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Metal CastingCasting means pouring molten metal into a mold with a cavity of the

shape to be made, and allowing it to solidify

Figure 1: Metal Cast parts

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1) Molten material can flow into very small sections so that intricateshapes can be made by this process. As a result, many other operations,such as machining, forging, and welding, can be minimized oreliminated.

2) It is possible to cast practically any material that is ferrous or nonferrous.

!) As the metal can be placed e"actly where it is re#uired, large savingin weight can be achieved.

$) %he necessary tools re#uired for casting molds are very simple andine"pensive. As a result, for production of a small lot, it is theideal process.

') %here are certain parts made from metals and alloys that can only beprocessed this way.

() ize and weight of the product is not a limitation for the castingprocess.

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Limitations

1)*imensional accuracy and surface finish of the castings madeby sand casting processes are a limitation to this techni#ue. Many new casting processes have been developed whichcan ta+e into consideration the aspects of dimensionalaccuracy and surface finish. ome of these processes are die

casting process, investment casting process, vacuumsealedmolding process, and shell molding process.

2) %he metal casting process is a labor intensive process

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$

asting Methods

Sand CastingHigh Temperature Alloy,

Complex Geometry,

ough Surface !inish

"nvestment CastingHigh Temperature Alloy,

Complex Geometry,

#oderately Smooth Surface

!inish

$ie CastingHigh Temperature Alloy,

#oderate Geometry,

Smooth Surface

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Casting Terms

1.Flask:A metal or wood frame, without fixed top or bottom, in which the mold is formed%

$epending upon the position of the flas& in the molding structure, it is referred to by various names

such as drag lower molding flas&, cope upper molding flas&, chee& intermediate molding flas& usedin three piece molding%

2.Pattern: "t is the replica of the final ob'ect to be made% The mold cavity is made with the help of

pattern%

3.Parting line( This is the dividing line between the two molding flas&s that ma&es up the mold%

4.Molding sand: Sand, which binds strongly without losing its permeability to air or gases% "t is a

mixture of silica sand, clay, and moisture in appropriate proportions%

5.Facing sand( The small amount of carbonaceous material sprin&led on the inner surface of themold cavity to give a better surface finish to the castings%

6.Core( A separate part of the mold, made of sand and generally ba&ed, which is used to create

openings and various shaped cavities in the castings%

7.Pouring basin( A small funnel shaped cavity at the top of the mold into which the molten metal is

poured%

.!"rue( The passage through which the molten metal, from the pouring basin, reaches the mold

cavity% "n many cases it controls the flow of metal into the mold%

#.$unner( The channel through which the molten metal is carried from the sprue to the gate%

1%.&ate( A channel through which the molten metal enters the mold cavity%

11.C'a"lets( Chaplets are used to support the cores inside the mold cavity to ta&e care of its own

weight and overcome the metallostatic force%

12.$iser( A column of molten metal placed in the mold to feed the castings as it shrin&s and

solidifies% Also &nown as feed head.

13.(ent( Small opening in the mold to facilitate escape of air and gases%

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Steps in Making SandCastings -atternma+ing

ore ma+ing

Molding

Melting and pouring leaning

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http://en.wikipedia.org/wiki/File:Haandform-e.png

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Pattern making %he pattern is a physical model of the casting used to ma+e

the mold. %he mold is made by pac+ing some readily formed

aggregate material, such as molding sand, around thepattern. hen the pattern is withdrawn, its imprint providesthe mold cavity, which is ultimately filled with metal tobecome the casting. If the casting is to be hollow, as in thecase of pipe fittings, additional patterns, referred to as cores,

are used to form these cavities.

Core making

ores are forms, usually made of sand, which are placed into

a mold cavity to form the interior surfaces of castings. %hus

the void space between the core and moldcavity surface iswhat eventually becomes the casting.

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Molding

Molding consists of all operations necessary to prepare amold for receiving molten metal. Molding usually involves

placing a molding aggregate around a pattern held with asupporting frame, withdrawing the pattern to leave themold cavity, setting the cores in the mold cavity andfinishing and closing the mold.

Melting and Pouring

%he preparation of molten metal for casting is referred tosimply as melting. Melting is usually done in a specificallydesignated area of the foundry, and the molten metal istransferred to the pouring area where the molds are filled.

