichrepresentsanoptimumpracticalcompromisewithrespecttothefullroundsection .Theeffectivecrosssectionoftherunnerisinthiscasethediameterofthefullcirclethatcanbeinscribedinit .

ForpartsofDelrin®tohavethebestphysicalproperties,therunnersnexttothegatemusthaveatleastaninscribeddiameterthepartthickness“T+1mm .”

Whenthemoldingsareverythin,however,thisrunnercannotbelessthanabout1 .5mm(0 .06in)inthickness .Therunnerthicknessisusuallyincreasedateachofthefirstoneortwoturnsfromthecavity,asshownintheexampleofFigure 20 .

Figure 21. Direct Gating (Left) and Indirect Gating to Break the Flow (Right), in a One-Cavity Mold

Figure 22. Balanced (Left) and Unbalanced (Right) Runner Systems in a 16-Cavity Mold

Single Cavity Mold

Thesimplestrunnerconfigurationforasinglecavitymoldcouldbedirectgating(seeFigure 21) .Inthiscase,however,itwouldbenecessarytohavea“coldslugcatcher”directlyonthepart,withassociatedsurfaceproblemsandlowermechanicalpropertiesinthatarea .Thepreferredsolutionisthento“breaktheflow”asindicatedinFigure 21 .

Runner Layout

Aperfectlybalancedlayout(withequalflowdistancefromthespruetoeachcavity)isbestachievedifthenumberofcavitiesisequaltoapowerof2(2,4,8,16,32,64,128,etc .) .Seeanexampleofa16-cavitymoldinFigure 22withbalanced(left)andunbalancedrunnersystems .Aperfectlybalancedlayoutmaybeimpracticalandexpensive .

D

d

2°

D1

T*

Figure 19. Details of a Three Plate Gate Design Adequate for Delrin® (Left Side). The One on the Right is Not Adequate for Crystalline Polymers and Would Give Problems with Delrin®. *Gate Length Should be <0.8 mm (0.03 in).

Figure 20. Correct Runner Thickness for an Eight Cavity Mold

Ø1 = D1 + 1 mm

Ø3 = D1

Ø2 = D1 + 0.5 mm

17

Figure 23. Examples of Unbalanced 16-Cavity Mold. The Solution on the Right is Provided with Overflow Wells to Trap Cold Slugs.

Figure 24. Example of “Spiral Effect” in a 32-Cavity Mold. Cavities 11, 14, 19, 22 Will Be Filled First and May Show Splays and Mold Deposits.

1

16 14

1917

32

24

9

22

11

25

8

Figure 25. Sprue and Nozzle Design Often Used with Delrin®. The Dimensions are Linked with the Dimensions of the Part and of the Runners.

Figure 26. Example of a Design of a Nozzle Without Sprue Used with 2 Plate Molds. Remember that for Delrin® the Nozzle Temperature Should Not Exceed 190°C (374°F).

D N1D N2

Ø1

D N1

D N2

5 mm

Whenanunbalancedrunnersystemisselected,thelayoutshowninFigure 23(left)couldpresentmorerisksofqualityproblems .Theflowtendstostopateachoftheearlygatesduetotherestrictionandthematerialstartstocrystallize .Then,astherunnercontinuestobefilled,thepressurerisesandthethecoldslugswhichstartedtobebuiltup,arepushedintothecavity .

Toreducesuchrisk,thesolutionshowninFigure 23(right)isrecommended .Insuchconfiguration,thecoldslugstendtobetrappedintoeachoverflowwell .

Incaseofmulti-cavitymolds(≥16cavities),theso-called“spiraleffect”couldtakeplaceinthe“internal”cavitiesofthelayout(seeforinstanceFigure 24),duetoover-heatingofthemeltinrunners,causedbylocalizedshear .Tominimizenegativeeffectlikesplaysormolddeposit,shearshouldbereducedbyusingappropriaterunnerdimensions .

Formulti-cavitymoldsforsmallthicknessparts(≤1mm[0 .04in]),thedesignofrunnersshouldbecheckedbyrunningadetailedflowanalysisstudy .

Nozzle and SprueNozzleandspruediametersaredirectlylinkedwiththedimensionsofthepartandoftherunners .Thedesignershouldfirstdecideifthesprueisneededornot .Ifyes,adesignliketheoneshowninFigure 25couldbeselected,onethatinmanycaseshasprovedtobethemosteffectivewithcrystallinematerialslikeDelrin® .Duetoitsparallelcylindricalshapeitiseasytomachineandpolish,allowslargenozzlediameters,anditiseasytoejectduetohighshrinkage .Guidelinesforthedimensionare:

• aspruediameter∅1atleastequaltotheinscribeddiameterofthemainrunner;

• anozzlediameter“DN1”equalto∅1minus1mm .

Incasethedesignerselectsadesignwithoutasprue,alongnozzlemayberequiredasshowninFigure 26fora2platetool,andinFigure 27fora3platetool .Again,thedimensionsarelinkedtothedimensionsofthepartandoftherunners(guideline:nozzlediameter“DN1”equalstothemainrunnerinscribeddiameterminus1mm) .

18

Areviewofthekeyrecommendationsrelatedtothesprueandrunnersystemfollows .Itcanbeusedasaquickreferencelisttochecktheirdesign .

1 . Cylindricalparallelspruepreferred:seeFigure 25andFigure 28-1 .

2 . Spruepullerfor2platemold:seeFigure 28-2.

3 . Coldslugwellfor3platemold:seeFigure 28-3.

4 . Perpendicularflowsplitswithcoldslugwellsateachsplit,seeFigure 28-4 .

Temperature

Mod

ulus

C

A T

BM

IM

Tg Tm

Figure 29. Softening/Melting Behaviour of Amorphous and Crystalline Polymers

5 . Noflowrestrictioncausedbyspruepullerin3platemold,seeFigure 28-5 .

6 . Runnerdimensions:

– forpartshavingthickness>1 .5mm(0 .06in),followgeneralrulesforcrystallinepolymers(Figure 20);

– forthinnerpartsandmulti-cavitymolds,aflowanalysismayberequiredtoselectdimensionsthatwillavoidover-shearing .

7 . Runnersshouldbeproperlyvented,seeFigures 28,29and30 .

8 . Balancedrunnersrecommended(seeFigure 24) .

9 . Forthinpartsandlargenumberofcavities,unbalancedrunnersmaybeacceptable .However,partsshouldneverbegateddirectlyontothemainrunner(seeFigure 23) .

Hot Runner Mold for Crystalline PolymersPreliminary Comments

Thissectionincludesallhotrunner,hotspruebush,andrunnerlessmolds .Thefollowingisnotintendedtorecommendanytrademarkorsystembuttopresentthebehaviorandtheneedsofcrystallinepolymersinsuchtools .

ThequestionthatfrequentlyarisesiswhentousehotrunnermoldswithcrystallinepolymerslikeDelrin® .Thechoicedependsonmanyfactors,andparticularlyonthequalityneeded,i .e .,mechanicalperformance,surfaceaspect,percentageofrejects .

Status

Allsuchmoldsgivetheobviousadvantagesoflessmaterialtoplastify,no(orminimum)regrindandshortercycles .Ontheotherhand,hotrunnermoldsaremoreexpensiveandheavier;theyneedmoremaintenanceandbetter-trainedoperatorsthanconventionalmolds .Inaddition,iftheyarenotproperlydesigned,theheatneededtorunthemcouldspreadtoallpartsofthemoldandcaninfactcausethecycletimetoincrease .

Figure 27. Example of a Design of a Nozzle without Sprue Used with 3 Plate Molds. Remember that for Delrin® the Nozzle Temperature Should Not Exceed 190°C (374°F).

Figure 28. Key Rules for the Design of the Sprue and Runners of a 2 Plate Mold (Top) and of a 3 Plate Mold (Bottom).

D N1

D N2

DSP 1

2°

5 mm

1

4

6

Ventchannel

2

35 6

19

Oneapproachistoevaluatetheexpectedincreaseoftheoreticalproductivityversusconventionalmolds .Ifsuchanincreaseislowerthan25%,itwouldbewisetostaywitha3platemoldthatwillbecheapertobuild,startandrun .

Thebreak-evenofabout25%appliestofullhot-runnersystems;forothermolds(withhotspruebushes,coldsub-runners)thebreak-evenpointismuchlower .

Direct Gating Versus Cold Sub-Runners for Crystalline Polymers

Whendesigningahotrunnermoldforcrystallinepolymers,itshouldbekeptinmindthatdirectgatingviahotrunnerismoredifficultwithcrystallinepolymersthanwithamorphousones .Thedifferencecomesfromthesofteningormeltingbehaviorofthesetwotypesofpolymers .

AnamorphousmaterialexhibitsagradualsofteningbehavioraboveTgfromthesolidstatetotheliquidstate,allowingawideprocessingwindowintermsoftemperatureandviscosity .Infact,asitstemperatureincreasesaboveTg(seeFigure 29)anamorphouspolymer(curve“A”)lendsitselffirsttothermoforming(“T”),thentoblowmolding(“BM”)andfinallytoinjectionmolding(“IM”) .

Vent LandEnd of flow

Vent channels

L 0.8 mm

W > 2 mm

D < 0.03 mm

* 0.3 mm

Figure 30. Recommended Venting of a Part and of its Runner System

Onthecontrary,theTghasusuallyalimitedornegligibleeffectonthestructureofcrystallinepolymers,whicharesolidaboveTg .AtthetemperatureTm,crystallinepolymersmeltsharplyandbecomeliquid(curve“C”) .

Suchbehaviorofacrystallinematerialmayinvolvetheriskof:

• Droolingaroundthegatewithconsequentproblemsofbadsurfaceaspectanddeformation .

• Pluggingofthegatesbysolidifiedmaterial,plugswhichwillbepushedintothecavities,withconsequentproblemsofsurfacedefectsandlowermechanicalperformances .ThebestwaytopreventsuchproblemsistouseCOLDSUB-RUNNERS .

Thermal Control of Hot Runner Molds

Thermalmanagementandstreamliningoftheflowareveryimportantforhotrunnertools .Itshouldbecheckedthatarelativelylowtemperaturesetting(≤190°C[≤374°F])givesaneasyflowofthematerialwithnohold-upspots .

Thereasonisthat,duetotheviscosityofthepolymer,itsflowisalwayslaminar .Thismeansthatthematerialwillremainagainstthesteelwallofthehotrunner,andresidencetimewillbeverylong .ForDelrin®,toavoidthermaldegradationwithprolongedtimes,thesteeltemperatureshouldneverexceed190°C(374°F) .Ifthehotrunnersystemsolidifiesatthattemperature,thenitmustbemodifiedtoimprovethermalinsulationandheatdistributiontoremovecoldspots .Degradationcanresultinsplays,odor,blackspecksandmolddeposit .

Conclusions

WithcrystallinepolymerssuchasDelrin®,werecommendthefollowing:

• Aminimumof25%theoreticalcostdecreaseshouldbeexpectedbeforeahotrunnerisconsidered .

• Highlytrainedmachineoperatorsandmoldmaintenancetoolmakersshouldbeavailable .

• Useofcoldsub-runners,neverdirectgatingstraightontothepart .

• UseofDelrin®Pgrades .

• Alltemperaturesinthehotrunnersystemmustnotexceed190°C(374°F) .

• Avoidtheuseofhotrunnermoldsifsurfacedefectsarenotacceptableandhighpartmechanicalperformanceisrequired .

• Avoidtheuseofhotrunnersfortoughenedgrades .

VentsVentingamoldforDelrin®isparticularlyimportant,andspecialattentionshouldbegiventothisfactorduringboththedesignofthemoldanditsinitialtrial .ThisattentionisrequiredbecauseburningofpartscausedbyinadequateventingisnoteasilyobservedwithDelrin® .Withotherresins,poorventingresultsinablackenedandburnedspotonthepart .WithDelrin®,however,theremaybeeithernovisibleflaworaninconspicuouswhitishmarkonthemolding .

