Casting
Casting is obtained by pouring the molten metal into the mould and allowing it to solidify. This process includes melting of the metal pouring the metal into the mould and solidification. The casting gets the shape of mould cavity.
Types of casting
1) sand casting
2) Gravity die casting
3) pressure die casting
a) hot chamber die casting b) cold chamber die casting
4) Centrifugal casting
5) Continuous casting
6) Chilled casting
7) malleable casting
8) Shell mould casting
1) Sand casting
In sand casting, after the molten metal is solidified, the casting is removed by destroying the mould. When the green sand mould is used, it is called green sand casting.
It is used to produce small and medium size castings. When the dry sand mould is used, it is called dry sand casting.
Large castings can be produced by this method. Defect like blow holes will not occur in this casting. Only one casting is produced by using a sand mould.
The mould cannot be reused. Production cost and time will be more in this process.
2) Gravity die casting
It is also called as permanent mould casting. Sand mould are destroyed to obtain casting. The sand mould cannot be reused.
So permanent mould are used for producing large number of castings. Permanent mould is mould is made by using heat resistance cast iron, alloy steel or graphite.
Permanent mould are made of several parts for easy removal of castings. Pouring cup, runner and riser are provided in the upper mould.
After placing the core, the parts of mould are assembled correctly. The mould or die cavity is coated with refractory material.
When the molten metals is poured, it fills into the the die due to the gravity. The core should be removed before the metal starts solidify.
After the metal is solidified, the die is split and the casting is removed.
Application
This method is used for producing carburetor body, hydraulic brake cylinder, etc.
Advantages
1) It is a fast process
2) Closer dimensional accuracy can be achieved
3) Good surface finish can be obtained
4) Less floor space is sufficient.
5) production cost is less
Disadvantages
1) The cost of die is more
2) It is not suitable for producing complicated castings
3) Removal of casting is difficult
4) It is suitable only for mass production
3) Pressure die casting
When the molten metal is forced into a die under pressure for producing casting, it is called pressure die casting. The die is made of two parts.
One part is stationary and the other is movable. the permanent mould made of metal is called die. This method is suitable for casting lead, magnesium, tin, zinc, brass.
The two types of die casting are:
1) Hot chamber die casting
2) Cold chamber die casting
4) Centrifugal casting
In centrifugal casting, a rotating metal mould is mounted on a trolley. Rails are provided to move trolley. the molten metal is poured into the mould through a long spout.
The mould is rotated as well as moved axially. Due to centrifugal force, the molten metal is thrown away from the center and deposits uniformly on the walls of the mould .
The outside of the mould is cooled by water. So the molten metal solidifies immediately. The mould may be rotated about a horizontal axis or vertical axis.
The casting is removed after splitting the mould. Hollow castings can be produced in this method without using core. The method is used for producing cylindrical and symmetrical castings.
Application
The casting like water pipes, gun barrels, fly wheel, bush bearings, gears, brake drum are produced by centrifugal casting.
Advantage
1) High rate of production
2) core is not required
3) casting defect are less
Disadvantage
1) The cost of the die is high
2) It is not suitable for large castings
3) It is suitable only for mass production
5) Continuous casting
In this process, casting are produced by pouring the molten metal continuously in to a vertical mould. The mould is made of brass or graphite.
This method is suitable for brass, bronze, copper and Aluminium. the metal is melted in a furnace which is just above the mould.
The mould metal continuously flows into the vertical mould through a valve. The mould is water cooled. so the molten metal solidifies immediately.
Two rollers are provided to pull the casting. A saw below the rollers cuts the casting to pieces of required length.
Application
Large bars or rods having round, square and hexagonal cross section can be produced. Also this method is used for producing pipes, slabs and flats.
Advantages
1) Good surface finish can be achieved
2) As this process is automatic, rate of production is high
3) Labour cost is less
Disadvantages
1) The cost equipment and mould is high
2) The cost of production and maintenance is also high
6) Chilled casting
Cooling a metal surface quickly is called chilling. This gives a hard and wear resistant surface. Chills are placed in the mould to cool the surface rapidly.
Chills are placed on the outside of the mould where the surface of the casting are to be hardened. The molten metal around the chill cools faster.
Due to the rapid cooling, the free carbon in casting becomes hard iron carbide. core chills are placed in the mould for hardening the inner surface of casting.
Application
This method is used for producing railway brake shoes, wheel, rims, crusher jaws, machines slide way.
7) Malleable casting
Hard casting cannot be easily machined. The toughness and strength of such casting are poor. To improve these properties annealing should be done.
The gives soft casting. the casting are placed in steel box and filled with sand. During this annealing process, iron carbide in the casting becomes free carbon.
These are called malleable castings. they have high tensile strength and good machinability.
Application
Malleable casting is used for producing wagon wheel hub, door hinges, pipes fittings, locks, levers, cranks, and textile machine part.
8) Shell mould casting
Shell mould casting is the modification of sand casting in which a relatively thin shell forms the mould cavity. It is used for producing precise castings which are not required for further machining.
The mould material consist of phenolic resin mixed with dry silica in the presence of alcohol. the pattern material is made of grey cast iron, aluminium or brass. It is machined accurately with good surface finish.
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