Mould Cooling System

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


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:


PL

Q + + = ACCUM

QAMB

Q TM

Q

PL

Q Heatflowsuppliedbythepolymer

AMB

Q

Heatflowtransferredfortheenvironment

TM

Q Heatflowtransferredforthecoolingfluid

ACCUM

Q Accumulatedenergyinthemouldmaterialpertimeunit



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.


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


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


Ifthecoolingchannelsarentcorrectlydesigned(fig.2),thecoreandcavitymouldwalltemperaturecanbedifferent.Ifthereisastronggradientinthecavitybetweenthetwohalvesthepartmaywarpanddistortitsshape[68].

Sothetargetsthatacorrectcoolingsystemhastofollowaretheuniformityofthewalltemperatureandagradualreductionofthepolymertemperature,inordertofindacompromisebetween thenecessityofreducingcycle timeandallowing forthecrystallization.

Ejected part

Last layer to cool

warpage

or

internal stresses Qcore


Inthiscontext,thedistancebetweenthecoolingchannelsandthemouldingsurface(h)and thedistancebetween cooling channels (e) are themainparameters tobeconsidered,asshownintheschemeofthefigure3.


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.


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.




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



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.


CaseStudy

Figure5Coolingsystemcasestudy.

Coolingsysteminthecavityside

a)Conventionalcoolingsystem

Figure6Temperaturedistributiononthepartssurfaces

Figure7Partsdeflection

Figure8Partscoolingtime Figure9Percentagefrozenlayer



b)Bafflecoolingsystem




c)Conformalcoolingsystem






Coolingsysteminthecavityandcoresides

a)Conventionalcoolingsystemsinthecavityandcoresides




b)Bafflecoolingsystemsinthecavityandcoresides






c)Conformalandbafflecoolingsystemsinthecavityandcoresides,respectively




d)Conformalcoolingsysteminthecavityandcoresides







References

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[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

Mould Cooling System

Documents

cooling andejection

cooling system

cooling channels

cooling fluid input

heat powers

includethe fluid

injection moulding process

usuallythetim injectedmaterial