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VentingproblemswithDelrin®acetalresinsmaybemademoreobviousbysprayingthemoldwithahydrocarbonorkerosene-basedsprayjustbeforeinjection .Ifventingispoor,thehydrocarbonwillcauseablackspotwheretheairistrapped .Thistechniqueisparticularlyusefulfordetectingpoorventsinmulti-cavitymolds .Aconvenientsourceofhydrocarbonsprayisarustpreventativespray .

Ventsshouldbelocatedat:

1 . theendofanyrunner;

2 . anyflowjunctionwhereairisentrappedandaweldlineresults .Thepositionofweldlinescanbedefinedbyshortshots .

OnlyNOventingtogetherwithexcessivefastinjectionspeedwillcausecorrosionofthetoolattheweldlineswithDelrin®(dieseleffect) .InadequateventingofmoldsforDelrin®maycauseagradualbuildupofmolddepositwhereventsshouldbelocatedandinmoldcrevicesthroughwhichlimitedventinghastakenplace .Thesedepositsconsistofawhitesolidmaterialformedfromthetracesofgasevolvedduringnormalmolding .Goodventsallowthisgastoescapewiththeairfromthecavities .

Poorventingmayalsoreducephysicalpropertiesatweldlines .

Ventingproblemsmaybeaggravatedbyhighmelttemperature,longholduptime,orholdupareasintheinjectioncylinder,whichwillgeneratemorethannormalamountsofgas .Fastinjectionfillspeedwillalsoaggravatetheseproblems .RemediesformolddepositproblemsarelistedintheTroubleshootingGuide(seepage39) .

Ventingusuallyoccursthroughthepartinglineofamoldandisprovidedbymachiningchannelsinthecavityplateandinserts .

Insomecases,ventingmaybeaccomplishedaroundanejectorpin .Thisventwillalsobeimprovedbygrindingflatsonthepinandrelievingtheventafterashortland .Pinsthatdonotmovewiththeejectionsystemtendtoclogandnolongerprovideventingafterashorttime .

Ventingtherunnersystemishelpfulinreducingtheamountofairthatmustbeventedthroughthecavities .Becauseflashisunimportantontherunner,theseventscanbeslightlydeeperthancavityvents,forexample,0 .06mm(0 .0024in) .

ThedrawingsinFigure 30showtherecommendeddimensionsforventsincavitiesforDelrin® .

Note:Duringmoldmaintenance,ventdepthand/orhobbingshouldbecarefullychecked .Ventsshouldbemodifiediftheventdepthislessthan0 .01–0 .015mm(0 .0004–0 .0006in) .

UndercutsGeneralsuggestionsforstrippingundercutswithDelrin®acetalresinsare:

• Theundercutpartmustbefreetostretchorcompress,thatis,thewallofthepartoppositetheundercutmustclearthemoldorcorebeforeejectionisattempted .

• Theundercutshouldberoundedandwell-filletedtopermiteasyslippageoftheplasticpartoverthemetalandtominimizestressconcentrationduringthestrippingaction .

• Adequatecontactareashouldbeprovidedbetweentheknockoutandplasticparttopreventpenetrationofthemoldedpartorcollapseofthinwallsectionsduringthestrippingaction .

• ThelengthofthemoldingcycleandspecificallytheHold(Pressure)Time(HPT)shouldbeoptimumtoavoidexcessiveshrinkagewithinsideundercuts .Sufficientpartrigiditymustbedevelopedwithoutcausingbindingduetoexcessiveshrinkagearoundpinsforminganinternalundercut .Ejectionofpartswithundercutsontheoutsidediameterwillbeaidedbymoldshrinkage .

• Highermoldtemperature,whichkeepstheparthotterandmoreflexiblewhenthemoldopens,mayaidejectionfromanundercut .

• Generally,partsofDelrin®acetalcanbemoldedwithamaximum5%undercut .CalculationofallowableundercutisillustratedinFigure 31 .Theallowableundercutvariessomewhatwithbothwallthicknessanddiameter .

A

R

B Inside undercuts

Outside undercuts

A

B

R

A

B

30°

A

B

R

R

Figure 31. Calculations for % Undercut (B–A)/B ≤ 5%

21

Notch radius, mm

Impa

ct s

tren

gth,

kJ/

m2

0,50,1 0,2 0,3 0,40

20

40

60

80

0

Delrin® 100P NC10

Delrin® 500P NC10

Figure 32. Impact Strength as a Function of Molded Notch Radius

Figure 33. Suggested Rib Dimensions Versus Wall Thickness

0.5-0.7T

R = 0.2-0.3T

T

Filling time, s

Tens

ile s

tren

gth

at y

ield

, MPa

Izod

unn

otch

ed, J

/m

5 10 15 200

50

60

70

1,000

800

600

400

200

0

80

40

Tensile strength at yield, MPa

Izod unnotched

Figure 34. Tensile Strength (Left Scale) and Unnotched Izod Impact (Right Scale) of a Delrin® 100P Test Bar, 4 mm thick, Molded at Both Ends with Different Filling Times.

Sharp CornersOneofmajorcausesoffailureofplasticpartsareinternalsharpcorners .Asharpcornerinapartactsasanotchandinitiatesbreakataverylowenergy .ThediagraminFigure 32showstheeffectofnotchradiusonimpactresistanceoftestbarsmoldedintwogradesofDelrin® .Notethatthenotcheshavebeenmolded(simulationofreallifeandnotmachinedasrequiredbythestandardIzodtest) .

Fromthisdiagramitcanbeseenthatanincreaseofaninternalradiusofcurvaturefrom0 .01(almostasharpcorner)to0 .2mmdoublestheimpactresistance .

Notealsothatsharpcornersarenotdesirableinplasticpartsbecausetheyareanimportantcontributingfactortowarpage .

Ribs DesignVeryoften,ribbedpartswillperformmuchbetterintermofcycletime,mechanicalperformancesandwarpagethanverythickunproperlypackedparts .Itiseconomicallyimpossibletopacksectionsabove6–8mm(0 .24–0 .32in)thicknessduringallthecrystallizationtime(solidification:seeFigure 30forHold[pressure]Timevspartthickness) .Howeveranimproperribdesigncouldalsocausedefectssuchassinkmarks .RecommendedribdimensionsareshowninFigure 33 .Notethattheradiusatthebaseoftheribshouldnotbetoosmalltopreserveparttoughness(seeFigure 32) .

Weld LinesWeldlinesoccurwheretwomeltflowsjointogether .Weldlinepositioncanbedefinedbyshortshots,orbyflowsimulations(ifthemolddoesnotexistyet) .Ifthemoldisprovidedwithproperventing(seepage18),theweldlinestrengthshouldbeatleast80–90%ofthenominalstrengthvalueoftheresin .

Tooptimizeweld-linestrength,twoparametersareimportant:

1 . optimumHold(Pressure)Time,toensuretheweldingoftheflowfrontsunderpressure(forthecorrectHPTseepage26);

2 . optimumfillingrate,whichwilldependonpartthickness(approximately1secondpermm(0 .04in)ofpartthickness) .

Figure 34showstheweldlinestrengthofa4mm(0 .16in)thicktestbarinDelrin®100Pgatedatbothends .Bothtensilestrengthandtoughnessareseriouslyaffectediffillingtimeisnotoptimized .

22

Mold MaintenanceAsageneralrule,moldsforprocessingDelrin®requirethesamecareasthoseforprocessingotherthermoplasticmaterials .Wipingthemoldandapplyingarust-preventingsolutionisusuallyadequateafteraproductionrun .

Vent Maintenance

Duetothecriticalnatureofthevents,theventdimensionsshouldbecheckedduringroutinemaintenance .Ventdepthand/orhobbing(deformationofthepartinglineoppositethevent)shouldbecarefullychecked .Ventsshouldbemodifiediftheventdepthislessthan0 .01mmto0 .015mm .Anyhobbingthatblockstheventsshouldbegroundoff .

Mold CleaningDependingonthetypeofdepositthecleaningprocedureisasfollows:

• White depositWhitedepositisduetotheaccumulationofparaformaldehyde .Thisdepositcanberemovedwithbenzylalcoholorisopropanol .Frequentcleaningofthetoolwiththesesolventsduringmoldingwillpreventtheaccumulationofthisdeposit .

• Translucent or colored depositThisdepositisnormallyobservednearthegate(incaseofovershearofthematerial),onpinsornearhotspots .Theuseofalessshearinggate(seegatedesignrecommendations)oramoreevenmoldtemperaturewillstoportremendouslydecreasethebuildupofthisdeposit .Itcanberemovedwithcommercialalkalinechemicalcleaners .Efficiencyofthecleaningagentcanbeimprovedwithanultrasonicbath .

23

Molding Process

InjectionmoldingofDelrin®acetalresinissimilartothatofotherthermoplasticresins .TheengineeringapplicationsforwhichDelrin®isused,however,frequentlyrequiretightspecificationsonstrength,dimensionsandsurfacecondition,sothatcontrolofthemoldingoperationbecomesmorecritical .

Theinformationdiscussedinthissectionincludessuggestionsfor:

• Start-upandshutdownprocedures,handlingprecautions

• OperatingconditionsforDelrin®

• Techniquesforoptimumproductivitymolding

Start-up and Shutdown ProceduresStart-up with Resin Change

Thesuggestedstart-upprocedurewithDelrin®isdesignedtopreventoverheatingoftheresinandcontaminationintheinjectionunitwithmaterialfrompreviousruns .

Tostartupamachinewhichcontainsanotherresin,theinjectionunitmustbepurgedwithcrystalpolystyreneuntilthecylinderandotherhightemperaturezoneshavebeencleared .Thiscannormallybedonewithcylindertemperaturesintherange210–250°C(410–482°F),ifappropriateforthepreviousmaterial .Thenozzleisquitedifficulttocleanbypurging,becausethelaminarflowinthisarealeadstoalayerofpolymerstickingtothemetal(thisisalsotrueforhotrunners) .Itisthereforerecommendedtoswitchoffthenozzleheater,removethenozzle,cleanittogetridalltracesofpreviouspolymer,andreassembleit .Thecylindertemperaturesshouldthenbeadjustedtoabout215°C(419°F),andthenozzletemperatureto190°C(374°F) .Whenbothcylinderandnozzlehavereachedtheexpectedtemperatures,Delrin®canbeaddedtothehopper .

Inunusualcircumstances,anintermediatepurgewithaharshercompoundmayberequiredtoremoveadherentdepositsfromthescrewandcylinder .Specialpurgecompoundsareusedforthispurpose .

ThesepurgecompoundsmustalsoberemovedfromthecylinderbypurgingwithpolyethyleneorpolystyrenebeforeDelrin®isintroduced .Intheworstcases,e .g .,afteruseofglass-reinforcedresinsorseveredegradationofpreviousmaterial,itmaybenecessarytopullthescrewandcleantheequipmentmanuallytopreventcontaminationofmoldings .

Safety point:PolystyreneischemicallycompatiblewithDelrin®,whereasevenatraceofpolyvinylchloride(PVC)isnot .ContaminationofDelrin®withsuchmaterialcancauseobjectionableodororevenaviolentblowback .

Start-Up From a Cylinder Containing Delrin®

Afterasafeshut-downprocedure,thescrewandthecylindershouldbeessentiallyempty .Torestart,thenozzleandcylindertemperaturesshouldbesetat190°C(374°F)topreheatthecylinderandtheresinitcontains .Whenthecylinderhasreachedthesettemperature,ensurethatthenozzleisopenandincreasethecylindersettingstonormaloperatingtemperatures .Whenalltemperaturesareintheoperatingrange,thehoppercanbefilledandmoldingcanbeginafterabriefpurgewithDelrin® .

Shutdown When a Restart with Delrin® is Planned

Shutoffthehopperfeedandcontinuemoldinguntilthecylinderisempty .Forlargemachines(withascrewdiameterabove40mm[1 .57in])itisrecommendedtopurgethecylinderwithcrystalpolystyrene,movethescrewfullyforward,thenswitchofftheheaterbands .Forsmallmachinesmovethescrewfullyforwardandswitchofftheheaterbands .

Shutdown When a Restart with Another Resin is Planned

Shutoffthehopperfeedandcontinuemoldinguntilthecylinderisempty .Purgewithcrystalpolystyrene,leavethescrewfullyforward,thenswitchofftheheaterbands .

