Casting

It is an ancient process, which enables complex shapes to be produced in a wide range of materials in a single-step operation. This involves the pouring of molten material into a shaped mould and allowing it to solidify to that shape. Cast components can range in size from the small teeth of a zip to large casings of several metres in diameter. Ocean-going ships’ propellers are produced this way. Modern casting techniques have resulted in:

• High quality (i.e. minimum porosity and reasonably defect-free products)
• High production rates
• Good surface finish
• Small dimensional tolerances
• The ability to cast a very wide range of materials

Moulds are made in a variety of materials including plaster and ceramics. But, the most widely used are those of sand and metal.

Casting defects vary to some extent, depending on the casting process used. The most common ones are stated below.

• Inclusions: e.g. sand or mould lining material sticking to the surface.
• Porosity: Usually caused by gas/vapour, which is unable to escape before solidification.
• Cold Shuts: It occurs when local areas of metal are not molecularly joined, due to solidification occurring too rapidly.
• Hot Tears: It occurs where the material is cracked by excessive tensile stresses, resulting from thermal contraction.