Mould Cooling System
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Transcript of Mould Cooling System
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMouldsSchoolofTechnologyandManagement,PolytechnicInstituteofLeiria
CoolingSystemsinInjectionMoulds1
The main phases in an injection moulding process involve filling, cooling andejection.Thecoolingphaseisthemostsignificantstepamongstthethree.Itdetermines the rate atwhich the parts are produced. In themoment of themeltedpolymer injection, ideally, themoulds temperature shouldbe likeof themeltedpolymerstemperatureandinthemomentofthepartsremovalthemouldmusttobe to the temperature of the environment. Of thisway, the polymerwould beinjectedwith theminimum of pressure and the difference between the surfacetemperature and thenucleus temperatureof the injectedpartswouldbe aminimum leading a slow cooling andminimising themouldings stresses.Notice thatthese technical advantages are not compatible with economical needs and thegeneralizedrule istoproducepartswiththebiggestpossiblespeed.Accordingtothisrule, themost important factor is thecapacityof thecoolingsystemremovesheatofthecavitiesofthemould.Usuallythetimeofcoolingisaround50%ofthetotalcycle.The injectedmaterial loses temperature in thecontactwith themouldsurfaces,transferringitselfheatthroughthemould.Forspeedingtheheattransferprocess, themoulddesignerdesign specificholes in the adjacent surfaces of themouldedpartinthemould.Theseholes,knownbylinesofwater(bythewateristhemorefrequentfluidofcooling),constitutethecoolingsystemofamould.
Thefundamentalrulesthatshouldbehadincountinthecoolingsystemdesignare:
Introduction
i)The circuitsof thewater shouldbe symmetricaland independent relativelytothefillingzonesandimpression(s)ofthemould;
ii) Thermal variations in thewalls of the impressions shouldnt be pronounced,sothelinesofwatershouldbedesignedinfunctionofitsdistancetotheimpressionwalls;
iii) The cooling fluid input and output should be placed for themouldbackwards (opposite side to the operator), or alternative for the breakslower;
iv)Itsimportanttoguaranteethatthecoolingflowinthechannelsbeturbulent.TheindexofturbulenceisgivenbyReynoldsnumber:
m
e
dvR =
Where,
vFlowsspeeddChanneldiameterFluiddensity
Dynamicviscosityofthefluidm
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CAEDSMouldandDieDesign
HeatTransferWhenitproceedstothepolymerinjectionforinsidetheimpressionofamouldtheremoval energy of thepolymer in themelted state is transmitted by conductionthrough themouldmaterialup to the channelsof the cooling system and to themouldexternalsurface.Theheatexchangemechanisms(fig.1)includetheconductionforthestructureoftheinjectionmouldingmachine,theforcedconvectionforthe fluid that circulates into the cooling channels and the thermal radiation andnaturalconvectionfortheairthatsurroundthewallsofthemould[1,2].
