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TECHNICAL INFORMATION
DuPontDelrin acetal resinMolding Guide
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Table o Contents
Pages
General Inormation 1
Description 1
Compositions 1
SaetyPrecautionstoObserve
WhenMoldingDelrinAcetalResins 2
Packaging 3
Polymer Structure and Processing Behavior 4
GlassTransitionandMelting 4
PVTDiagrams 4
Heating-CoolingBehavior 6
ViscosityandRheologicalBehavior 6
Injection Molding Unit 8
ScrewDesign 9
ELCeeScreworOptimumProductivity 10
CylinderTemperatureControl 10
CylinderAdaptor 10
Non-ReturnValve(BackFlowValveBFV) 10
Nozzle 11
EvaluationoMeltQuality 11
Molds 13
AbilitytoFill 13
Gates 14
RunnerSystem 16
NozzleandSprue 17
HotRunnerMoldorCrystallinePolymers 18
Vents 19
Undercuts 20
SharpCorners 21RibsDesign 21
WeldLines 21
MoldMaintenance 22
MoldCleaning 22
(continued)
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Table o Contents (continued)
Pages
Molding Process 23
Start-upandShutdownProcedures 23
OperatingConditionsorDelrin
TemperatureSettings 24
OperatingConditionsorDelrin
MoldingCycle 26
OptimumProductivityMolding 29
StandardMoldingConditions
orISOTensileBars 30
HoldPressureTime
viaIn-cavityPressureMeasurement 30Dimensional Considerations 31
FundamentalsoDimensional
Control 31
MoldShrinkage 31
FactorsAectingMoldShrinkage 31
MoldShrinkageoFilledResins 31
EectoPigments 33
Post-MoldingShrinkage 34
InsertMolding 35Annealing 35
EnvironmentalChanges 35
DimensionalTolerances 36
Auxiliary Operations 37
MaterialHandling 37
RegroundResin 37
Drying 37
Coloring 38
Disposal 38
Troubleshooting Guide 39
Index 42
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1
General Inormation
Description
Delrinacetalresinsaresemi-crystalline,thermoplasticpolymers
madebythepolymerizationoormaldehyde,andarealso
commonlyreerredtoaspolyoxymethylene(POM)Theyhave
gainedwidespreadrecognitionorreliabilityinmanythousands
oengineeringcomponentsallovertheworldSincecommercial
introductionin1960,Delrinhasbeenusedintheautomotive,
appliance,construction,hardware,electronics,andconsumer
goodsindustries,amongothers
Delrinisnotedor:
Highmechanicalstrengthandrigidity
Excellentdimensionalstability
Naturallubricity
Fatigueendurance
Highresistancetorepeatedimpacts
Excellentresistancetomoisture,gasolines,solventsand
manyotherneutralchemicals
Toughnessatlowtemperature(downto50C[58F])
Wideuseultemperaturerange(inair:50to90C
[58to194F],withintermittentuseupto120C[248F])
Goodelectricalinsulatingcharacteristics
Easeoabrication
Delrinacetalresinsareavailableinavarietyocompositionsto
meetdierentend-useandprocessingrequirements
CompositionsThemainavailableDelrincompositionscanbe
classiedasollows:
a Standard
b Toughened
c Lowwear/Lowriction
d Glasslled/Reinorced
e UV-stabilized
Thestandardcompositionscoverabroadrangeomelt
viscositiesThehighestviscositycomposition,solike
Delrin
100P,areotenmoldedwhenmaximumtoughnesspropertiesareneededTheintermediatemeltviscosityDelrin
500Pisusedorgeneral-purposeinjectionapplicationsThe
resinshavinglowermeltviscosity,Delrin900Pisusuallychosen
orinjectionmoldingapplicationswithhard-to-llmolds
AsummaryothemaincompositionsisshowninTable 1
Safety Precautions to ObserveWhen Molding Delrin Acetal Resins
Delrinaswellasmanyotherthermoplasticpolymers
decomposestogaseousproductswhenheatedoraprolonged
timeThesegasescangeneratehighpressuresiconnedI
materialisnotreetoexitromaninjectioncylinderthroughthe
nozzle,itmayblowbackthroughthehopper
InthecaseoDelrinacetalresin,decompositionisalmost
entirelytogaseousproducts,sopressurebuild-upcanberapid
Theproductodecompositionisormaldehyde
Aswithanyacetalpolymer,Delrin,whenoverheated,can
discolorandormgaseousdecompositionproducts,whichare
largelyormaldehydeLowlevelsoormaldehydeemissions
canalsooccurattypicalprocessingtemperaturesRepeated
exposuretoormaldehydemayresultinrespiratoryandskin
sensitizationinsomeindividualsFormaldehydeisapotential
cancerhazard
ProcessingtoughenedgradesoDelrin
canreleaselowlevelsoisocyanatesRepeatedexposuretoisocyanatesmayresultin
respiratoryandskinsensitizationinsomeindividuals
Lowlevelsoormaldehydemayoccurintheheadspaceo
bagsasreceivedorincontainersoormedpartsater
processingBagsoresinorormedpartsshouldbeopenedin
wellventilatedareas
Useadequatelocalexhaustventilationinprocessareasto
maintainexposuresbelowrecommendedcontrollimitsDropair
shotsintowatertoreduceemissions
WhenmoldingDelrin,itisimportantthattheoperatorbe
amiliarwiththeactorsthatcancausedecomposition,withthe
dangersignalsthatwarnothisproblem,andwiththeactionthat
shouldbetakenThisinormationissummarizedonacardor
displayatthemoldingmachine
Theinormationgivenhereisbasedonourexperiencetodate
Itmaynotcoverallpossiblesituationsanditisnotintendedasa
substituteorskillandalertnessotheoperator
Follow correct start-up, operating and shut-down procedures
as described later in this guide.
Be aware o troublemakerscauses o decomposition:
Hightemperaturestickingtemperaturecontroller,aultythermocoupleconnections,incorrectreading,burned-out
heaterorheaterwithahotspot,heatsurgesonstart-up
DonotoverheatMaintainpolymermelttemperaturebelow
230C(446F)
CycledelayAvoidprolongedexposureatorabovethe
recommendedprocessingtemperatureRecommendedmelt
temperaturesaregiveninTable 5
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High Viscosity Grades
Delrin100/II100
POMhomopolymerHighviscositymoldingmaterialExcellent
tensilestrengthandresistancetocreepoverawidetemperature
range,evenunderhumidambientconditionsHighatigue
enduranceandimpactresistanceApplications:moldedparts
suchashighlyloadedgears,plainbearingsandsnap-ts
Delrin100P
SamecharacteristicsandapplicationsasDelrin100,plus
improvedthermalstabilityorlowdepositmoldingindemanding
processingconditions,eg,hotrunnertools
Delrin111P
Characteristics:Delrin100Pwithenhancedcrystallinity
Resistancetocreepandatigueenduranceimprovedover
Delrin100PTypicalapplications:Highlyloadedgears,
bearings,snap-ts
Medium-High Viscosity Grades
Delrin311DP
POMhomopolymerOptimizedstabilizerormulationwithenhancedcrystallinity;excellentdimensionalstability
Excellentresistancetouels,lubricants,solventsandmost
neutralchemicals
Medium Viscosity Grades
Delrin500P
POMhomopolymerGeneralpurposemoldingresinwith
improvedprocessingstabilityorlowdepositmoldingin
demandingprocessingconditions,eg,hotrunnertools
Delrin511P
Characteristics:Delrin500Pwithenhancedcrystallinity,
goodresistancetocreepandatigue
Applications:uelsystemcomponents,gears,astenersLow Viscosity Grades
Delrin900P
Characteristics:Lowviscosity,astmoldingresinplusimproved
processingstabilityorlowdepositmoldingindemanding
processingconditions,eg,hotrunnertoolsApplications:
Multicavitymoldsandpartswiththinsections,eg,consumer
electronicsparts,zippers
Toughened Grades
Delrin100ST
POMhomopolymer,SuperToughHighviscosity,super
toughmaterialorinjectionmoldingandextrusionExcellent
combinationosuper-toughness,impactatigueresistance,
solventandstresscrackresistance,aswellashightensile
elongationatlowtemperatureApplications:Mainlyusedor
partsrequiringresistancetorepeatedimpactsandloads,
suchasautomotiveasteners,helmets,hosesandtubing
Delrin100T
POMhomopolymer,ToughenedHighimpactresistance
Delrin500T
Mediumviscosity,toughenedresinorinjectionmoldingand
extrusionExcellentnotchedIzodandtensileimpactstrength
Applications:Mainlyusedorpartssubjectedtorepeated
impactsandalternatingloads,suchasautomotiveasteners,
helmets,hosesandtubing
Low-Wear/Low-Friction Grades
Delrin100AL
Highviscositylubricatedgradewithpackageoadvanced
lubricants
Delrin100KM
Delrin100PmodiedwithKevlararamidresinorabrasivewearreductionApplications:Specialtyrictionandwear
Delrin500AF
Mediumviscositygradewith20%TefonPTFEbers,
outstandingrictionandwearpropertiesApplications:Specialty
rictionandwear,conveyorsystems
Delrin520MP
Delrin500Pwith20%TefonPTFEmicropowder,with
low-wearandlow-rictionpropertiesApplications:Specialty
rictionandwear
Delrin500TL
Delrin500with15%Tefonpowder,withlow-wearandlow-
rictionpropertiesApplications:Specialtyrictionandwear,
conveyorsystems
Delrin500AL
Mediumviscosityresinwithadvancedlubricantsystem,very
goodlow-rictionandlow-wearpropertiesApplications:Gears,
drivetrains,slidingdevices
Delrin500CL
ChemicallylubricatedDelrin500,verygoodlow-rictionandlow
wearpropertiesApplications:Gears,drivetrains,slidingdevices
Delrin500MP
MediumviscositygradewithTefonPTPEmicropowder
Delrin911AL
Lowviscsoitygradewithadvancedlubricants;excellent
dimensionalstability
Glass-Filled/Glass-Reinorced Grades
Delrin570
Mediumviscosityresin,with20%glassberllerApplications:
Wherehighstinessandcreepresistancearerequired
Delrin510GR
10%glass-reinorcedresinApplications:Partsrequiringhigh
stinessandstrength,andcreepresistance
Delrin525GR
25%glass-reinorcedresinApplications:Partsrequiringvery
highstinessandstrength,andcreepresistance
UV-Stabilized GradesDelrin127UV
Delrin100PwithUVstabilizerApplications:Automotiveinterior
partswithmaximumUVperormancerequirements,skibindings
seatbeltrestraintparts
Delrin327UV
Medium-highviscositygradewithUVstabilizerandenhanced
crystallinity
Delrin527UV
Delrin500PwithUVstabilizerApplications:Automotiveinterior
partswithmaximumUVperormancerequirements,interiortrim
seatbeltrestraintparts
Table 1
Main Compositions o Delrin Acetal Resins
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Hold-upareasincylinder,adapter,nozzle,screwtip,hot
