An exact wax model is used for both the precision casting and the investment casting. Such a model is produced in an aluminum mould through injection moulding (extrusion). These models are combined to form a wax casting unit, the so called casting tree. In several steps this casting unite is coated with ceramics. After the ceramic shell has dried the wax is burned out. The shell is baked at 900°C and the liquid metal alloy is then poured into the hot ceramic shell. Trapped gas escapes through the permeable ceramic layer. The feeding via the central core prevents the forming of shrink marks and air locks (cavities) in the hardening casting.
The ceramic shells of the cooled-down casting unit are removed and the castings are separated from the casting unite. They are sandblast or even etched in order to remove remaining ceramics on the inside, e.g. in bore holes or undercuts. Then the ingate on the surface is ground off. The visual check and the dimensional check of the castings are very costly since they have to be carried out by hand. Depending on the further use of the castings the following steps can be taken: the parts can be processed mechanically (threads, fits) or be heat-treated (tempering, case hardening, nitrating). Even a surface treatment may be required (hand polishing, powder coating, cathodic dip-coating, fibration polishing etc.).
The quality assurance for initial samples and serial parts can be carried out using different procedures. It is possible to perform chemical analyses, mechanical tests, ultrasonic tests, leakage tests, dimensional checks and radiographic examinations.
Volker Groitl Dipl.-Ing. (FH)
Lohwiesenweg 5 A
Telephone: +49(0)9172 669004