U2 p3 core, core prints and chaplets

m,m.

1. Manufacturing Technology II
(ME-202)
Sheet Metal
Operations
Dr. Chaitanya Sharma
PhD. IIT Roorkee

2. Core, Core prints & Chaplets
LessonObjectives
In this chapter we shall discuss the following:
Core: Need, characteristics, types
Core prints
Chaplets
Learning Activities
1.Look up
Keywords
2.View Slides;
3.Read Notes,
4.Listen to
lecture
Keywords:

3. Cores
•Full-scalemodelofinteriorsurfacesofpart
•Itisinsertedintothemoldcavitypriortopouring
•Themoltenmetalflowsandsolidifiesbetweenmoldcavity
andcoretoformthecasting'sexternalandinternalsurfaces
•Mayrequiresupportstoholditinpositioninthemoldcavity
duringpouring,calledchaplets
Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b)
possible chaplet design, (c) casting with internal cavity.

4. Core Parts
A core consists of two portions:
a)Thebodyofthecoreand
b)oneormoreextensionscalledprints
Thebodyofthecoreissurroundedbymoltenmetal
duringcastingprocess.
Bodyofcorehasallthefeatureswhicharerequiredin
finalinternalsurface(e.g.hole)ofthecastings.
Theprintsarenecessarytosupportcoreinthemould.
Theyalsoconducttheheat(andgasesproducedbya
sandcore)tothemould.

5. CORE, CORE PRINT & CORE BOX
CORE:asandshapethatis
insertedintothemoldto
producetheinternalfeatures
ofacasting,suchasholesor
passagesforwatercooling
COREPRINT:regionaddedto
thepattern,core,ormoldwhich
isusedtolocateandsupport
thecorewithinmold
COREBOX:themoldordie
usedtoproducecastingcores

6. Essential Characteristics of
Core (Sand)
A good core must possess followings:
High permeability to allow an easy escape to gases formed.
High refractoriness to withstand high temperature of
molten metal
Smooth surface.
High collapsibility i.e. it should be able to disintegrate
quickly after the solidification of the metal is complete.
Sufficient strength to support itself.

7. Functions (Purposes) of Cores
Cores are required for following :
The cores are used to form the internal cavities.
Cores are used to form a part of a green sand mould.
Cores are used to strengthen the moulds.
Cores are used as a part of the gating system.

8. Desired Characteristics of A Core
Cores are the materials used for making cavities and internal
features which cannot be produced by the pattern alone.
Coresaregenerallymadeofthesandandareevenusedin
permanentmolds.
Ingeneral,coresaresurroundedonallsidesbymeltandtherefore
subjectedtomuchmoreseverethermalandmechanicalconditions
coresandshouldbeofhigherstrengththanmoldingsand.
Followingarethedesiredcharacteristicsforacore
1.Greenstrength:acoremadeofgreensandshouldbestrong
enoughtoretaintheshapetillitgoesforbaking.
2.Drystrength:coreshouldhaveadequatedrystrengthsothat
whenthecoreisplacedinthemold,itisabletoresistthecast
materialpressureactingonit.
3.Permeability:thegasesevolvingfromthemeltandfromthemold
mayhavetogothroughthecoretoescapeoutofthemold.Hence
coresarerequiredtohaveadequatepermeability.

9. 4.Refractoriness:inmostofthecases,coreissurroundedallaroundby
themelt,itisdesirablethatthecorematerialshouldhavehigher
refractoriness.
5.Collapsibility:asthecastingcools,itshrinks,andsothecoreshouldhave
goodcollapsibility(abilitytodecreaseinsize).Lackofcollapsibilitymay
provideresistanceagainstshrinkageandcancausethecastingdefectof
hottears.
6.Smoothness:surfaceofthecoreshouldbesmoothsoastoprovidea
goodfinishtothecastingsurfacesincontactwiththecores.
7.Friability(abilitytocrumble):afterthecastingiscompletelycooled,the
coreshouldberemovedfromthecastingbeforeitisprocessedfurther.
Hencethefriabilityisalsoanimportantconsideration.
8.Lowgasemission:becausethecoresaresubjectedtoveryhigh
temperature,theevolutionofgasesfromtheinsideareveryhighat
thattemperature.Thesegasesareotherwiselikelytoproducegas
inclusiondefects.Sothecoresshouldbemadesuchthattheevolution
ofgasesisminimum.
Desired Characteristics For
A Core

