METAL CASTING

C.Manivannan

C.I.T

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Casting Definition

Casting is the process of pouring

molten metal into a mould

containing a cavity, which

represents the required product

shape.

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Casting steps

Steps in casting are three:

–Melt the metal

–Pour it into a mold

–Let it freeze– Take the solidified casting out of the mould

– Clean the Casting

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Patterns

Configuration of casting

Number of casting required

Single-piece pattern Split pattern Follow board pattern Cope and drag pattern Match plate pattern Loose-piece pattern Sweep pattern Skeleton pattern

Choice of pattern depends on:

Pattern Types

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Single piece pattern

The one piece or single pattern is the most

inexpensive of all types of patterns.

• This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems.

• It is also used for application in very small-scale production or in prototype development.

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Split Pattern or Two Piece Pattern

Split or two piece pattern is most widely used type of pattern for intricate

castings.

It is split along the parting surface, the position of which is determined by the

shape of the casting.

One half of the pattern is molded in drag and the other half in cope.

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Gated Pattern

This is an improvement over the simple pattern where the gating

and runner system are integral with the pattern.

This would eliminate the hand cutting of the runners and gates and

help in improving the productivity of a moulder.

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Cope and Drag Pattern

• It is similar to split patterns. In addition to splitting the pattern, the cope and drag halves of the pattern along with the gate and riser systems are attached separately to the metal or wooden plates along with the alignment pins.

• Used for castings which are heavy and inconvenient for handling as also for continuous production.

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Match piece pattern

the cope and drag patterns along with the gate

and riser are mounted on a single matching

plate on either side.

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Loose Piece Pattern

• It is used when the contour of the part is such that withdrawing the pattern from the mould is not possible.

• Moulding with loose pieces is a highly skilled job and is generally expensive

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Follow Board Pattern

It is adopted for those castings where there are

some portions which are structurally weak

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Sweep Pattern

It is used to sweep the complete casting by means of a plane

sweep.

used for generating large

shapes which are axi-symmetrical or prismatic in nature such

as bell shaped or cylindrical.

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Skeleton Pattern

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Pattern allowances

Shrinkage or contraction allowance

Draft or taper allowance

Machining or finish allowance

Distortion or camber allowance

Rapping allowance

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Pattern allowances

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Rate of Contraction of Various Metals

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Draft or taper allowance

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Draft Allowances of Various Metals

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Machining Allowances

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Machining Allowances of Various Metals

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Distortion or camber allowance

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Types of Casting Sand

• Green Sand• Green sand is made up of water, clay and sand.• It is the least expensive and easiest to use.

• Oil Tempered Sand• Oil tempered sand contains a binder instead of clay.• It is a sand that reacts with oil instead of water. • This type of sand is good for high quality metals, and

metals that you would to prefer have a very smooth appearance.

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• Resin Bonded Sand• Resin bonded sand is often referred to by professionals as "no

bake" sand, because it will harden at room temperature without heat.

• Resin bonded sand is created by bonding polymeric resin with silica sand.

• Core Sand• Core sand, also known as sodium silicate sand, is created by

mixing sodium silicate with plain sand and carbon dioxide. • This is an inexpensive type of sand used often used to create

decorative pieces such as metal vases.

Types of Casting Sand

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• Synthetic Sand• synthetic sand is "clean, graded sand in which clay can be

added as desired." Synthetic sand allows professionals to control its strength by adding as much or as little clay as necessary. Otherwise, this sand is used the same way green sand is used.

Types of Casting Sand

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Core

A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in injection molding.

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Core

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• In the green condition there must be adequate strength for handling.

• In the hardened state it must be strong enough to handle the forces of casting; therefore the compression strength should be 100 to 300 psi (0.69 to 2.1 MPa).

• Permeability must be very high to allow for the escape of gases.• As the casting or molding cools the core must be weak enough to

break down as the material shrinks. Moreover, they must be easy to remove during shakeout.

• Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.

• A smooth surface finish.• A minimum generation of gases during metal pouring.

Requirements

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Core making process

• Sodium Silicate/ CO2 Process• It is one of the easiest modern core-making processes. In this

process, sodium silicate (4 -6%) is mixed with silica sand • The sand is rammed into a core-box and cured by passing

CO2 through the core. CO2 dissolves in the water of sodium silicate and forms carbonic acid.

• Carbonic acid reacts with sodium silicate and forms silica jel.• The silica gel that is formed binds individual sand grains

together.

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• Molasses Process• 7-10 % molasses is mixed with silica sand and filled in mould or

core boxes then put them in ovens to dry at about 200oC. • It takes time and is not suitable for mass production.

• Shell process• In shell process Silica and is coated with Phenol formaldehyde. • The color would become light brown. • This sand is Thermoplastic. • There are two processes of coating of Phenol formaldehyde.

• 1)Warm at 80-100oC: Phenol formaldehyde is added in liquid form about 3-3.5%

• 2)Hot at120-130oC: Phenol formaldehyde is added in form of solid flakes about 3-3.5%

Core making process

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• Green sand• It is mixed with sand and clay. To bind together

• Cold-box• Cold box process consists of two parts:

1. Polyol Phenolic Resins (1%)2. Isocynate (0.4%)

Core making process

Shell Molding

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Shell Molding

• A match plate or cope-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

• Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell.

• Box is repositioned so that loose, uncured particles drop away.• Sand shell is heated in oven for several minutes for complete curing.• Shell mold is stripped from the pattern• Two halves of the shell mold are assembled, supported by sand or

metal shot in a box, and pouring is accomplished. The finished casting with sprue is removed.

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• Shell thickness typically 9 mm is used• Surface of shell mold cavity is smoother than sand mold.• Easy flow of molten metal, good surface quality• Finish is of the order of 2.5 micrometer.• Good dimensional accuracy• Can be mechanized for mass production and is very

economical• Gears, valve bodies, bushings, and cam shafts are typical

products• Expensive metal pattern as compared to sand casting• Difficult to justify for small quantities manufacturing• Possible on small to medium size parts• Suitable for steel castings less than 10 kg.

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Shell MoldingAdvantages & Limitations

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Investment Casting

• Parts of greater complexity and intricacy can be cast• Close dimensional control ±0.075mm• Good surface finish• The lost wax can be reused• Additional machining is not required in normal course• Al, Cu, Ni, Carbon and alloy steels, tool steels etc. are the

common materials• Preferred for casting weight less than 5 kg, maximum

dimension less than 300 mm, Thickness is usually restricted to 15mm

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Investment CastingAdvantages & Limitations

Die Casting

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Die Casting Machine

• In Die casting the molten metal is forced to flow into a permanent metallic mold under moderate to high pressures, and held under pressure during

• Solidification• This high pressure forces the metal into intricate details,

produces smooth surface and excellent dimensional accuracy

• High pressure causes turbulence and air entrapment. In order to minimize this larger in-gates are used and in the beginning, pressure is kept low

• and is increased gradually40

Die Casting

• A permanent mold made of metal or ceramic is rotated at high speed (300 to 3000 rpm). The molten metal is then poured into the mold cavity and due to centrifugal action the molten metal conform to the cavity provided in the mould.

• Castings are known for their higher densities in the outer most regions.

• The process gives good surface finish• Applications: pipes, bushings, gears, flywheels etc. 41

Centrifugal Casting