Cleaning

leaning refers to all operations necessary to the removalof sand, scale, and e"cess metal from the casting. /urnedon sand and scale are removed to improved the surfaceappearance of the casting. Excess metal, in the orm oins, !ires, parting line ins, and gates, is removed"Inspection of the casting for defects and general #uality is

performed.

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Pattern%he pattern is the principal tool during the casting process. #t isthe replica o the o$%ect to $e made $& the casting process,

!ith some modiications. %he main modifications are theaddition of pattern allowances, and the provision of core prints. Ifthe casting is to be hollow, additional patterns called cores are usedto create these cavities in the finished product. %he #uality of thecasting produced depends upon the material of the pattern, itsdesign, and construction. %he costs of the pattern and the relatede#uipment are reflected in the cost of the casting. %he use of an

e"pensive pattern is 0ustified when the #uantity of castings re#uiredis substantial.

Functions o the Pattern A pattern prepares a mold cavity for the purpose of ma+ing a

casting.

A pattern may contain pro0ections +nown as core prints if thecasting re#uires a core and need to be made hollow.

unner, gates, and risers used for feeding molten metal in the moldcavity may form a part of the pattern.

-atterns properly made and having finished and smooth surfacesreduce casting defects.

A properly constructed pattern minimizes the overall cost of thecastings.

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Pattern Materialome materials used for ma+ing patterns are wood, metalsand alloys, plastic, plaster of -aris, plastic and rubbers,wa", and resins.

%o be suitable for use, the pattern material should be 3asily wor+ed, shaped and 0oined 4ight in weight trong, hard and durable esistant to wear and abrasion

esistant to corrosion, and to chemical reactions *imensionally stable and unaffected by variations in

temperature and humidity Available at low cost

%he usual pattern materials are wood, metal, and plastics. %he most commonly used pattern material is wood, since itis readily available and of low weight. Also, it can be easilyshaped and is relatively cheap. 'he main disadvantageo !ood is its a$sorption o moisture, !hich cancause distortion and dimensional changes" 5ence,proper seasoning and up+eep of wood is almost a prere#uisite for largescale use of wood as a pattern material.

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A t&pical pattern attached !ith gating andrisering s&stem

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Pattern Allo!ances -attern allowance is a vital feature as it affects the

dimensional characteristics of the casting. %hus,

when the pattern is produced, certain allowancesmust be given on the sizes specified in the finishedcomponent drawing so that a casting with theparticular speciication can be made. %heselection of correct allowances greatly helps toreduce machining costs and avoid re%ections"

%he allowances usually considered on patterns andcore bo"es are as follows

hrin+age or contraction allowance

*raft or taper allowance

Machining or finish allowance

*istortion or camber allowance

apping allowance

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Shrinkage or Contraction Allo!ance

All most all cast metals shrin+ or contract volumetrically oncooling. %he metal shrin+age is of two types

Li(uid Shrinkage it refers to the reduction in volume!hen the metal changes rom li(uid state to solidstate at the solidus temperature. %o account for thisshrin+age6 riser,which feed the li#uid metal to the casting,are provided in the mold.

Solid Shrinkage:it reers to the reduction in volumecaused !hen metal loses temperature in solid state.

%o account for this, shrin+age allowance is provided on thepatterns.

%he rate of contraction with temperature is dependent on

the material. 7or e"ample steel contracts to a higherdegree compared to aluminum. %o compensate the solidshrin+age, a shrin+ rule must be used in laying out themeasurements for the pattern. A shrin+ rule for cast iron is189 inch longer per foot than a standard rule. If a gear blan+of $ inch in diameter was planned to produce out of castiron, the shrin+ rule in measuring it $ inch would actually

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)ate o Contraction o *ariousMetals

Material +imension !'rinkage allo)ance

*inc'+,t-

:rey ast Iron )p to * feet* feet to + feetover + feet

%-*.

%-.

%/0ast teel )p to * feet

* feet to 1 feet

over 1 feet

%*.-

%-2-

%-..

Aluminum )p to + feet

+ feet to 1 feet

over 1 feet

%-..

%-+0

%-*.

Magnesium )p to + feet

3ver + feet

%-40

%-..