10. Core Sands
CORESANDCONSTITUENTS :
•Coresandshouldcontainthesandgrains,bindersandotheradditivestoprovide
specificproperties.
Sand:
•Silicasandwhichiscompletelydevoidofclayisgenerallyusedformakingcore
sands.
Coarsesilica(becauseofitshigherrefractoriness)isusedinsteelfoundries
Finersandsareusedforcastironsandnon-ferrousalloys.
Binders:
Coresandsneedtobestrongerthanthemoldingsand.
Clayusedasbinderinmoldingsandsisnotenough&soorganicbindersare
used.
Generallyusedbindersare,linseedoil,coreoil,resins,dextrin,molasses,etc.
Coreoilisamixtureoflinseed,soya,fishandpetroleumoilsandcoaltar.
Thesebindersareburntawaybytheheatofthemeltandthusmakethecore
collapsibleduringthecoolingofthecasting.
Amountofbinderrequireddependstoagreatextentonthefinenessofsand
grains.Amountofclayleftinthesandincreasestheconsumptionofbinder.

11. Organicbindersdevelopstrengthbymeansofpolymerizationand
Cross-linking.
Toeffectthis,thecoresafterpreparationneedtobeBaked.
Apropercombinationofbakingtimeistobechosensoasto
Optimizethecoreproperties(asshowninfig).
Generalcompositionofacoresandmixturecouldbecoreoil(1%)
andwater(2.5to6%).
Core Sands

12. Classification of Cores
•Theselectionofthecorrecttypeofcoredependsonproduction
quantity,productionrate,requiredprecision,requiredsurface
finish,andthetypeofmetalbeingused.
Core can be classified as follows:
1.Based on material used for making cores
a)Sand cores b) Metal cores
2.Based on nature of use
a)Dispensable (in sand casting) b) Permanent (in die casting)
3.Based on shapes and positions of the cores in prepared moulds
a)Horizontal core b) Vertical core
c) Balanced core d) Hanging or cover core
e) Drop core or stop off core f) Ram up core
g) Kiss core.

13. Metal And Sand Cores
•MetalCoresareusedin
permanentmouldcasting.
•Metalcoresshouldbe
paralleltothemouldparting
line,orcanberemoved
beforethecastingis
removedfromthemould,and
shapedsothatisreadily
freedfromthecasting.
•Metalcoresaretypically
madefromcastironorsteel.
•Sandcoresaremade
frommaterialssimilarto
thoseusedforchemically
bondedsandmoulds.
•Thesecoresareformedin
coreboxes-similarto
patternboxesusedto
makemoulds.
•Sandcorearechemically
bondedsandofcomplex
shapes,andusedinall
mouldtypes.
Based on the material used for making cores are of two types:
Metal cores and sand cores.

14. Types of Cores
Cores are generally made of sand & are even used in permanent molds.
BASED ON THE TYPE OF SAND USED:
1.Greensandcore:theseareobtainedbythepatternitselfduring
molding.
Thisisusedonlyforthosetypeofcavitieswhichpermitthe
withdrawalofthepattern.
Thoughthisisthemosteconomicalwayofpreparingcore,the
greensandbeinglowinstrengthcannotbeusedforfairlydeep
holes.
Alargeamountofdraftistobeprovidedsothatthepatterncan
bewithdrawn.
2.Drysandcores:arethosewhicharemadebymeansofspecial
coresandsinaseparatecorebox,bakedandthenplacedinthe
moldbeforepouring.
Green Sand Core

