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Casting Design Optimization driven by SimulationDr.B.Ravi,ProfessorofMechanicalEngineering,I.I.T.Bombay(b.ravi@iitb.ac.in)

hemethodslayoutofacastingisanimportantactivity

intoolingdevelopment.Itinvolvescriticaldecisions

regardingpartorientationinmold,partingline,cores,

cavitylayout,feeders,feedaidsandgatingsystem.An

improperlayoutleadstoeitherpoorqualityorlowyield,

affectingmanufacturingcostsandproductivity1.

Methodsdesignisusuallycarriedoutmanuallyon2D

drawingsofthecastpart.Thentoolingisfabricated,trial

castingsareproducedinthefoundry,andinspected.If

samplecastingscontaindefects(suchasshrinkageorgas

porosity),thenthemethodslayoutismodifiedandthe

processisrepeated.Eachsuchiterationcantakeupseveral

days,affectingregularproduction.Afterafewiterations,

thefoundrymayresorttoasafemethodsdesign(implying

lowyield),orcontinuewithhighrejectionrates(implying

highscraporrepaircost).Thisisespeciallytrueinthecase

oflargecastings,wherethecostofatrialorrepaircanbe

prohibitive.

Assumingatypicalfoundrydevelops50newcastingsevery

year,eachcastingrequiresatleast2trials,andtheaverage

costofeachtrial(toolingmodification,melting&pouring,

inspection,andeffectonregularproduction)asRs.20,000,

theeconomiclossworksouttobetwomillion(20lakh)

rupeesperyearperfoundry.

Further,takingtheaveragedifferenceinthepriceofa

saleablecastingandscrapmetalasRs.10/kg,andassuming

averagerejectionsinafoundryas5%,theeconomicloss

causedbydefectivecastingsworksouttoRs.500pertonne

ofproduction(inrealitythiscanbemuchhigher,with

transport,warranty,andfailuresduringproductlife).

Castingsimulationcanovercometheaboveproblems:

virtualtrialsdonotinvolvewastageofmaterial,energyand

labour,anddonotholdupregularproduction.However,

mostofthesimulationprogramsavailabletodayarenot

easytouse,takeasmuchtimeasrealtrials,andtheir

accuracyisaffectedbymaterialpropertiesandboundary

conditionsspecifiedbyusers.Thebiggestproblemisthe

preparationof3Dmodelofthemoldcavitywithcores,

feedersandgatingforeveryiteration,whichrequiresCAD

skillsandtakesconsiderabletimeforevensimpleparts.

Thisalsopreventsearlymanufacturability evaluationand

improvementbyproductdesigners,whichcanbenefit

severaltimesmorethantoolingandprocesschanges.

TheAutoCASTsoftwaredevelopedbyAdvancedReasoning

Technologies,MumbaiincollaborationwithI.I.T.Bombay

providesasingleintegrateduserfriendlyenvironmentfor

castingmethods

design,

solid

modeling,

and

simulation2.

Ithandlesbothferrousandnonferrousparts,andsandas

wellasmetalmolds.Release10incorporatesmulticavity

moldlayout,automaticmodelingandoptimization,anda

costingmodeltocomparevariouslayouts(Fig.1).

Fig.1.Casting

methods

design

and

simulation

software.

T

Computersimulationprovidesaclearinsight

regardingthelocationandextentofinternaldefects,

ensuringcastingsarerightfirsttimeandeverytime.It

however,requiresa3DCADmodelofthemethod

layout(withmoldcavities,cores,feeders,andgating

channels),proper

setting

of

boundary

conditions

for

eachvirtualtrial,andcorrectinterpretation ofresults.

AutoCASTsoftwareintegratesandautomatesthe

abovetasks,andprovidesanextremelyeasytouse

graphicaluserinterfacesuitableforevenfirsttime

computerusers.Themoldcavities,feedersandgating

systemareautomaticallyoptimized,drivenbythe

criteriaandconstraintsspecifiedbyuser.Thisreduces

thetotaltimeformethodsdesignandsimulationofa

typicalcastingtolessthanonehour.

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Computer-aided Methods Design

Themaininputisthe3DCADmodelofanascastpart:

withoutdrilledholes,andwithdraft,shrinkageand

machiningallowance(Fig.2).Themodelfilecanbeobtained

fromtheOEMfirm,orcreatedbyalocalCADagency.

Variousdisplayoptionssuchaspan,zoom,rotate,

transparencyand

measure,

are

provided

to

view

and

understandthepartmodel.Thecastmetalandprocessare

selectedfromadatabase.Thicknessmapisgenerated.Part

manufacturability (compatibilitywiththeselectedprocess)

iscomputedandpictoriallydisplayed(Fig.3).