Figure1Heatexchangeinamouldofinjection
EnergyBalanceIntheinjectionmouldingcycle,theheatcorrespondingtotheenthalpyvariationofthemouldingmaterialduringthecycle,isexchangedforthemouldingzonesurface(or impressionsurfaceofthemould)andofthisforhisoutside.Todefinetheenergy swing, is established an equilibrium between the heat powers that areintroduced in themould, theheatpoweraccumulated ineverysinglemoment intheir interior and the heat powers removed from themould, being positive ornegativethosethatrespectivelyincreaseordiminishtheirinternalenergy[1,3].Inaprocess analysiswith accumulationof internal energy, theheat flow that is suppliedtothemouldandtheheatflowthatisremovedfromthemouldshouldbeinthermal equilibrium, in every singlemoment,with the heat accumulated in thestructureofthemould:
CoolingSystemsinInjectionMoulds2
PL
Q + + = ACCUM
QAMB
Q TM
Q
PL
Q Heatflowsuppliedbythepolymer
AMB
Q
Heatflowtransferredfortheenvironment
TM
Q Heatflowtransferredforthecoolingfluid
ACCUM
Q Accumulatedenergyinthemouldmaterialpertimeunit
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds3
Simplifiedhypothesestoobtainresults
i)Quasistaticprocess
ii)During the cycles the temperaturesand thermal flows fluctuationsaredespised
iii)Duringthedifferentperiodsmediumvaluesareconsidered
PL
Q + + =0AMB
Q TM
Q
Where,
arref
PLPL t
mh=Qarref
PLPL t
Vh = Q or,
Where,
;hh = hi- he i Polymerenthalpyattheinjectiontemperature;he Polymerenthalpyattheejectiontemperature;m Polymermassinjectedinthemould;PL PLPolymermediumdensitybetweentheinjectiontemperatureandtheejectiontemperature;tarrefCoolingtimeoftheplasticpart;VVolumeoftheplasticpart
AMB
Q = + + CONV
Q COND
Q RAD
Q
Where,
CONV
Q Heatflowbyconvectiononthemouldlateralwalls
COND
Q Heatflowbyconductionontheinjectionmouldingwalls
RAD
Q Heatflowbyconductiononthemouldlateralwalls
CONV
Q =ALxhx(TambT )mould
Where,
A Mouldexposedarea;hHeattransfercoefficient,naturalconvection;TL ambEnvironmentTemperature;TmouldMouldtemperature.
COND
Q =Afixxx(TambT )mould
Where,
AfixContactareaMould/Fixingsystem;Proportionalityfactor
RAD
Q
44100100
TmoldeTamb=ALxxradx
Where,
StefanBoltzmanconstant;Materialemissivityrad
When thematerial is inside themould cools supplying him heat, by thatQPL isalwayspositive.Theheatchangedwiththeenvironment,canbepositiveornegativedependingonthetemperatureofthemould.
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CAEDSMouldandDieDesign
CoolingConditions
An efficient system of cooling,with optimal cooling conditions, leads to a partuniformdistributionof temperatures,minimizing theundesired effects appearedduringdecoolingprocess,thecycletimeandtherateofrejections.Theconceptionofanefficientcoolingsystemisnotasimpletrial,becausetherearedifferentfactorsthatcancontributeforthefinalintendedresults.Someofthefactorsthatinfluencethecoolingprocessare:thegeometryofthepart,thetemperatureofthemould,thearchitectureofthecoolingchannels,thecoolingfluidtemperatureandthespeedoftheflow.
Itcanbeidentifiedtworeferencetermsforaniterativeprocessofcharacterization
ofthemouldcoolingsystem[3]:
i)Theincreaseoftheheattransferrate ii)Uniformtemperaturedistributioninthemouldingsurface
Whereas the increase of the heat removal rate between the plastic part and the
mould is important in the economical point of view, the uniformization of thetemperaturesdistributiononthepartssurfaceswillprovidetheobtainingofpartswithestatesandqualityimproved.
CoolingTimeTheWubkenequationallowustoestimatethecoolingtime[3]
=
bW
aWK TT
TTst22
2 8ln
Where
CoolingSystemsinInjectionMoulds4
isthematerialthermaldiffusivity;s isthepartthickness;Ta istheinjection temperature; Tb is the ejection temperature and Tw is the medium mouldtemperature.
Themediummouldtemperatureisconsideredoneofthemostsignificantvariablesinthecoolingtimedetermination[4,5].Somedeterminationsusethetemperatureofthecoolingfluidforcalculatingthemediummouldtemperaturevariable.However, such utilization ignores the temperature increases of the melted plasticmaterialinthemoldingzones,duringtheinjectionphase.Duringthemoldingcyclethemould temperature increasewhiletheplasticmaterial is injected,diminishingprogressively up to the following injection.Also the flow regime of the coolingfluid,thetemperatureofthecoolingfluid,thearchitectureofthechannels,thekindofthecoolingfluid,andthemouldmaterialproperties(namelythemouldmaterialthermalconductivity),influencethemouldtemperature.
Table1Propertiesofatypicalresin,Aluminiumandsteel,usedinthemanufactureofinjectionmoulds.
