Processing technologies: Investment casting
Investment casting is often referred to as the lost-wax process. This method of casting is also based on the “expendable mould” principle. The patterns are made of wax or a special plastic using injection moulding in metal moulds. The patterns are then nested together like a bunch of grapes. They are dipped repeatedly in a ceramic slurry until they have a coating that is at least 7 mm thick, and which can consist of up to ten layers. The coating should be thick enough to ensure that it has sufficient inherent strength. The wax is removed by melting during subsequent baking (sintering) of the coating. The aluminium alloy is then cast into the ceramic shell while it is still hot. This has the advantage that there is very good mould filling and that casting can produce contours that are sharply defined. After cooling, the shell of the mould is removed from the casting using vibration or pressurised water.
The unsplit mould enables thin-walled shaped parts with very close dimensional tolerances to be produced and thus provides a high degree of freedom of design for the casting. Parts can be designed with a large amount of ribbing, a large surface area and cut-out areas having a good surface quality and good property levels.
Investment castings are often used as complex structural parts in the aerospace industry.