Casting

Casting is the process of pouring a liquid material into a mould of the required shape in which it becomes solid. Plastics, metals and ceramics can all be cast and the casting of metals is one of the oldest techniques known, dating back to around 600 BC.

Sand-casting

Damp sand will retain its shape when compressed and this is the basis of the sand casting process which can be used with any metal. The mould or pattern is a replica of the shape to be cast and can be made from any number of materials although hardwood, epoxy resin and metal are popular choices. The pattern must have a good surface finish so that it can be removed from the sand easily. The different stages are shown below.

Pattern is placed in the moulding box, covered in facing sand and compacted underneath coarse moulding sand.

Moulding box is flipped over and another moulding box placed on top with 2 conical shapes. These become holes for the molten metal to flow in (runner) and provide an indication that the metal has reached all parts of the mould (riser).

The patterns are removed leaving an impression or hollow.

Casting continued

A variation on sand casting is the full-mould process in which the whole pattern is buried in the sand and cannot therefore be removed intact. The pattern itself could be made from wax or polystyrene which is then burnt out when the molten metal is poured in. This may sound uneconomic but, in fact, casting is very economic for any production quantity because the equipment used is relatively low-tech and inexpensive. The disadvantage with casting is that the accuracy is not very good ~ typically + 1.5mm on smallish castings, + 6mm on a metre long casting ~ because of the different rates of cooling, and the surface finish tends to be poor.

Quantity Manufacture

Both conventional and full-mould sand-casting processes have been developed for mass-production by mechanising all the operations, using metal patterns and pneumatic ramming equipment.
Other variations include Shell-moulding in which a resin cast is taken from both halves of the pattern, clamped together and the molten metal poured in. This method is more accurate and gives a better finish than sand-casting but is only economic for quantities greater than 500.


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