SL Vantico 5260
Aluminium Steel P20 AlZn5Mg3Cu
Young modulus 600 - 800 MPa 72 MPa 2500 GPa Tensile strength 40 - 65 MPa 540 MPa 965-300 MPa Thermal conductivity 0.2 W.m-1 -1K 120-150 W.m-1K-1 29-34 W.m-1 -1KCoefficient of thermal expansion (at 20C)
10510-6 K-1 23,610-6 -1 K 12,810-6 -1 K
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CAEDSMouldandDieDesign
Ifthecoolingchannelsarentcorrectlydesigned(fig.2),thecoreandcavitymouldwalltemperaturecanbedifferent.Ifthereisastronggradientinthecavitybetweenthetwohalvesthepartmaywarpanddistortitsshape[68].
Sothetargetsthatacorrectcoolingsystemhastofollowaretheuniformityofthewalltemperatureandagradualreductionofthepolymertemperature,inordertofindacompromisebetween thenecessityofreducingcycle timeandallowing forthecrystallization.
Ejected part
Last layer to cool
warpage
or
internal stresses Qcore
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CAEDSMouldandDieDesign
Inthiscontext,thedistancebetweenthecoolingchannelsandthemouldingsurface(h)and thedistancebetween cooling channels (e) are themainparameters tobeconsidered,asshownintheschemeofthefigure3.
CoolingSystemsinInjectionMoulds6
molding
d
qmin
s/2
qmx
h
emould
Coolingchannels
Figure3Heatflowprofile[13].
Inthepracticalone,iscommontoconsider:e=2,5a3,5deh=0,8a1,5e
On the issue ofdimensional criteria indesigning cooling channels, threedimensionshavetobeconsidered:thediameterofthecrosssection(orthecrosssectionareaifnotcircular),thedistancebetweenchannelsandthedistancebetweenchannel andwall of themould. Themain problems that arisewhen choosing thesedimensions concerns thepressure lossesderived from the choiceof thediameterand thedesignof thechannel.Aheating/cooling relationship reported inZollner[14]givesaguidelineonthechannelspositioning.Thisstatesthatthevalueresultingfromthesolutionoftherelationshipshouldstaybetween2.5and5%forsemicrystallinethermoplasticsandbetween5and10%foramorphousthermoplastics.
ConformalCooling
In the injectionmoldingprocess themainpartof thecycle time isdeterminedbythecoolingprocess.Therefore,itisimportanttooptimizethecoolingcycleinordertoreducethecoolingtime.Conformalcoolingchannels(i.e.channelsthatfollowthegeometricshapeofthepart)havebeenusedforthispurposeallowingasignificantcoolingtimereduction.AccordingtoWohlers[15]itispossibletoreducethecooling cycle by 20% using conformal cooling channel. Similarly, Dimla et al. [10]considers that cycle time can be significantly reducedwith cooling taking placeuniformly in all zones if the cooling channels aremade to conform to thepartsshapeasmuchaspossible.Someinvestigationshaverelatedthemouldscycletimereductionwithconformalcooling; themostrelevantresultassociated to itsuse isthemouldsurfacetemperatureuniformity.Furthermore,ifthepartisejectedwiththesametemperatureineverypointthesubsequentshrinkageoutsidethemouldisalsouniform,whichavoidspostinjectionwarpageofparts.Thiswasalsopointedout byVoet et al. [16],whichmentioned that the goal of cooling amould is toobtain auniform temperature at themould surface andwithin the final injectedproducttoavoidinternalstresses.
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CAEDSMouldandDieDesign
Amethodthatutilisesacontourlikechannel(fig.4),constructedascloseaspossible to the surface of themould to increase the heat absorption away from themoltenplastic,ensuresthatthepartiscooleduniformlyaswellasmoreefficiently.
Figure4Conformalcoolingchannels
Whenmoltenplasticisinjectedinthemoulditmustbesolidifiedtoformtheobject.Themouldtemperatureisregulatedbycirculationofaliquidcooler,usuallywateroroilthatflowsinsidechannelsinsidethemouldparts.