runnerandcheckvalveassembly
Pluggednozzleromscrapmetalorhighermeltingpoint
resin,orromclosednozzlevalve
Foreignmaterials
Additives,llersorcolorantsotherthanthosespecically
recommendedoruseinDelrin
Contaminants(especiallythosecontainingchlorineorgeneratingacidmaterials)suchaspolyvinylchlorideresin
orfameretardants
Copper,brass,bronzeorothercopperalloysincontactwith
moltenDelrin(notinmoldswheretheresinsolidiesater
eachcycle)
Copper-basedlubricantsorgreaseorthreads
Contaminatedreworkespeciallyreworkorreprocessed
resinromoutsideorunknownsources
DonotmixDelringradeswithotherDelringrades,nor
withanyotherresins,withoutrstconsultingDuPont
AvoidprocessingDelrinonequipmentthatisalsoused
orincompatibleresins,particularlyhalogenatedpolymers,suchasPVCorhalogenatedthermoplasticelastomers,or
UVstabilizedorfameretardantmaterials
Watch or Danger Signals
Frothynozzledrool
Spittingnozzle
Pronouncedodor
Discoloredresinbrownorblackstreaking
Badlysplayedpartswhitishdepositonmoldingormold
Screwpushbackromgaspressure
Action Required When Any o the Danger Signals Occur AVOIDPERSONALEXPOSUREWhenDANGERSIGNALS
arepresent,DONOTlookintohopperorworkaroundnozzle
asviolentejectionomeltispossible
MINIMIZEPERSONALEXPOSURETODECOMPOSITION
GASESbyusinggeneralandlocalventilationInecessary,
leaveareaomachineuntilventilationhasreduced
concentrationoormaldehydetoacceptablelevelsPersons
sensitizedtoormaldehydeorhavingexistingpulmonary
disabilitiesshouldnotbeinvolvedinprocessingDelrin
FREENOZZLEPLUGbyheatingwithtorchIthisails,cool
downcylinder,makesurePRESSUREISRELIEVED,and
CAREFULLYREMOVENOZZLEandclean TAKEAIRSHOTStocooltheresinPURGEWITHCRYSTAL
POLYSTYRENEDROPALLMOLTENDelrinINTOWATERto
reduceodorlevel
Turnocylinderheaters
Checktemperaturecontrolinstruments
Discontinueautomaticmoldingandrunmanuallyuntiljobis
runningsmoothly
Provideadequatemeansoventingeedmechanismincase
oblowback
Useexhaustventilationtoreduceormaldehydeodor
Idecompositionoccurs:
1 Shutoandpurgemachine
2 Minimizepersonalexposuretodecompositiongases
byusinglocalandgeneralventilation
3 Inecessary,leaveareaomachineuntilventilationhas
reducedconcentrationoormaldehydetoacceptablelevels
Packaging
Delrinacetalresinissuppliedassphericalorcylindricalpellets
approximately3mm(012in)indimensionsTheyarepackaged
in1,000kg(2,200lb)netweightbulkcorrugatedboxes,
500kg(1,100lb)netweightfexiblecontainer,or25kg(5516lb)
moistureprotected,tearresistantpolyethylenebagsThebulkdensityotheunlledresingranulesisabout08g/cm3
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Polymer Structure andProcessing Behavior
Thebehavioroapolymerduringthemoldingprocessandthe
behavioroamoldedpartduringitswholeend-useliearehighly
dependentonthetypeostructurethatthepolymertendstoormduringsolidication
Somepolymersexhibitinthesolidstateroughlythesame
moleculararrangementasinthemelt,ie,arandommass
oentangledmoleculeswithnoorderThisclassisnamed
amorphouspolymersandincludesorexampleABS,
polycarbonateandpolystyrene
Otherpolymerstendtosolidiyinanorderedmanner:the
moleculesarrangingintocrystallineorms(lamellae,spherulites)
Becauseothelengthothemacromolecules,partsothem
cannotbelongtocrystals(duetolackospaceandmobility)and
createanamorphousinter-crystallinezoneThesepolymersarethereorepartiallycrystallineorsemi-crystalline;orsimplicity,in
thistextwewillreertothemascrystalline(asopposed
toamorphous)
TypicalcrystallinematerialsareDelrin(acetalresins),Zytel(polyamideresins),RynitePETandCrastinPBT(thermoplasticpolyesterresins),polyethyleneandpolypropylene
Table 2summarizessomeundamentaldierencesbetween
amorphousandcrystallinepolymersThesepointsare
describedinmoredetailintheollowingparagraphsThis
inormationisessentialtounderstandwhytheoptimization
othemoldingprocessissubstantiallydierentorthetwocategoriesopolymers
Table 2Comparison of Amorphous and Crystalline Polymers
Resin type Amorphous Crystalline
PropertiesThermal parameters T
gT
g, T
m
Maximum T in use* Below Tg
Below Tm
Specifc volume vs. T Continuous Discontinuity at Tm
Melt viscosity vs. T High dependence Low dependence
ProcessingSolidication Cooling below T
gCrystallization below T
m
Hold pressure Decreased during cooling Constant during
crystallization
Flow through gate Stops ater dynamic lling Continues until end o
crystallization
Deects i bad process Over-packing, stress-cracking, Voids, deormations,
sink marks sink marks
* For typical engineering applications
Glass Transition and MeltingAmorphous Polymers
Theoverallbehavioroamorphouspolymersislargely
determinedinrelationtotheirglasstransitiontemperatureTg
Belowthistemperature,themoleculesareessentiallyblocked
inthesolidphaseThematerialisrigidandhasahighcreep
resistance,butitalsotendstobebrittleandsensitivetoatigue
Whenthetemperatureisincreasedabovetheglasstransition
temperatureTg,themoleculeshavesomereedomtomove
byrotationaroundchemicalbondsTherigiditydecreases
graduallyandthematerialshowselastomericproperties,lending
itseltoprocesseslikethermoorming,blowmoldingand(at
temperatures120150C[248302F]aboveTg)injectionmolding
AmorphouspolymersusedinengineeringapplicationshaveTg
abovetheambienttemperature,andthemaximumtemperature
orend-useshouldbebelowTg;orexamplepolystyrenehasT
g=
90100C(194212F),andisinjectionmoldedbetween210and
250C(410and482F)
Crystalline Polymers
Incrystallinepolymers,theonsetomolecularmovementinthe
materialalsodenestheglasstransitiontemperatureTg
WhenthetemperatureisincreasedaboveTg,thecrystalline
polymersmaintainrigidityappropriateorengineering
applications(orexamplewithDelrinapartcaneasilywithstand
temperatureswellabovetheTg)
Uponurtherheatingthematerialreachesitsmelting
temperatureTm,wherethecohesionothecrystallinedomains
isdestroyedWithinaewdegrees,thereisaconsiderablechangeomechanicalpropertiesromsolidtoliquidbehavior
AboveTm,thecrystallinepolymersbehaveashighviscosity
liquids,andcangenerallybeprocessedbyinjectionmolding,
typicallyattemperatures3060C(86140F)abovetheirmelting
temperatureAsaconsequence,thetemperaturedomainorthe
useocrystallinepolymersisnotlimitedbytheglasstransition
temperatureTg,butbythemeltingtemperatureT
mForDelrin,
theeectotheTgisnegligibleandverydiculttomeasure,due
toitsverylowamorphouscontentTherearetwotransitionsor
Delrin,aweakonearound015C(3259F)andastrongerone
at80C(112F)Thetransitionjustbelowroomtemperatureis
soweakthereisminimaleectonpropertiesForDelrinacetalhomopolymrs,T
m=178C(352F)andthetypicalprocessing
rangeis210220C(410446F)
PVT Diagrams
ThePVTdiagramisacondensedpresentationothe
interrelationsothreevariablesthataecttheprocessingoa
polymer:Pressure,VolumeandTemperature
Theeectothetemperature(T)orvolume(V)isillustratedin
Figure 1oranamorphousandacrystallinepolymerWhenthe
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temperatureothematerialisincreased,itsspecicvolume
(theinverseodensity)alsoincreasesduetothermalexpansion
Therateoincreasebecomeshigherattheglasstransition
temperature,becausethemoleculeshavemorereedomto
moveandtheyoccupymorespaceThischangeoslopeis
observedwithbothamorphousandcrystallinepolymersAt
highertemperature,themeltingocrystallinepolymersis
markedbyasuddenincreaseothespecicvolume,whenthewell-orderedandrigidcrystallinedomainsbecomerandomly
orientedandreetomoveThespecicvolumeisthereore
asignatureothechangesostructureothepolymerasa
unctionotemperature
APVTdiagramissimplythepresentationotheseriesocurves
obtainedwhenthemeasurementospecicvolumeversus
temperatureisrepeatedatdierentpressuresThePVTdiagram
oatypicalamorphouspolymer(polystyrene)isshownin
Figure 2,andthePVTdiagramoDelrinisshowninFigure 3
Themoldingprocesscanbeillustratedbyacycleotransitions
onthePVTdiagramForsimplication,itwillbeassumedintheollowingdescriptionthatheatingtakesplaceatconstant
pressure(alongisobarlines)andthatapplicationopressureis
isothermal(verticallines)
Foranamorphousmaterialthemoldingcycleisas
ollows(seeFigure 2):
Startingromroomtemperatureand1MPapressure(point
A)thematerialisheatedinthebarrelThespecicvolume
increasesaccordingtotheisobarat1MPatoreachthe
moldingtemperature(pointB)
Thematerialisinjectedintothecavityandthepressureis
appliedThisprocessisroughlyisothermal(topointC),and
thespecicvolumeisdecreasedtoavalueclosetothatat
1MPaandTg
Theresiniscooledinthemold,andatthesametimethe
holdpressureisdecreased,toollowahorizontallineinthe
PVTdiagramandreachpointDwherethepartcanbeejected
whenitisat1MPapressureandatemperaturebelowTg
Ideally,thereshouldbenofowomaterialthroughthegate
duringthiscoolingphasetoproduceastress-reepart
Foracrystallinematerial,thepictureisdierent(seeFigure 3):
thematerialisheatedat1MPapressureromroom
temperature(pointA)uptotheprocessingtemperature
(pointB)Thisresultsinalargechangeovolume(almost25%orDelrin);
theresinisinjectedandcompressedinthecavityThespecic
volumeisdecreasedtopointC,whereitsvalueisstillmuch
higherthanat1MPa/23C(73F);
crystallizationtakesplaceinthemoldunderconstant
holdpressureWhenthecrystalsbuildupromtheliquid
phase,alargedierenceovolumeoccurs,whichmustbe
compensatedbyinjectionoadditionalliquidresinthroughthe
gate(otherwisevoidsarecreatedwithinthepart);
Temperature, C
Specificvolum
e,
cm3/g
Tg
Temperature, C
Specificvolume,
cm3/g
Tm
"Liquid" phase
"Solid" phase
Tg
AMORPHOUS
CRYSTALLINE
Figure 1. Specic Volume as Function o Temperature orAmorphous And Crystalline Polymers
Figure 2. Pressure-Volume-Temperature (PVT) Diagram orPolystyrene. Points A, B, C, and D Reer to DierentSteps o the Molding Process (see text).