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Exercise1%he casting shown is to be made in cast iron using a

wooden pattern. Assuming only shrin+age allowance,calculate the dimension of the pattern. All*imensions are in Inches

%he shrin+age allowance for cast iron for size up to 2 feet is o 12' inch

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%he shrin+age allowance for cast iron for size up to 2 feet is o.12' inchper feet7or dimension 19 inch, allowance ; 19X

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+rat or 'aper Allo!ance

/y draft is meant the taper provided by the pattern

ma+er on all vertical suraces o the pattern so thatit can be removed from the sand without tearing awaythe sides of the sand mold and without e"cessiverapping by the molder.

7igure ! a) shows a pattern having no draft allowancebeing removed from the pattern. In this case, till thepattern is completely lifted out, its sides will remain incontact with the walls of the mold, thus tending tobrea+ it.

7igure ! b) is an illustration of a pattern having properdraft allowance. 5ere, the moment the pattern liftingcommences, all of its surfaces are well away from thesand surface. %hus the pattern can be removed withoutdamaging the mold cavity.
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*raft allowance varies with the comple"ity of the sand 0ob. /ut ingeneral inner details of the pattern re#uire higher draft than outersurfaces. %he amount of draft depends upon the length of the verticalside of the pattern to be e"tracted6 the intricacy of the pattern6 themethod of molding6 and pattern material.

%able 2 provides a general guide lines for the draft allowance.

Pattern material Height of the givensurface (inch)

+rat angleExternalsurace-

+rat angle#nternalsurace-

5ood -

- to ** to ++ to // to 0*

0%

-%.-%%4.%.

0%

*%.-%.-%-%

#etal and plastic -

- to ** to ++ to // to 0*

-%.

-%%4.%.%.

0%

*%-%-%%4.
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Machining or Finish Allo!ance

%he finish and accuracy achieved in sand casting are

generally poor and therefore when the casting isfunctionally re#uired to be of good surface finish ordimensionally accurate, it is generally achieved bysubse#uent machining. Machining or finish allowances aretherefore added in the pattern dimension. 'he amount omachining allo!ance to $e provided or is aected$& the method o molding and casting used vi." handmolding or machine molding, sand casting or metalmold casting"

%he amount of machining allowance is also affected by thesize and shape of the casting6 the casting orientation6 themetal6 and the degree of accuracy and finish re#uired.

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Allo!ances o Machining *ariousMetals

Metal Dimension (inch) Allowance (inch)

Cast iron Up to 1212 to 2020 to 40

0.120.200.25

Cast steel Up to 66 to 2020 to 40

0.120.250.30

Non ferrous Up to 8

8 to 1212 to 40

0.09

0.120.16

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+istortion or Cam$er Allo!ance ometimes castings get distorted, during solidification, due to their typical shape. 7or

e"ample, if the casting has the form of the letter B, C, %, or 4 etc. it will tend tocontract at the closed end causing the vertical legs to loo+ slightly inclined. %his canbe prevented by ma+ing the legs of the B, C, %, or 4 shaped pattern converge slightly

inward) so that the casting after distortion will have its sides vertical 7igure $). 'he distortion in casting ma& occur due to internal stresses" tresses are

caused on account of une#ual cooling of different section of the casting and hinderedcontraction. Measure ta+en to prevent the distortion in casting include

Modification of casting design.

-roviding sufficient machining allowance to cover the distortion affect.

-roviding suitable allowance on the pattern, called camber or distortion allowance

inverse reflection) Figure /: +istortions in Casting

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Shake or )apping allo!ance

7or the easy withdrawal of the pattern from the mouldingsand , the pattern is slightly rapped or shoo+ around thevertical faces , which leads to a slight enlargement in themould cavity. o the final casting will be slightly over sized .

%he sha+e allowance is given to overcome this problem , by

ma+ing the pattern slightly smaller in size.

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Core and Core Prints

Castings are oten re(uired to have holes, recesses,etc" o various si.es and shapes. %hese impressions can

be obtained by using cores" o where coring is re#uired,provision should be made to support the core inside themold cavity. ore prints are used to serve this purpose.

%he core print is an added pro0ection on the pattern and itforms a seat in the mold on which the sand core rests

during pouring of the mold. %he core print must be ofade#uate size and shape so that it can support the weightof the core during the casting operation. *epending uponthe re#uirement a core can be placed horizontal, verticaland can be hanged inside the mold cavity. A typical 0ob, its

pattern and the mold cavity with core and core print isshown in 7igure '.