Temporary Interruption

AmoldingmachinewithDelrin®inthecylinderatmoldingtemperaturesshouldnotbeallowedtostayidle .Themaximumrecommendedcylinderresidencetime,undernormalmoldingconditions,is10minforpigmentedmaterialand20minfornaturalstandardmaterial .Inexcessofthesetimes,resindecompositionmayoccur .

If,duringthetemporaryinterruption,thecylinderresidencetimereachestheabovelimits,closethehopperfeed,emptythecylinderandleavethescrewforward .Thecylindertemperaturesshouldbereducedtoabout150°C(302°F)(atthesetemperaturesDelrin®willbestableevenforaweekendshutdown) .

Action to Follow When the Nozzle Heater Band Breaks Down

Retracttheinjectionunit,closethehopperandslideitoutoftheway .Ifthenozzleisstillopen,followthenormalshutdownprocedures .Ifthenozzleisfrozen,heatthenozzlewithagastorchtomeltthefrozenmaterialinsidethenozzleandthenpurge .

Start-up after Emergency Shutdown

Adifferentprocedureshouldbeusedafteranemergencyshutdownduetolossofpowerorothercauses .Inthiscase,thescrewmaybefullofDelrin®thatcooledslowlyandwasexposedtomelttemperaturesforaprolongedperiod .ThescrewmayevenbeintheretractedpositionwithalargequantityofDelrin®infrontofthescrew .Inordertoventgasesfromresinthatmaybedegraded,itisessentialthatthenozzlebeopenandheatedtooperatingtemperatureandDelrin®inthisareabecompletelymeltedbeforethecylinderreachesmelttemperature .Thecylinderzonesshouldbeheatedtoanintermediatetemperature

24

belowthemeltingpointofDelrin®andthemachineallowedtoequilibrateatthattemperature .Cylindertemperaturesof150–175°C(300–350°F)aresuggested .Afterallzoneshavebeenatthistemperaturefor30min,cylindertemperaturesshouldberaisedto195°C(380°F) .AssoonastheDelrin®hasmelted,itshouldbepurgedfromthecylinderwithfreshDelrin® .Thepartlydegraded,hotpurgeresinshouldbeplacedinapailofwaterifitemitsanodor .Whentheoldresinispurgedfromthecylinder,thecylindertemperaturesmayberaisedtonormalproductionsettings .

Operating Conditions for Delrin® — Temperature SettingsIntroduction

Thebasicpurposeoftheinjectionunitistodelivertothemoldthenecessaryamountofahomogeneousmelt(nounmeltandnodegradedmaterial) .Therulesofconstructionoftheinjectionunitformoldingacrystallinematerialhavebeenpresentedin“InjectionMoldingUnit,”(seepage7);therulesforthesettingsarepresentedbelow .

Note:Tworoughbutpracticalmethodstoevaluatethepresenceofunmeltandofdegradedmaterialweredescribedonpage11andcanbeusedhereaswell .

Delrin®acetalresinisacrystallinepolymerwithameltingpointof178°C(352°F) .FormostgradesofDelrin®homopolymerthepreferredmelttemperaturerangeis215°C(419°F)±5°C*,asmeasuredwithaneedlepyrometerinthemelt .ThecaloriesneededtoheatandmeltDelrin®willbeprovidedbyshear(fromscewrotation)andthebalancebyconductionintheheatedcylinder(slowheattransferduetotheinsulatingcharacterofpolymers) .

Cylinder temperature

Themainparameterinfluencingthetemperatureprofileofthecylinderistheresidencetime(orHold-UpTime—HUT)ofthepolymerintheplastificationunit(seepage8tocalculateHUT) .

WithashortHUT(<3minutes,shortcycletime,highmeltoutput),higherthannormalcylindersettingsmayberequired .WithalongHUT(>5minutes,longcycletime,lowmeltoutput),lowersettings,especiallyintherearzone,maybeused .Sincegeneralizationofcylindertemperaturesettingsisdifficult,itisoftenwisetobeginwithalevelprofileandadjustasneeded .ThediagramshowninFigure 35canbeusedasaguidelineforinitialtemperaturesettings .

Notes:

1 . AsthepreferredmelttemperatureforDelrin®100STandDelrin®500Tisabout10°C(18°F)lower,thezonesettingsshouldbe10°C(18°F)lowerthanshowninFigure 35 .

2 . HoppercoolingisnotneededandshouldnotbeusedforDelrin® .AsdescribedinChapter3,excessivehoppercoolingmaycreateproblemsofscrewdepositandblackspecks .

3 . Withverysmallinjectionunitsand/orshortresidencetime(HUT),pre-heatingthegranules(e .g .,withaheatedhopper)mayhelptoachieveanhomogeneousmelt .

Nozzle Temperature

Thenozzletemperatureisadjustedtocontroldroolandfreezing(see“Nozzle”page11),butitshouldneverbesetabove190°C(374°F)inordertopreventpolymerdegradation(thelaminarflowandhighviscosityofthemoltenpolymerresultinverylongcontacttimewiththemetalwall) .Ifthenozzlefreezeswithasettingof190°C(374°F),itsinsulationfromthespruebushingshouldbeimproved,oritsinsidediametershouldbeincreasediffeasible .

Notes:

1 . Practically,itisalwayseasiertosetthenozzletemperaturecorrectlybyusingspruebreak .Theinjectionunitispulledbackafterscrewrotationandthenthenozzleisinsulatedfromthecoldmold .Thisallowsthecaloriestoflowtothetipofthenozzlewithouthavingtosettoohighatemperature,andreducestheriskofstringingfromthenozzle .

2 . Hotrunner .Byanalogy,ahotrunnersystemisanozzletransferringthemoltenresinfromtheinjectionunittothepart .Hencetheprinciplesandrecommendationsfornozzles

*ThepreferrredmelttemperatureforDelrin®100STandDelrin® 500Tisabout205°C(401°F) .

Figure 35. Cylinder Temperatures Profile Versus Residence Time for a Given Recommended Melt Temperature. Recommended Nozzle Temperature is 190°C (374°F) for All Delrin® Grades.

Residence time

215

220

225

195

180

FRONT ZONE CENTER ZONE REAR ZONE

< 3 min

> 5 min

Hopper

no cooling or 80–90°C (176–194°F) min

3 5 minRecommendedmelttemperature,°C

25

arealsovalidforhotrunners .Inparticular,thelaminarflowandhighviscosityofthemoltenpolymeragainresultinverylongcontacttimeswiththemetalwall;sothetemperatureofthemetalinthehotrunnershouldneverexceed190°C(374°F),inordertopreventdegradationofthepolymer .

Screw Rotation Speed

Screwrotationspeedbehavesasa“thermalsetting,”becausetherotationofthescrewwill“shear”thematerialandsupplyaroundhalfofthecaloriesneededtomeltandheatDelrin®totherecommendedmelttemperaturerangeof215°C(419°F)±5°C(205°C[401°F]±5°CforDelrin®TandST) .Aswithallpolymers,Delrin®issensitivetoshearandamaximumof0 .2to0 .3m/sofscrewperipheralspeedisrecommended .Figure 36showstheoptimumscrewrotationspeedforhighviscosityDelrin®(type100)andlowviscosityDelrin®(types500to900)asafunctionofscrewdiameter .

Back Pressure

Backpressurealsobehaveslikeathermalsetting .Increasingbackpressureincreasestheworkdonebythescrewonthemelt .

Theuseoftheoptimumscrewdesignforcrystallinematerials,suchasDelrin®,shouldprovidethenecessaryworktomeltandbringDelrin®totherecommendedmelttemperaturewiththeminimumbackpressure .OnlymeltingofhighlyviscousDelrin®suchasDelrin®100Pmayrequiresomebackpressuretoavoidthescrewwormingback(leadingtoinconsistentshotvolumeandpad) .

Figure 36. Maximum Screw Rotation Speed as Function of Screw Diameter. The Curve for Delrin® 500P is Also Valid for the Low Viscosity Grades Delrin® 900.

Screw diameter, mm

Max

. scr

ew ro

tatio

n sp

eed,

rpm

80 1006040200

50

100

150

200

250

300

0

Delrin® 500PDelrin® 100P

Theuseofaninappropriatescrewmayrequiresomebackpressuretoincreasetheworkdonebythescrewonthemelt,toincreasethemelttemperatureanditsuniformity .Higherbackpressuremaybeusedtoeliminateunmeltedparticlesandtoimprovecolormixingwhencolorconcentratesareused .Increasingbackpressure,however,tendstoreduceglassfibrelengthandchangepropertiesoffilledresinssuchasDelrin®570 .Moreimportantly,increasingbackpressurealwaysdecreasestheoutputofthescrew,leadingtolongercycletimesandlowerproductivity .Thisincreasesthebuildupofscrewdepositleadingtocontaminationandlowpartperformance .

Therefore,backpressureshouldbeusedonlywhenincreasingcylindertemperatureorotherchangesarenoteffectiveorpossible .

Forallmaterials,thebackpressureused(specificorinherenttotheinjectionunit)willcreatesomepressureonthemeltinfrontofthescrew .Tocontroldroolattheendofthescrewrotation,somesuckbackisrequired .Thisshouldbekepttoaminimum .

Mold Temperature

ThebestmoldtemperatureforlongtermpartperformancewouldbejustbelowthecrystallizationtemperatureofDelrin®,e .g .,155°C(311°F) .Thistemperaturewouldallowthepolymertocrystallizeinanoptimumstateandeliminateanyriskofre-crystallization(postmoldingshrinkage) .Obviouslyitiseconomicallyimpossibletosetthemoldatthattemperatureasthecrystallizationtimebecomesalmostinfinitealongwiththecycletime .

Practically,alowermoldtemperatureisused,leadingtoshortercrystallizationtime(HPT),henceshortercycletime,lowermoldshrinkagebuthigherpostmoldshrinkage(especiallyifpartsarethenexposedtoelevatedtemperatures) .Acompromiseshouldbefounddependingonthetemperatureinuseandtherequireddimensionalprecisionofthemoldedpartshortandlongterm .

ForstandardDelrin®,amoldtemperatureof80–100°C(176–212°F)isagoodcompromisefornormaluse,givingrelativelyshortcrystallizationtime,highshrinkagebutlowpostmoldshrinkage(see“DimensionalConsiderations,”page31) .Ahighermoldtemperaturewillleadtohighermoldshrinkage,longercycletimebutlowerpostmoldshrinkage .Itisspeciallyrecommendedforhighprecisionpartsusedathightemperature .Alowermoldtemperatureleadstoshortercycletime,lowermoldshrinkagebutmuchhigherpostmoldshrinkageleadingtostressesanddistortion .

FortoughenedresinssuchasDelrin®100STand500T,theuseofalowermoldtemperature(50°C[122°F]±10°C[50°F])isacceptablewithoutendangeringlongtermpartperformances .

26

Note 1:“Moldtemperature”isalwaysthetermusedbuttheimportantparameteristhesurfacecavitytemperature .Withfastcyclingoperations,itmaybenecessarytousealowermoldcoolanttemperaturetomaintainthemoldsurfacetemperatureintherecommendedrange .Chilledwaterisoftenusedforveryshortcyclesortocoolcorepinsandothermoldsectionsthattendtorunveryhot .

Note 2:Coolant:Closedcoolingcircuitsarethemostcommontypesusedtoday .Coolantsforclosedcircuitsneedtoresistheat,freezing,pressureandvacuum .Theyshouldneitherleavedepositsinthecircuit,norcorrodethecoolingchannelsandtubes(tubescanbeinsteel,copper,plastic,rubberetc .) .Byanalogy,thesituationissimilartoautomotiveenginecoolingsystems,andhenceitisrecommendedtousethesamefluid(anti-freeze+corrosioninhibitor)butinlowerconcentration .Initiallythethermalexchangeislessefficientthanwithwater,asthefluidismoreviscousduetotheglycol(morepowerisneededforturbulentflow) .However,forlongtermuse,acoolant(suchasthoseusedincars)isthemosteffectivesolution(nocorrosionordeposit,lowerosionfromcavitation) .