Table2Heatconvectioncoefficientoftheair,waterandoil.
Air Water Oil HeatconvectioncoefficientWm2k1
50 900 400
Whenthepart issufficientlycooled itcanbeejected.Most(95%)oftheshrinkagehappensinthemouldanditiscompensatedbytheincomingmaterial;theremainderoftheshrinkagetakesplacesometimefollowingtheproductionofthepart[17].
Ifthechannelscarryingthewatercouldbeconformedtotheshapeofthepartand
their cross section changed to increase the heat conducting area then a moreefficientmeans of heat removal could be realised.Thismay also help to reducewarpagewhenthepartisejected,astheplasticwouldbecooledmoreuniformly.
Anotheradvantageisthatamouldequippedwithconformalchannelsreachestheoperation temperature quicker than a normal one equipped with standard (ordrilled)coolingchannels[18,19].
Modelling
The analysis toolsutilization for the cooling systems conception that assures theuniformityofthecoolingalongthepart,drivethesignificantimprovementsinthemouldproductionanddefinitionof theprocessconditions to thespecificationsoftheproductdemanded.
Themain resistance to the transferenceofheat in thecoolinghappenof theownmaterialduetothelowthermaldiffusivityoftheplasticmaterial.So,itsessentialtoconsiderthedependenceofthematerialwiththetemperatureinthemodulationoftheheatconduction.
CoolingSystemsinInjectionMoulds7
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds8
Inthecoolingprocessitsessentialtoconsiderthethermalpropertiesofthemouldmaterialandappropriateborderconditions(e.g.theheattransferbyforcedconvectioninthecoolingchannels).
Inisotropicdomaintheheattransferisdescribedbytheenergyconservationequation[20]:
( ) += QTK
tTCP
Where,CPandkrepresentthedensity,thespecificheatandthethermalconductivity of the material, respectively. T represents the local temperature in each
instantmomenttand ineachspatialcoordinate,whereas representstheenergygenerated/dissipatedbyunitof timeandbyunitofvolume in thematerial. Thisdifferentialequationwithderivedpartial forbidimensionalheatconduction,notstationary,inCartesianscoordinatesandinasimplifiedform,takestheform:
Q
+
+
=
QyTK
yxTK
xtTCP
Thetemperatureprofile inagivenzoneofthematerialandhisvariationwiththetimeareabletobeobtainedresolvingthisequation.However,itisnecessaryspecifythetemperatureprofileintheinitialinstantandtheborderconditions.
Tooptimise thedesignandconstructionof themould,withattentionon refiningthe tooldesign through application of finite element and thermal flow analyses,specific commercial software for injectionmouldinghave beenused. In thenextsection itwillbemadeabriefdescriptionabouttheheattransferprocessanalysisusingsomecommercialsoftware.
ThelatestcommercialsoftwareofCAEallowsthreedimensionalsimulationoftheinjection molding process. This software has modules for conception efficientcoolingsystems.Thecoolinganalysisisbasedinthemethodoftheborderelementsapproach.
InthecoolingmoduleofthecommercialCAEsoftware,thetransferenceofheatinthepolymer is treatedasonedimensionalconduction located in transientregime.Theheatexchangebetweenthesurfaceofthecoolingchannelsandthecoolingfluidareconsidered instationaryregime,consideringthecorrelationfortheheattransferencecoefficientbyconvection.Tosolvesimultaneouslytheprominentequationsoftransferenceofheatinthisprocess,theprogramutilizesahybridschemewherethe transference of heat is calculated by the approachmodified analyzes of theelement of three dimensional border for the region of the mould, and onedimensionalheat transferenceanalysis,along thepart thickness for the regionofmeltedplastic.Thesetwoanalysesareconjugatedofformitequalthetemperatureandtheheatflowintheinterfacepolymer/mould.
Coolingsystemsimulation
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds9
Theequationsfortheflowofthefluidinacircuitofcoolingareresolvedthroughthe iterative approach ofNewtonRaphson, to obtain the torrent and the fall ofpressureineachchannelofthecoolingsystem.Then,theheattransferencecoefficientsbetweenthesurfacesofthechannelsandthecoolingfluidarecalculated.