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A '&pical 0o$, its Pattern and the Mold Cavit&

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'&pes o Pattern

-atterns are of various types, eachsatisfying certain castingre#uirements.

ingle piece pattern

plit or two piece pattern

Match plate pattern

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Single Piece Pattern

%he one piece or single pattern is the most ine"pensive of

all types of patterns. %his type of pattern is used only incases where the 0ob is very simple and does not create anywithdrawal problems. It is also used for application in verysmallscale production or in prototype development. %histype of pattern is e"pected to be entirely in the drag and

one of the surface is is e"pected to be flat which is used asthe parting plane. A gating system is made in the mold bycutting sand with the help of sand tools. If no such flatsurface e"ists, the molding becomes complicated.

S lit ' Pi P tt

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Split or '!o Piece Patternplit or two piece pattern is most widely used type of pattern for

intricate castings. It is split along the parting surface, the position ofwhich is determined by the shape of the casting. Dne half of the

pattern is molded in drag and the other half in cope. %he two halvesof the pattern must be aligned properly by ma+ing use of the dowelpins, which are fitted, to the cope half of the pattern. %hese dowelpins match with the precisely made holes in the drag half of thepattern.

A typical split pattern of a cast iron wheel 7igure = a) is shown in

7igure = b).

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!!

S67"T C38 93:

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!$

haplets

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Classiication o casting Processes

1" Conventional Molding Processes :reen and Molding

*ry and Molding

7las+ less Molding

" Chemical Sand Molding Processes hell Molding

odium ilicate Molding

Eo/a+e Molding

2" Permanent Mold Processes :ravity *ie casting

4ow and 5igh -ressure *ie asting

/" Special Casting Processes 4ost a"

eramics hell Molding

3vaporative -attern asting

Cacuum ealed Molding

entrifugal asting

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3reen Sand Molding

:reen sand is the most diversified molding method used inmetal casting operations. %he process utilizes a mold made

of compressed or compacted moist sand. %he term FgreenFdenotes the presence of moisture in the molding sand. %hemold material consists of silica sand mi"ed with a suitablebonding agent usually clay) and moisture.

Advantages

Most metals can be cast by this method. -attern costs and material costs are relatively low.

Eo 4imitation with respect to size of casting and type ofmetal or alloy used

+isadvantages

urface 7inish of the castings obtained by this process isnot good and machining is often re#uired to achieve thefinished product.

Sand Mold Making Procedure

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g

%he first step in ma+ing mold is to place the pattern on the molding board.

%he drag is placed on the board 7igure 9a)).

*ry facing sand is sprin+led over the board and pattern to provide a non stic+ylayer.

Molding sand is then riddled in to cover the pattern with the fingers6 then the dragis completely filled.

%he sand is then firmly pac+ed in the drag by means of hand rammers. %heramming must be proper i.e. it must neither be too hard or soft.

After the ramming is over, the e"cess sand is leveled off with a straight bar +nownas a stri+e rod.

ith the help of vent rod, vent holes are made in the drag to the full depth of theflas+ as well as to the pattern to facilitate the removal of gases during pouring andsolidification.

%he finished drag flas+ is now rolled over to the bottom board e"posing the pattern.

ope half of the pattern is then placed over the drag pattern with the help oflocating pins. %he cope flas+ on the drag is located aligning again with the help of

pins 7igure 9 b)). %he dry parting sand is sprin+led all over the drag and on the pattern.

A sprue pin for ma+ing the sprue passage is located at a small distance from thepattern. Also, riser pin, if re#uired, is placed at an appropriate place.

%he operation of filling, ramming and venting of the cope proceed in the samemanner as performed in the drag.

%he sprue and riser pins are removed first and a pouring basin is scooped out at thetop to pour the li#uid metal.

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Molding Material and Properties

A large variety of molding materials is used in foundries formanufacturing molds and cores. %hey include molding sand, systemsand or bac+ing sand, facing sand, parting sand, and core sand. %hechoice of molding materials is based on their processing properties.

%he properties that are generally re#uired in molding materials are

)eractoriness

It is the ability of the molding material to resist the temperatureo the li(uid metal to $e poured so that it does not get used

!ith the metal"%he refractoriness of the silica sand is highest. Permea$ilit&

*uring pouring and subse#uent solidification of a casting, a largeamount of gases and steam is generated. %hese gases are thosethat have been absorbed by the metal during melting, air absorbedfrom the atmosphere and the steam generated by the molding andcore sand. If these gases are not allowed to escape from the mold,they would be entrapped inside the casting and cause castingdefects. %o overcome this pro$lem the molding material must$e porous" -roper venting of the mold also helps in escaping thegases that are generated inside the mold cavity.