Inthecaseofcoolantsforopentowercircuits,thereisaneedforchemicaltreatmenttopreventbuild-upofmicro-biologicalorganismsthatcouldcausediseaseandrespiratoryproblems .

Operating Conditions for Delrin®—Molding CycleIntroduction

Aspreviouslymentioned,thefactthatDelrin®isacrystallinematerialleadstoamoldingcycledifferentfromthatofamorphouspolymers .ForDelrin®,amoldingcyclegenerallyconsistsofthefollowingphases(seeFigure 37):

A=Mold Open Time .ThisincludestheOpeningTime,theEjectionTimeandtheClosingTime .

B=Filling Time or Injection Time .Moltenresinisintroducedintothemoldina“dynamicfillingphase .”

C=Hold (Pressure) Time .Duringthis“packingphase,”theresinissolidifiedunderpressure,whileadditionalresinisintroducedintothemoldtocompensateforvolumeshrinkageoccurringwithinthemold .

D=Screw Retraction Time .Thescrewrotatesandpreparesnewmoltenmaterialforthenextshot .

E= Cooling Time .Sincethepartiscrystallized(solid)andreadytobeejectedattheendoftheHPT,thereisnoneedforacoolingtime;hencethecoolingtimeisonlytheScrewRetractionTimeplusashortsafetytime .

TheOverallCycleTime(OAC)forDelrin®istheadditionofthevarioustimessetforeachofthemoldingoperations .

Note:FrequentlythesumoftheFilling(Injection)TimeandtheHold(Pressure)TimeisdefinedasScrewForwardTime(SFT),asoftenstatedinpreviousDelrin®literature .

ThecycleestimationgraphinFigure 38showsarangeoftotalcycletimesthathavebeenusedforgoodqualitymoldingofDelrin®inpartsofvariousthickness .TheactualcyclewillbeclosetothelowerlimitwhenahighproductivityresinsuchasDelrin®900Pisusedandwhenend-userequirementsarelessstringent .

B. Filling time

E.Cooling time

C. Hold (pressure)time

A. Mold opened

D. Screw retraction time

Figure 37. The Molding Cycle for Delrin®

Figure 38. Estimation of Overall Molding Cycle Times for High Quality Molding of Delrin® Parts

Wall thickness of the part, mm

Tota

l cyc

le ti

me

OA

C, s

3 4 5 61 20

30

50

70

90

110

10

In this arearisk of porosity,voids, warpage

27

Filling Phase

Injection Time

Theoptimumfillrateforamolddependsonthepartgeometryandthickness,therunnersize,thesizeandlocationofthegate .

Asruleofthumb,afillingtimeof1secpermm(0 .04in)ofpartthicknessisagoodstartingpointforfillspeedsetting .Thesurfaceaspectwillgovernthisadjustment .Higherandmoreuniformsurfaceglosscanbeobtainediftheinjectionrateisfastenoughtoallowthecavitytobefilledbeforetheresinbeginstosolidify,althoughlocalizedsurfaceflaws,suchasjettingandgatesmear,areoftenreducedbydecreasingtheinitialinjectionrate .

Ifmaximumparttoughnessisrequiredfortheapplication,theshearappliedtothematerialintherunner(s)andpartshouldbecheckedtoensureoptimummoldingperformanceandpartproperties .Figure 39showstheimpactperformanceofa2mm(0 .08in)partversustheshearduringfilling .Ifneeded,pleasecontactyourDuPontrepresentativetoanalyzeyourspecificcase .

Note:Minimizingtheshearinthegatecanalsobeanimportantfactortowardsoptimumpartperformance .

Withnon-optimumgatedesigns(conical,excessivegatelength),shearinthegatemaybecomeanimportantlimitingfactorforparttoughness .

Withtheoptimumgatedesignpresentedinthemolddesignsection(dimensionsthatallowoptimumpackingduringcrystallization,gatelength<0 .8mm),inmostcasestheshearatthegatehasnoeffectonpartperformance .Flowanalysisshouldbeperformedtochecktheshearatthegateformoldingverylargepartsonly .

Filling time, s3 4 5210

0

Impact resistance, J/m-2

Shear rate at wall, s-1

Impact

Delrin® 500

Figure 39. Shear Strain Rate at the Wall (s–1) and Impact Resistance (J/m2) as Function of the Filling Time. This Data was Obtained with the Sample Shown (180 by 27 mm with 2 mm thickness). For Impact Resistance, the Part is Clamped Under the Rib and Hit by a Pendulum.

Injection Pressure

Thisterminologyoftenleadstomisunderstanding .

Theso-called“injectionpressure”servestomovethescrewandpushthematerialinthemold .Duringthisdynamicfillingphase,thepressurebuiltupinfrontofthescrewisonlyequaltothepressuredropinthemold,fromthenozzletothepositionoftheflowfront .Thereisnopressureattheflowfrontitselfduringthisdynamicfillingphase .

Beforethefrontoftheflowreachestheendofthemold(whenaround95%ofthepartvolumeisfull),themachineshouldswitchfromdynamicfilling(undervelocitycontrol)toquasi-staticfeeding(controlledbytheHOLDpressure) .ThisistheV-Pswitchingpoint .Theholdpressurewillthenbeappliedeverywhereinthemoldduringthewholepackingphase .Foracrystallinematerial,morematerial(≈14%forDelrin®)shouldbeaddedtotheparttocompensatethecrystallization,leadingtoaslowscrewforwardmovementduringHold(Pressure)Time .

Withthatdefinition,theinjectionfillpressurecanbesettowhatevervalueneededbythegeometryofmold(includingrunners),aslongasthefillingrateisadequateforthepartperformance .

IncasetheV-Pswitchisincorrectlyset(noswitchortoolateofaswitch),theinertiaofthesystemwillcreateapressurepeakattheendoffilling,leadingtomolded-instressesandflash .Forthatreason,inmostpracticalcasesitissafertosettheV-Pswitchpointwithdistanceratherthanwithpressure(aswouldtypicallybedoneforanamorphousmaterial) .

Packing Phase

Hold (Pressure) Time (HPT)

TherecommendedHold(Pressure)Time(HPT)forDelrin®isthetimeforthemoltenpolymertofullycrystallizeinthemoldcavity .

Asthecrystallization(solidification)leadstoalargevolumedrop(≈14%,seepage4),moremeltedmaterialhastobepushedintothecavityduringalltheHPT .Thisleadstospecialdesignrulesforthegateandrunners,asdiscussedintheMoldssection,sothatthegatewillnotfreezebeforethecavityisproperlypacked .

TheHPTisobviouslyafunctionofthepartthickness .Figure 40showstheoptimumHPTforDelrin®500Pasafunctionofthepartthickness(withtherecommendedHPTof85MPa(12 .3kpsior12,300psi)andtherecommendedmoldtemperatureof90°C[194°F]) .

Note:FortheDelrin®ElevenSeriesresins,theenhancedcrystallizationleadstoashorterHPTofup10% .

28

TochecktheefficiencyoftheHPTforagivenpartgeometry,thetraditionalmethodistoplotthepartweightasafunctionoftheHPT .ThemaximumpartweightshouldcorrespondtotheoptimumHPTthatcanbereadinFigure 40forthepartthicknessatthegate .Atthistime,thepartissolidifiedandnomorematerialcanbeaddedtothepart .Asanexample,Figure 41showstheeffectofHPTonpartweightfora4mmthickISOtestbar .Figure 41alsoshowstheevolutionofpartshrinkagewiththeHPT,whichwillbediscussedinmoredetailin“DimensionalConsiderations”(seepage31) .

AnothertechniquetodefineoptimumHPT,usinginstrumentedmolds,hasbeendevelopedandispresentedasanappendixlaterinthisguide .

AlltheaboveconsiderationsonHPTanditseffectsassumethatthenonreturnvalvefunctionsproperlyandmaintainsacushionofmeltinfrontofthescrewasdiscussedearlierinthe“InjectionMoldingUnit”section .

TooshortorinefficientHPTleadstohigherthannormalanduncontrolledshrinkage .Additionalside-effectssuchasvoids,porosity,warpage,sinkmarksshouldbeexpected(see“DimensionalConsiderations”) .

Hold Pressure

OptimumholdpressuresforDelrin®acetalresinslieinarangeof60–110MPa(8 .7–16kpsi)toachieveanhomogeneouscrystallization .Ifhigherorlowerpressuresareusedinspecialconditions,theytendtoleadtolowerpartperformance .ThefollowingtableshowstheholdpressurerangerecommendedforthevariousDelrin®types .

Hold pressure, Resin type Grades of Delrin® MPa (kpsi)

Highviscosity 100,100P,111P 90–110(13–16)

Medium-and 500P,511P,low-viscosity 900P 80–100(11 .6–14 .5)

Toughened 100ST,500T 60–80(8 .7–11 .6)

Toobtainahomogeneouscrystallization,theholdpressureshouldremainconstantuntilthepartisfullypacked(solidified) .

Clamping Force

Thisdoesnotreallybelongtothedescriptionofthemoldingcycle,butitisdirectlycorrelatedtotheholdpressureandforthatreasonitisdiscussedhere .

Theclampingforceistheforcerequiredtokeepthemoldclosedduringfillingandhold(pressure)time .Thisforceiscalculatedbymultiplyingtheprojectedareaofthecavity(cavities),includingrunnersystem,bythemaximuminternalpressure(theholdpressure) .

Commonly,moldsaresetusingthemaximumclampingforceofthemoldingmachine .Howeverinmanycases,themachineusedhasamuchhigherclampingforcethanactuallyneeded .Intheseconditions,itisrecommendedtolowertheclampingtotheforceactuallyneededbythemold(seecalculationfollowing) .Thiswillpreventexcessivepressureatthepartingline(compressionofthevents,hobbingofthepartingline,deformationofthemoldcomponents),leadingtolongerlifetimeofthemoldandlesscostlymoldmaintenance .

Estimatingthemaximuminternalpressurecanbedonebycarryingoutaflowanalysis .However,forpartswithaflowlengthtothicknessratiolessthan100to1,normallytheinternalpressureisequaltotheholdpressure .

Figure 40. Hold (Pressure) Time Versus Part Thickness of Delrin® 500P

Figure 41. Hold (Pressure) Time Versus Part Weight and Mold Shrinkage of Delrin® 500P

Hold (pressure) time for part crystallization, s

Part

thic

knes

s, m

m

Part

thic

knes

s, in

10060 8040200

Mold temperature, 90°C (194°F)

1

2

3

4

5

6

7

0

0.280

0.200

0.100

0.040

Hold (pressure) time, s

Part

wei

ght,

g

Mol

d sh

rink

age,

%

40 50 6020 30100

17.2

17.4

17.0

16.8

16.6

Test bar thickness, 4 mm

16.4

16.2

16.0

3.2

3.0

2.8

2.6

2.4

2.2

2.0

15.8

29

Thefollowingguidelinescanbeused:

1 . Forpartsneedingoptimummechanicalproperties,thespecificclampingpressuremustbe1ton/cm2forDelrin®100P,and0 .85ton/cm2forotherDelrin®grades .

Examplecalculation:

• projectedareaofpart(orparts),includingrunnersystem=115cm2

• material=Delrin®500

• machineclampingforcerequired=115×0 .85=98tons

2 . Forpartsnotrequiringoptimummechanicalproperties,itmaybepossibletomoldacceptablepartswithlowerspecificholdpressures(andlowerclampingforces) .

Plastification Phase

Screw Retraction Time

Givenafixedamountofresintoplasticizeforthenextshot,thescrewretractiontimeisdirectlydependentonthescrewrotationspeed .

Itiscrucialtocheckthattheappliedscrewrotationspeedislowenoughtoavoidover-shearingtheresininthebarrel(whichmayleadtodegradation),buthighenoughtoprovideahomogeneousmelt(withnounmeltedparticles) .Thiscanbedonewiththetwopracticaltestsforthepresenceofunmeltanddegradedmaterial,asdescribedonpage11 .