Thechangeofheatbynaturalconvectionbetweentheenvironmentandthewallsofthemouldarealsocalculated.Forthiscalculation,commercialsoftwareconsiderstheexteriorsurfaceofthemouldasaspherewithanareaequivalenttothesurfaceofabox,inthatthechannelsofcoolingwillbeincluded,thefeedingsystemandthemoldingzones.
Theprocesssimulationstarts inthephaseofthemouldfilling. Whenthecoolingmoduleof cooling isused, thepolymer injection temperature isassumedasconstant. This assumption has some associated errors; therefore the injectiontemperature can be a superior due to the heating by viscous dissipation of thematerialinthesprue.Thattemperaturewouldbeabletogoupuntil30Cdependingonthespeedofinjectionandofthematerialproperties[21].
The thermal resistance in the interfacepolymer/moulddefines theheat transmissioncoefficient(hint)intheinterfacebetweenthepolymerandthemoldingsurfaces.Thiscoefficientisusedforsimulatetheresistancetotheexistingheatinthecontactbetweenthetwomaterialsbythefollowingequations:
CAESoftwareSimplifications
( )bx
bxM nTkTTh
==
=intint
( )bx
bxM nTkTTh
+=+=
+
=intint
where,Tintisthemelttemperatureintheinterfaceofthetwomaterials; and arethemoldingzonestemperatures,onthecavityside(negativeside)andonthecoreside(positiveside),respectively.Theindicesband+bindicatethepositiveandnegativesideofthedistancerelativelytothecenterofthepart(equivalentthehalfofitsthickness).
MT
+MT
If the thermalconductivityassumes thezerovalue, (thermal isolatedborder), thechangesbetween the twomaterialsdonotexist. If itassumesanelevatedvalue,existaperfectthermalcontactbetweenthematerialsandtheinterfacetemperatureis considered equivalent at the mould wall temperature. Many times, and bydefect,thisvalueisof25000w/m2C,incommercialsoftware.
The case study presented shows some important aspectswhendifferent coolingsystemsareconsidered.
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CAEDSMouldandDieDesign
CaseStudy
Figure5Coolingsystemcasestudy.
Coolingsysteminthecavityside
a)Conventionalcoolingsystem
Figure6Temperaturedistributiononthepartssurfaces
Figure7Partsdeflection
Figure8Partscoolingtime Figure9Percentagefrozenlayer
CoolingSystemsinInjectionMoulds10
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CAEDSMouldandDieDesign
b)Bafflecoolingsystem
Figure10Temperaturedistributiononthepartssurfaces
Figure11Partsdeflection
Figure12Partscoolingtime Figure13Percentagefrozenlayer
c)Conformalcoolingsystem
Figure14Temperaturedistributiononthepartssurfaces
Figure15Partsdeflection
Figure16Partscoolingtime Figure17Percentagefrozenlayer
CoolingSystemsinInjectionMoulds11
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CAEDSMouldandDieDesign
Coolingsysteminthecavityandcoresides
a)Conventionalcoolingsystemsinthecavityandcoresides
Figure18Temperaturedistributiononthepartssurfaces
Figure19Partsdeflection
Figure20Partscoolingtime Figure21Percentagefrozenlayer
b)Bafflecoolingsystemsinthecavityandcoresides
Figure22Temperaturedistributiononthepartssurfaces
Figure23Partsdeflection
Figure24Partscoolingtime Figure25Percentagefrozenlayer
CoolingSystemsinInjectionMoulds12
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CAEDSMouldandDieDesign
c)Conformalandbafflecoolingsystemsinthecavityandcoresides,respectively
Figure26Temperaturedistributiononthepartssurfaces
Figure27Partsdeflection
Figure28Partscoolingtime Figure29Percentagefrozenlayer
d)Conformalcoolingsysteminthecavityandcoresides
Figure30Temperaturedistributiononthepartssurfaces
Figure31Partsdeflection
Figure32Partscoolingtime Figure33Percentagefrozenlayer
CoolingSystemsinInjectionMoulds13
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds14
References
[1]MENGES,G.;MOHREN,P.How toMake InjectionMoulds.2nded,HanserPulishers,1993.ISBN3446163050
[2] LIMA, S. P. Evaluation of the rapid prototyping incorporation in injectionmoulds,MasterThesis,October2002.