15. Types of Cores
3.Horizontalcore:themostcommontype.
Usuallyinacylindricalformlaidhorizontallyinthemold.
Endsofcorerestinseatsprovidedbythecoreprintsonpattern.
Horizontalcoremaybemadeinonepieceusingasplitcorebox,orin
twohalvesusingahalfcorebox.
4.Verticalcore:
Thecoreisplacedalongaverticalaxisinthemould.
Theendsofthecoreattopandthebottomfitintotheseatsprovided
inthecopeanddraghalvesofthemold.
Bothhorizontalandverticalcoresareusedmorefrequentlythan
othercoresinthefoundrywork.Forthisreasontheyarecalledstock
coresandarekeptreadyinvariousdiametersandlengths.
Horizontal core Vertical core

16. Types of Cores
5.Balancedcore:
Balancedcoreissuitablewhenthecastinghasanopening
onlyononesideandonlyonecoreprintisavailableonthe
pattern.
Coreprintinsuchcasesshouldbesufficientlylargeto
supporttheweightoftheCore,whichextendsintothemold
cavity,anditshouldbeabletowithstandtheforceof
buoyancyofthemeltsurroundingit.
Tosupportcoreinmoldcavity,chapletsareofteninserted.
Balanced core

17. Types of Cores
6.CoverCore:
Covercoreisusedwhentheentirepatternis
rammedinthedragandthecoreisrequiredtobe
suspendedfromthetopofthemold.
Unlikethebalancedcore,whichextendshorizontally
inthemoldcavity,thecovercorestretches
verticallydownwards.
Cover core

18. 7.Hanging Core:
Ifthecorehangsfromthecopeanddoesnothave
anysupportatthebottominthedrag,itisreferred
toasahangingcore.
Inthiscase,itmaybenecessarytofastenthecore
withawireorrod,whichextendsthroughthecope
toafasteningonthetopsideofthecope.
Types Of Cores
Hanging core

19. 8.Wingcore"orstop-off:
Wingcoremaybeusedwhenaholeorrecessistobeobtained
inthecastingeitheraboveorbelowthepartingline.
Wingcoreisnecessitatedwhenitisnotpossibletoplacethe
patterninthemoldsuchthattherecesscanbecored
directlyorwiththeothertypesofcores.
Sinceapartofthecoreplacedinseatbecomesastop-offand
formsasurfaceofcasting,itisalsoreferredasstop-offcore.
Itisalsoknownastailcore,chaircore,andsaddlecore
accordingtoitsshapeandpositioninthemold
Types of Cores
Wing core

20. 9.Ram-UpCore:
Sometimes,thecoreissetwiththepatterninthemold
beforethemoldisrammed.Suchacoreiscalledram-upcore
itisfavoredwhenthecoredetailislocatedinan
inaccessibleposition.
Itmaybeusedforbothinteriorandexteriorportionsofa
casting.
Types of Cores
Ram Up core

21. 10.KissCores:
Whenthepatternisnotprovidedwithcoreprintsandno
seatisavailableforrestingthecore,thecoreisheldin
positionbetweenthecopeanddragsimplybythepressure
ofthecope.
Kisscoresareusefulwhenanumberofholesarerequired
inthecasting
Dimensionalaccuracywithregardtotherelativelocationof
theholesisnotimportant.
Types of Cores
Kiss Core

22. Core Making
Coresforsandcastingaremanufacturedbypacking
speciallypreparedsandinCoreboxes.
Core-makingprocessesincludesandpreparation,core
shooting,coating/treatmentandplacementinmould.
Thecavityinacoreboxisanegativereplicaofthe
correspondingpartfeature.
Thecoreboxismadeintwosegments(withaparting)to
enableremovalofthecore.
Complexcoresarepreparedbyassemblingorgluingtwoor
morecoresofsimplershapes.
Thecore-relatedactivitiesconsumesignificantresources.
Thusthenumberandvolumeofcoresmustbeminimized
totheextentpossible,toreducetoolingcostand
manufacturingtime.