Temperature, C
Spe
cificvolume,
cm3/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)
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attheendocrystallization(pointD),thepartissolidandit
canbeejectedimmediately;themoldingshrinkageisthe
dierencebetweenthespecicvolumesatthecrystallization
temperature(pointD)andatroomtemperature(pointA)
Thisdierenceinbehaviorhasimportantimplicationsor
injectionmoldingDuringthesolidicationprocess(ater
dynamiclling):
theholdpressureisdecreasedwithtimeoramorphous
polymers,whereasitismaintainedconstantorcrystalline
polymers;
thefowthroughthegateisstoppedoramorphouspolymers,
whileitcontinuesuntiltheendothecrystallizationor
crystallinepolymersThisimpliesthatorcrystallinematerials
thedesignoparts,gates,runnersandsprueshouldollow
specialrulesthatwillbedescribedintheMoldssection
Heating-Cooling Behavior
Foranysubstance,theenergyneededtoincreasethe
temperatureo1gmaterialby1C(18F)isdenedasits
specicheatThisquantityisgenerallydeterminedbyDierential
ScanningCalorimetry,andtheresultsorDelrin,polyamide
6-6andpolystyreneareshowninFigure 4Thetwocrystalline
polymers,Delrinandpolyamide6-6,showalargepeakthatis
duetotheadditionalheatrequiredtomeltthecrystallinephase
(latentheatousion)Theamorphouspolymerdoesnotshow
suchapeak,butexhibitsachangeoslopeatTg
Thetotalenergytobringeachmaterialuptoitsmolding
temperatureisgivenbytheareaunderthecurveFrom
Figure 4itisclearthatthecrystallinepolymersneedmore
energythantheamorphousonesThisexplainswhythe
designoascreworacrystallinepolymerlikeDelrin
shouldbedierent(andusuallymorecritical)thanoran
amorphouspolymer
Temperature, C
Specificvolume,
cm3/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
80
120140
180
P (MPa)
Figure 3. Pressure-Volume-Temperature (PVT) Diagram ForDelrin 500. Points A, B, C, and D Reer to DierentSteps o the Molding Process (see text).
Figure 4. Specic Heat versus Temperature or Delrin 500,PA66 and Polystyrene
Specific
heat,
kJ
kg
1K
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 Behavior
Meltviscositydeterminestoalargeextenttheabilitytollthe
moldcavityHighviscositymeansdicultfowthroughthin
sectionsandhigherinjectionllpressure
Temperatureandshearratearecrucialparameterswhen
consideringtheviscosityomoltenpolymers,andtheyshould
alwaysbespeciedtogetherwithavalueormeltviscosity
ForpolymersconsistingolinearmoleculeslikeDelrin,
theviscosityisalsoindirectrelationtotheaverage
molecularweight
Inuence o Temperature
Thegeneralrulethatliquidsbecomelessviscouswhen
increasingtemperatureisalsotrueormoltenthermoplastics
Howevercrystallineandamorphouspolymersbehavedierently
asshowninFigure 5ThecurvesorDelrinandpolystyrene
werebothobtainedbyreducinggraduallythetemperatureothe
materialsrom230to100C(446to212F)Twodierencesare
worthmentioningFirst,attemperaturesabove180C(356F),
thedependenceoviscosityontemperatureismorepronounced
ortheamorphouspolystyrenethanorDelrin;thereore,
increasingthemelttemperatureoDelrindoesnotgreatly
improveitsabilitytofowthroughathinsectionSecond,below
170C(338F)theviscosityoDelrinrisessharplybecausethe
materialcrystallizeswithinaewdegreesothattemperature
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Figure 5. Viscosity/Temperature Curves for Delrin 500Pand or Polystyrene at a Constant Shear Rate o1000 s1 (Temperature Reduced rom 230 to 100C[446 to 212F])
Temperature, C
Apparentvisco
sity,
Pa.s
180 220 240200120 140 160100
600
400
200
0
Delrin 500
Polystyrene
Thermoforming
Extrusion
Injection molding
Y
V (Y)
Figure 6. Approximate Shape o the Velocity DistributionBetween Two Parallel Plates. The Shear Rate is theDerivative dv(y)/dy.
Figure 7. Viscosity versus Shear Rate o Delrin 500P atThree Temperatures
Figure 8. Viscosity versus Shear Rate or Various Grades oDelrin at a Constant Temperature o 215C (419F)
Inuence o Shear Rate
Theshearratecharacterizestherateodeormationothe
materialandisdenedasthederivativeothevelocityoverthe
directionperpendiculartofow(seeFigure 6);inotherwords,the
shearrateisproportionaltothevariationospeedwithinthepart
thicknessSoitdependsonthevelocityothefowandonthe
geometryothefowchannels
Shear rate, s-1
Viscosity,
Pas-1
200C
215C
230C
102 103 104
100
1000
10
Table 3Viscosity, Flow and Molecular Weight (Mw)
o the Delrin Grades
Spiral fow lengthMFR MFR (215C/100 MPa/2 mm
(190C/ (190C/ Ease Mw, 90C moldGrade 1.06 kg) 2.16 kg) o fow 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,themeltviscositydecreasesconsiderablywhenthe
shearrateincreases,asshowninFigure 7Thiseectismore
importantthanthedierencesresultingromvariationsothe
melttemperaturewithintheprocessingwindoworinjection
molding
Inuence o Molecular Weight
DelrinisavailableinourgradesomolecularweightTheyare
codedaccordingtotheirabilitytofow,asmeasuredbyMFRor
meltfowrate(seeTable 3)Highvaluesmeaneasyfowand
abilitytollthinparts,whereaslowvaluesmeanhighviscosity,
highmolecularweightandhightoughness(impactresistance,
elongationatbreak)
Shear rate, s-1
Viscosity
,Pas-1
100 1,000 100,000
100
1,000
10
900500
100
MFRisameasurementperormedatlowshearrate,butthe
relativedierencesbetweenthegradesaremaintainedathigh
shearrates,asshowninFigure 8
Amoredirectcomparisonotheabilitytollcanbeobtained
usinganopen-endedsnake-fowmoldResultsorthedierent
gradesoDelrinarepresentedintheMoldssection
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Hold-up time, min
Meltte
mperature,
C
60 80 10040200
200210
220
230
240
250
190
Minimum recommended
melt temperature
Recommended operatingzone
Figure 9. Eect o Temperature on Hold-up Timeo Delrin 500
Withascrewstrokeo1diameter(asmallshot)andacycletime
o1min(averylongone),theaverageHUTisequalto8min
AccordingtothedegradationcurveshowninFigure 9,Delrin
shouldbestableenoughorinjection-moldingwiththisHUTat
amelttemperatureo240C(464F)Somecustomershave
experiencedmoldingDelrinsuccessullyatthattemperature
Attherecommendedmelttemperatureo215C(419F),the
maximumHUTisover30minandDelrin(standardgrades)is
thermallystableevenundertheseextremeconditions
Thereare3mainpotentialcausesodegradation:
Material trapped in Hold-up spotsIntheinjectionunit,
trappedmoltenmaterialwillstayorverylongtimesinany
deadspotsandwillstarttodegradeSoalltheinjectionunit
(screw,backfowvalve,adaptor,nozzleandhotrunners)
shouldbedesignedtoavoidHold-upspots(seeollowing
recommendeddesign)
Material sticking to hot steelDuetothehighviscosityo
polymers,thespeednexttothesteelotheinjectionunit
(screw,backfowvalve,adaptor,nozzleandhotrunner)isalmostzeroandtheresidencetimeisalmostinnite(as
evidencedbyhowlongittakestochangecolorsinaninjection
unit)Whereasinsidethebarrelthemoltenpolymeriscleaned
bythescrewandthevalve,insideallotherareasthematerial
willsticktothewallsTowithstandaverylongresidencetime
thesteelincontactshouldbecontrolledatatemperature
lowerthan190C(374F)(seeFigure 9)
Chemical degradationContamination(eg,PVC,fame
retardantresins,acidgeneratingresins),incompatible
coloringsystems(acidorbasicpigments),contactwith
copper(pure,alloys,grease)willacceleratethethermal
degradationomoltenDelrinintheinjectionunitNotethat
moldcomponentsincopperorcopperalloys(suchascopper-
beryllium)donotcauseanydegradationandhavebeenused
oryearswithoutproblems
Injection Molding Unit
Delrinacetalresinsaremoldedthroughouttheworldina
widevarietyotypesanddesignsoinjectionandextrusion
equipment
Therstpurposeotheinjectionunitormoldingacrystalline
materialistodelivertothemoldthenecessaryamountoa
homogeneousmelt(withnounmeltandnodegradedmaterial)
Therulesoconstructionotheinjectionunitarethendependent
onthemoldingmaterialrequirementsintermothermal
behaviorandheatneededTherstpointtotakeintoaccountor
acrystallinematerialisthethermalstabilityatmelttemperature,
toavoiddegradationThen,screw,nozzle,backfowvalve,
adaptor,shouldbedesignedtoprovideecientmeltingo
crystallinematerialanddeliveryomoltenpolymertothemold
Tworoughmethodstoevaluatethepresenceounmeltand
odegradedmaterialwillbepresentedinEvaluationoMelt
Quality(seepage11)
Thermal Stability During Processing
Aspresentedinthepreviouschapter,onedierencebetween
amorphousandcrystallinematerialisthemeltingbehavior
TheamorphouspolymerstartssoteningjustaterTgand
presentsacontinuouschangeinviscosityThisgivesavery
largetemperaturerangetooperate(butalargevariationo
viscositywithtemperature)Incontrast,thecrystallinepolymer
stayssoliduptothemeltingpointandsuddenlymeltstothe
liquidphaseathightemperatureThislimitstheprocessing
rangeotemperaturebetweenunmeltandthermaldegradation
(specicallyorDelrin
190250C[374482F])
Thesecondactoristhetimethematerialstaysatthat
temperatureForallpolymers,themoleculescanwithstanda
certaintimeatacertaintemperaturebeoredegradationcan
startObviouslythisacceptabletimelimitbecomesshorter
whenthetemperatureishigherThetypicalbehavioroDelrin
ispresentedinFigure 9DegradationoDelrinwillresultin
generationogaseswhichcausebubblesinthemelt,splayson
