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
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
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.
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
CAEDSMouldandDieDesign
Ifthecoolingchannelsarentcorrectlydesigned(fig.2),thecoreandcavitymouldwalltemperaturecanbedifferent.Ifthereisastronggradientinthecavitybetweenthetwohalvesthepartmaywarpanddistortitsshape[68].
Sothetargetsthatacorrectcoolingsystemhastofollowaretheuniformityofthewalltemperatureandagradualreductionofthepolymertemperature,inordertofindacompromisebetween thenecessityofreducingcycle timeandallowing forthecrystallization.
Ejected part
Last layer to cool
warpage
or
internal stresses Qcore
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.
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
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
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.
CAEDSMouldandDieDesign
CaseStudy
Figure5Coolingsystemcasestudy.
Coolingsysteminthecavityside
a)Conventionalcoolingsystem
Figure6Temperaturedistributiononthepartssurfaces
Figure7Partsdeflection
Figure8Partscoolingtime Figure9Percentagefrozenlayer
CoolingSystemsinInjectionMoulds10
CAEDSMouldandDieDesign
b)Bafflecoolingsystem
Figure10Temperaturedistributiononthepartssurfaces
Figure11Partsdeflection
Figure12Partscoolingtime Figure13Percentagefrozenlayer
c)Conformalcoolingsystem
Figure14Temperaturedistributiononthepartssurfaces
Figure15Partsdeflection
Figure16Partscoolingtime Figure17Percentagefrozenlayer
CoolingSystemsinInjectionMoulds11
CAEDSMouldandDieDesign
Coolingsysteminthecavityandcoresides
a)Conventionalcoolingsystemsinthecavityandcoresides
Figure18Temperaturedistributiononthepartssurfaces
Figure19Partsdeflection
Figure20Partscoolingtime Figure21Percentagefrozenlayer
b)Bafflecoolingsystemsinthecavityandcoresides
Figure22Temperaturedistributiononthepartssurfaces
Figure23Partsdeflection
Figure24Partscoolingtime Figure25Percentagefrozenlayer
CoolingSystemsinInjectionMoulds12
CAEDSMouldandDieDesign
c)Conformalandbafflecoolingsystemsinthecavityandcoresides,respectively
Figure26Temperaturedistributiononthepartssurfaces
Figure27Partsdeflection
Figure28Partscoolingtime Figure29Percentagefrozenlayer
d)Conformalcoolingsysteminthecavityandcoresides
Figure30Temperaturedistributiononthepartssurfaces
Figure31Partsdeflection
Figure32Partscoolingtime Figure33Percentagefrozenlayer
CoolingSystemsinInjectionMoulds13
CAEDSMouldandDieDesign
CoolingSystemsinInjectionMoulds14
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[3] POUZADA,A.S. Heat transfer in injectionmoulds Support texts to theMouldDesignandManufacturingMasterDegree
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[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.
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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.
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Cooling Systems in Injection Moulds Case StudyCooling system in the cavity side Cooling system in the cavity and core sides
References
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