Fig.2.Partpropertycomputation.

Fig.3.Partthicknessdistributionwithsensor.

Fig.4.Methods

design

and

its

automatic

modeling.

Methodsdesigninvolvescores,feedersandgatingsystem.

Holesinthepartmodelareautomaticallyidentifiedfor

coredesign,orpluggediftheyaredrilled.Evenintricate

holescanbeidentifiedbyspecifyingtheiropenings.The

print

length

is

computed

based

on

core

diameter

and

length(theusercanchangetheirvaluesifrequired),and

theentirecoremodelisautomaticallycreated.The

programsuggeststhenumberofcavitiesdependingon

themoldsize(selectedfromacustomizablelibrary),

consideringbothcavitycavityandcavitywallgaps.Then

thepartmodelisautomaticallyduplicatedinthecorrect

locationsasperthedesiredcavitylayout(Fig.4).

Tofacilitatefeederlocation,aquicksolidificationanalysisis

carriedoutthatidentifiesfeedingzones.Theuserselectsa

suitableconnectionpointclosethehottestzone,andthe

sizeofthefeederiscomputedusingmodulusprinciple

(solidificationtimeoffeederslightlymorethanthatofthe

feedingzone).Standardfeedershapesincludecylindrical,

oval,sphericalbottom,cruciform,etc.Othershapescanbe

imported.Thefeedermodelisautomaticallycreated;the

usercanchangeitsdimensionsorapplyfeedaidssuchas

insulatingsleevesandexothermiccovers.Chills,padding

andfinscanalsobecreated.Morefeedersorfeederswith

multipleneckscanbecreatedbyspecifyingtheirpositions.

Thegatingchannelsarealsocreatedsemiautomatically.

First,theuserindicatesgatepositionsonthepartorfeeder

model.Thenthespruepositionisdecided,andconnected

tothegatesthroughrunners.Runnerextensionsarealso

automaticallycreated.Eitherhorizontalorverticalgating

systemcanbedesignedandmodifiedwithinminutes.The

programsuggestsasuitablefillingtime(whichcanbe

changedbyuser),accordinglycomputesthedimensionsof

thegatingchannels,andcreatestheirsolidmodel.

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Fig.5.Meltjetpathandmoldfilling.

Fig.6.Castingsolidification simulation.

Fig.7.Feed

metal

paths

(temperature

gradients).

Automatic Optimization

Themoldcavitylayout,feeders,andgatingmodelsare

automaticallyoptimizedwithinminutesbasedonquality

requirementsandotherconstraints3.Formoldcavity

layout,theprimarycriterionistheweightratioofcast

metaltomoldmaterial.Ahighratiosuchas1:2(cavities

tooclose

to

each

other)

can

reduce

the

heat

transfer

rate

andleadtoshrinkageporositydefects.Alowratiosuchas

1:8(cavitiestoofarfromeachother)impliespoor

utilizationofmoldmaterialandreducedproductivity.The

programtriesoutvariouscombinationsofmoldsizesand

numberofcavitiestofindthecombinationthatisclosestto

thedesiredvalueofmetaltomoldratio.

Thegatingoptimizationisdrivenbytheidealmoldfilling

time,whichdependsoncastmetal,castingweightand

minimum

wall

thickness.

Fast

filling

leads

to

turbulence

relateddefects(molderosion,airaspirationandinclusions).

Slowfillingmaycausedefectsrelatedtopremature

solidification(coldshutsandmisruns).Tooptimizethe

gatingdesign,moldfillingissimulatedandtotalfilltimeis

computed(Fig.5).Alayerbylayerfillingalgorithmtakes

intoaccounttheinstantaneousvelocitythroughthegates

(consideringbackpressure),andthelocalcrosssectionof

themoldcavity.Thisgivesafairlyaccurateestimationof

fillingtime,whilebeingcomputationallyfast.Ifthe

differencebetweentheidealandsimulatedfillingtimeis

morethanaspecifiedlimit,theprogramautomatically

changesthegatingdesign,createsitssolidmodel,and

verifiesthefillingbysimulation.