parts,molddeposit,yellowandbrownmarksontheparts
Theaverageresidencetime(orHold-UpTime,HUT)intheinjection
unitislinkedtotheamountopolymerinthecylinder,theshot
weightandthecycletimeandcanbecalculatedwiththeollowingequation:
AverageHUT= weightoresinincylindercycletime
shotweight
Aquickapproximationcanbedoneby:
AverageHUT=maximumscrewstroke2cycletime
currentscrewstroke*
*Eectivescrewstroke=distancethescrewtravelsduringrotationonly
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Screw Design
Screwdesignisakeyparameterorproductivity,becauseor
crystallinematerialsthescrewrotationtimeisaninherentparto
thecycletime
Asmentionedabove,itshouldtakeinconsiderationthespecic
meltingbehaviorothecrystallinematerial,ie,soliduptothe
meltingpoint,highdemandoheatduringmeltingandlow
viscosityothemoltenmaterial
Althoughgeneral-purposescrewsarewidelyusedormolding
Delrin,optimumproductivitywillrequireaspecicdesign
Exceedingtheoutputcapabilityoaninadequatelydesigned
screwwillcausewidetemperaturevariationsandunmelted
particles(sometimesunmeltanddegradedmaterialhavebeen
observedatthesametime)Theresultislossotoughness,
variabilityinshrinkageanddimensions,warping,surace
deects,pluggedgates(leadingtoshortshots)orothermolding
problems
Duetothespecicsothemeltingprocessoacrystalline
polymer,ascrewdesignedorDelrinwillhaveshallowfight
depthsinthemeteringsectionandaslightlyhighercompression
thanageneral-purposescrewSpecicsuggestionsaregivenor
variousscrewdiametersandcompositionoDelrinacetalresin
inTable 4Compressionratioistheratioovolumeooneturn
intheeedsectiontothatinthemeteringsection(canbeap-
proximatedtotheratioothedepthothetwozones)
Thelengthothescrewwillalsoaectthemeltquality(an
insulatingmaterialneedssometimetogetthethermalenergy
transerredevenitheshearcontributestotheheatingprocess)
Thepreerredlengthisabout20timesthescrewdiameteror
20turnswhenthepitchanddiameterareequalThescrew
shouldbedividedasollows:3040%(68turns)eedsection,
3545%(79turns)transitionand25%(5turns)metering
Table 4Screw Design or Delrin Acetal Resins
Medium and Low Viscosity Grades: Delrin
500P, 900P, 500T High Viscosity Grades of Delrin
: Delrin
100P, 100STNominal diameter (D) Depth o eed section (h
1) Depth o metering section (h
2) Depth o eed section (h
1) Depth o metering section (h
2)
mm mm mm mm mm
30 5.4 2.0 5.2 2.645 6.8 2.4 6.5 2.860 8.1 2.8 7.5 3.090 10.8 3.5 8.7 3.6
120 13.5 4.2
(in) (in) (in) (in) (in)
(1-12) (0.240) (0.087) (0.230) (0.105)(2) (0.290) (0.100) (0.270) (0.115)
(2-12) (0.330) (0.110) (0.300) (0.120)(3-12) (0.420) (0.140) (0.340) (0.140)(4-12) (0.510) (0.160)
Dh1
FEED SECTION
Pitch h2
METERINGSECTION
TRANSITION
(20/1 Length/Diameter Ratio)
sectionScrewswith20turnsarecommonlydividedinto7turns
eed,8turnstransitionand5turnsmeteringInscrewslessthan
16diameterslong,itmaybenecessarytoreducethepitchtoget
upto20turnsDenitively,theeedsectionshouldneverbeless
than6turns
TherelativelyhighcompressionratioscrewssuggestedorDelrin
aredesignedtoincreasetheheatinputbymechanicalworking
otheresinBecausetheenergyorthisincreasecomesrom
thescrewmotor,additionalhorsepowermustbeavailableian
increaseinmeltingcapabilityistoberealized
Screw Size
Theidealscrewsizeisdeterminedbythevolumeothecurrent
shotOptimumproductivitywillbeachievedwhentheshotsize
requiresascrewtravelduringplasticizationequaltoorlower
than50%othecapacityotheinjectionunitOtherwise,screw
rotationspeedwillhavetobedecreasedattheendothe
traveltoguaranteeanhomogeneousmelt,leadingtoalossin
productivityPractically,optimumproductivityisachievedwitha
screwtravelobetween1and2diametersothescrew
Thermalsettingsotheinjectionunitwillbedependentonthe
residencetime(HUT)andhencedependentonthecycletime
RuleswillbepresentedunderMoldingProcess
Screw Design or the Use o Color Concentrate
Afowanalysisshowsthatthemajorpartothefowinthe
screwislaminar,thendividedinthebackfowvalve(duetothe
changesinfowdirection),andstilllaminarintheadaptor,nozzle,
sprue,etcTogetoptimummeltquality,todispersepigments
andcolorconcentrates,itisstronglyrecommendedtoadda
mixingheadThepurposeoaproperlydesignedmixingheadis
nottomixmaterialbyturbulence(turbulentfowisimpossible
withhighlyviscousmoltenpolymer),butbyorcedchangesin
fowdirection
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ELCee Screw for Optimum Productivity
Anothersolutiontoachieveoptimumproductivitywhenmolding
highlycrystallineresinssuchasDelrin,istouseaspecial
screwdesignedbyDuPontcalledtheELCeescrew(patent
appliedor)TheELCeescrewisdesignedtocapitalizeonthe
rheologicalcharacteristicsoresinswithbettercontroloshear
onthemelt,makingthescrewmoreecientThisallowsthe
moldertorunthemoldingmachinesatastercyclesandproduce
partsohigherquality(ConsultyourlocalDuPontrepresentative
ormoreinormation)
Cylinder Temperature Control
Thisisdeterminedbythemachinemanuacturer,buttwo
commentsshouldbemade
Thetemperaturecontrolshouldprovideatleastthree
independentzones,withthermocouplesplacednear
thecenteroeachzoneBurn-outooneormoreheater
bandswithinazonemaynotbereadilyapparentrom
thetemperaturecontrollers,sosomemoldershaveused
ammetersineachzonetodetectheaterbandmalunctions
UsuallyorDelrinthereisnoneedtocooltheeedthroat,
butincasesuchaneedexists,thewaterfowshouldbekept
toaminimumOvercoolingtheeedthroathasbeenobserved
asamajorreasonorcontaminationbyblackspecksThese
aregeneratedinthebarrel,between therstandsecond
heatingzones,withtheollowingmechanism(seeFigure 10)
ThethermocoupleTC1isinfuencedbythelowtemperature
duetoexcessivecooling,andthesystemwillrespondby
switchingONtheheatingbandsHB1andHB2Thiscausesno
problemwithHB1,butresultsinoverheatinganddegradation
intheareaunderHB2Toreducetheriskoormationoblack
specks,theollowingrecommendationsshouldbeobserved:
a) theeedthroatcoolingshouldbelimitedtoaminimum
temperatureo8090C(176194F);
b) theheaterbandHB2shouldbecontrolledbyTC2,orTC1
shouldbeplacedinthemiddleoHB2,orHB2shouldhave
halthepowerdensityoHB1
HB1 HB2
TC1
Cooling channels Contamination
source
TC2 TC3 TC4
HB3 HB4 HB5 HB6 HB7
HB8
Figure 10. The Risk o Black Specks ContaminationThat Could Arise From the Presence o a CoolingSystem o the Feed Throat
Figure 11. Design o Adaptor and Non-Return Valve
Cylinder Adaptor
TheadaptorshowninFigure 11isdesignedtoavoidholdup
areasandfowrestrictions,thetwomaincausesodegradation
andproblemslinkedtothisareaNotethattheconceptisthe
sameorscrewedadaptorsasrepresentedinFigure 11(used
orsmallscrews40mm)andorboltedadaptors(usedor
largerscrews)Theadaptorshasshortcylindricalsections(Aand
B)whereitjoinsboththenozzleandthecylindertomaintain
accuratematchingothesediameters,eveniitbecomes
necessarytoreacethematingsuracesThematingsuraces(C)
shouldbenarrowenoughtodevelopagoodsealwhenthenozzle
oradaptoristightenedandyetwideenoughtoavoiddeormation
Inadditiontoitsmechanicalunctionoreducingthediameter,
theadaptoractstoisolatethenozzlethermallyromtheronto
thecylinderorbettercontrolonozzletemperatureAseparate
adaptor,madeosotersteelthantheoneusedorthecylinder,
iseasierandlessexpensivetorepairandchangethanacylinder
Italsoprotectsthecylinderromdamageduetorequent
changingothenozzleWiththeboltedadaptor,specialcare
shouldbetakenduringassemblytoensureparallelism(dont
overtightenscrewsromonesideonly)
Nozzle
Adaptor
A
D H
E
B
G FC
Non-Return Valve (Back Flow ValveBFV)
Thenon-returnvalveorcheckringshowninFigure 11prevents
meltromfowingbackwardduringinjectionThisunitis
requentlynotproperlydesignedtoeliminateholduporesin
andfowrestrictionsMalunctioningthatallowsresinbackfowisalsoacommonexperienceandiscausedbypoordesign
ormaintenanceAleakingnon-returnvalvewilladdtoscrew
retractiontime,whichcanincreasecycle,anditwillalsocause
poorcontrolopackinganddimensionaltolerances
Thenon-returnvalvemustmeettheollowingrequirements:
Noholdupspots
Nofowrestrictions
Goodseal
Controlowear
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Figure 12. Reverse Taper Nozzle
Heater Band
Thermocouple well
A
B
Thermocouple wellHeater BandA B
Figure 13. Straight Bore Nozzle, Only or Machines WithoutScrew Decompression
Theserequirementsareprovidedorinthenon-returnvalve
showninFigure 11
Theslotsorfutes(D)inthescrewtiparegenerously
proportioned,andthespace(E)betweenthecheckringandtip
issucientorresinfow
Theseatingothexedringiscylindricalwhereitjoinsboththe
endothescrew(F)andthescrewtip(G)topermitaccurate
matchingothesediametersandavoidholdup
Thescrewtipthreadhasacylindricalsection(H)aheadothe
threadsthattscloselyinamatchingcounterboreorsupport
andalignmentothescrewtipandseatring
Thescrewtipandcheckringseatshouldbeharder(about
Rc52)thanthefoatingring(Rc44),becauseitislessexpensive
toreplacethefoatingringwhenwearoccurs
CorrosionresistantsteelissuggestedorthetipGoodmatching
ocylindricaldiametersisessentialtoavoidholdupspots
NozzleAswithothersemi-crystallinepolymers,Delrinmaydroolrom
thenozzlebetweenshotsithenozzleistoohot,oritmayreeze
itoomuchheatislosttothespruebushing