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+r& Sand Molding

hen it is desired that the gas forming materials arelowered in the molds, air4dried molds are sometimespreerred to green sand molds"%wo types of drying ofmolds are often re#uired.

+in drying and

omplete mold drying. In s+in drying a firm mold face is produced. ha+eout of the

mold is almost as good as that obtained with green sandmolding. %he most common method of drying the refractorymold coating uses hot air, gas or oil flame. +in drying of

the mold can be accomplished with the aid of torches,directed at the mold surface.

h

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3reen Strength

%he molding sand that contains moisture is termed asgreen sand. %he green sand particles must have the abilityto cling to each other to impart sufficient strength to the

mold. %he green sand must have enough strength so thatthe constructed mold retains its shape.

+r& Strength

hen the molten metal is poured in the mold, the sandaround the mold cavity is #uic+ly converted into dry sand

as the moisture in the sand evaporates due to the heat ofthe molten metal. At this stage the molding sand mustposses the suicient strength to retain the exactshape o the mold cavit& and at the same time itmust $e a$le to !ithstand the metallostatic pressure

o the li(uid material" 5ot Strength

As soon as the moisture is eliminated, the sand would reachat a high temperature when the metal in the mold is still inli#uid state. 'he strength o the sand that is re(uired

to hold the shape o the cavit& is called hot strength"

C ll i$ilit

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Collapsi$ilit&

%he molding sand should also have collapsibility so that during thecontraction o the solidiied casting it does not provide an&resistance, !hich ma& result in cracks in the castings"

6esides these specific properties the molding material should becheap, reusable and should have good thermal conductivity.

Molding Sand Composition

%he main ingredients of any molding sand are

/ase sand,

/inder, and

Moisture

6ase Sand

ilica sand is most commonly used base sand. Dther base sandsthat are also used for ma+ing mold are zircon sand, hromite sand,and olivine sand. ilica sand is cheapest among all types of basesand and it is easily available.

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6inder

/inders are of many types such as

lay binders,

Drganic binders and Inorganic binders

lay binders are most commonly used binding agents mi"ed withthe molding sands to provide the strength. %he most popular claytypes are

Gaolinite or fire clay Al2D!2 iD22 52D) and /entonite Al2D!$ iD2n52D)

Df the two the /entonite can absorb more water which increases itsbonding power.

Moisture

lay ac#uires its bonding action only in the presence of the re#uiredamount of moisture. hen water is added to clay, it penetrates themi"ture and forms a microfilm, which coats the surface of eachfla+e of the clay. %he amount of water used should be properlycontrolled. %his is because a part of the water, which coats thesurface of the clay fla+es, helps in bonding, while the remainder

helps in improving the plasticity. A typical composition of moldingsand is given in %able $).

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A '&pical Composition o Molding Sand

Molding Sand Constituent Weight Percent

Silica sand 92

Clay (Sodiu !entonite" 8

#ater 4

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Shell Molding Process It is a process in which, the sand mi"ed with a

thermosetting resin is allowed to come in contact with aheated pattern plate 2

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Shell Molding Process

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Molding Sand in Shell Molding Process

%he molding sand is a mi"ture of fine grained #uartz sand andpowdered ba+elite. %here are two methods of coating the sandgrains with ba+elite. 7irst method is old coating method andanother one is the hot method of coating.

In the method of cold coating, #uartz sand is poured into the mi"erand then the solution of powdered ba+elite in acetone and ethylaldehyde are added. %he typical mi"ture is @2 #uartz sand, 'ba+elite, ! ethyl aldehyde. *uring mi"ing of the ingredients, the

resin envelops the sand grains and the solvent evaporates, leavinga thin film that uniformly coats the surface of sand grains, therebyimparting fluidity to the sand mi"tures.

In the method of hot coating, the mi"ture is heated to 1'

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Sodium Silicate Molding Process

In this process, the refractory material is coated with a sodiumsilicatebased binder. 7or molds, the sand mi"ture can becompacted manually, 0olted or s#ueezed around the pattern in theflas+. After compaction, D 2 gas is passed through the core ormold. %he D 2 chemically reacts with the sodium silicate to cure,or harden, the binder. %his cured binder then holds the refractory inplace around the pattern. After curing, the pattern is withdrawnfrom the mold.

