A device for vacuum casting in plaster moulds

A very interesting technical solution in the scope of special casting technologies is that of making castings in plaster moulds. Technical literature and industrial practice give some information on this process, which in reality consists in the use of plaster mixed in different ratios with e.g. chamotte.

Pouring of alloys into moulds made from pure plaster, which is a mineral from the sulphate family, is very difficult. Two water molecules chemically bonded with calcium sulphate make the ceramic material lose under the rapid effect of temperature 1 ½ of its water molecules, which penetrate next into molten metal and give castings with porosity defects. A method to remove the chemically bonded water has been developed in an Antioch process, the essence of which consists in cyclic preheating of a plaster mixture to about 850 0C followed by cooling to room temperature. This operation changes the internal structure of plaster. As a next step in mould preparation, thus prepared plaster is mixed with make-up water and poured over a pattern. In practice, the required ratios of water and plaster are determined and mixed in advance to obtain a mixture of homogeneous consistency, which is next poured over the pattern.

The bonding process is an exothermic reaction (the volume of the precipitated heat amounts to about 30 kcal/kg); when setting, plaster increases its volume by about 1%. With bonding completed, the process of the mixture setting starts. Plaster acquires its full strength after complete drying. It absorbs water and dissolves in water (2,4 g/l), and therefore, after hardening and before pouring, moulds should be additionally dried to remove the make-up water. There are companies that prepare the ready-for-use mixtures for moulds. Complex solutions in this respect are available from Indutherm Germany.