ThenozzledesignshowninFigure 12cansolvetheseproblems
Theollowingshouldbeconsidered:
1 Theheaterband(A)shouldextendasclosetothenozzletip
aspossibleandcoverasmuchotheexposedsuraceas
practicalThiscounteractsanyheatloss,especiallyheatloss
tothespruebushing
2 ThethermocouplelocationisimportantAnappropriate
location(B)isshowninthesamepicture
3 Adequatetemperatureuniormityisrequiredsothatlocal
overheatingorprematurereezingisavoided
4 Topreventpolymerdegradationthesteeltemperatureshould
notexceed190C(374F)
5 Thenozzleheatershouldhaveitsownindependent
temperaturecontroller
Screwdecompressionorsuckbackisrequentlyusedtomake
controlodrooleasierwiththeseopennozzlesThiseatureis
availableinmostmachines
Whennotavailable,adesignsuchastheoneillustratedin
Figure 13shouldbeused
Althoughshutonozzleshaveoccasionallybeenused
successullywithDelrin,theytendtocauseholduporesinthat
resultsinbrownstreaksorgassing,especiallyatersomewear
hasoccurredinthemovingpartsothenozzleThesenozzlesare
notgenerallyrecommendedorDelrinonsaetygroundsalone
Note:Withalongnozzle,thethermocouplewellBshouldbe
positionedinthemiddleothenozzleandnotatthebackothe
nozzle
Evaluation of Melt Quality
Belowarepresentedtwoquickandeasyteststoevaluatethe
meltqualitydeliveredbytheinjectionunitAlthoughtheresultis
linkedwiththetemperaturesettingotheinjectionunit,itisalso
highlydependentonthedesignotheinjectionunit
Foaming Test
Theoamingtestisrecommendedtodeterminethequalityo
theresinatermeltingintheinjectionunit,ie,thequalityothe
resinANDthequalityotheinjectionunit
Procedure:
1 Whenthemachineisrunningincycle,stop
themachineaterscrewretractionor3minorpigmentedDelrin(10minornaturalmaterial)
2 Purgeatlowspeed(toavoidhotsplashes)intoacupand
observethemoltenmaterialor1or2minThenputthe
moltenmaterialinabucketowater
3 Thenrechargethescrewandwait2moreminutes
(10moreminutesornaturalmaterial)
4 Repeatoperation2
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Anunstablemeltwillgrow(oam)duringtheobservationand
foatinthebucketAstablemeltwillstayshinywithatendency
toshrinkduringtheobservation,andwillsinkinthebucket
Foamingresinwillquicklycausemolddepositandwillaccelerate
screwdeposit,whichmayleadtoblackspeckcontamination
Thistechniqueisuseultoevaluatenon-DuPontcolorsystems
(colormasterbatches,liquidcoloring)
Theoamingtestcanalsobeusedtodetectinadequatequality
otheinjectionunit(eg,problemsothroatcoolingand
consequentoverheating,excessivenozzletemperature,hold-up
spots,etc)
Unmelt Test
Theunmelttestisrecommendedtoevaluatemelthomogeneity:
Whenthepressisrunningoncycle,stopattheendoacycle
andpurgeoneshot;
chargethescrewimmediatelywiththeshotvolumeusedand
purgeagain;
repeattheoperationuntildetectionolumps/irregularitiesin
thepurgecomingoutothenozzle
Isuchlumps/irregularitiesappearaterlessthan3purges,the
riskounmeltisveryhighandshouldbedealtwithbyincreasing
cylindertemperature,byloweringscrewRPMandbyincreasing
backpressureIsuchchangeslengthenthecycletimetoo
much,amoreappropriatescrewdesignshouldbeused(see
Table 4)Ilumps/irregularitiesappearater3purgesbutbeore
6,thesituationisacceptable,butthereisnotmuchsaety
marginItheyappearater6purges,thereisaverylowriskounmelt
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Figure 14. Exploded View o Mold
LocatingRing
SprueBushing
FrontClampingPlate
FrontCavityPlate(APlate)
LeaderPins
LeaderPinBushings
RearCavityPlate(BPlate)
SupportPlate
Cavity
SpacerBlock
EjectorRetainerPlate
ReturnPin
EjectorPlate
KnockOutPins
RearClampingPlate
SpruePuller
SupportingPuller
StopPin
Ability to Fill
Meltviscositylargelygovernstheabilityoaresintollamold
DelrinacetalresinsrangeinmeltviscosityromDelrin900P,
thelowestinviscosityormostfuid,toDelrin100P,thehighest
TheviscosityoDelrindoesnotdecreaserapidlyasmelt
temperatureincreases,incontrasttoamorphousthermoplastic
resinsIncreasingmelttemperaturewillnotgreatlyimprovethe
abilityoDelrintollathinsection
Inadditiontothepropertiesotheresin,themoldingconditions
andcavitythicknessdeterminethedistanceofowFigure 15
showsthemaximumfowdistancesthatcanbeexpectedat
twocavitythicknessesorDelrinacetalresinsasaunctiono
injectionllpressureThesecomparisonsweremadeinanopen-
endedsnakefowmoldwithnogaterestrictionObstructionsin
thefowpath,suchassuddenchangesinfowdirectionorcore
pins,cansignicantlyreducethefowdistance
Molds
Delrinacetalresinshavebeenusedinmanytypes
omolds,andmoldershaveawealthoknowledgeconcerning
molddesignorDelrinMoldsorDelrinarebasicallythesame
asmoldsorotherthermoplasticsThepartsoatypicalmoldare
identiedinFigure 14
Thissectionwillocusontheelementsomolddesignthat
deservespecialconsiderationorprocessingDelrinandcanlead
tohigherproductivityandlowercostorthemolderThesetopics
are:
Abilitytoll Undercuts
Gates Runnerlessmolds
Runners Moldmaintenance
Vents
Moldshrinkageandotheraspectsomoldsizingarediscussedin
DimensionalConsiderations(seepage29)
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Gates
Thegatesoamoldplayamajorroleinthesuccessorailure
oamoldingjobThelocation,design,andsizeoagatearekey
actorstoallowoptimumpackingObviously,thedesignwillbe
dierentthantheoneusedormoldingamorphousmaterialIn
thatcasethefowshouldstopassoonaspossibleaterllingthe
cavitytoavoidoverpacking(fowin)andsinkmarksatgate(fow
back)Withcrystallinematerial,thelocation,designandsizeo
thegateshouldbesuchthatitwillallowacontinuousfowduring
ALLthepackingphase(Holdpressuretimeseepage27)
Gate Location
Asakeyrule,whenapartisnotuniorminwallthickness,the
gatemustbelocatedinthethickestsectionTherespecto
thisbasicprincipleplaysanessentialroleinobtainingoptimum
packingandconsequentlythebestmechanicalproperties,dimensionalstabilityandsuraceaspectOcourseevery
bottleneck(reducedsectionalongthefowothemelt)shouldbe
avoidedbetweenthegateandallareasothepart
Anareawhereimpactorbendingwilloccurshouldnotbe
chosenasthegatelocation,becausethegateareamayhave
residualstressandbeweakenedsinceitworksasanotch
Similarly,thegateshouldnotcauseaweldlinetooccurina
criticalarea
Thegateshouldbepositionedsothattheairwillbeswept
towardapartinglineorejectorpinwhereconventionalvents
canbelocatedForexample,aclosed-endtubesuchasapen
capshouldbegatedatthecenterotheclosedend,soairwill
beventedatthepartinglineAnedgegatewillcauseairtrapping
attheoppositesideneartheclosedendWhenweldlinesareun
avoidable,orexamplearoundcores,anescapeorgasesmust
beprovidedtoavoidseriousweaknessandvisualfawsSpecicrecommendationsorventingaregivenlaterinthissection
AnotherconsiderationinchoosingagatelocationorDelrin
issuraceappearanceGatesmearorblush,aswellasjetting,
areminimizedbylocatingthegatesothatthemeltenteringthe
cavityimpingesagainstawallorcorepin
Acentralgatelocationisotennecessarytocontrolroundnesso
gearsandothercriticalcircularpartsMultiplegates,usuallytwo
toour,arecommonlyusedwhenthereisacentralholetoavoid
adicult-to-removediaphragmgate
Gate Design
Asmentionedabove,orcrystallinematerialslikeDelrinthe
thicknessothegateoritsdiameter(orapin-pointgateor
tunnelgate)determinesthereeze-otime,andthereore
alsodetermineswhetheritispossibletopackthepart(to
compensatethevolumereductionduetocrystallization)and
maintainthepressureduringsolidicationThegateshould
remainopenuntilthepartdensityismaximumoraspecic
materialThethickness(ordiameter)othegateshouldamount
to5060%othewallthicknessatthegateThewidthothe
gateshouldalwaysbeequalorgreaterthanthegatethickness
Thelengthothegateshouldbeasshortaspossibleandnever
exceed08mm(003in)ThegateareaothepartshouldnotbesubjectedtobendingstressesduringactualserviceImpact
stressesareparticularlyliabletocauseailureinthegatearea
ThemostcommontypesogatesaresummarizedinFigure 16
DIAPHRAGMGATE:Circulargateusedtollasingle
symmetricalcavityTheadvantagesareareductionoweld
lineormationandimprovementollingratesHoweverthe
parthastobemachinedtoremovethegate
DIRECTGATE:Thesprueeedsdirectlyintothemold
cavitywithoutrunnersThisdesignmayotenleadtosurace
deectscomingromthenozzle(eg,coldslug,coldskin,
entrappedair)
EDGEGATE:UsualtypeogatewithtwoplatemoldsItisno
seldegating
FANGATE:ThisgateisusedtoenlargethefowrontUsually
itleadstoareductionostressconcentrationsinthegate
areaLesswarpageopartscanusuallybeexpectedbythe
useothisgatetype
PINPOINTGATE:Thisgateisusedwiththreeplatemolds
Itisseldegating
Injection pressure, MPa
Flowd
istance,mm
Flowd
istance,
in
12080 100
Injection pressure, psi
17.40011.600 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 o Delrin Acetal Resins
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Diaphragm gate
Submarine gate(bucket type)
Direct gate
Edge gate
Fan gate
Fan gate
Pin point gate
Ring gate
Figure 16. Schematic View o the Most CommonTypes o Gates
Figure 17. Details o a Typical Edge Gate Suitable or 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 o a Submarine Gate (Tunnel Gate)Adequate or Delrin (let side). The One on theRight is Not Adequate or Crystalline Polymersand Would Give Problems with Delrin.
