Essential Manufacturing

8Casting

8.1 Introduction

Casting is the process of pouring or injecting molten metal into a mould and then allowing it to solidify. Products weighing many tonnes or just a few grams can be produced in a variety of surface finishes and accuracies; internal cavities are also possible. It may be that casting is only the first process in a series of operations that will lead to the finished product, or the desired item may be cast with such precision that it can be fully utilised immediately in the ‘as cast’ condition.

The process is probably one of the earliest, dating from about 4000 BC. However, in recent years much refinement has taken place and a variety of techniques have been devised to satisfy different needs. There are still some problems experienced generally with castings due to their metallurgical structure, as relative to other processes they tend to have lower toughness and ductility and porosity can occur.

As the metal cools in the mould from the molten state and solidifies, it also begins to contract. This means that, if parts of the casting cool before others, depressions can appear on the surface and cavities can occur internally. The problem can be decreased by ensuring that there is always molten metal available to fill the spaces as they begin to grow. This is achieved by providing a ‘head’ of metal in reservoirs in the case of gravity fed moulds and injecting the metal under pressure in the die casting processes.

8.2 Ingot Casting

This is a preliminary process ...