Sand casting of metals - Gating system for sand casting mould

Topics covered: Sand Casting of Metals
Design and manufacture of gating system
Pouring basin
Sprue
Runners
In-gates
Riser

1. Sand Casting of Metals
Prof.Ms.AmrutaA.Rane
AssistantProfessor,
DJSCE,UniversityofMumbai,
Mumbai.

2. Sand Casting of Metals
Topicscovered:SandCastingofMetals
•Designandmanufacturingofgatingsystem
•Pouringbasin
•Sprue
•Runners
•In-gates
•Riser

3. GatingSystem
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4. GatingSystem
•Thegatingsystemreferstoallthoseelementswhichareconcernedwiththeflow
ofmoltenmetalfromladletotheMouldBasincavity.
•Thevariouselementsthatcomesundergatingsystemare:
oPouringbasin/cup
oSprue
oSpruebasewell
oRunner
oRunnerextension
oIn-gate
oRiser
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5. Requirements/functionofthegatingsystem
Agatingsystemshould,
•Fillthemoldcavitycompletelybeforefreezing.
•Introducetheliquidmetalintothemoldcavitywithlowviscosityandlittle
turbulence,sothatmolderosion,metaloxidationandgaspickupisprevented.
•Helptopromotetemperaturegradientfavorableforproperdirectional
solidification.
•Incorporatetrapsforseparationofnonmetallicminclusionswhichareeither
introducedwiththemoltenmetalorarepresentinthegatingsystem.

6. PouringBasin
•Themoltenmetalispouredintotheapouring
basinwhichactsasareservoirfromwhichit
movessmoothlyintothesprue.
•Thepouringbasinmaybecutintothecope
portiondirectlyoraseparatedrysand
pouringbasinmaybepreparedandused.
•Themoltenmetalinthepouringbasinshould
befullduringthepouringoperationtoavoid
theatmosphericairandslagfromentering
intomoldcavity.
•Themoltenmetalisnotdirectlypouredinto
themoldcavitybecauseitmaycausethe
molderosion.
•Thepouringbasinalsostopstheslagfrom
enteringthemoldcavitybymeansof
skimmerorskimcore.Itholdsbacktheslag
ordirtwhichfloatsonthetopandallowsonly
cleanmetalunderneathitintothesprue.
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7. PouringBasin
Astrainercore:
•Itisastrainerorscreenwithmanysmallholes.
•Itisutilizedtomaintaintheconstantconditionsofflow.
•Thestrainerrestrictstheflowofmoltenmetalintothesprue,thushelpsinquick
fillingofthepouringbasinandrestrictstheflowofslagintothemold.
•Itallowsonlycleanmetaltoenterintothesprue.
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8. PouringBasin/Cup
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9. Sprue
•Sprueisthechannelthroughwhichthemoltenmetalisbroughttotheparting
planewhereitenterstherunnersandgatestoultimatelyreachthemoldcavity.
•Ifthesprueisstraightandcylindrical,thenmetalflowwouldnotbefullat
bottom,butsomelowpressureareawouldbecreatedaroundthemetalinthe
sprue.Atmosphericairwouldbebreathedintothislowpressureareawhich
wouldbethencarriedtothemoldcavity.
•Toeliminatethisproblemtaperedsprueisused.
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10. SprueBaseWell
•Thisisreservoirofthemetalatthebottomofthespruetoreducethe
momentumofthemoltenmetal.
•Themoltenmetalgainsvelocitywhilemovingdownthesprue,someofwhichis
lostinthespruebasewellbywhichthemolderosionisreduced.
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11. Runner
•Itislocatedinpartingplaneandconnectsthespruetothein-gates.
•Therunnersarenormallymadetrapezoidalincross-section.
•Theslagtrappingtakesplaceintherunner,whenrunnerflowsfull.Iftheamount
ofmoltenmetalcomingfromspruebaseismorethantheamountflowing
throughthein-gates.
•Apartiallyfilledrunnercausesslagtoenterthemoldcavity.
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12. Runner
•Whiledesigningtherunnersystem,careshouldbetakentoreducethesharp
cornersorsuddenchangeofsections.
•Fromheat-lossfactorcircularcross-sectionrunnersarepreferable.
•Alsorunnerisgenerallycutincopeandin-gateindragtotraptheslag.
•Itisalsogoodpracticetohavehalfoftherunnerinthecopeandtherestinthe
dragwhicheffectivelyreducestheslaginclusion.
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13. RunnerExtension
•Therunnerisextendedlittlefurtherafteritencountersthein-gate.
•Thisextensionisprovidedtotraptheslaginthemoltenmetal.
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14. GatesorIn-gates
•Theseareopeningsthroughwhichmolten
metalentersthemoldcavity.
TopGate:
•Inthistypeofgatemetalentersthecavity
fromtop.
•Cavityisfilledveryquickly.Therefore,top
gatesarenotadvisableforthosematerials
whicharelikelytoformdross(turbulence,
waste,slag,etc.).
BottomGate:
•Thistypeofgateisusedwhenthemolten
metalentersthemoldcavityfrombottom
ofthecavity.
•Ittakesmoretimetofillthemold.
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15. GatesorIn-gates
Bottom Gates

