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  • SAND, SAND ADDITIVES, SAND PROPERTIES and SAND

    RECLAMATION Edited by Avinash Juriani M.Tech Manufacturing IIT DHN

    MSE432

    Prof. Foundry TechnologyDr. Altan Turkeli

  • SANDADDITIVES Casting Sand

    SAND(SiO2) CLAYS(Bentonite..) WATER

    CELLULOSE(Wood

    flour,Cob

    flour..)

    OILCHEMICALS(Sodaash,Polymers.) REFRACTORIES(Alumina,Silica

    flour)

    SOLUBLES(Corn

    flour,Corn

    sugar)

  • Flow

    Chart

    ofaMetalCasting

    System

  • SAND

  • SAND

  • Casting Sands

    Silica

    Sands

    Zircon Olivine Chromite Aluminum

    Silicates

  • Silica Sands

    Mostgreensandmoldsconsistofsilicasands bondedwithabentonitewatermixture.(The termgreenmeansthatthe

    mold,whichis

    temperedwithwater,isnotdriedorbaked.) Thecomposition,size,sizedistribution,purity,

    andshapeof

    thesandareimportanttothe successofthemold

    makingoperation.

  • Sandsaresometimesreferredtoasnaturalor synthetic.

    Natural

    or

    Synthetic

    Naturalsandscontainenoughnaturallyoccurringclays thatthey

    canbemixedwithwaterandusedforsand

    molding. Syntheticsandshavebeenwashedtoremoveclayand

    other

    impurities,carefullyscreenedandclassifiedto giveadesiredsizedistribution,andthenreblended

    withclaysandother

    materialstoproducean optimizedsandforthecastingbeingproduced.

    Becauseofthedemandsofmodernhighpressure moldingmachinesandthenecessitytoexerciseclose

    controlovereveryaspectofcastingproduction,most foundriesuse

    only

    synthetic

    sands.

  • Composition

    Foundrysandsarecomposedalmostentirely ofsilica(SiO2)intheformofquartz.Some

    impuritiesmay

    bepresent,suchasilmenite (FeOTiO2),magnetite(Fe3O4),orolivine,

    whichiscomposedofmagnesiumandferrous orthosilicate

    [(Mg,Fe)SiO4].Silicasandisused

    primarilybecauseitisreadilyavailableand inexpensive.

  • Quartzundergoesaseriesofcrystallographic transitionsasitisheated.Thefirst,at573C, isaccompaniedby

    expansion,whichcan

    causemoldspalling.Above870C,quartz transformstotridymite,andthesandmay actuallycontractuponheating.Atstillhigher

    temperatures(>1470C),tridymite transformstocristobalite.

  • ShapeandDistributionofSandGrains

    Thesize,sizedistribution,andshapeofthesand grainsareimportantin

    controllingthequalityofthe

    mold.Mostmoldaggregatesaremixturesofnew sandandreclaimedsand,whichcontain

    notonly

    reclaimedmoldingsandbutalsocoresands.In determiningthesize,shape,anddistributionofthe

    sandgrains,it

    isimportanttorealizethatthegrain shapecontributestotheamountofsandsurface

    areaandthatthegrainsize

    distributioncontrolsthe permeabilityofthemold.

  • Asthesandsurfaceareaincreases,theamountofbonding material(normallyclayandwater)mustincreaseifthesandis

    tobeproperlybonded.Thus,achangeinsurfacearea, perhapsduetoachangeinsandshapeorthepercentageof

    core

    sandbeingreclaimed,willresultinacorresponding changeintheamountofbondrequired.

    Roundedgrainshavealowsurfaceareatovolumeratioand arethereforepreferredformakingcoresbecausethey

    require

    theleastamountofbinder.However,whentheyarerecycled intothemoldingsandsystem,theirshapecanbea

    disadvantageifthemoldingsystemnormallyusesahigh percentageofclayandwatertofacilitaterapid,automatic

    molding.Thisisbecauseroundedgrainsrequirelessbinder thantherestofthesystemsand.

  • Angularsandshavethegreatestsurfacearea(exceptforsands thatfractureeasilyandproducealargepercentageofsmall

    grainsandfines)andthereforerequiremoremulling,bond, andmoisture.Theangularityofasandincreaseswithuse

    becausethesandisbrokendownbythermalandmechanical shock.

    Theporosityofthemoldcontrolsitspermeability,whichisthe abilityofthemoldtoallowgasesgeneratedduring

    pouringto

    escapethroughthemold.Thehighestporositywillresultfrom grainsthatareallapproximatelythesamesize.

    Asthesizedistributionbroadens,therearemoregrainsthat aresmallenoughtofillthespacesbetweenlargegrains.As

    grainsbreakdownthroughrepeatedrecycling,therearemore andmoreofthesmallergrains,andtheporosityofthemold

    decreases.

  • However,iftheporosityofthemoldistoogreat,metalmay penetratethesandgrainsandcauseaburnindefect.

    Therefore,itisnecessarytobalancethebasesand distributionandcontinuetoscreenthesandandusedust

    collectors

    duringrecyclingtoremovefinesandtodetermine theproperbondaddition.

