In this article we will discuss about the process and moulds of centrifugal casting.
Process of Centrifugal Casting:
i. True Centrifugal Casting:
In this process, the castings are made in a hollow, cylindrical mould rotated about an axis common to both casting and mould; the axis may be horizontal, vertical or inclined. Usually the mould is rotated in horizontal plane. (Refer Fig. 3.67). The most commonly cast parts by this process are cast-iron pipes, liners, bushes and cylinder barrels etc.
The mould used for this process may be either of permanent type or a Sand lined mould. Spinning speeds are such as to give a centrifugal force of the order of 50 to 100 times gravity. The wall thickness of cylindrical hole is governed by the quantity of metal that is introduced into the spinning mould.
This process thus does not require any use of sand or permanent cores as metal remains near the boundary of mould due to centrifugal forces. As no gates or risers are used, the yield is increased to nearly 100 per cent and no material is wasted. In a horizontal mould, a true cylindrical inside surface is produced.
But in vertical rotation, a parabolic inside surface is produced and this 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, bearings bushings, etc.
ii. Semi-Centrifugal Casting:
This is also known as profiled centrifugal casting. It is nearly similar to true centrifugal casting with the only difference that a central core is used to form the inner surface. The particular shape of the casting is produced by the mould and core shapes and not by centrifugal force. However centrifugal force aids in proper feeding of mould cavities like rim sections and eliminates the possibility of porosity in the castings.
Rotational speeds for this form of casting process are lower than those for true centrifugal process. Symmetrical objects like wheels having arms etc. are good examples of this casting. In this process the axis of spin is always vertical. This process is used only for symmetrical objects and its yield is not as high as that of true centrifugal casting. Parts produced by the semi-centrifugal method include gears, fly-wheels and track wheels.
iii. Centrifuged or Pressure Casting:
This process is used for non-symmetrical castings having intricate details and also for precision castings. The centrifugal force provides high fluid pressure to the force the molten metal into mould cavity.
A number of similar components can be cast simultaneously. The mould cavities are positioned about its own axis and also about the axis of a central down sprue. The metal is introduced at the centre and fed into the moulds through radial in-gates. Centrifuging is possible only in vertical direction.
Moulds Used for Centrifugal Casting:
The moulds for centrifugal casting can be made of:
i. Temporary or Sand Moulds:
These are prepared by conventional moulding techniques in the special moulding boxes. Green sand and dry sand mixtures are used depending on product to be manufactured and other factors.
In sand moulds, the rate of solidification is low but the metallurgical defects due to chilling of metal are eliminated. The complex shapes may be produced by it, but the mould has to be made again and again for casting, and is thus not much economical.
ii. Permanent or Metal Moulds:
These are generally made of commercial grades of carbon steel, or alloy steel, and have the following advantages:
(a) These are more economical for mass production of identical castings.
(b) The production rate is high as metallic portion increases rate of solidification.
(c) The smooth casting surfaces can be produced by proper coating of mould surface with some refractory material and thus increasing the life of mould. This is most commonly used for cast-iron pipes.
iii. Graphite Moulds:
These are made by machining the commercially available stock of compressed graphite or used electrodes in furnaces etc. These can be used several times before replacing. The graphite is very economical and has sufficient strength and is easily machinable.
The graphite moulds do not pose any problem of wetting of molten metal with their surface and are thus more durable. The cooling rate is neither very high nor very low. The limitation of it is that it can’t be used for ferrous alloys which react with carbon and dissolve it.
Generally the surface of all moulds is given adhesive refractory material wash to improve finish, increase mould life, and facilitation in removal of casting. The wash generally consists of fine silica and clay, graphite or oil.