Page 15 - Curriculum Visions Dynamic Book

Cast form
Molten metal is poured into mould and solidifies
Mould
(Above) To cast something, a mould is first made to the exact shape that the final casting will take. A pattern is needed for this. The pattern is often made from metal or wood. Fine sand or some other appropriate material is then packed around the pattern and the pattern then removed. Alternatively, the pattern can be made from
a plastic which will melt and then evaporate as the hot metal flows in. This kind of pattern does not need to be removed before the pouring. In either case a small tube is fitted to allow the metal to be poured into the mould.
When the mould is complete, liquid metal is then poured into the mould. The metal flows into all of the spaces in the mould by gravity.
As the metal flows in, air flows out. Once the mould
is full, the metal is allowed to cool. As the metal cools, it shrinks a little. By placing the tube (called a gate) at the top of the casting, more liquid metal can flow in to fill any gaps left by shrinkage. Once the metal is solid, the mould can be broken open and the casting removing.
(Below) Casting is a very old technique. This Civil War bronze cannon was cast and then the barrel machined out.
(Right) The body of this car water pump is cast.
Bronze castings
Bronze is an ideal material for castings. That is because bronze expands slightly as it solidifies and so pushes against the surface of the mould, thereby reproducing any fine detail. Then, as it cools, it contracts and so pulls away from the mould, making it easy to get out.
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