cpt 2-sand sand

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  1. 1. SAND,SANDADDITIVES,SAND PROPERTIES,and SAND RECLAMATION Prof.Dr.AltanTurkeli MSE432 Foundry Technology
  2. 2. 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)
  3. 3. Flow Chart ofaMetalCasting System
  4. 4. SAND
  5. 5. SAND
  6. 6. Casting Sands Silica Sands Zircon Olivine Chromite Aluminum Silicates
  7. 7. Silica Sands Mostgreensandmoldsconsistofsilicasands bondedwithabentonitewatermixture.(The termgreenmeansthatthe mold,whichis temperedwithwater,isnotdriedorbaked.) Thecomposition,size,sizedistribution,purity, andshapeof thesandareimportanttothe successofthemold makingoperation.
  8. 8. 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.
  9. 9. Composition Foundrysandsarecomposedalmostentirely ofsilica(SiO2)intheformofquartz.Some impuritiesmay bepresent,suchasilmenite (FeOTiO2),magnetite(Fe3O4),orolivine, whichiscomposedofmagnesiumandferrous orthosilicate [(Mg,Fe)SiO4].Silicasandisused primarilybecauseitisreadilyavailableand inexpensive.
  10. 10. Quartzundergoesaseriesofcrystallographic transitionsasitisheated.Thefirst,at573C, isaccompaniedby expansion,whichcan causemoldspalling.Above870C,quartz transformstotridymite,andthesandmay actuallycontractuponheating.Atstillhigher temperatures(>1470C),tridymite transformstocristobalite.
  11. 11. ShapeandDistributionofSandGrains Thesize,sizedistribution,andshapeofthesand grainsareimportantin controllingthequalityofthe mold.Mostmoldaggregatesaremixturesofnew sandandreclaimedsand,whichcontain notonly reclaimedmoldingsandbutalsocoresands.In determiningthesize,shape,anddistributionofthe sandgrains,it isimportanttorealizethatthegrain shapecontributestotheamountofsandsurface areaandthatthegrainsize distributioncontrolsthe permeabilityofthemold.
  12. 12. 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.
  13. 13. 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.
  14. 14. 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.
  15. 15. Zircon Zirconiszirconiumsilicate(ZrSiO4).Itishighlyrefractoryand possessesexcellentfoundrycharacteristics.Its primary advantagesareaverylowthermalexpansion,highthermal conductivityandbulkdensity(whichgivesita chillingrate aboutfourtimesthatofquartz),andverylowreactivitywith moltenmetal.Zirconrequireslessbinderthan othersands becauseitsgrainsarerounded.Theveryhighdimensional andthermalstabilitiesexhibitedbyzirconarethe reasonsitis widelyusedinsteelfoundriesandinvestmentfoundries makinghightemperaturealloycomponents.
  16. 16. 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.
  17. 17. 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.
  18. 18. 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.
  19. 19. Bulk density =The mass ofpowdered or granulated solid material per unit ofvolume
  20. 20. SANDADDITIVES Water Water H20
  21. 21. SANDADDITIVES Clays Clays Bentonite,Southern (CaBentonite) Bentonite,Western(NaBentonite) Fireclay KaolinClay
  22. 22. Themostcommonclaysusedinbondinggreensandmolds arebentonites,whichareformsofmontmorillonite or hydratedaluminumsilicate.Montmorillonite isbuiltupof alternating tetrahedra ofsiliconatomssurroundedbyoxygen atoms,andaluminumatomssurroundedbyoxygenatoms,as showninFig.1.Thisisalayeredstructure,anditproduces clayparticlesthatareflatplates.Waterisadsorbedonthe surfacesoftheseplates,andthiscausesbentonite toexpand in thepresenceofwaterandtocontractwhendried.
  23. 23. SANDADDITIVESClays
  24. 24. SANDADDITIVESClays
  25. 25. SANDADDITIVESClays
  26. 26. 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.
  27. 27. SANDADDITIVESClays
  28. 28. SANDADDITIVESClays
  29. 29. 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
  30. 30. GENERALPROPERTIESOFMOLDINGSANDS Fromageneralviewpoint,themoldingsandmust bereadilymoldableandproducedefectfree castingsifitistoqualifyasagoodone.Certain specificpropertieshavebeenidentified,and testingproceduresadaptedfortheirquantitative description. TheAFS"FoundrySandHandbook"1 setsforth thestandardconditionsoftestingthesand properties.Thosepropertiesofmostobvious importanceinclude:
  31. 31. GENERALPROPERTIESOFMOLDINGSANDS 1. Greenstrength.Thegreensand,afterwater hasbeenmixedintoit,must haveadequate strengthandplasticityformakingandhandling ofthemold. 2. Drystrength.Asacastingispoured,sand adjacenttothehotmetal quicklylosesitswater assteam.Thedrysandmusthavestrengthto resisterosion,andalsothemetallostatic pressureofthemoltenmetal,or elsethemold mayenlarge.
  32. 32. GENERALPROPERTIESOFMOLDINGSANDS 3. Hotstrength.Afterthemoisturehasevaporated, thesandmayberequiredtopossessstrengthat someelevatedtemperature,above100oC. Metallostatic pressureoftheliquidmetalbearing againstthemoldwalls maycausemold enlargement,orifthemetalisstillflowing,erosion, cracks, orbreakagemayoccurunlessthesand possessesadequatehotstrength. 4. Permeability.Heatfromthecastingcausesa greensandmoldtoevolve agreatdealofsteam andothergases.Themoldmustbepermeable,i.e. porous,topermitthegasestopassoff,orthe castingwillcontaingasholes.
  33. 33. GENERALPROPERTIESOFMOLDINGSANDS 5.T