%he sodium silicate process is one of the most environmentallyacceptable of the chemical processes available. %he ma0ordisadvantage of the process is that the binder is very hygroscopicand readily absorbs water, which causes a porosity in the castings..Also, because the binder creates such a hard, rigid mold wall,sha+eout and collapsibility characteristics can slow down

production. ome of the advantages of the process are A hard, rigid core and mold are typical of the process, which gives

the casting good dimensional tolerances6

good casting surface finishes are readily obtainable6

Permanent Mold Process

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Permanent Mold Process In all the above processes, a mold need to $e prepared or each o the

casting produced. 7or largescale production, ma+ing a mold, for everycasting to be produced, may be difficult and e"pensive. %herefore, apermanent mold, called the die may be made from which a large number of

castings can be produced. , the molds are usually made of cast iron or steel,although graphite, copper and aluminum have been used as mold materials.

%he process in which we use a die to ma+e the castings is called permanentmold casting or gravity die casting, since the metal enters the mold undergravity. ome time in diecasting we in0ect the molten metal with a highpressure. hen we apply pressure in in0ecting the metal it is called pressuredie casting process.

Advantages

-ermanent Molding produces a sound dense casting with superiormechanical properties.

%he castings produced are #uite uniform in shape have a higher degree ofdimensional accuracy than castings produced in sand

%he permanent mold process is also capable of producing a consistent#uality of finish on castings

+isadvantages

%he cost of tooling is usually higher than for sand castings

%he process is generally limited to the production of small castings of simplee"terior design, although comple" castings such as aluminum engine bloc+sand heads are now commonplace.

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Centriugal Casting

In this process, the mold is rotated rapidly about its centrala"is as the metal is poured into it. /ecause of the

centrifugal force, a continuous pressure will be acting onthe metal as it solidifies. %he slag, o"ides and otherinclusions being lighter, get separated from the metal andsegregate towards the center. %his process is normally usedfor the ma+ing of hollow pipes, tubes, hollow bushes, etc.,

which are a"isymmetric with a concentric hole. ince themetal is always pushed outward because of the centrifugalforce, no core needs to be used for ma+ing the concentrichole. %he mold can be rotated about a vertical, horizontal oran inclined a"is or about its horizontal and vertical a"essimultaneously. %he length and outside diameter are fi"ed

by the mold cavity dimensions while the inside diameter isdetermined by the amount of molten metal poured into themold.7igure @Certical entrifugal asting), 7igure 1

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5orizontal entrifugal *ieasting

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emi entrifugal asting

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Advantages

7ormation of hollow interiors in cylinders without cores

4ess material re#uired for gate

7ine grained structure at the outer surface of the castingfree of gas and shrin+age cavities and porosity

+isadvantages

More segregation of alloy component during pouring underthe forces of rotation

ontamination of internal surface of castings with nonmetallic inclusions

Inaccurate internal diameter

#nvestment Casting Process

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%he root of the investment casting process, the cire perdue or lost?wa" method dates bac+ to at least the fourth millennium /.. %he?artists and sculptors of ancient 3gypt and Mesopotamia used therudiments of the investment casting process to create intricately

detailed 0ewelry, pectorals and idols. %he investment castingprocess alos called lost wa" process begins with the production ofwa" replicas or patterns of the desired shape of the castings. Apattern is needed for every casting to be produced. %he patternsare prepared by in0ecting wa" or polystyrene in a metal dies. Anumber of patterns are attached to a central wa" sprue to form a

assembly. %he mold is prepared by surrounding the pattern withrefractory slurry that can set at room temperature. %he mold is thenheated so that pattern melts and flows out, leaving a clean cavitybehind. %he mould is further hardened by heating and the moltenmetal is poured while it is still hot. hen the casting is solidified,the mold is bro+en and the casting ta+en out.