Note:sinceDelrin®isahighlycrystallinepolymer,itsthermalrequirementsaredifferentfromthoseofamorphousmaterials .ScrewsspecificallydesignedforDelrin®andanappropriateratioofshotweighttomachinecapacityprovideanefficientplastification .Formoredetailsaboutscrewdimensions,seepages8–9 .

Cooling Time

Thecoolingtimeisanimportantparameterfortheinjectionmoldingofamorphouspolymers .ThesituationiscompletelydifferentwithDelrin®(seealsopages5–6) .Attheendofacorrectlysetandefficientholdpressuretime(HPT),theDelrin®partiscrystallizedandsolid .Thereisnoneedforfurthercoolingtime,andthepartcouldinprinciplebeejectedimmediatelyfromthemold .ThiscanbedemonstratedbystoppingthecycleattheendoftheHPTandejectingthepartimmediately .

Inmostpracticalcasesthepartisejectedafterthescrewretractiontime,sothecoolingtime(asdefinedinFigure 37)issimplythescrewretractiontimeplusasmallsafetymargin .Anexceptionisthecaseofmachineswithshut-offnozzles,wherepartejectioncantakeplaceduringthescrewrotation .Thistheoreticallygivesshortercycles,althoughpracticalproblemsmayariseandlimitproductivity(seepage11formoredetailsonshut-offnozzles) .

Optimum Productivity MoldingEconomicconstraintsarepushingforlowerpartcost,whichcanbereachedbyincreasingtheyieldofqualitypartsand/orshorteningtheOverAllCycletime .Thisguiderecommendstheparameterstoachieveoptimumpartpropertiesintheshortandinthelongterm,leadingtoanoptimumOverAllCycletime(OAC) .

Anymodificationtothecycleshouldbedoneonlyafterrealisticevaluationofpartperformanceshortandlongterm .Decreasingthecycletoomuchmayleadtoa)lowerpartpropertiesandotherqualityproblems(especiallyshrinkage,warpageandpostshrinkage),andb)processnotrunninginarobustarea,whichcouldleadtoloweryieldofqualitypartsandhigherpartcost .

BeforetryingtoshortenthecurrentOAC,thefollowingitemsshouldbeconsidered:

• Thedesignofthepartmaynotbeoptimum,i .e .,thepartmaybetoothick .Changestothedesign(addingribs,useofpins)arecostlybutmayallowsignificantreductionofthecycletime .

• Thedesignofinjectionunitmaynotbeoptimum .WithDelrin®thecoolingtimecanbeminimizedtothetimerequiredforproperscrewretraction .Optimumscrewsizeanddesignwillfacilitatethis .

• Thesortingofpartsfromrunnersmaynotbeoptimized .

HavingdecidedtodecreasetheOAC,thefollowingactionsmaybecarriedout(inorderofincreasingrisk):

• Investigateobviousbottlenecksinthecycle .

• MinimizetheMoldOpenstroke .

• MinimizetheMoldOpentimebyincreasingtheopening/closingspeeds .Rubberbumpersorspringscanbeusedtopreventbangingofthefloatingplatein3platemolds,givingnoeffectonpartquality .

• Minimizetimebetweenthescrewstoppingandthemoldopening .Noeffectonpartquality .

• MinimizeFillingTime(fasterinjectionfill) .Itmayresultinovershearinganddecreasedweldlinestrength .Largernozzleandrunners,aswellasimprovedventingmayberequired .

• DecreaseScrewRetractionTime:

1 . Usealargerscrewandlimitthestroketobetween1and2screwdiameters,noeffectonpartquality .

2 . UseanoptimizedscrewdesignforDelrin®(screwforcrystallinepolymerswithcorrectdepthofmeteringzone,mixinghead) .Thisensuresahomogeneousmeltevenathighscrewrotationspeeds,hencetherearenoeffectsonpartquality .UsinghigherscrewrotationspeedswithageneralpurposescrewwouldreduceSRT,butwiththeriskofpoormeltqualityandpartfailure .

Note:asthereisnoneedforcoolingtime,shutoffnozzles(wherethescrewcanberotatedduringmoldopening)havebeentried .Unfortunately,problemssuchaswear,contamination,holdupspotsetc .havebeenobserved,andnolong-termsatisfactorysolutionwasfound .

30

• DecreaseHold(Pressure)Time .WithalowerthanoptimumHPT,highermoldshrinkageanddeformationleadingtowarpagewillbeseen .Voidswillalsobeformedinthecenterofthepart,leadingtolowermechanicalproperties(lowerelongationatbreak),andqualitycontrolshouldbecarriedoutonlargerlotsofmoldedparts .IfthemoldtemperatureisdecreasedasanattempttocompensatefortheshorterHPT,thisactionwillleadtoalowermoldshrinkagebutwillresultinveryhighpostmoldingshrinkage,deformationandwarpage .

Standard Molding Conditions for ISO Tensile BarsStandardprocessingparameterstoinjectionmoldDelrin®intotensilebarsISO294-1areshowninTable 5 .TheycanhelpmoldersinestablishingmoldingparameterswhenprocessingDelrin®acetalresins .However,itmustbeemphasizedthatforpartsofdifferentshapesanddimensionssuchparametersshouldbemodifiedusingtheinformationpresentedonpages29–34 .

Hold Pressure Time via In-cavity Pressure MeasurementThistechniquehasbeendevelopedduringrecentyears,particularlyforamorphousresins .Themainobjectivewastooptimizeandcontroltheholdpressureprofileinordertoreduceinternalstresses,whichhavebeenafrequentcauseoffailureofmoldedarticlesinamorphouspolymers .

EvenifsuchproblemsofinternalstressesdonotapplytoacrystallinepolymerlikeDelrin®,thistechniqueisprovingtobeaneffectivemethodtodeterminethecrystallizationtime(HPT)ofapartmoldedwithaspecificpolymergradeatgivenprocessingparameters .

AflexibledataacquisitionsystemhasbeensetupbyDuPont .ItconsistsofacomputerwithadataacquisitioncardandproprietarysoftwareCAVAN(CAVityANalysis),andallowsallavailableanalogsignals(e .g .,injectionfillspeed,hydraulicpressure,etc .)tobeacquired,displayedandanalyzed .Thesystemmeasuresthecrystallizationtimeofeachcyclewithaprecision,dependingonthesensorlocation,downto0 .1sec .

AsinglepressuresensorclosetothegateisusuallysufficienttodeterminethecrystallizationtimeofaDelrin®part .Thisisdonewithinasinglemoldingcycle,byanalysingthepressurechangesduringthepackingphase .Figure 42showsatypicalCAVANcurvefromwhichtheHold(Pressure)Timeofa2mm(0 .08in)thickDelrin®partcanbedetermined .

Table 5 Processing conditions for ISO 294 tool (insert type A)

Delrin® 100, Delrin® 500P, Resin grade 100P, 111P 511P, 900P Delrin® 100ST Delrin® 500T

Medium and Super tough Toughened High viscosity low viscosity high viscosity medium viscosity polyacetal polyacetal polyacetal polyacetal Characteristics homopolymer homopolymer homopolymer homopolymer

Pretreatment         Moisture level for processing  <0.2%  <0.2%  <0.05%  <0.05%  Drying temperature, °C (°F)  80 (176)  80 (176)  80 (176)  80 (176)  Drying time, hr  2 hr  2 hr  4 hr  4 hr

General parameter         Type of screw  HC screw  HC screw  HC screw  HC screw  Max. screw tangential speed, m/s  0.2  0.3  0.15  0.3  Melt temperature, °C (°F)  215 (419) ± 5  215 (419) ± 5  205 (401) ± 5  205 (401) ± 5  Mold temperature, °C (°F)  90 (194) ± 10  90 (194) ± 10  50 (122) ± 10  50 (122) ± 10  Hold pressure, MPa  90–110  80–100  60–80  60–80  Back pressure, MPa  <1.0  <0.25  <1.0  <0.25 

Specific parameters, insert A         Injection fill time, s  1–5  0.5–2  0–5  0.5–2  Flow front velocity, mm/s  40–200  100–400  40–200  100–400  Hold (Pressure) Time, s  35–45   35–45  25–35  25–35  Cycle time, s  40–60   40–60   35–50   35–50

Time, s

00

In-c

avity

pre

ssur

e in

MPa

20

40

2 4 16 1812 148 106

20

80

60

Hold (Pressure) Time

Dynamicfillingtime

Single pressure transducer Delrin® 500 wall thickness: 2 mm

Figure 42. Cavity Pressure Measured During the Filling and Packing (1 sensor)

31

Mold ShrinkageMoldshrinkageistheshrinkagethatoccurswithin24hrofmolding .Itisdefinedasthedifferencebetweencavityandactualpartdimension,bothmeasuredatroomtemperature .ItisduetothedifferenceinspecificvolumeofDelrin®atthecrystallizationtemperatureanditsspecificvolumeatroomtemperature(see“PVTDiagrams,”pages5,6) .

ThetypicalmoldshrinkageofDelrin®resinsisbetween1 .7and2 .2%,exceptforthesupertoughandfiber-containinggrades(Delrin®100ST,500AF,570)whichhavealowershrinkage .Table 6summarizestheaveragemoldshrinkageofa2mm(0 .08in)thickpartmoldedinthespecificrecommendedconditions .Thesevaluesshouldbeconsideredasanapproximateguideonly,becausetheshrinkageforanactualpartdependsonitsdesignandonthemoldingconditions,asdescribedinmoredetailbelow .

Table 6 Average Mold Shrinkage for Various Grades of Delrin®

Average mold shrinkage

Delrin® grade Parallel (% ± 0.2%) Normal (% ± 0.2%)

100P, 111P, 127UV 2.1–2.2 1.9

311DP, 327 UV 1.9 1.8

500P, 527UV 2.0–2.1 1.9–2.0

511P 1.8 1.7

900P 1.9 1.9

colors* 1.8–2.1 1.7–2.0

500T 1.7 1.7

100T 2.1 1.9

100ST 1.3 1.4

500TL 1.8 1.7

500AF 2.0 1.4

500CL, 500AL 1.8–1.9 1.7–1.8

570 1.2 2.1

510GR 1.0 1.4

525GR 0.4 1.2

* depends on the pigments

Factors Affecting Mold ShrinkageMoldshrinkageisdependentonthefactorsthataffectthecrystallinityofDelrin® .Theseinclude:

• holdpressure

• hold(pressure)time

• moldtemperature

• partthickness

• gatedimensions

Dimensional Considerations

Delrin®acetalresinshavegooddimensionalstability,comparedtootherpolymers,overawiderangeoftemperaturesandinthepresenceofmoisture,lubricantsorsolvents .Theyfindextensiveuseinindustryforthefabricationofprecisiongears,bearings,housingsandsimilardevices,becauseoftheiruniquecombinationofdimensionalstabilitywithotherproperties,suchasfatigueresistanceandtensilestrength .However,aswithallmaterialsofconstruction,therearefactorsaffectingthedimensionalstabilityofDelrin®whichmustbeconsideredwhenclosetolerancesareessential .

Fundamentals of Dimensional ControlThedimensionsofamoldedpartaredeterminedprimarilybythedimensionsofthecavity,andsecondlybyallthosevariablesthataffectresinpackingandcrystallinity(forexampleholdpressure,HPT,moldtemperature) .Itmayseemobvioustomentioncavitydimensionsasthemainfactorforpartdimensions;howeverexperiencehasdemonstratedthatdimensionalproblemsareoftenaddressedbychangesinmoldingconditions,generallywithalimitedsuccess .Isotropicdimensionalproblemscaninprinciplebecorrectedbychangestoholdpressure .Inthemorefrequentcaseswhereafewdimensionsareoutofspecification,attemptstocorrectwiththemoldingparametersgenerallygreatlyreducetheacceptableprocessingwindow,leadingtoahigherriskofrejects .

Moldshrinkageandpost-moldshrinkageoccurasnaturalconsequencesofthemoldingprocess .Theyinfluencethetolerancesthatcanbeobtainedformoldedparts .Dataontheseeffectsarepresentedinthissection .

FurtherdimensionalvariationsinmoldedpartsofDelrin®canarisefromchangesinthetemperatureornatureofthesurroundings .Reversiblechangesresultfromthermalexpansionorcontractionandfromabsorptionofwaterorothersolvents .Thesearediscussedlaterinthissection,under“EnvironmentalChanges .”