16. GatesorIn-gates
PartingGate:
•Themetalentersthemoldatthe
partingplanewhenapartofthe
castingisinthecopeandapartof
thecastingisindrag.
StepGate:
•Theyareusedforheavyandlarge
castings.
•Themoltenmetalentersmold
cavitythroughanumberofin-
gates,whicharearrangedin
verticalsteps.
•Thesizeofin-gatesisnormally
increasedfromtoptobottom.
•Thisensuresthegradualfillingof
themoldwithoutmolderosion
andproducessoundcasting.
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17. GatesorIn-gates
•Thegatesaregenerallymadewidercomparingtodepth,uptoarationof1:4.
•Thegatesshouldbeplacednearthecore-printorchill
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18. Riser
•Mostofthefoundrymetalsandalloysshrinkduringsolidification,asaresultof
volumetricshrinkage,thevoidsarelikelytoforminthecasting.
•Additionalmoltenmetalisfedintotheseplaceswhichistermedashotspots
sinceitremainshottilltheend.
•Hence,areservoirofmoltenmetalismaintainedfromwhichthemetalcanflow
readilyintothemoldcavitywhentheneedarises,thisreservoiriscalledriser.
•Differentmaterialshavedifferentshrinkageshencetheriseringrequirements
varyforthematerials.
e.g.Greycastironsometimesmayhavenegativeshrinkage.Thishappens
becausewithhighercarbonandsiliconcontents,graphitizationoccurswhich
increasesthevolumeandcounteractsthemetalshrinkage.Thusriseringmayb
verycriticalinsuchsituations.
Formetalslikealuminiumandsteel,thevolumetricshrinkagebeingveryhigh,
elaborateriseringisrequired.
•The solidifiedmetal in the riser is cut
offfromthemaincastingandmeltedforreuse.
•Thehighertheriservolume,theloweristhecastingyield.
•Therequirementoftheriserdependsonthetypeofmetalpouredandthe
complexityofthecasting.
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19. Riser
•Duringsolidificationmetalexperienceshrinkagewhichresultsinvoidformation
creationofhotspots.
•Thiscanbeavoidedbyfeedinghotspotduringsolidification.
•Riserareusedtofeedcastingduringsolidification.
•Riser must solidify after casting.
•Riser should be located so that directional solidification occurs from the
extremities of mold cavity back toward the riser.
•Thickest part of casting–last to freeze, riser should feed directly to these regions.
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20. Riser
Theshrinkageoccursinthreestages,
1.Whentemperatureofliquidmetaldropsfrompouringtofreezingtemperature.
2.Whenthemetalchangesfromliquidtosolidstate,and
3.Whenthetemperatureofsolidphasedropsfromfreezingtoroomtemperature.
•Theshrinkageforstage3iscompensatedbyprovidingshrinkageallowanceonpattern,
whiletheshrinkageduringstages1and2arecompensatedbyprovidingrisers.
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21. Riser
TypesofRisers
1.TopRisers:Theyareopentoatmosphere.TheyTaremostconventionaland
convenienttomake.
2.BlindRiser:arecompletelyconcealedinsidethemoldcavity.Itlosesheatslowly
sinceitissurroundedbythemoldingsandandthuswouldbemoreeffective.
3.InternalRisers:Theyareenclosedonallsidesbythecasting.Theyarenormally
usedforthecastingswhicharecylindricalinshapeorhavehollowportions.
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22. Chills
•Thechillsareusedtoprovideprogressivesolidificationandtoavoidthe
shrinkagecavities.
•Chillsarelargeheatsinks.
•Whenthegeometryofthemoldingcavitypreventssolidificationfromoccurring
naturally,achillcanbestrategicallyplacedtohelppromoteit.
•Wheneverthethicknessofthewallsofthecastingisunequal,thechillsare
placedclosetothewallwithlargerthickness,sothattheheatisquicklyabsorbed
bythechillfromlargermassmakingthecoolingrateequaltothatofthin
sections.
•Itdoesnotpermittheformationofshrinkagecavity.
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23. MaterialsofChills
•Chillscanbemadeofmanymaterials,includingiron,copper,bronze,aluminium,
graphiteandsiliconcarbide.
•Sandmaterialswithhigherdensities,thermalconductivityorthermalcapacity
canalsobeusedasachill.
•Forexample,chromitesandorzirconsandcanbeusedwhenmolding
withsilicasand.
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24. TypesofChills
Internalchills:
•Theyarepiecesofmetalthatareplacedinsidethemoldingcavity.
•Whenthecastingcavityisfilled,partofthechillwillmeltandultimatelybecome
partofthecasting,thusthechillmustbemadeofthesamematerialasthatof
thecasting.
Externalchills:
•Theyaremassesofmaterialthathaveahighheatcapacityandthermal
conductivity.
•Theyareplacedontheedgeofthemoldingcavityandeffectivelybecomepartof
thewallofthemoldingcavity.
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25. DefectsinSandCasting
DefectsoccurringduetoimproperdesignofGatingSystem:
1.Oxidationofmetal
2.Inclusionofslag,drossandotherforeignmatter
3.Coldshuts
4.Molderosion
5.Roughsurfaces
6.Shrinkage
7.Porosity
8.Entrappedgases
9.Misruns
10.Penetrationofliquidmetalintomoldwalls

