Molding medium, method for making same and evaporative pattern casting

Abstract

A molding medium and process for making it, preferably for use in the evaporative pattern casting process, is disclosed. The molding medium comprises, in one embodiment, a base granular molding material having spherically shaped grains wherein the individual grains of the material are coated with a refractory material. Preferably the grains are coated first with a binding agent and then mixed with a refractory material, which may be zirconium oxide. After coating, the material is fired at a high temperature, crushed and screened to size, according to one method. Alternatively, the base molding material may itself be a refractory material, in which case the refractory material is mixed with a binding agent to agglomerate the base material into substantially spherical particles and a refractory coating need not be applied. In either embodiment, substantially spherical free-flowing particles are produced having a low angle of repose. This allows the molding medium to come into close contact with the pattern of the object to be cast. Furthermore, the use of a refractory coating for the particles of the molding medium or a refractory material for the molding medium itself eliminates the need for a refractory wash or coating on the pattern.