Irreversiblechangesindimensionoccurwhenpolymerchainsfrozeninanunstableconditionmovetowardsamorestablestate .Anexampleiswhenpartsmoldedinatoolatlowmoldtemperatureareexposedtoelevatedtemperatures .Thesechangesarediscussedunder“Post-MoldShrinkage”and“Annealing .”

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Table 7summarizestheeffectoftheseparametersonmoldshrinkage .Theyarediscussedinmoredetailbelow .

Furthermore,moldshrinkageisalsohighlydependentonthegeometryofthepartandontheflowpatternoftheresin .Experimentshavebeendoneinourlaboratorywith180×27mmplaqueswiththicknessesfrom1 .5to6mm .Fourvaluesofshrinkageweremeasured,closetoandfarfromthegate,parallelandperpendiculartotheflow .FormostDelrin®gradesitisobservedthattheshrinkageishigherfarfromthegatethanclosetothegate(typicallyby0 .1to0 .3%),andthattheshrinkageintheflowdirectionisabout0 .1%higherthantransversetotheflow .

Hold Pressure

Injectionpressurehastwofunctionsinthemoldingprocess:

1 . Transferthemoltenpolymerfromtheinjectionunitintothemold .This“injectionfillpressure”isneededonlytoovercometheresistancetoflowofthepolymerfromtheinjectionunittothecavity .Usuallythisisahighspeedprocess(dynamicphaseofthescrew) .

2 . Controlthepackingandcrystallizationprocess .Theholdpressurepushesmorematerialintothecavitytocompensateforthevolumereductionthatoccursinthepolymerduringcrystallization .Thisisalowspeedprocess(slowmotionofthescrew) .Thisphaseismoreimportantforthedimensionalstabilitysinceithelpsmaintainauniformandgradualcrystallization .Whenalowerholdpressureisused,itwillpacklessmaterialintothecavityandtheshrinkagewillbehigher .ThisisshowninFigure 43forthreemoldtemperatures .

Smallchangesofholdpressuremaybeusedtohelpfinetunethedimensionsofapart,becausethisparameterisessentiallyindependentandhasrelativelysmalladverseeffects .

Notethatholdpressureshouldbeconstantduringthewholepackingtime .

Hold (Pressure) Time (HPT)

Hold(Pressure)Timeisthetimeduringwhichtheholdpressureisapplied .TheHPTisimportantforthevalueofshrinkageanditsuniformityoverthepart .

Figure 44showstheeffectofHPTonmoldshrinkageforDelrin® .

WhentheHPTisbelowtheoptimumvaluerequiredforthespecificpart(seepages27,28),thepackingprocessisinterruptedbeforecompletionandmoldshrinkageishigherthanitsoptimumvalue .Additionalside-effectsofashortHPTareporosity,voids,warpage,sinkmarks,lowermechanicalproperties .

Table 7

Effect on Parameter Shrinkage Remarks

Hold (Pressure) Time   ➘  up to optimum HPT,  (HPT)    then no effect

Hold Pressure  ➘

Mold Temperature (cavity)  ➚  but post-mold shrinkage ➘

Part Thickness  ➙ or ➘  if all settings optimized

Gate Thickness  ➘  up to optimum thickness,      then no effect

Melt Temperature  ➙  if mold temperature is kept      constant and HPT is optimized

Key parameters affecting mold shrinkage: Symbol ➚ means that the shrinkage increases  when the value of the parameter increases, and the opposite for the symbol ➘. Symbol ➙ means that there is no effect on shrinkage provided that the conditions listed  under “Remarks” are met.

Hold pressure, MPa

Mol

d sh

rink

age,

%

140100 12080

1.6

1.7

1.8

1.9

2

2.1

2.2

2.3

1.5

Part thickness 4 mm (0.16 in)Melt temperature 215°C (419°F)

HPT optimized for thickness and mold temperature

Mold temp. = 100°C (212°F)HPT = 43 s

Mold temp. = 90°C (194°F)HPT = 39 s

Mold temp. = 80°C (176°F)HPT = 36 s

Figure 43. Effect of Hold Pressure on Mold Shrinkage at Three Mold Temperatures, for Delrin® 500P. Hold Pressure Can be Used for Small Adjustments of Part Dimensions, as it has Negligible Effect on Post-Molding Shrinkage.

Figure 44. Effect of Hold (Pressure) Time on Mold Shrinkage of Delrin® 500P

Hold (Pressure) Time HPT, s

Mol

d sh

rink

age,

%

10 20 30 40 50 60 70 80 900

2

2.2

2.4

2.6

2.8

3

3.2

3.4

1.8

6 mm

4 mm

1.5 mm

Optimum HPT1.5 mm 10 seconds 4 mm 40 seconds 6 mm 80 seconds

Molding conditions:Hold pressure 90 MPa (13 kpsi)Mold temperature 90°C (194°F)Melt temperature 215°C (419°F)

33

Onthecontrary,anyincreaseofHPTaboveitsoptimumvaluewouldhavenoeffectonmoldshrinkage,becausethepart(andthegate)arealreadysolidified .

Mold Temperature

Moldtemperatureinfluencesmoldshrinkagethroughitseffectoncoolingrateandcrystallizationtemperatureofthemoltenpolymer .TheeffectofmoldtemperatureonshrinkageisalsoshowninFigure 43 .

Athighmoldtemperatures,thepolymercrystallizesslowly .Insuchconditionsthemoldshrinkageishigh,butsincethecrystallizationismorecomplete,abetterlong-termdimensionalstabilityistobeexpectedforthemoldedparts(lesspost-moldshrinkage) .

Lowmoldtemperatures,ontheotherhand,tendtocoolthepolymerataveryhighrate .Thisresultsinalowermoldshrinkageandbettertoughness .However,inthelongterm,higherdimensionalvariationsleadingtobuildupofinternalstresseswilloccur,particularlyifthepartisexposedduringitsend-uselifetotemperaturesexceedingthemoldtemperatureatwhichthepartwasmolded .

Part Thickness

AsshowninFigure 44forDelrin®500P,thethicknesshasaminorinfluenceonmoldshrinkage,providedthatthegatedimensionsandtheHold(Pressure)Timearecorrectforeachthickness .Figure 45showstheshrinkageofvariousDelrin®compositionsvs .partthickness,asmeasuredwithcorrectHPT .Notethat,tooptimizetoughness,themoldtemperatureisreducedfrom90°C(194°F)forthestandardgradesto50°C(122°F)forthetoughenedgrades(withoutleadingtoahighpost-moldingshrinkage) .

Forpartsofuniformwallthickness,themoldshrinkagetendstobeuniform .Inthecaseofvariablethickness,shrinkagewilltendtobenearlyuniformifthepartisgatedintothethickestsection,ifthegateisproperlysizedandiftheHold(Pressure)Timeequalsorexceedsthegatefreezetime .Whenthesecriteriaarenotmet,themoldshrinkagetendstobegreaterforlargersections,withpossibleproblemsofvoids,warpage,sinkmarksandlowermechanicalproperties .

Gate Dimensions

Adequategatedimensionsarerequiredtoensureproperpackingofthepart(see“Gates,”pages14–16) .

Whenthethicknessofthegateissmallerthanitsoptimumvalue,moldshrinkagewillincreaseduetotheprematuresolidificationoftheresinatthegate .ThissituationisthenequivalenttoashorterHold(Pressure)Time,andtheapproximateeffectonshrinkagecanbeobservedinFigure 44 .Inthisrangethemoldshrinkageisnotstable,anditisverydifficulttocontrol .Theresultingwarpagecouldevenmakedifficultthemeasurementofcertaindimensionsofthepart .

Figure 45. Average Mold Shrinkage Versus Thickness, for Various Delrin® Compositions

1.6

Mol

d sr

hink

age,

%

Thickness, mm0

1.2

1

2.2

2

82 4

100P, 500P

500T

100ST

6

1.4

1.8

Melt Temperature

Melttemperaturehasaneffectonmoldshrinkage .Itishoweverlimitedbythenarrowrangeofmelttemperaturesneededtomaintainaconsistentqualityofthemoldedpart .Consequently,themelttemperatureshouldnotbeconsideredasavariabletoadjustmoldshrinkage .

Mold Shrinkage of Filled ResinsThemoldshrinkageofcompositionscontainingfibrousfillers,suchasDelrin®570(glass)andDelrin®500AF(Teflon®),islesspredictable,becauseofthefiberorientationeffects .Theshrinkageinthedirectionofflowtendstobesignificantlydifferentfromthatinthetransversedirection(seeFigure 45) .

Ingeneral,themoldshrinkageofDelrin®500AFintheflowdirectionissimilartothatofDelrin®500P .Themoldshrinkageinthetransversedirection,however,rangesupto50%oftheshrinkageofDelrin®500P .

Incontrast,themoldshrinkageofDelrin®570intheflowdirectionisabouthalfofthatforDelrin®500P .Inthetransversedirection,themoldshrinkageofDelrin®570approachesthatofDelrin®500P .

Effect of PigmentsThepresenceinthemeltofcrystallizationnucleisuchaspigmentsandregrindcanhaveaninfluenceoncrystallizationandconsequentlyonmoldshrinkage .

AnaccuratestudyhasbeencarriedouttoevaluatetheeffectofvarioustypesofpigmentsonthemoldshrinkageofDelrin® .Itappears,asdepictedinFigure 46,thatpigmentsystemsgivingthesameresincolormayhaveadifferenteffectonmoldshrinkageandpartdimensions .

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Note:Thisstudyhasbeencarriedoutonstandardbarsandintypicalmoldingconditions .Theshrinkagevaluesshownhereshouldnotbeconsideredvalidforallpartsofdifferentgeometryand/ormoldedindifferentmoldingconditions .

Post-Molding ShrinkagePost-moldingshrinkageisdefinedastheshrinkagewhichtakesplacemorethan24hraftermolding .Itisaconsequenceofcontinuedcrystallizationandrelaxationofmolded-instresses,wheretheresinmovestowardsamorestablestate .

Thepost-moldshrinkageofpartsmoldedinDelrin®canbeestimatedfromFigure 47 .

Partsmoldedwiththerecommendedmoldtemperature(90°C[194°F])orhigherwillhavealowpost-moldshrinkage,whichensuresgooddimensionalstabilityoverthelifetimeofthepart .

However,partsmoldedwithacoldmold(<80°C[<176°F])willhaveahigherpost-moldshrinkage,becausefastcoolingleavestheDelrin®inanunstablecrystallinestateandresultsinmoresignificantrecrystallization .IfsuchDelrin®partsarethenexposedtohightemperatures,therecrystallizationcausesahighandrapidpost-moldshrinkage .

Remarks:

1 . Forpartsrequiringtighttolerancesandexposuretoelevatedtemperaturesforprolongedperiodsoftime,itisstronglyrecommendedtousehighmoldtemperatures(upto120°C[248°C]) .Thisprovidesamoreeffectivesolutionthanannealingapartmoldedatlowmoldtemperature .

2 . Forexposureatmoderatetemperatures,gooddimensionalstabilityandpartperformancecanbeachievedusinga90°C(194°C)moldtemperature .