26. GatingRatio
•Thegatingrationreferstoproportionofthecrosssectionalareasbetweenthesprue,runnerand
in-gatesandgenerallydenotedas:
Sprue area : runner area : in-gate area
•Dependingonchokearea,therecanbetwotypesofgatingsystems:pressurizedandnon-
pressurized

27. Anon-pressurizedgatingsystem
•Anon-pressurizedgatingsystemhavechokeareaatthebottomofthespruebase,total
runnerareaandin-gateareashigherthanthespruearea.
•Inthissystemnopressureisexistinginthemetalflowsystemandthusithelpsto
reduceturbulence.
•Itishelpfulforcastingdrossymetalsandalloyssuchasaluminiumandmagnesium.
•Thegatingsystemshouldbedesignedtoseeallthepartsflowfull.Otherwisesome
elementsofthegatingsystemmayflowpartiallyallowingforairaspiration.
•Taperedspruces
•Runnersindrag
•Lowercastingyield

28. Apressurizedgatingsystem
•Thein-gateareaissmallest
•Backpressureistherethroughoutthesystem
•Metalismoreturbulentanddrossformation
•Gatingsystemflowsfull
•Straightspruecanbeused
•Highercastingyield
•Usedforferrouscastings

29. Caine’sMethod
•Solidificationofcastingoccursbylosingheatfromthesurfaceandamountofheatisgivenbythe
volumeofthecasting.Thecoolingcharacteristicsofacastingcanberepresentedbysurface-area-
to-volumeratioofthecasting.
•SinceRiserisalsosameinsolidificationbehaviorascasting,risercoolingcharacteristicscanalsobe
specifiedbytheratioofitssurfaceareatovolume.
•Ifthisratioofcastingishigherthanitisexpectedtocoolfaster.

30. Chvorinov’sRule
•Solidificationtimeofacastingisproportionaltosquareoftheratioofvolumetosurfaceareaof
thecasting.
t
s= K ( V / SA )
2
•t
s=solidificationtime
•K=mouldconstantdependsonpouringtemp.,castingandmouldthermalcharacteristics
•V=Volumeofthecasting
•SA=Surfacearea

31. FreezingRatio
•Thefreezingratio(X)ofthemouldisdefinedastheratioofcoolingcharacteristicsof
castingtotheriser.
X = (SA / V)
casting/ (SA / V)
riser
•Inordertofeedthecastingtherisershouldsolidifylastandhencefreezingratioshould
begreaterthanunity.
•SpherehaslowestSAtoVratioandhencethatshouldbeusedasriser.Insphere
hottestmaterialbeingatcenter,itisdifficulttouseitforfeedingthecasting.
•Thenextbestisthecylindricaltypeofriser.

32. Caine’sformulaforfreezingratio
•BasedonChvorinov’srule,Cainedevelopedarelationshipempiricallyforthefreezingratioas
follows:
X = a / (Y –b) –c
•Y=Riservolume/Castingvolume
•Theaboveequationwhenplottedwillbeshownasfollowinggraph.Thelineshowsthelocusofthe
pointsthatseparatesthesoundcastingsandcastingswithshrinkages.

33. References
•PrinciplesofMetalCasting,RWHeine,CRLoper,P.C.Rosenthal.
•MetalCasting,T.V.RamanaRao.
•ManufacturingTechnology,P.N.Rao.
•FoundryEngineering,P.L.Jain.

34. Thank You!