Thefeederoptimizationisdrivenbycastingquality,defined

asthepercentageofcastingvolumefreefromshrinkage

porosity.Theuserindicatesatargetquality.Theprogram

automaticallychangesthefeederdimensions,createsits

solidmodel,carriesoutsolidificationsimulation(Fig.6),and

estimatesthecastingquality.Thesolidificationsimulation

employstheVectorElementMethod,whichcomputes

temperaturegradients(feedmetalpaths)insidethe

casting,andfollowstheminreversetoidentifythelocation

andextentofshrinkageporosity(Fig.7).Thishasbeen

foundtobemuchfasterthanFiniteElementorVolume

Method,andusuallymoreaccuratetoo.Feederdesign

iterationsarecarriedoutuntilthedesiredqualityis

achieved,orthenumberofiterationsexceedsasetlimit.

Theusercanaccepttheresults,orcanmodifythefeeder

designinteractively.

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Fig.8.Costanalysisandmethodsreportgeneration.

Finally,thecostofthecastingiscomputedintermsof

amortizedtooling,castmetal,othermaterials(mold,core,

etc.),energy,andlabour.Variouscostratesandparameters

canbesetbytheuser.Thisenablescomparingdifferent

castinglayoutsintermsoftoolingandmanufacturingcost.

Adetailedmethodsdesignreportalongwithanimageof

theentirecastingisautomaticallygenerated,whichcanbe

printedorstoredforfuturereference(Fig.8).

Themetaldatabasecoversallmajoralloys(aluminum,

copper,castiron,ductileiron,steel,andpreciousmetals)

andprocesses(sand,shell,investment,diecasting).Itcan

becustomizedtoanynewmetalprocesscombination.

ThesoftwarehasbeendevelopedforstandardWindowsXP

computers,andperformswellonevenportablecomputers.

Thegraphicalinterfaceisdesignedtominimizethelearning

andoperationtime,andtheuserisgentlyguidedthrough

forgottenorwrongsteps.Eventhosewithoutanyprior

exposuretocomputersareabletousethesoftwareaftera

singledayoftraining.Allstepsstartingfrompartmodel

importingto

mold,

core,

feeder

and

gating

system

design,

simulationandoptimizationarecompletedwithinonehour

fortypicalcastings.

Directbenefitsincludeatleast50%reductionincasting

developmenttimeandporositydefects.Otherbenefits

includeyieldimprovement,fasterquotation,handlingmore

complexpartsandknowledgereuseforfutureprojects.

ContinuousinteractionoftheR&Dteamwithlocalindustry

hasmadeitpossibletoincorporatehundredsofuseful

improvementsover

the

last

20

years.

TodayAutoCASTisthemostwidelyusedcastingsoftwarein

Indiawith50licenses(foundries,engineeringandR&D

institutes,andconsultants)coveringallmajorcastmetals

andprocesses.Manyothershaveusedthesoftwarefor

benchmarking.Simulationconsultantsareavailableacross

thecountrytoprovidelocaltechnicalsupport,ensuringa

smoothtransitiontocomputeraidedmethoding.

Summary and Future

Castingsimulationcanminimizethewastageofresources

requiredfortrialproduction.Inaddition,theoptimization

ofqualityandyieldimplieshighervalueadditionandlower

productioncost,improvingthemargins.Forwidespread

application,simulationprogramsmustbefast,reliable,and

easytouse.Thishasbeenachievedbyintegratingmethods

design,solidmodeling,simulationandoptimizationina

single

software

program,

and

automating

many

tasks

that

otherwiserequirescientificknowledgeandcomputational

skills.Inmanybenchmarkingexercisesandsimulation

clinics(Fig.9),thesoftwarehasconsistentlyprovenits

reliabilityinpredictinginternaldefects(ex.shrinkage

porosity)withinminutes,oftenbyseniorengineerswhoare

firsttimecomputerusers.Thesimulationcostsarea

fractionofthecostsoffoundrytrials,whileprovidingbetter

andfasterinsightforcastingoptimization.Anetworkof

localtechnicalsupportcentresandsimulationconsultants

acrossthecountryensuresthatevenSMEfoundriesin

remoteareascannowtakeadvantageofthetechnology.

Thegoalofcastingsrightfirsttime,everytime,inthe

shortesttime,iswithinthereachofeveryfoundry.

Fig.9.CastingsimulationtrainingandclinicatMumbai.

References

1. B Ravi, Metal Casting: ComputerAided Design and Analysis, PHIIndia,NewDelhi,20052008,ISBN8120327268.

2. AdvancedReasoningTechnologies,AutoCASTSoftwareUserManualandCaseStudies,http://www.autocast.co.in,2009.

3. BRavi,CastingSimulationandOptimisation:Benefits,Bottlenecks,andBestPractices,IndianFoundryJournal,54(1),Jan2008.