RINGGATE:SeeDIAPHRAGMGATE
SPRUEGATE:SeeDIRECTGATE
SUBMARINEGATE:Atypeoedgegatewheretheopening
romtherunnerintothemoldisnotlocatedonthemold
partinglineItisusedtoseparatethegateromthepartwith
atwoplatemold(sel-degating)
TUNNELGATE:SeeSUBMARINEGATE
DetailsoatypicaledgegatesuitableorDelrinareshownin
Figure 17
Figure 18showsdetailsoasubmarinegateadequate
orDelrin(let),comparedtoasimilartypeogatenot
recommendedorcrystallinematerials(right)
Design criteria:
alwaysgateinthickestareaothepart;
diameterothegatedmustbeatleasthalthepart
thicknessThelengthmustbeshorterthan08mm(003in)
topreventprematuregatereezingduringpacking;
theinscribeddiameterDothetunnelnexttothegatemustbeatleast12thepartthicknessT
ThegateshownontherightsideoFigure 18isnot
recommendedorcrystallinematerialslikeDelrin,because
suchconicalgatesectionscrystallizebeoretheendocomplete
partpackoutThisresultsinlowmechanicalperormanceand
uncontrolledshrinkage
Figure 19showsdetailsoathreeplategatedesignadequate
orDelrin(let),comparedtoasimilartypeogatenot
recommendedorcrystallinematerialsThedesigncriteria
illustratedabovearealsoapplicabletothiskindogate
Note:Restrictionsaroundthespruepullerwillleadtoincomplete
partpackoutSo,thediameterD1 inFigure 19shouldbeatleast
equaltodiameterD
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Runner SystemGuidelines
Keyguidelinestoollowwhendesigningarunner
systeminclude:
a runnersshouldstayopenuntilallcavitiesareproperlylled
andpacked;
b runnersshouldbelargeenoughoradequatefow,minimum
pressurelossandnooverheating;
c runnersizeandlengthshouldbekepttotheminimum
consistentwithpreviousguidelines
Eachotheseactorscanaectqualityandcostomolded
partsFactor(a)shouldberegardedasthemostcritical
Thecrosssectionotherunnersismostotentrapezoidal,which
representsanoptimumpracticalcompromisewithrespectto
theullroundsectionTheeectivecrosssectionotherunner
isinthiscasethediameterotheullcirclethatcanbeinscribed
init
ForpartsoDelrintohavethebestphysicalproperties,
therunnersnexttothegatemusthaveatleastaninscribed
diameterthepartthicknessT+1mm
Whenthemoldingsareverythin,however,thisrunnercannotbelessthanabout15mm(006in)inthicknessTherunner
thicknessisusuallyincreasedateachotherstoneortwo
turnsromthecavity,asshownintheexampleoFigure 20
Figure 21. Direct Gating (Let) and Indirect Gating to Breakthe Flow (Right), in a One-Cavity Mold
Figure 22. Balanced (Let) and Unbalanced (Right) RunnerSystems in a 16-Cavity Mold
Single Cavity Mold
Thesimplestrunnercongurationorasinglecavitymoldcould
bedirectgating(seeFigure 21)Inthiscase,however,itwould
benecessarytohaveacoldslugcatcherdirectlyonthe
part,withassociatedsuraceproblemsandlowermechanical
propertiesinthatareaThepreerredsolutionisthentobreak
thefowasindicatedinFigure 21
Runner Layout
Aperectlybalancedlayout(withequalfowdistanceromthe
spruetoeachcavity)isbestachievedithenumberocavities
isequaltoapowero2(2,4,8,16,32,64,128,etc)Seean
exampleoa16-cavitymoldinFigure 22withbalanced(let)and
unbalancedrunnersystemsAperectlybalancedlayoutmaybe
impracticalandexpensive
D
d
2
D1
T*
Figure 19. Details o a Three Plate Gate Design Adequate orDelrin (Let Side). The One on the Right is NotAdequate or Crystalline Polymers and WouldGive Problems with Delrin. *Gate Length Shouldbe
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Figure 23. Examples o Unbalanced 16-Cavity Mold. TheSolution on the Right is Provided with OverfowWells to Trap Cold Slugs.
Figure 24. Example o Spiral Eect in a 32-Cavity Mold.Cavities 11, 14, 19, 22 Will Be Filled First and MayShow Splays and Mold Deposits.
1
16 14
1917
32
24
9
22
11
25
8
Figure 25. Sprue and Nozzle Design Oten Used withDelrin. The Dimensions are Linked with theDimensions o the Part and o the Runners.
Figure 26. Example o a Design o a Nozzle Without SprueUsed with 2 Plate Molds. Remember that orDelrin the Nozzle Temperature Should NotExceed 190C (374F).
D N1
D N2
1
D N1
D N2
5 mm
Whenanunbalancedrunnersystemisselected,thelayout
showninFigure 23(let)couldpresentmorerisksoquality
problemsThefowtendstostopateachotheearlygates
duetotherestrictionandthematerialstartstocrystallize
Then,astherunnercontinuestobelled,thepressurerises
andthethecoldslugswhichstartedtobebuiltup,arepushed
intothecavity
Toreducesuchrisk,thesolutionshowninFigure 23(right)isrecommendedInsuchconguration,thecoldslugstendtobe
trappedintoeachoverfowwell
Incaseomulti-cavitymolds(16cavities),theso-calledspiral
eectcouldtakeplaceintheinternalcavitiesothelayout
(seeorinstanceFigure 24),duetoover-heatingothemeltin
runners,causedbylocalizedshearTominimizenegativeeect
likesplaysormolddeposit,shearshouldbereducedbyusing
appropriaterunnerdimensions
Formulti-cavitymoldsorsmallthicknessparts(1mm[004in]),
thedesignorunnersshouldbecheckedbyrunningadetailed
fowanalysisstudy
Nozzle and Sprue
Nozzleandspruediametersaredirectlylinkedwiththe
dimensionsothepartandotherunnersThedesignershould
rstdecideithesprueisneededornotIyes,adesignlike
theoneshowninFigure 25couldbeselected,onethatin
manycaseshasprovedtobethemosteectivewithcrystalline
materialslikeDelrinDuetoitsparallelcylindricalshapeitis
easytomachineandpolish,allowslargenozzlediameters,
anditiseasytoejectduetohighshrinkageGuidelinesorthe
dimensionare:
aspruediameter1atleastequaltotheinscribeddiameter
othemainrunner;
anozzlediameterDN1equalto1minus1mm
Incasethedesignerselectsadesignwithoutasprue,along
nozzlemayberequiredasshowninFigure 26ora2platetool,
andinFigure 27ora3platetoolAgain,thedimensionsare
linkedtothedimensionsothepartandotherunners(guideline
nozzlediameterDN1equalstothemainrunnerinscribed
diameterminus1mm)
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AreviewothekeyrecommendationsrelatedtothesprueandrunnersystemollowsItcanbeusedasaquickreerencelistto
checktheirdesign
1 Cylindricalparallelspruepreerred:seeFigure 25and
Figure 28-1
2 Spruepulleror2platemold:seeFigure 28-2.
3 Coldslugwellor3platemold:seeFigure 28-3.
4 Perpendicularfowsplitswithcoldslugwellsateachsplit,see
Figure 28-4
Temperature
Modulus
C
A T
BM
IM
Tg
Tm
Figure 29. Sotening/Melting Behaviour o Amorphous andCrystalline Polymers
5 Nofowrestrictioncausedbyspruepullerin3platemold,see
Figure 28-5
6 Runnerdimensions:
orpartshavingthickness>15mm(006in),ollowgenera
rulesorcrystallinepolymers(Figure 20);
orthinnerpartsandmulti-cavitymolds,afowanalysis
mayberequiredtoselectdimensionsthatwillavoid
over-shearing7 Runnersshouldbeproperlyvented,seeFigures 28,
29and30
8 Balancedrunnersrecommended(seeFigure 24)
9 Forthinpartsandlargenumberocavities,unbalanced
runnersmaybeacceptableHowever,partsshouldneverbe
gateddirectlyontothemainrunner(seeFigure 23)
Hot Runner Mold for Crystalline PolymersPreliminary Comments
Thissectionincludesallhotrunner,hotspruebush,and
runnerlessmoldsTheollowingisnotintendedtorecommend
anytrademarkorsystembuttopresentthebehaviorandthe
needsocrystallinepolymersinsuchtools
Thequestionthatrequentlyarisesiswhentousehotrunner
moldswithcrystallinepolymerslikeDelrinThechoicedepends
onmanyactors,andparticularlyonthequalityneeded,ie,
mechanicalperormance,suraceaspect,percentageorejects
Status
Allsuchmoldsgivetheobviousadvantagesolessmaterial
toplastiy,no(orminimum)regrindandshortercyclesOnthe
otherhand,hotrunnermoldsaremoreexpensiveandheavier;
theyneedmoremaintenanceandbetter-trainedoperators
thanconventionalmoldsInaddition,itheyarenotproperly
designed,theheatneededtorunthemcouldspreadtoallparts
othemoldandcaninactcausethecycletimetoincrease
Figure 27. Example o a Design o a Nozzle without SprueUsed with 3 Plate Molds. Remember that orDelrin the Nozzle Temperature Should NotExceed 190C (374F).
Figure 28. Key Rules or the Design o the Sprue and
Runners o a 2 Plate Mold (Top) and o a3 Plate Mold (Bottom).
D N1
D N2
DSP 1
2
5 mm
1
4
6
Vent
channel
2
3
5 6
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Oneapproachistoevaluatetheexpectedincreaseotheoretical
productivityversusconventionalmoldsIsuchanincreaseis
lowerthan25%,itwouldbewisetostaywitha3platemoldthat
willbecheapertobuild,startandrun
Thebreak-evenoabout25%appliestoullhot-runnersystems;
orothermolds(withhotspruebushes,coldsub-runners)the
break-evenpointismuchlower
Direct Gating Versus Cold Sub-Runners or CrystallinePolymers
Whendesigningahotrunnermoldorcrystallinepolymers,it
shouldbekeptinmindthatdirectgatingviahotrunnerismore
dicultwithcrystallinepolymersthanwithamorphousonesThedierencecomesromthesoteningormeltingbehaviorothese
twotypesopolymers
Anamorphousmaterialexhibitsagradualsoteningbehavior
aboveTgromthesolidstatetotheliquidstate,allowingawide
processingwindowintermsotemperatureandviscosityIn
act,asitstemperatureincreasesaboveTg(seeFigure 29)an
amorphouspolymer(curveA)lendsitselrsttothermoorming
(T),thentoblowmolding(BM)andnallytoinjection
molding(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 o a Part and oits Runner System
Onthecontrary,theTghasusuallyalimitedornegligibleeect
onthestructureocrystallinepolymers,whicharesolidabove
TgAtthetemperatureT
m,crystallinepolymersmeltsharplyand
becomeliquid(curveC)
Suchbehavioroacrystallinematerialmayinvolvetherisko:
Droolingaroundthegatewithconsequentproblemsobad
suraceaspectanddeormation
Pluggingothegatesbysolidiedmaterial,plugswhich
willbepushedintothecavities,withconsequentproblems
osuracedeectsandlowermechanicalperormances
ThebestwaytopreventsuchproblemsistouseCOLD
SUB-RUNNERS
Thermal Control o Hot Runner Molds
Thermalmanagementandstreamliningothefowarevery
importantorhotrunnertoolsItshouldbecheckedthata
relativelylowtemperaturesetting(190C[374F])givesan
easyfowothematerialwithnohold-upspots
Thereasonisthat,duetotheviscosityothepolymer,itsfowis
alwayslaminarThismeansthatthematerialwillremainagainst
thesteelwallothehotrunner,andresidencetimewillbevery
longForDelrin,toavoidthermaldegradationwithprolonged
times,thesteeltemperatureshouldneverexceed190C(374F)
