Investment-Casting (Lost Wax)

This is a very old method of casting (which was used by the ancient Chinese), but it only became of great industrial importance in the 1950's, when gas turbine manufacturing began to increase. The process was ideally suited to the production of complex-shaped nozzle guide vanes and turbine blades which, often, contained tortuous inner passages, very thin sections and had to be cast in exotic materials. The basic process is as follows:

1. A master die is made first from an easily worked metal such as brass.
2. Under pressure, hot wax is then injected into the die, to produce a wax pattern.
3. The wax pattern is then removed from the die and coated with a layer of investment material (a ceramic slurry or paste), usually by dipping a number of times.
4. When the investment coating is set, it is then heated to allow the wax to run out, and molten metal is then poured into the investment mould.
5. Once cooled, the investment coating is then broken away from the cast, metallic component.

For obvious reasons, this investment-casting process is often referred to as the ‘Lost Wax’ process. It is a technique, which is capable of producing precision castings with a dimensional accuracy of less than 0.1 mm. Surface finish is also excellent, but the major advantage is the ability to produce accurate, complex shapes that would be impossible by machining.