1. Field of the Invention
The present invention relates generally to a method for casting and, more particularly, to an improved shell mold assembly.
2. Description of the Prior Art
In shell mold casting, the shell mold comprises two sections, the cope and drag, which abut together along substantially planar mating surfaces. With the cope and drag mating surfaces in abutment with each other, the cope and drag define the casting cavity of the part which is to be formed.
The cope and drag are typically made from a dry mixture of silica or other refractory oxide sand and a minor amount of plastic or resin binder. The dry mixture of molding sand is then applied to a hot casting pattern for a relatively short period of time which melts the binder and bonds the sand together to accurately reproduce the details of the casting pattern. The cope and drag are thereafter secured together along their mating surfaces and molten metal is introduced into the casting pattern through a sprue to form the desired part.
One previously unsolved problem with shell mold casting has been the formation of openings in the casting which extend in a direction other than substantially normal to the mating surfaces of the cope and drag and particularly openings through the casting which extend substantially parallel to the cope and drag mating surfaces. Such openings cannot be formed in the cope or drag by the casting pattern since, to do so, would bond the shell mold to the casting pattern and render it impossible to remove the shell mold without breaking it.
As a result of the above described limitations of shell mold casting, it has been the previous practice to machine the desired openings in the casting which are not normal to the mating surfaces after the casting has been removed from the shell mold. The subsequent machining of the casting, however, is expensive both in machine and labor costs which accordingly increases the overall price of the final part.