    Most

    foundries

    intheUnitedStatesusethe

    American Foundrymens'Society(AFS)grainfinenessnumberasa generalindicationofsandfineness.TheAFSgrain

    fineness

    numberofsandisapproximatelythenumberofopeningsper inchofagivensievethatwouldjustpassthe

    sampleifits

    grainswereofuniformsize,thatis,theweightedaverageof thesizesofgrainsinthesample.Itis

    approximately

    proportionaltothesurfaceareaperunitweightofsand exclusiveofclay.

  • Zircon

    Zirconiszirconiumsilicate(ZrSiO4).Itishighlyrefractoryand possessesexcellentfoundrycharacteristics.Its

    primary

    advantagesareaverylowthermalexpansion,highthermal conductivityandbulkdensity(whichgivesita

    chillingrate

    aboutfourtimesthatofquartz),andverylowreactivitywith moltenmetal.Zirconrequireslessbinderthan

    othersands

    becauseitsgrainsarerounded.Theveryhighdimensional andthermalstabilitiesexhibitedbyzirconarethe

    reasonsitis

    widelyusedinsteelfoundriesandinvestmentfoundries makinghightemperaturealloycomponents.

  • Olivine

    Olivineminerals(socalledbecauseoftheircharacteristic greencolor)areasolidsolutionofforsterite

    (Mg2SiO4)and

    fayalite

    (Fe2SiO4).Theirphysicalpropertiesvarywiththeir chemicalcompositions;therefore,thecompositionofthe

    olivineusedmustbespecifiedtocontrolthereproducibilityof thesandmixture.Caremustbetakentocalcine

    theolivine

    sandbeforeusetodecomposetheserpentinecontent,which containswater.

    Thespecificheatofolivineissimilartothatofsilica,butits thermalexpansionisfarless.Therefore,olivineis

    usedfor

    steelcastingtocontrolmolddimensions.Olivineissomewhat lessdurablethansilica,anditisanangular

    sand.

  • Chromite

    Chromite

    (FeCr2O4),ablack,angularsand,ishighlyrefractory andchemicallyunreactive,andithasgoodthermal

    stability

    andexcellentchillingproperties.However,ithastwicethe thermalexpansionofzirconsand,anditoften

    contains

    hydrousimpuritiesthatcausepinholing

    andgasdefectsin castings.Itisnecessarytospecifythecalciumoxide

    (CaO)and

    silicondioxide(SiO2)limitsinchromite

    sandtoavoidsintering reactionsandreactionswithmoltenmetalthat

    cause

    burnin.

  • Aluminum Silicates Aluminumsilicate(Al2SiO5)occursinthreecommonforms:

    kyanite,sillimanite,andandalusite.Allbreakdownathigh temperaturestoformmullite

    andsilica.Therefore,aluminum

    silicatesforfoundryuseareproducedbycalcining

    these minerals.Dependingonthesinteringcycle,thesilicamaybe

    presentascristobalite

    orasamorphoussilica.The

    grainsare highlyangular.Thesematerialshavehighrefractoriness,low thermalexpansion,andhighresistancetothermal

    shock.

    Theyarewidelyusedinprecisioninvestmentfoundries,often incombinationwithzircon.

  • Bulk

    density

    =The

    mass

    ofpowdered

    or

    granulated

    solid

    material

    per

    unit

    ofvolume

  • SANDADDITIVES Water

    Water H

    2

    0

  • SANDADDITIVES Clays

    Clays Bentonite,Southern

    (CaBentonite)

    Bentonite,Western(NaBentonite) Fireclay KaolinClay

  • Themostcommonclaysusedinbondinggreensandmolds arebentonites,whichareformsofmontmorillonite

    or

    hydratedaluminumsilicate.Montmorillonite

    isbuiltupof alternating

    tetrahedra

    ofsiliconatomssurroundedbyoxygen

    atoms,andaluminumatomssurroundedbyoxygenatoms,as showninFig.1.Thisisalayeredstructure,anditproduces

    clayparticlesthatareflatplates.Waterisadsorbedonthe surfacesoftheseplates,andthiscausesbentonite

    toexpand

    in

    thepresenceofwaterandtocontractwhendried.

  • SANDADDITIVESClays

  • SANDADDITIVESClays

  • SANDADDITIVESClays

  • Fireclay

    Fireclayconsistsessentiallyofkaolinite,ahydrousaluminum silicatethatisusuallycombinedwithbentonites

    inmolding

    sand.Itishighlyrefractory,buthaslowplasticity.Itimproves thehotstrengthofthemoldandallowsthewatercontent

    to

    bevariedovergreaterranges.Becauseofitshighhot strengthpotential,itisusedforlargecastings.Itisalsoused

    to

    improvesieveanalysisbycreatingfineswheneverthe systemdoesnothaveanoptimumwidesievedistributionof

    the

    basesand.However,becauseofitslowdurability,itsuse isgenerallylimited.Inaddition,theneedforfireclaycan

    usuallybeeliminatedthroughclosecontrolofsandmixesand materials.

  • SANDADDITIVESClays

  • SANDADDITIVESClays

  • Sand Properties Green

    Compression

    Strength

    Dry

    Compression

    Strength HotCompression

    Strength

    Moisture

    (water) Permeability Flowability Refractoriness Thermal

    Stability

    Collapsibility Produces

    good

    casting

    finish

    Mold

    Hardness Deformation Isreusable Remove

    heat

    from

    the

    cooling

    casting

  • GENERALPROPERTIESOFMOLDINGSANDS

    Fromageneralviewpoint,themoldingsandmust bereadilymoldableandproducedefectfree