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Casting Design Optimization driven by SimulationDr.B.Ravi,ProfessorofMechanicalEngineering,I.I.T.Bombay([email protected])
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.