Color

Shri

nkag

e, %

NC-

10

Yello

w

Beig

e

Mau

ve

Whi

te

Red

(1)

Blue

(1)

Blue

(2)

Blac

k

Purp

le

Gree

n (1

)

Gree

n (2

)

Pink

Brow

n

Red

(2)

1.80

1.90

2.00

2.10

2.20

1.70

Figure 47. Post-Molding Shrinkage of Delrin® Acetal ResinsFigure 46. Effect of Selected Pigments on Mold Shrinkage of Delrin® 500P. Part Thickness 2 mm

1.0

Post

-mol

ding

shr

inka

ge, %

Exposure temperature, °C (°F)

50 80 11020

0.2

0

1.2

1.4

1009040 706030

0.4

0.6

0.8

40°C mold

65°C mold

95°C mold

120°C mold

AnnealedAll mold temperatures

0.6

Post

-mol

ding

shr

inka

ge, %

Exposure temperature, °C (°F)

50 80 10020

0.2

0

1.0

11060 70 9030 40

0.4

0.8 40°C

mold

65°C

mold

95°C m

old

120°C mold

AnnealedAll mold temperatures

Thickness – 0.8 mm (0.03 in)

Thickness – 1.6 mm (0.06 in)

0.7

50 80 11020

0.4

01007060 9030 40

0.5

0.6

0.3

0.1

0.2

Thickness – 3.2 mm (0.125 in)

Exposure temperature, °C (°F)Exposure time = 1,000 hr

Post

-mol

ding

shr

inka

ge, %

40°C mold

65°C mold

95°C mold

120°C mold

AnnealedAll mold temperatures

(104) (176)(68) (212)(140)(122) (194)(86) (230)(158)

(104) (176)(68) (212)(140)(122) (194)(86) (230)(158)

(104) (176)(68) (212)(140)(122) (194)(86) (230)(158)

35

Chan

ge in

leng

th, %

12040 8020 100600(248)(104) (176)(68) (212)(140)(32)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

Immersion

100% RH

80% RH

50% RH

0% RH

Temperature, °C (°F)

Figure 48. Environmental Dimensional Change of Delrin® 100 and 500

Insert MoldingAlmostalltheproblemsofinsertmoldingarelinkedwithshrinkagearoundtheinsert,moldshrinkageandpost-moldingshrinkage .Tominimizetotalshrinkage,thefollowingshouldbetakenintoconsideration:

• Highmoldtemperaturesshouldbeused(90°C[194°F]orabove)inordertominimizethetotalshrinkage(sumofmoldshrinkageandpost-moldshrinkage) .Atlowertemperaturesthemoldshrinkageisindeedsmaller,butthepost-moldshrinkageismuchhigher .

• OptimumHold(Pressure)Timeforthepartthickness,tominimizepartshrinkage .TheshrinkageincreasesdramaticallywithshorterHPT(seeFigure 44) .

• Insertsshouldbepreheatedtothesametemperatureasthemold .Thisisveryimportantforlargeinserts .

• Insertsshouldbefreeofsharpcornersandcontamination .

• Tominimizecracking,high-viscosityDelrin®isrecommendedduetoitshigherelongation .

Note: Ifacrackingproblemcannotbeovercomebyusingtheabovemeasures,otherinsertingtechniquesshouldbeevaluated,suchasinsertionaftermoldingbypress-fitting,insertionbysonicenergy,oraself-tappinginsert .

AnnealingAnnealingisoccasionallyusedtoacceleratestressrelaxationanddimensionalstabilizationofparts .Itisacomplexprocessandshouldonlybeusedwhenmoldedpartsrequireverytighttolerancesandexposuretohightemperaturesforprolongedperiods .

Annealingisalsosuggestedasatestprocedureinsettingupmoldingconditionsonanewmold,toevaluatepost-moldingshrinkageandmolded-instresses .Thechangesindimensionsduringannealingwillcloselyrepresenttheultimatechangeinpartsizeinuse .

Whendimensionalprecisionisaprimerequirement,theuseofahighmoldtemperature(90–120°C[194–248°F])isstronglyrecommended .Attemptstoreachgooddimensionalstabilitybyannealingpartsmoldedinacoldmold(<80°C[<176°F])willleadtohighpost-moldingshrinkageandmayintroducestressesduringthere-crystallizationprocess,resultinginuncontrolleddeformation .

Annealing Procedure

Annealingshouldbeperformedinairorininertmineraloilsat160±3°C(320°F),for30minutes+5minutespermmofwallthickness .Overheatingandhotspotsshouldbeavoided,andpartsshouldneithercontacteachothernorthewallsoftheoven/bath .Partsshouldbeleftintheoventocoolslowlyuntil80°C(176°F)isreached .Stackingorpiling,whichmaydeformtheparts

whiletheyarehot,shouldbedelayeduntilthepartsarecooltothetouch .ThisprocedurewasusedtoobtaintheresultsshowninFigure 47,andpermitsevaluationoftheultimatedimensionalchangesthatapartislikelytoexperienceinnormaluse .

Tosimplystabilizepartsforcontinuoushightemperatureuse(<90°C[<194°F]),partsmaybeheatedto90°C(194°F)forupto24hr .Post-moldingshrinkageofaround0 .1to0 .2%willthenbeseenifthepartsweremoldedinamoldat90°C(194°F)±10°C .

Environmental ChangesPartdimensionsofDelrin®acetalresinchangewiththeenvironmentaltemperatureandwiththeabsorptionofsmallamountsofwater .DataconcerningdimensionsforvariousDelrin®acetalresinsareplottedinFigure 48,whichcombinestheeffectsofmoisturecontentandtemperature .Thegraphshowsseverallinesrepresentingdifferentexposureconditionswithrespecttomoisture(50%RH,80%RH,100%RH,andimmersion) .

36

Dimensional TolerancesGeneral

Takingintoaccountmolddimensionsandprocessingvariability,experiencesuggeststhatthefollowingdimensionaltolerancesareachievablewithgoodmoldingpractice:

• dimensionsupto150mm(5 .9in): ±0 .15%forprecisionmolding ±0 .3%fortechnicalmolding

• dimensionsabove150mm(5 .9in):±0 .25%forprecisionmolding±0 .4%fortechnicalmolding

Molds

Formulti-cavitymolds,thetoolmakingtolerancesareimportant .Theyhaveadirecteffectonthedimensionaltoleranceofthepart .Asanexample,foramolddimensionof30mm(1 .2in)manufacturedtowithin±0 .01mm,experiencehasshownthatdimensionalconsistencybetterthan±0 .03–0 .04mmcannotbeexpectedforpartsfromdifferentcavitiesinasingleshot .

Molding Conditions

Partsmoldedunderrecommendedconditions(gate,runner,nozzle,screw,machineparameters)asdefinedinthemoldingguidearesubjecttosmallshot-to-shotvariationsindimensions .Anychangeinmachineparametersorconditionswilleffectdimensionaltolerance .Forexample,acoldermoldleadstohigherpost-moldingshrinkage,tooshortHold(Pressure)Timeleadstoinconsistentshrinkage,deformationandlargervariabilityinpartdimensions .

37

Auxiliary OperationsSeveralauxiliaryoperationsassociatedwiththemoldingofDelrin®acetalresinsarediscussedinthissection .Theyincludethefollowingsubjects:

• Materialhandling

• Drying

• Regroundresin

• Coloring

• Disposal

Material HandlingDelrin®acetalresinisshippeddryandneednotbedriedbeforemolding .Resinthathasbeenstoredinacoldwarehouseareashouldbebroughttoroomtemperaturepriortomolding .Thiswillpreventmoisturecondensationandvariationsinheatrequiredtomeltandthusinmelttemperature .

ParticularcareisrequiredforthetoughenedcompositionsofDelrin® .BagsofDelrin®500T,100Tand100STshouldnotbeopeneduntiltheyarereadytobeused .Ifabagisopenedforanysignificantperiodoftimeandtheresinhaspickedupmoisture,thematerialshouldbedriedbeforeitismolded .

PelletsofDelrin®aresurfacelubricatedwithethylenedi-stearamide .Furtherlubricationofthesecompositionsisnotnecessary .

Opencontainerofresinonlyinwell-ventilatedareas .Minimizethegenerationandaccumulationofdust .Additivesinthisproductdonotpresentarespiratoryhazardunlesstheproductisgroundtoapowderofrespirablesizeandthedustisinhaled .Alldustsarepotentiallyinjurioustotherespiratorytractifrespirableproductsaregeneratedandinhaledinsufficientlylargeconcentration .Goodexecutionofhygienepractices,aswithalldusts,shouldincludeprecautiontopreventinhalationofrespirableparticles .

Reground ResinRecommendations to Regrind Delrin®

TheuseofcontaminationfreeanduniformlyregroundDelrin®hasminimalinfluenceonmechanicalpropertiesandmoldingperformanceofstandardgrades(seedetailsbelow) .Toregrindthematerialproperly,thefollowingshouldbeconsidered:

• Donotregrindmoldedparts,spruesorrunnersthatarediscoloredorsplayed—theseconditionsmayindicatethattheresinwasdegradedduringprocessing .

• Avoidaccumulationofregroundresinwheneverpossiblebycontinuousreuseofspruesandrunners .Ideallyregrindatthemoldingmachineandfeedbackimmediatelyusingacloseloopsystemtoavoidanycontamination .Ifgrindingisdone

inabatchprocessawayfromthemoldingmachine,careshouldbetakentoavoidcontaminationofspruesandrunners .Protectregroundresinfromcontaminationanddirtbystoringinclean,dry,clearlylabelled,coveredcontainers .

• Maintainaconstantratioofvirgintoregroundresin,andmixadequatelypriortomolding .Asuitableratiodependsuponthequalityoftheregroundresinandtherequirementsofthepart .A3to1ratioofvirgintoregroundresiniscommon,althoughlargerquantitiesofregroundresincanbeusedsuccessfully .

• Ideallyusealowspeedgrinder,buthigherspeedgrindersareacceptableifknivesarewellsharpenedandifholesinthescreenarelargeenough(4mm)toavoidfines .Thegrindershouldbethoroughlycleanedbeforegrindingadifferentmaterial .

• Excessivefinesshouldberemoved .

• Avoidreprocessedresinfromoutsidesources .

• Foroptimumpropertiesoftoughenedgrades,spruesandrunnersshouldberegroundandusedassoonaspossible,asmoisturepick-upisfastfortheseresins(seepreviousparagraph) .Thefractionofregrindforthesecompositionsshouldnotexceed25%inthefeed,anditshouldbefedimmediatelybackintothehopper .

Effect on Mechanical Properties

Figure 49showstheresultsofa10passregrindstudywhichhasbeenrunusingeither100%or50%regrindwithDelrin®500 .A10pass50%additionregrindstudyisequivalenttoamoldercontinuouslyregrinding50%oftheshotweight .Excellentretentionofmechanicalpropertiesisobservedintheseconditions .

DryingAsageneralrule,Delrin®doesnotrequiredrying .Howeverdryingisrecommendedinsomecases .

Standard Grades

• Whenaresincontainerstaysopenforasignificanttime,dryingat80°C(176°F)fortwohoursmayimprovethemeltquality .ThewaterabsorptionrateofDelrin®acetalresinsatvarioushumiditylevelsisshowninFigure 49 .

• Whenusingmorethan50%ofthecapacityofthemachine,preheatingtheresinat80°C(176°F)fortwohoursmayimprovethehomogeneityofthemeltanddecreasethetorqueonthescrew .

• Whenthermalstabilityisaconcern(e .g .,withsomedifficultcolors),blowingairat80°C(176°F)throughDelrin®mayhelp .Thiswillresultinlessmolddepositandbettersurfacefinish .

Toughened Grades

MoldingoftoughenedDelrin®compositionswithexcessivemoisture(>0 .05%)hasanegativeeffectontoughness .Therefore,itisrecommendedthattheresinisdriedfor4hrat80°C(176°F)inadehumidifieddryer(seethedryingbehaviorofDelrin®100STinFigure 50) .

38

Figure 50. Drying Behavior of Delrin® 100ST

Time, hr

Moi

stur

e, %

53 41 20

0.1

0.2

0.3

0.4

0.5

0.6

0

Hopper dryertemperature : 80°C (176°F)

Dew point : –23°C (–9°F)

Figure 49. Rate of Water Aborption at Various Conditions

Measurements were made on a shallowlayer of Delrin® molding powder.

Note:

Time, days

Wat

er b

y w

eigh

t, %

10 20 30 40 50 600

0.2

0.4

0.6

0.8

0

100% RH, 25°C (77°F)

83% RH, 25°C (77°F)

50% RH, 25°C (77°F)

12% RH, 25°C (77°F)

Delrin® 100, 500, and 900

At23°C(73°F)and50%RH,Delrin®100STpicksup0 .1%moisturein4hr;at30°C(86°F)and85%RHitwillpickup0 .3%moisturein2hr .Forthisreasonrunnersandspruesshouldberegroundandreusedassoonaspossible .