'he $asic steps o the investment casting process are Figure 11- :

-roduction of heatdisposable wa", plastic, or polystyrene patterns

Assembly of these patterns onto a gating system

Investing, or covering the pattern assembly with refractory slurry

Melting the pattern assembly to remove the pattern material

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'he 6asic Steps o the #nvestment CastingProcess

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Advantages

7ormation of hollow interiors in cylinders without cores

4ess material re#uired for gate

7ine grained structure at the outer surface of the castingfree of gas and shrin+age cavities and porosity

+isadvantages More segregation of alloy component during pouring under

the forces of rotation

ontamination of internal surface of castings with nonmetallic inclusions

Inaccurate internal diameter

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Ceramic Shell #nvestment Casting Process

%he basic difference in investment casting is that in theinvestment casting the wa" pattern is immersed in a

refractory aggregate before dewa"ing whereas, in ceramicshell investment casting a ceramic shell is built around atree assembly by repeatedly dipping a pattern into a slurryrefractory material such as zircon with binder). After eachdipping and stuccoing is completed, the assembly isallowed to thoroughly dry before the ne"t coating isapplied. %hus, a shell is built up around the assembly. %hethic+ness of this shell is dependent on the size of thecastings and temperature of the metal to be poured.

After the ceramic shell is completed, the entire assembly isplaced into an autoclave or flash fire furnace at a hightemperature. %he shell is heated to about @92 o to burnout any residual wa" and to develop a hightemperaturebond in the shell. %he shell molds can then be stored forfuture use or molten metal can be poured into themimmediately. If the shell molds are stored, they have to be

preheated before molten metal is poured into them.

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Advantages

e"cellent surface finish tight dimensional tolerances

machining can be reduced or completelyeliminated

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Casting +eects

%he following are the ma0or defects,which are li+ely to occur in sandcastings

:as defects

hrin+age cavities

Molding material defects

-ouring metal defects

Mold shift

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3as +eectsA condition e"isting in a casting caused by the trapping ofgas in the molten metal or by mold gases evolved during

the pouring of the casting. %he defects in this category canbe classified into $lo!holes and pinhole porosit&"6lo!holes are spherical or elongated cavities present in thecasting on the surface or inside the casting. Pinholeporosit& occurs due to the dissolution of hydrogen gas,which gets entrapped during heating of molten metal.

Causes

%he lower gaspassing tendency of the mold, which may bedue tolo!er venting, lo!er permea$ilit& o the moldor improper design o the casting" %he lowerpermeability is caused by finer grain size of the sand, highpercentage of clay in mold mi"ture, and e"cessive moisturepresent in the mold.

Metal contains gas

Mold is too hot

-oor mold burnout

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Shrinkage Cavities

%hese are caused by li#uid shrin+age occurring during thesolidification of the casting. %o compensate for this, properfeeding of li#uid metal is re#uired. 7or this reason risers areplaced at the appropriate places in the mold. prues maybe too thin, too long or not attached in the proper location,

causing shrin+age cavities. It is recommended to use thic+sprues to avoid shrin+age cavities.

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Molding Material +eects

%he defects in this category are cuts and washes, metalpenetration, fusion, and swell.

Cut and !ashes

%hese appear as rough spots and areas of e"cess metal,and are caused by erosion of molding sand by the flowingmetal. 'his is caused $& the molding sand not having

enough strength and the molten metal lo!ing athigh velocit&"%he former can be ta+en care of by theproper choice of molding sand and the latter can beovercome by the proper design of the gating system.

Metal penetration

hen molten metal enters into the gaps between sandgrains, the result is a rough casting surface. %his occursbecause the sand is coarse or no mold wash was applied onthe surface of the mold. %he coarser the sand grains morethe metal penetration.

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Fusion

%his is caused by the fusion of the sand grains with themolten metal, giving a brittle, glassy appearance on thecasting surface. %he main reason for this is that the clay orthe sand particles are of lower refractoriness or that thepouring temperature is too high.

S!ell

Bnder the influence of metallostatic forces, the mold wallmay move bac+ causing a swell in the dimension of thecasting. A proper ramming of the mold will correct thisdefect.

#nclusions-articles of slag, refractory materials, sand or deo"idationproducts are trapped in the casting during pouringsolidification. %he provision of cho+e in the gating systemand the pouring basin at the top of the mold can preventthis defect.

Pouring Metal +eects

%he li+ely defects in this category are

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%he li+ely defects in this category are

Misruns and

old shuts.