Ithehotrunnersystemsolidiesatthattemperature,then
itmustbemodiedtoimprovethermalinsulationandheat
distributiontoremovecoldspotsDegradationcanresultin
splays,odor,blackspecksandmolddeposit
Conclusions
WithcrystallinepolymerssuchasDelrin,werecommend
theollowing:
Aminimumo25%theoreticalcostdecreaseshouldbe
expectedbeoreahotrunnerisconsidered
Highlytrainedmachineoperatorsandmoldmaintenance
toolmakersshouldbeavailable
Useocoldsub-runners,neverdirectgatingstraight
ontothepart
UseoDelrinPgrades
Alltemperaturesinthehotrunnersystemmustnotexceed
190C(374F)
Avoidtheuseohotrunnermoldsisuracedeectsarenot
acceptableandhighpartmechanicalperormanceisrequired
Avoidtheuseohotrunnersortoughenedgrades
Vents
VentingamoldorDelrinisparticularlyimportant,andspecial
attentionshouldbegiventothisactorduringboththedesign
othemoldanditsinitialtrialThisattentionisrequiredbecause
burningopartscausedbyinadequateventingisnoteasily
observedwithDelrinWithotherresins,poorventingresultsin
ablackenedandburnedspotonthepartWithDelrin,however,
theremaybeeithernovisiblefaworaninconspicuouswhitish
markonthemolding
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VentingproblemswithDelrinacetalresinsmaybemade
moreobviousbysprayingthemoldwithahydrocarbonor
kerosene-basedsprayjustbeoreinjectionIventingispoor,the
hydrocarbonwillcauseablackspotwheretheairistrappedThis
techniqueisparticularlyuseulordetectingpoorventsinmulti-
cavitymoldsAconvenientsourceohydrocarbonspray
isarustpreventativespray
Ventsshouldbelocatedat:
1 theendoanyrunner;
2 anyfowjunctionwhereairisentrappedandaweldline
resultsThepositionoweldlinescanbedenedby
shortshots
OnlyNOventingtogetherwithexcessiveastinjectionspeedwill
causecorrosionothetoolattheweldlineswithDelrin(diesel
eect)InadequateventingomoldsorDelrinmaycausea
gradualbuildupomolddepositwhereventsshouldbelocated
andinmoldcrevicesthroughwhichlimitedventinghastaken
placeThesedepositsconsistoawhitesolidmaterialormed
romthetracesogasevolvedduringnormalmoldingGoodventsallowthisgastoescapewiththeairromthecavities
Poorventingmayalsoreducephysicalpropertiesatweldlines
Ventingproblemsmaybeaggravatedbyhighmelttemperature,
longholduptime,orholdupareasintheinjectioncylinder,which
willgeneratemorethannormalamountsogasFastinjection
llspeedwillalsoaggravatetheseproblemsRemediesormold
depositproblemsarelistedintheTroubleshootingGuide(see
page39)
Ventingusuallyoccursthroughthepartinglineoamoldandis
providedbymachiningchannelsinthecavityplateandinserts
Insomecases,ventingmaybeaccomplishedaroundanejector
pinThisventwillalsobeimprovedbygrindingfatsonthepin
andrelievingtheventaterashortlandPinsthatdonotmove
withtheejectionsystemtendtoclogandnolongerprovide
ventingaterashorttime
Ventingtherunnersystemishelpulinreducingtheamounto
airthatmustbeventedthroughthecavitiesBecausefashis
unimportantontherunner,theseventscanbeslightlydeeper
thancavityvents,orexample,006mm(00024in)
ThedrawingsinFigure 30showtherecommendeddimensions
orventsincavitiesorDelrin
Note:Duringmoldmaintenance,ventdepthand/orhobbing
shouldbecareullycheckedVentsshouldbemodiedithevent
depthislessthan0010015mm(0000400006in)
Undercuts
GeneralsuggestionsorstrippingundercutswithDelrinacetal
resinsare:
Theundercutpartmustbereetostretchorcompress,that
is,thewallothepartoppositetheundercutmustclearthe
moldorcorebeoreejectionisattempted
Theundercutshouldberoundedandwell-lletedtopermit
easyslippageotheplasticpartoverthemetalandto
minimizestressconcentrationduringthestrippingaction
Adequatecontactareashouldbeprovidedbetweenthe
knockoutandplasticparttopreventpenetrationothemolded
partorcollapseothinwallsectionsduringthestripping
action
ThelengthothemoldingcycleandspecicallytheHold
(Pressure)Time(HPT)shouldbeoptimumtoavoidexcessive
shrinkagewithinsideundercutsSucientpartrigiditymust
bedevelopedwithoutcausingbindingduetoexcessive
shrinkagearoundpinsorminganinternalundercutEjection
opartswithundercutsontheoutsidediameterwillbeaided
bymoldshrinkage
Highermoldtemperature,whichkeepstheparthotterand
morefexiblewhenthemoldopens,mayaidejectionrom
anundercut
Generally,partsoDelrinacetalcanbemoldedwitha
maximum5%undercutCalculationoallowableundercut
isillustratedinFigure 31Theallowableundercutvaries
somewhatwithbothwallthicknessanddiameter
A
R
BInside undercuts
Outside undercuts
A
B
R
A
B
30
A
B
R
R
Figure 31. Calculations or % Undercut (BA)/B 5%
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Notch radius, mm
Impactstrength,
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 o MoldedNotch Radius
Figure 33. Suggested Rib Dimensions Versus Wall Thickness
0.5-0.7T
R = 0.2-0.3T
T
Filling time, s
Tensilestrengthatyield,
MPa
Izodunnotched,
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 (Let Scale) and Unnotched IzodImpact (Right Scale) o a Delrin 100P Test Bar,4 mm thick, Molded at Both Ends with DierentFilling Times.
Sharp Corners
Oneomajorcausesoailureoplasticpartsareinternalsharp
cornersAsharpcornerinapartactsasanotchandinitiates
breakataverylowenergyThediagraminFigure 32showsthe
eectonotchradiusonimpactresistanceotestbarsmolded in
twogradesoDelrinNotethatthenotcheshavebeenmolded
(simulationoreallieandnotmachinedasrequiredbythe
standardIzodtest)
Fromthisdiagramitcanbeseenthatanincreaseo aninternal
radiusocurvaturerom001(almostasharpcorner)to02mm
doublestheimpactresistance
Notealsothatsharpcornersarenotdesirableinplasticparts
becausetheyareanimportantcontributingactortowarpage
Ribs Design
Veryoten,ribbedpartswillperormmuchbetterintermo
cycletime,mechanicalperormancesandwarpagethanvery
thickunproperlypackedpartsItiseconomicallyimpossibleto
packsectionsabove68mm(024032in)thicknessduring
allthecrystallizationtime(solidication:seeFigure 30or
Hold[pressure]Timevspartthickness)Howeveranimproper
ribdesigncouldalsocausedeectssuchassinkmarks
RecommendedribdimensionsareshowninFigure 33Note
thattheradiusatthebaseotheribshouldnotbetoosmallto
preserveparttoughness(seeFigure 32)
Weld Lines
WeldlinesoccurwheretwomeltfowsjointogetherWeldline
positioncanbedenedbyshortshots,orbyfowsimulations(i
themolddoesnotexistyet)Ithemoldisprovidedwithproper
venting(seepage18),theweldlinestrengthshouldbeatleast
8090%othenominalstrengthvalueotheresin
Tooptimizeweld-linestrength,twoparametersareimportant:
1 optimumHold(Pressure)Time,toensuretheweldingothe
fowrontsunderpressure(orthecorrectHPTseepage26);
2 optimumllingrate,whichwilldependonpartthickness
(approximately1secondpermm(004in)opartthickness)
Figure 34showstheweldlinestrengthoa4mm(016in)
thicktestbarinDelrin100PgatedatbothendsBothtensile
strengthandtoughnessareseriouslyaectedillingtimeisnot
optimized
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Mold Maintenance
Asageneralrule,moldsorprocessingDelrinrequirethe
samecareasthoseorprocessingotherthermoplastic
materialsWipingthemoldandapplyingarust-preventing
solutionisusuallyadequateateraproductionrun
Vent Maintenance
Duetothecriticalnatureothevents,theventdimensionsshouldbecheckedduringroutinemaintenanceVentdepth
and/orhobbing(deormationothepartinglineoppositethe
vent)shouldbecareullycheckedVentsshouldbemodiedi
theventdepthislessthan001mmto0015mmAnyhobbing
thatblockstheventsshouldbegroundo
Mold Cleaning
Dependingonthetypeodepositthecleaningprocedureisas
ollows:
White deposit
Whitedepositisduetotheaccumulationo
paraormaldehydeThisdepositcanberemovedwithbenzyl
alcoholorisopropanolFrequentcleaningothetoolwith
thesesolventsduringmoldingwillpreventtheaccumulationothisdeposit
Translucent or colored deposit
Thisdepositisnormallyobservednearthegate(incaseo
overshearothematerial),onpinsornearhotspotsTheuse
oalessshearinggate(seegatedesignrecommendations)
oramoreevenmoldtemperaturewillstoportremendously
decreasethebuildupothisdepositItcanberemoved
withcommercialalkalinechemicalcleanersEciencyothe
cleaningagentcanbeimprovedwithanultrasonicbath
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Molding Process
InjectionmoldingoDelrinacetalresinissimilartothatoother
thermoplasticresinsTheengineeringapplicationsorwhich
Delrinisused,however,requentlyrequiretightspecications
onstrength,dimensionsandsuracecondition,sothatcontrolo
themoldingoperationbecomesmorecritical
Theinormationdiscussedinthissectionincludes
suggestionsor:
Start-upandshutdownprocedures,handlingprecautions
OperatingconditionsorDelrin
Techniquesoroptimumproductivitymolding
Start-up and Shutdown ProceduresStart-up with Resin Change
Thesuggestedstart-upprocedurewithDelrinisdesigned
topreventoverheatingotheresinandcontaminationintheinjectionunitwithmaterialrompreviousruns
Tostartupamachinewhichcontainsanotherresin,theinjection
unitmustbepurgedwithcrystalpolystyreneuntilthecylinder
andotherhightemperaturezoneshavebeenclearedThiscan
normallybedonewithcylindertemperaturesintherange
210250C(410482F),iappropriateorthepreviousmaterial
Thenozzleisquitediculttocleanbypurging,becausethe
laminarfowinthisarealeadstoalayeropolymersticking
tothemetal(thisisalsotrueorhotrunners)Itisthereore
recommendedtoswitchothenozzleheater,removethe
nozzle,cleanittogetridalltracesopreviouspolymer,and
reassembleitThecylindertemperaturesshouldthenbe
adjustedtoabout215C(419F),andthenozzletemperatureto
190C(374F)Whenbothcylinderandnozzlehavereachedthe
expectedtemperatures,Delrincanbeaddedtothehopper
Inunusualcircumstances,anintermediatepurgewithaharsher
compoundmayberequiredtoremoveadherentdepositsrom
thescrewandcylinderSpecialpurgecompoundsareusedor
thispurpose
Thesepurgecompoundsmustalsoberemovedromthecylinder
bypurgingwithpolyethyleneorpolystyrenebeoreDelrin is
introducedIntheworstcases,eg,ateruseoglass-reinorced
resinsorseveredegradationopreviousmaterial,itmaybe
necessarytopullthescrewandcleantheequipmentmanuallyto
preventcontaminationomoldings
Saety point:Polystyreneischemicallycompatiblewith
Delrin,whereasevenatraceopolyvinylchloride(PVC)is
notContaminationoDelrinwithsuchmaterialcancause