[3] POUZADA,A.S. Heat transfer in injectionmoulds Support texts to theMouldDesignandManufacturingMasterDegree
[4]BARROS,I.;TEIXEIRA,S.F.C.;TEIXEIRA,J.C.;CUNHA,A.M.EvaluationofthethermalBehaviourofInjectionMoulds.Intern.PolymerProcessing,Vol.15,No.1(2000),pp.95102.
[5]BOELL,K.M.Predicting thecooling timeofna injectionmouldedpart,Proceedings of the 53th Annual Technical Conference & Exhibition, ANTEC 1995,Boston,711May1995,pp.42424246.
[6]MALLOY,ROBERTA.Plastic partdesign for injectionmolding.NewYork:HanserPublishers,1994.460p.ISBN1569901295.
[7]WANG,T.J.;YOON,C.K.Shrinkageandwarpageanalysisofinjectionmoldedparts.Orlando:SPEANTEC2000,p.687692.
[8] JOHANNABER, F. Injectionmoldingmachines. Third Edition.New York:HanserPublishers,1994.315p.ISBN1569901694.
[9]MARTINHO,P.G.Warpagestudy in injectionmouldingparts.UniversityofMinho,Guimares,2002.98p.MasterThesis
[10]DIMLA,D.E.;CAMILOTTO,M.;MIANI,F.Designandoptimisationofconformalcoolingchannelsininjectionmouldingtools.JournalofMaterialsProcessingTechnology,164165,pp.12941300,2005.
[11]SINGH,K.J.MoldCooling.InBERNHARDT,E.C.CAE:ComputerAidedEngineeringforInjectionMolding.Munich:CarlHanserVerlag,1983.ISBN3446139508.p.326347.
[12]YANG,S.Y.;CHANG,H.C.Studyontheperformanceofcoolingsystemsinprecisioninjectionmolds.Intern.Polym.Proc.Vol.10,n2(1995),p.255261.
[13] POTSCH,G.;MICHAELI,W. Injectionmolding: an introduction.Munich:CarlHanserVerlag,1995.195p.ISBN1569901937.
[14]ZOLLNER,O.Optimisedmouldtemperaturecontrol,Appl.Technol.Inform.(1997)1104.
[15]WOHLERS,T.,WohlersReport2006RapidprototypingandmanufacturingStateoftheIndustry.AnnualWorldwideProgressReport,WohlersAssociates,Inc.,2006.
[16]VOET,A.;PEE,B.V.;MINGNEAU,J.;CARDON,L.;HOUTEKIER,R.Optimizationofconformalcoolingbyuseofdesignofexperiments:industrialcasestudyof an injectionmolded product, RPD 2006 Building the future by innovation,MarinhaGrande,1317November2006.
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CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds15
[17]BRYCE,D.M.PlasticInjectionMoulding,SocietyofManufacturingEngineers,Dearborn,MI,1996.
[18] SACHS,E.;WYLONIS,E.;ALLEN, S.;CIMA,M.;GUO,H. Production ofinjectionmoldingwith conformal cooling channels using the three dimensionalprintingprocess,Polym.Eng.Sci.,2000,40(5),12321247.
[19]DELGARNO,K.,W., Layermanufactured production tooling incorporatingconformalheatingchannelsfortransfermouldingofelastomercompounds,PlasticRubberCompos.30(8)(2001)384388.
[20]HOLMAN,J.P.HeatTransfer,NewYork:MacGrawHill,Inc,1989.ISBN0071004874.
[21]CMOLDusersmanual,ACTechnology,IthacaNewYork,1997.
Cooling Systems in Injection Moulds Case StudyCooling system in the cavity side Cooling system in the cavity and core sides
References