A misrun is caused when the metal is unable to fill the mold cavity

completely and thus leaves unfilled cavities. A misrun results!hen the metal is too cold to lo!to the e"tremities of themold cavity before freezing. 4ong, thin sections are sub0ect to thisdefect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold

cavity, do not fuse together properly thus forming a discontinuity inthe casting. hen the molten metal is poured into the mold cavitythrough morethanone gate, multiple li#uid fronts will have to flowtogether and become one solid. If the flowing metal fronts are toocool, they may not flow together, but will leave a seam in the part.uch a seam is called a cold shut, and can be prevented by assuring

sufficient superheat in the poured metal and thic+ enough walls inthe casting design.

%he misrun and cold shut defects are caused either by a lo!erluidit& o the mold or when the section thic+ness of the castingis very small. 7luidity can be improved by changing the compositionof the metal and by increasing the pouring temperature of the

metal.

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3 ti S t

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3ating S&stem

%he assembly of channels which facilitates the moltenmetal to enter into the mold cavity is called the gatingsystem 7igure1=). Alternatively, the gating system refersto all passage ways through which molten metal passes toenter into the mold cavity. %he nomenclature of gatingsystem depends upon the function of different channelswhich they perform.

*own gates or sprue

ross gates or runners

Ingates or gates

%he metal flows down from the pouring basin or pouring

cup into the down gate or sprue and passes through thecross gate or channels and ingates or gates before enteringinto the mold cavity.
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Schematic o 3ating S&stem

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3oals o 3ating S&stem

%he goals for the gating system are

%o minimize turbulence to avoid trapping gasses into themold

%o get enough metal into the mold cavity before the metalstarts to solidify

%o avoid shrin+age 3stablish the best possible temperature gradient in the

solidifying casting so that the shrin+age if occurs must be inthe gating system not in the re#uired cast part.

Incorporates a system for trapping the nonmetallic

inclusions5&draulic Principles used in the 3ating S&stem

)e&nold7s 8um$er

Eature of flow in the gating system can be established bycalculating eynoldJs number

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E ; eynoldJs number C ; Mean Celocity of flow

* ; diameter of tubular flow

m ; Ginematics Ciscosity ;*ynamic viscosity 8 *ensity

r ; 7luid density

hen the eynoldJs number is less than 2

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(

h ; height of li

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'&pes o 3ating S&stems Figure1a,19b-

%he gating systems are of two types

-ressurized gating system

Bnpressurized gating system Pressuri.ed 3ating S&stem

%he total cross sectional area decreases towards the moldcavity

/ac+ pressure is maintained by the restrictions in the metal

flow 7low of li#uid volume) is almost e#ual from all gates

/ac+ pressure helps in reducing the aspiration as the spruealways runs full

/ecause of the restrictions the metal flows at high velocityleading to more turbulence and chances of mold erosion

;n4Pressuri.ed 3ating S&stem

%he total cross sectional area increases towards the moldcavity

estriction only at the bottom of sprue

'&pes o 3ating S&stems )iser
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'&pes o 3ating S&stems )iser

iser is a source of e"tra metal which flows from riser tomold cavity to compensate for shrin+age which ta+es placein the casting when it starts solidifying. ithout a riserheavier parts of the casting will have shrin+age defects,either on the surface or internally.

isers are +nown by different names as metal reservoir,feeders, or headers.

hrin+age in a mold, from the time of pouring to finalcasting, occurs in three stages.

during the li#uid state

during the transformation from li#uid to solid

during the solid state

7irst type of shrin+age is being compensated by the feedersor the gating system. 7or the second type of shrin+agerisers are re#uired. isers are normally placed at thatportion of the casting which is last to freeze. A riser muststay in li#uid state at least as long as the casting and must

be able to feed the casting during this time.

Functions o )isers

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Functions o )isers

-rovide e"tra metal to compensate for the volumetricshrin+age

Allow mold gases to escape -rovide e"tra metal pressure on the solidifying mold to

reproduce mold details more e"act

+esign )e(uirements o )isers

1")iser si.e:7or a sound casting riser must be last to

freeze. %he ratio of volume 8 surface area)2 of the risermust be greater than that of the casting. 5owever, whenthis condition does not meet the metal in the riser can be

+ept in li#uid state by heating it e"ternally or usinge"othermic materials in the risers.

")iser placement: the spacing of risers in the castingmust be considered by effectively calculating the feedingdistance of the risers.

2")iser shape: cylindrical risers are recommended formost of the castings as spherical risers although considers

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89940010 Metal Casting