Centrifugal Casting :

In this process the molten metal is poured in a mould and allowed to solidify while the mould is revolving. The mould which will be rotating on its axis at high speeds 300 – 3000 RPM throws the metal centrifugally towards the inner wall of the mould , where it solidifies after cooling. The centrifugal force cause the metal to take up the impression of the mould cavity.the pressure is selective, in that the greater force is exerted on the denser components. This is of considerable benefit in eliminating non metallic and gasses during casting. This process is best suited for mass production.Cylindrical parts and pipes are most adaptable to this process.

In centrifugal casting the molten metal is subjected to the centrifugal process, due to which the molten metal flows in mould cavities easily and results in production of high density castings. Thin parts with high strength can be cast using this process.The castings are produced with promoted directional solidification as the colder metal is thrown to outer side of castings and the hotter metal nearer the axis of rotation which further acts as a feeder during the solidification of metal.

Centrifugal castings are used when large no of identical parts are to be produced.

The three recognized centrifugal casting process are true centrifugal casting, semi centrifugal casting and centrifuges or pressure castings.

True Centrifugal Castings: In this process the axis of casting and mould are same, the mould which is hollow and cylindrical rotates on this axis. The axis may

be horizontal, vertical or inclined. Usually the mould is rotated in horizontal plane.

The most commonly cast parts by this process are cast iron pipes, liners, bushes and cylindrical barrelsThe mould used for this process may be either of permanent type or sand lined mould.The spinning speed which gives the centrifugal force is of the order of 53- 100 times of gravity. The wall thickness and diameter of the cylindrical holes are governed by the qty of metal that is introduced into the spinning mould. This process thus does not require any sand or permanent cores as metal remains near the boundary of the mould due to centrifugal force.

Since the gates and runners are not used, the yield is almost 100%.

In horizontal mould, a true cylindrical inside surface is produced, where as in vertical mould, a parabolic inside surface is produced which involves material wastage for exact inside circular parts. The use of this process is limited only for symmetrical shaped objects such as pipes, rolls, cylinder sleeves, and liners, piston ring stock and bearings and bushes.

Semi centrifugal castings: Also known as profiled CC. the only diff between this and true CC is a central core is used to form an inner surface. The particular shape of the casting is produced by a mould and core shapes and not by centrifugal forces. However the C force aid in giving the proper feeding of mould cavities like rim sections and eliminates the risk of porosity in castings.

Symmetrical objects like wheels having arms etc are good examples of this casting.In this process the axis of spin is always

vertical. The yield is not as high as true CC. gears , fly wheels and track wheels are some of the parts produced by this process.

Centrifuged or pressure casting : This process is used for non symmetrical castings having intricate details and also for precision castings. The CF provides high fluid pressure to force the molten metal into the mould cavity.A no of similar components can be cast simultaneously. The mould cavities are positioned about its own axis and also on the axis of a central down sprue. The matl is introduced at the centre and fed into the moulds through radial ingates. Centrifuging is possible only in vertical direction.

Mould for Centrifugal Castings :Sand( temprory) , metal(Permamnet),graphite and combination of 3 materials.

1. Temp or Sand moulds : Prepared by conventional mldg tecqniques in spl mldng

boxes Green sand and dry sand are used dpndg on the product

to be mfd and other fctrs. Rate of solidification is low in these mlds bt

metallurgical defects due to chilling of metal are eliminated.

Complex shapes may be produced by this bt vthe mld hs to be made again and again for casting and hence not economical.

Permamnet or Metal moulds :Made of commercial grades of carbon steels or alloy steel and have the follwing advantages:

1. Economical for mass prdn of identical castings

2. Solidifies fast hence prdn rate is high.3. The life of the mould may be increased if

the metal surface is coated with proper refractory material.

Most commonly used for production of cast iron pipes.

Graphite Moulds : These are made from machining commercially available compressed graphite . these can be used for several times before replacing. The graphite is very economical. Have sufficient strength and easily machinable.The graphite mould does not pose problem of wetting of molten metal with their surface and thus more durable.The cooling rate is neither very high or very low.The limitation is that it cannt be used for ferrous alloys which react with carbon and dissolve it .

Generally the surface of all moulds is given adhesive refreactory material wash to improve finish. increase mould life and facilitation in removal of castings.The wash generally consist of fine silica and clay , graphite or oil.

Selection of proper speed in centrifugl castingThe CF exerted on the molten metal is directly proportional to square of the speed.In normal practice the CF is 60-75 times of gravity for sand moulds with horizontal axis, 40 – 60 times of gravity for metal moulds and about 100 times for mouds revolving abt a vertical axis. If CF is more,then longitudinal hot tears in the outer surface of the casting are produced,whereas low CF slipping or raining of the molten metal during casting is experienced.

Exact spinning speed is dependent on several factors like application and shape of casting. Thus it is very difficult to determine precisely the exact value of rotation.

Material Considerations : Grey cast iron is centrifugally cast in large tonnages because of its relatively low pouring temps, and fluidity.Centrifugal casting of steel has replaced the forging methods.The heavy non ferrous alloys, easpeccially the copper base alloys,are readily formed by CC.Light metals have been Centrifugally cast to some degree,although some cases where the oxides are as dense as the metal,CF has little or no value eliminating the non metalics.CC has proved to be efficient and economical method for producing annualr components in spl compositions and heavy walled tubings of the common alloys, particularly for those alloys which are very difficult to forge or roll.the bonding between 2 metakls by this process is said to be complete and continous and thus it is best suited to be complete and contious and thus it is best suited for producing parts having soft lining on hard metals and vice versa.

Advantages :

1. The castings produced are sounder with dense structure, cleaner and the foreign inclusions are elimianated completely. This calls for simplified inspection tecqniques.

2. Mass prdn is possible with less rejections3. Use of runners and risers and cores is eliminated.4. Mechanical and physical properties of castings are improved.5. Parts are produced closer to finished dimensions with

consequent savings in machining.

Limitations : 1. The process is limited to only cylindrical and annular parts

with a limited range of sizes.2. It involves high initial cost and requires skilled lobour for its

maintenance.