ColoringDelrin®isavailableinarangeofstandardandcustomcolors .

WhenmoldingnaturalDelrin®withacoloringsystemfromamanufacturerotherthanDuPont,thefollowingshouldbenoted:

• Thepigmentormasterbatchmanufacturer’ssafehandlingproceduresmustbeapplied .

• Smallscaletestsshouldberuninitiallytocheckmeltstability(seepage12,foamingtest),assomeacidic,basicormetallicpigmentswilldecomposeDelrin® .

• Differentcoloringsystems(eventhosegivingthesamecolor)couldcausedifferentshrinkages,ascanbeseenfromFigure 46 .Partdimensionsshouldbecheckedinthesmallscaletests .

• Flowalonginjectionunitscrewsislaminarandcolordispersioncouldbeunsatisfactory .Apropermixingheadshouldbeused(seepage9) .

• Totalpigmentloadingshouldbeaslowaspossibletomaintainresinproperties .

DisposalWastedisposalmustbeinaccordancewithallapplicableregulations .Preferredoptionsfordisposalare:

1 . recycling,

2 . incinerationwithenergyrecovery,and

3 . landfill .

Recyclingofsprueandrunnersisbestdonedirectlyatthemoldingmachine(seeRegroundResin,pages34–35) .Mechanicalrecyclingofpost-consumerpartsisrarelyattractive .Sinceresinstabilityandmechanicalpropertiescanbeseverelyaffectedbycontamination,theseparationandcleaninglogisticsbecomecomplicatedandexpensive .Chemicalrecyclingistechnicallypossible,butagainitispresentlylimitedbywastecollectionandseparation .

Thehighfuelvalueofacetalresinsmakesoption(2)verydesirableformaterialthatcannotberecycled .However,partsorregrindofresinscontainingTeflon®(suchasDelrin®500AFand520MP)shouldnotbeincinerated .

39

Troubleshooting GuideProblem Suggested remedies (listed in order of convenience)

Melt quality problemsMold deposit  •  Decrease injection fill rate  •  Decrease melt temperature  •  Avoid resin contamination  •  Correct hold-up spots in cylinder, screw, nozzle assembly  •  Increase gate size, flare gate  •  Enlarge vents  •  Change vent location  •  Use hopper drier to improve the resin’s thermal stability in extreme casesOdor  •  Observe melt appearance (gassing) and measure melt temperature  •  Reduce cylinder temperatures if melt temperature is high  •  Avoid resin contamination  •  Reduce overall cyle to decrease holdup time  •  Correct holdup spots in cylinder, adaptor, nozzle, screw tip, and check valve assembly  •  Use smaller injection unitUnmelted particles  •  Increase cylinder temperatures  •  Increase back pressure  •  Reduce screw rpm  •  Use hopper drier to preheat resin  •  Increase overall cycle  •  Use screw designed for Delrin®

  •  Use larger machine or injection unitScrew deposit  •  Lessen severity of screw (esp. for Delrin® 100 flow grades)—within recommendations  •  Avoid overcooling the feed throat  •  Check % of feed/transition/metering—within recommendationsBlack spots or brown streaks  •  Decrease residence time in injection unit (smaller screw)  •  Avoid resin contamination  •  Correct holdup spots in cylinder, screw, nozzle assembly  •  Check hopper cooling (80–90°C [176–194°F])Pigment streaks  •  Increase back pressure to improve dispersion  •  Use a mixing head screw  •  Evaluate other coloring systems  •  Evaluate fully precompounded colorFilling problemsShort shots  •  Maintain uniform pad  •  Repair leaking back flow valve if pad cannot be maintained  •  Increase injection fill pressure  •  Increase injection fill rate  •  Increase melt temperature  •  Increase mold temperature  •  Enlarge vents  •  Change vent location  •  Increase overall cycle  •  Use screw designed for Delrin®

  •  Use larger machine or injection unitNote: Minimize nozzle length when molding at or near limit of injection pressure capacity of molding equipment. This will be particularly true for Delrin®100 type resins having high melt viscosity.

Voids in parts  •  Increase hold pressure  •  Increase hold (pressure) time  •  Locate gate in thickest area  •  Decrease injection fill rate  •  Decrease melt temperature; improve melt uniformity  •  Repair leaking back flow valve if pad cannot be maintained  •  Enlarge vents  •  Improve gate thickness or location  •  Eliminate any restrictions in runner or nozzleWeak weld lines  •  Increase hold pressure  •  Adjust injection fill rate (around 1 s per mm of part thickness)  •  Increase melt temperature, but avoid excessive temperature  •  Enlarge vents  •  Increase mold temperature  •  Avoid mold release spray  •  Change vent or gate location  •  Use larger machine or injection unit

40

Troubleshooting Guide (continued)

Problem Suggested remedies (listed in order of convenience)

Ejection problemsParts sticking in mold  •  Increase hold (pressure) time  •  Correct mold defects (undercuts)  •  Change or add ejector pin locations  •  Decrease hold pressure  •  Decrease injection fill rate  •  Increase cycle (possibly only temporarily)  •  Use mold release temporarilySprue sticking  •  Remove burrs on sprue  •  Correct alignment between sprue and nozzle  •  Radius sharp corners where the sprue meets the runner (or the part)  •  Increase hold (pressure) time  •  Increase nozzle temperature  •  Increase mold cooling time  •  Use nozzle orifice smaller than sprue bushing  •  Improve sprue puller  •  Increase taper of sprue  •  Use mold release temporarilyDimensional problemsShot-to-shot dimensional variations  •  Increase injection hold pressure  •  Maintain uniform pad (cushion)  •  Repair leaking back flow valve if pad cannot be maintained  •  Increase hold (pressure) time  •  Increase gate thickness and/or location  •  Maintain uniform cycle  •  Eliminate unmelted particles (see below)  •  Use larger machine or screw designed for Delrin®

Warpage  •   Balance mold temperature  •  Locate gate in thickest area  •  Increase hold (pressure) time  •  Increase gate thickness and/or location  •  Round sharp corner  •  Clean water channels in mold; improve mold cooling system  •  Improve part design (e.g., avoid bottlenecks in melt flow)  •  Change or add ejector pin locationsSurface problemsBlush, frost, and folds  •  Decrease injection fill rate  •  Increase mold temperature  •  Change gate locationGate smear  •  Decrease injection fill rate  •  Flare gate  •  Increase gate size  •  Change gate locationJetting  •  Increase or decrease injection fill rate  •  Increase gate size, flare gate   •  Increase mold temperature  •  Change gate locationPits, orange peel, wrinkles  •  Increase hold pressure  •  Increase injection fill rate  •  Increase hold (pressure) time  •  Increase mold temperature  •  Increase melt temperature  •  Enlarge vents  •  Increase gate sizeSink marks  •  Repair leaking back flow valve if pad cannot be maintained  •  Increase hold pressure  •  Increase hold (pressure) time  •  Increase gate size  •  Change gate location  •  Decrease melt temperature if it is too highSplay  •  Decrease melt temperature if it is too high  •  Avoid resin contamination  •  Decrease injection fill rate  •  Correct holdup spots in cylinder screw, nozzle assembly  •  Increase size of small gate

41

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42

E

Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,20,26,29,39,40Elongation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,7,30,35EmergencyShutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23EnvironmentalDimensionalChange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

F

FillRate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27,39,40Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,27Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . .4–11,13,14–21,23–28,29,32,33FoamingTest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,12,38Formaldehyde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,3,22

G

Gassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,39GateDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14,15,16,22,27GateFreezeTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33GateLocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14,39,40GateSize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39,40GateSmear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26,34,39,40Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6,9,13,14,17,19,32GlassTransitionTemperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,5Gloss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

H

HandlingPrecautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23HeatofFusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Hold-upareas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Hold-UpTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,24

I

InjectionPressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27,32,39InjectionRate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27InsertMolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35ISO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29,30

J

Jetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14,27,39,40

L

Lubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,31

Index

A

Adaptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8,9,10,39AirShots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,3AirTrapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31,34,35

B

BackFlowValve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8,9,10,39,40BackPressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12,25,30,39Blowback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,23BrownStreaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,38Burning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

C

CAVANCavityAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30CavityPressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30CheckRing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10,11ClampingForce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28,29Coloring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8,12,36,38,39Compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,33,37Compressionratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Contamination . . . . . .8,10,11,12,23,25,29,35,36,37,38Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10,20,26Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Cushion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28,38,40Cycle . . . . . . . . . . . . . . . . . . . . . . . . .1,3,5,8,9,10,11,12,13,18,20,21, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24,25,26,28,29,30,39,40CycleEstimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26CylinderTemperatureControl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

D

Decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,3,23Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8–11,19,22,25,29Delrin®100P . . . . . . . . . . . . . . . . . . .1,2,9,13,21,25,28,30,3539Delrin®100ST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,24,25,30,31,37,38Delrin®111P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Delrin®500AF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,33,38Delrin®500CL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Delrin®500P . . . . . . . . . . . . . . . . . . . . .1,2,7,9,25,27,28,30,33,34Delrin®500T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,24,30,36Delrin®511P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Delrin®570 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,25,33Delrin®900P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2DimensionalStability . . . . . . . . . . . . . . . . . . . . . . . . . . 1,4,33,34,34,35Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36,38Drool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3,11,19,24,25Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30,36,37,38

43

M

MaterialHandling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36MeltOutput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24MeltTemperature . . . . . . . . . . . . . . . . . . 1,6–9,12,13,23–25,33,36MeltViscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,4,6,7,13,39MoldDeposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,12,17,19,20,37,39MoldDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,12,25Molded-inStresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27,34,35MoldMaintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13,19,20,22,28MoldOpenTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26,29MoldShrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13,20,25,28,30–35MoldTemperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20,22,25–27,30–35

N

Non-ReturnValve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10,11Nozzle . . . . . . . . . . . . . . . . . . . . . .1,3,–12,14,17,22,23,24,27,29,36

O

Odor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3,19,22,24,39OpenNozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11OptimumProductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9,10,22,29

P

Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3PartWeight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,9,30,31,34,38Post-moldingshrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32–36ProjectedArea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28,29Purging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

R

Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38RegroundResin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37,38Runner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3,9,13,15–20,24,27,29,36RunnerlessMolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13,18Runners . . . . . . . . . . . . . . 6,8,13,14,16–19,23,25,27,29,36–39

S

ScrewDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . .8,9,10,12,25,29,39,40ScrewForwardTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26ScrewRetractionTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10,26,29ScrewRotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,24,25,29ScrewRotationSpeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,25,29Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Sinkmarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4,14,21,28,32,33,39,40SnakeFlowMold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Splay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3,8,17,19,36,39,40Start-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,22,23

T

ThermalStability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,37,39Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10,31,34,35,36Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1,7,9,21,27,33,37Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,39,40

U

Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12,20,39,40UnmeltedParticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,25,29,39,40

V

Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2,14,19,20,21,22,29Voids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,5,28,30,32,33,39

W

Warping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9WaterAbsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37WeldLine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14,20,21,29

Index (continued)

Copyright © 2006 DuPont. The DuPont Oval Logo, DuPont™, The miracles of science™, Delrin® , Kevlar®, Teflon®, and ELCee™ Screw are registered trademarks or trademarks of E. I. du Pont de Nemours and Company or its affiliates. All rights reserved.

H-78783-1 (11/06) Printed in the U.S.A.

The data listed here fall within the normal range of properties, but they should not be used to establish specification limits nor used alone as the basis of design. The DuPont Company assumes no obligations or liability for any advice furnished or for any results obtained with respect to this information. All such advice is given and accepted at the buyer’s risk. The disclosure of information herein is not a license to operate under, or a recommendation to infringe, any patent of DuPont or others. Since DuPont cannot anticipate all variations in actual end-use conditions, DuPont makes no warranties and assumes no liability in connection with any use of this information.

CAUTION: This product is not permitted to be sold for use in medical applications involving any implantation in the human body or where contact with internal body fluids or tissues will equal or exceed 24 hours. For applications involving contact of less than 24 hours, see “DuPont Medical Caution Statement”, H-50102 or contact your DuPont sales representative.

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