objectionableodororevenaviolentblowback
Start-Up From a Cylinder Containing Delrin
Aterasaeshut-downprocedure,thescrewandthecylinder
shouldbeessentiallyemptyTorestart,thenozzleandcylinder
temperaturesshouldbesetat190C(374F)topreheatthe
cylinderandtheresinitcontainsWhenthecylinderhasreached
thesettemperature,ensurethatthenozzleisopenandincrease
thecylindersettingstonormaloperatingtemperaturesWhenall
temperaturesareintheoperatingrange,thehoppercanbelledandmoldingcanbeginaterabriepurgewithDelrin
Shutdown When a Restart with Delrin is Planned
Shutothehoppereedandcontinuemoldinguntilthecylinder
isemptyForlargemachines(withascrewdiameterabove
40mm[157in])itisrecommendedtopurgethecylinderwith
crystalpolystyrene,movethescrewullyorward,thenswitch
otheheaterbandsForsmallmachinesmovethescrewully
orwardandswitchotheheaterbands
Shutdown When a Restart with Another Resin is Planned
Shutothehoppereedandcontinuemoldinguntilthecylinder
isemptyPurgewithcrystalpolystyrene,leavethescrewully
orward,thenswitchotheheaterbands
Temporary Interruption
AmoldingmachinewithDelrininthecylinderatmolding
temperaturesshouldnotbeallowedtostayidleThemaximum
recommendedcylinderresidencetime,undernormalmolding
conditions,is10minorpigmentedmaterialand20minor
naturalstandardmaterialInexcessothesetimes,resin
decompositionmayoccur
I,duringthetemporaryinterruption,thecylinderresidence
timereachestheabovelimits,closethehoppereed,emptythecylinderandleavethescreworwardThecylindertemperatures
shouldbereducedtoabout150C(302F)(atthesetemperatures
Delrinwillbestableevenoraweekendshutdown)
Action to Follow When the Nozzle Heater Band Breaks Down
Retracttheinjectionunit,closethehopperandslideitouto
thewayIthenozzleisstillopen,ollowthenormalshutdown
proceduresIthenozzleisrozen,heatthenozzlewithagas
torchtomelttherozenmaterialinsidethenozzleandthen
purge
Start-up ater Emergency Shutdown
Adierentprocedureshouldbeusedateranemergency
shutdownduetolossopowerorothercausesInthiscase,the
screwmaybeulloDelrinthatcooledslowlyandwasexposed
tomelttemperaturesoraprolongedperiodThescrewmay
evenbeintheretractedpositionwithalargequantityoDelrin
inrontothescrewInordertoventgasesromresinthatmay
bedegraded,itisessentialthatthenozzlebeopenandheated
tooperatingtemperatureandDelrininthisareabecompletely
meltedbeorethecylinderreachesmelttemperatureThe
cylinderzonesshouldbeheatedtoanintermediatetemperature
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belowthemeltingpointoDelrinandthemachineallowed
toequilibrateatthattemperatureCylindertemperatureso
150175C(300350F)aresuggestedAterallzoneshavebeen
atthistemperatureor30min,cylindertemperaturesshouldbe
raisedto195C(380F)AssoonastheDelrinhasmelted,it
shouldbepurgedromthecylinderwithreshDelrinThepartly
degraded,hotpurgeresinshouldbeplacedinapailowateri
itemitsanodorWhentheoldresinispurgedromthecylinder,thecylindertemperaturesmayberaisedtonormalproduction
settings
Operating Conditions for Delrin Temperature SettingsIntroduction
Thebasicpurposeotheinjectionunitistodelivertothemold
thenecessaryamountoahomogeneousmelt(nounmeltand
nodegradedmaterial)Therulesoconstructionotheinjection
unitormoldingacrystallinematerialhavebeenpresentedin
InjectionMoldingUnit,(seepage7);therulesorthesettings
arepresentedbelow
Note:Tworoughbutpracticalmethodstoevaluatethepresence
ounmeltandodegradedmaterialweredescribedonpage11
andcanbeusedhereaswell
Delrinacetalresinisacrystallinepolymerwithameltingpoint
o178C(352F)FormostgradesoDelrinhomopolymerthe
preerredmelttemperaturerangeis215C(419F)5C*,as
measuredwithaneedlepyrometerinthemeltThecalories
neededtoheatandmeltDelrinwillbeprovidedbyshear(rom
scewrotation)andthebalancebyconductionintheheated
cylinder(slowheattranserduetotheinsulatingcharactero
polymers)
Cylinder temperature
Themainparameterinfuencingthetemperatureproleothe
cylinderistheresidencetime(orHold-UpTimeHUT)othe
polymerintheplasticationunit(seepage8tocalculateHUT)
WithashortHUT(5minutes,longcycletime,lowmeltoutput),
lowersettings,especiallyintherearzone,maybeusedSince
generalizationocylindertemperaturesettingsisdicult,itis
otenwisetobeginwithalevelproleandadjustasneededThediagramshowninFigure 35canbeusedasaguidelineorinitial
temperaturesettings
Notes:
1 AsthepreerredmelttemperatureorDelrin100STand
Delrin500Tisabout10C(18F)lower,thezonesettings
shouldbe10C(18F)lowerthanshowninFigure 35
2 Hoppercoolingisnotneededandshouldnotbeusedor
DelrinAsdescribedinChapter3,excessivehoppercooling
maycreateproblemsoscrewdepositandblackspecks
3 Withverysmallinjectionunitsand/orshortresidencetime
(HUT),pre-heatingthegranules(eg,withaheatedhopper)
mayhelptoachieveanhomogeneousmelt
Nozzle Temperature
Thenozzletemperatureisadjustedtocontroldroolandreezing
(seeNozzlepage11),butitshouldneverbesetabove190C
(374F)inordertopreventpolymerdegradation(thelaminar
fowandhighviscosityothemoltenpolymerresultinverylong
contacttimewiththemetalwall)Ithenozzlereezeswitha
settingo190C(374F),itsinsulationromthespruebushing
shouldbeimproved,oritsinsidediametershouldbeincreased
ieasible
Notes:
1 Practically,itisalwayseasiertosetthenozzletemperature
correctlybyusingspruebreakTheinjectionunitispulledbac
aterscrewrotationandthenthenozzleisinsulatedrom
thecoldmoldThisallowsthecaloriestofowtothetipo
thenozzlewithouthavingtosettoohighatemperature,and
reducestheriskostringingromthenozzle
2 HotrunnerByanalogy,ahotrunnersystemisanozzle
transerringthemoltenresinromtheinjectionunittothe
partHencetheprinciplesandrecommendationsornozzles
*ThepreerrredmelttemperatureorDelrin100STandDelrin 500Tisabout205C(401F)
Figure 35. Cylinder Temperatures Prole VersusResidence Time or a Given Recommended MeltTemperature. Recommended Nozzle Temperatureis 190C (374F) or All Delrin Grades.
Residence time
215
220
225
195
180
FRONT ZONE CENTER ZONE REAR ZONE
< 3 min
> 5 min
Hopper
no cooling or 8090C (176194F) min
3 5 minRecommendedmelt
temperature,C
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arealsovalidorhotrunnersInparticular,thelaminarfow
andhighviscosityothemoltenpolymeragainresultinvery
longcontacttimeswiththemetalwall;sothetemperature
othemetalinthehotrunnershouldneverexceed190C
(374F),inordertopreventdegradationothepolymer
Screw Rotation Speed
Screwrotationspeedbehavesasathermalsetting,because
therotationothescrewwillshearthematerialandsupplyaroundhalothecaloriesneededtomeltandheatDelrinto
therecommendedmelttemperaturerangeo215C(419F)
5C(205C[401F]5CorDelrinTandST)Aswithall
polymers,Delrinissensitivetoshearandamaximumo
02to03m/soscrewperipheralspeedisrecommended
Figure 36showstheoptimumscrewrotationspeedorhigh
viscosityDelrin(type100)andlowviscosityDelrin(types
500to900)asaunctionoscrewdiameter
Back Pressure
BackpressurealsobehaveslikeathermalsettingIncreasing
backpressureincreasestheworkdonebythescrewon
themelt
Theuseotheoptimumscrewdesignorcrystallinematerials,
suchasDelrin,shouldprovidethenecessaryworktomeltand
bringDelrintotherecommendedmelttemperaturewiththe
minimumbackpressureOnlymeltingohighlyviscous
DelrinsuchasDelrin100Pmayrequiresomebackpressure
toavoidthescrewwormingback(leadingtoinconsistentshot
volumeandpad)
Figure 36. Maximum Screw Rotation Speed as Function
o Screw Diameter. The Curve or Delrin
500Pis Also Valid or the Low Viscosity GradesDelrin 900.
Screw diameter, mm
Max.s
crewr
otationspeed,rpm
80 1006040200
50
100
150
200
250
300
0
Delrin 500P
Delrin 100P
Theuseoaninappropriatescrewmayrequiresomeback
pressuretoincreasetheworkdonebythescrewonthemelt,
toincreasethemelttemperatureanditsuniormityHigher
backpressuremaybeusedtoeliminateunmeltedparticles
andtoimprovecolormixingwhencolorconcentratesareused
Increasingbackpressure,however,tendstoreduceglassbre
lengthandchangepropertiesolledresinssuchasDelrin570
Moreimportantly,increasingbackpressurealwaysdecreasestheoutputothescrew,leadingtolongercycletimesandlower
productivityThisincreasesthebuilduposcrewdepositleading
tocontaminationandlowpartperormance
Thereore,backpressureshouldbeusedonlywhenincreasing
cylindertemperatureorotherchangesarenoteectiveor
possible
Forallmaterials,thebackpressureused(specicorinherentto
theinjectionunit)willcreatesomepressureonthemeltinronto
thescrewTocontroldroolattheendothescrewrotation,some
suckbackisrequiredThisshouldbekepttoaminimum
Mold Temperature
Thebestmoldtemperatureorlongtermpartperormance
wouldbejustbelowthecrystallizationtemperatureoDelrin,
eg,155C(311F)Thistemperaturewouldallowthepolymer
tocrystallizeinanoptimumstateandeliminateanyrisko
re-crystallization(postmoldingshrinkage)Obviouslyitis
economicallyimpossibletosetthemoldatthattemperatureas
thecrystallizationtimebecomesalmostinnitealongwiththe
cycletime
Practically,alowermoldtemperatureisused,leadingtoshorter
crystallizationtime(HPT),henceshortercycletime,lowermold
shrinkagebuthigherpostmoldshrinkage(especiallyipartsare
thenexposedtoelevatedtemperatures)Acompromiseshould
beounddependingonthetemperatureinuseandtherequired
dimensionalprecisionothemoldedpartshortandlongterm
ForstandardDelrin,amoldtemperatureo80100C
(176212F)isagoodcompromiseornormaluse,giving
relativelyshortcrystallizationtime,highshrinkagebutlowpost
moldshrinkage(seeDimensionalConsiderations,page31)
Ahighermoldtemperaturewillleadtohighermoldshrinkage,
longercycletimebutlowerpostmoldshrinkageItisspecially
recommendedorhighprecisionpartsusedathightemperature
Alowermoldtemperatureleadstoshortercycletime,lowermoldshrinkagebutmuchhigherpostmoldshrinkageleadingto
stressesanddistortion
FortoughenedresinssuchasDelrin100STand500T,theuse
oalowermoldtemperature(50C[122F]10C[50F])is
acceptablewithoutendangeringlongtermpartperormances
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Note 1:Moldtemperatureisalwaysthetermusedbutthe
importantparameteristhesuracecavitytemperatureWith
astcyclingoperations,itmaybenecessarytousealowermold
coolanttemperaturetomaintainthemolds