23. Core Boxes
Coreboxesareusedformakingcores.Acoreboxisawooden
ormetallictypeofpatternandaremadeeithersingleorin
twoparts.
Theymaybeclassifiedaccordingtothemethodofmakingthe
coreorshapeofcore.
Thecommontypesofcoreboxesaredescribedbelow:
1.HalfCoreBox
•Halfcoreboxisusedwhenasymmetricalcoreispreparedin
twoidenticalhalveswhicharelateronpastedorcemented
togethertoformacompletecore.
Half Core Box

24. Core Boxes
2.SplitCoreBox
Itismadeintwopartslikeasplitpattern.
Boththepartsarejoinedtogetherbymeansofdowel
pinstoformthecompletehollowcavityformakingthe
coreasshowninfig.
Split Core Box

25. Core Boxes
3.DumpCoreBox
Formakingtheslaborrectangularshapeofcore,
dumpcoreboxisused.
Inconstruction,itissimilartohalfcorebox.Thebox
ismadewithsideopening.
Dump Core Box

26. Core Boxes
4.LoosePieceCoreBox
Itisusedforthepreparationofcorewiththe
provisionsofboxesorhubs.
Thisisusedwhenthetwohalvesofacoreofwhich
thehalvesarenotidenticalinshapeandsizeisto
bepreparedinthesamecore-boxasshowninfig.
Loose Piece Core Box

27. Core Boxes
5.StrickleTypeCoreBox
Usedformakingunsymmetricalorirregularshapesofcores.
Astricklecoreboxisusedwhenthecoreisrequiredtohavean
irregularshapewhichcannotbeeasilyrammedbyother
method.
Thedesiredirregularshapeisachievedbystrikingoffthe
corefromthetopofthecoreboxwithapieceofwoodcalled
strickleboard.
Strickleboardishavingsamecontourasthatofthecore.
Strickle Core Box

28. Core Prints
Coreprintsareprovidedsothatthecoresaresecurely
andcorrectlypositionedinthemoldcavity.
Designofcoreprintstakescareoftheweightofthecore
beforepouringandtheupwardmetallostaticpressureof
themeltafterpouring.
Coreprintsshouldalsoensurethatthecoreisnotshifted
duringtheentryofthemeltintothemoldcavity.
Mainforceactingonthecorewhenmeltispouredintothe
moldcavityisduetobuoyancywhichisthedifferencein
theweightoftheliquidmetalto
thatofthecorematerialof
thesamevolumeasthatofthe
exposedcore.

29. Design Of Core Prints
Coreprintsshouldbeabletotakecareofweightofcorebeforepouring&
upwardmetallostaticpressureofmoltenmetalafterpouring.
Thecoreprintshouldensurethatcoreisnotshiftedduringtheentryof
metalintomouldcavity
Themainforceactingonthecorewhenmetalispouredintomouldcavityis
duetobuoyancy.
Buoyantforceisthedifferenceintheweightoftheliquidmetaltothatof
thecorematerialofthesamevolumeasthatoftheexposedcore.
Mathematically
For horizontal core P = V(ρ-d)
P = Buoyant force, N
V = Volume of the core in the mould cavity, cm
3
(Volume = 0.25 πD
2
H)
ρ= Weight density of the liquid metal, N/cm
3
d = weight density of core material= 1.65x 10
-2
N/cm
3
For vertical core, Buoyant force P= [0.25 π(D
1
2
-D
2
) H ρ–Vd]
Where V= total volume of the core in the mould
A core should be able to support a load of 35 N/cm
2
of surface area to keep core in
position . A core must satisfy following condition A= surface area
If above condition is not satisfied than provide additional support by using chaplets.

30. TheRussianpracticeofdimensioningthecoreprintistomakethe
pressureactingonthecorebearingarea(i.e.thecoreprint
surfacearea)tobelessthan50-75%ofthemouldingsand
compressionstrengthHence

31. Core Print Dimensions
•Coreprintdimensionsaretabulatedbelowwith
referencetofigonnextslide
Table 1: Core Print Dimensions

33. Core Print Sizes

34. Effect
of
moisture,
specimen
weight,
permeability
and
green
strength
on
process
parameters