Investment Casting uses a mold that has been produced by surrounding an expendable pattern with a refractory slurry that sets at room temperature. The pattern (usually of wax or plastic) is then melted or burned out, leaving the mold cavity. Investment casting is also known as the “lost-wax process”and as “precision casting”.
In sand casting, wood or metal pattern are used to make the impression in the molding material. The pattern can be re-used, but the mold is expendable. In Investment casting, a metal pattern die is used to produce the patterns, which, in turn, are used to produce ceramic molds. Both the pattern and molds are expendable. Ceramic cores are used, as required, and these also are expendable.
Investment casting is the most flexible of all the precision casting process with respect to attainable intricacy,precision and the variety of alloys that may be cast within its inherent size limitations. Of the various casting techniques, investment casting is both the newest and the oldest, depending on whether you consider it from the standpoint of industrial history or total genealogy.
Many exaggerated claims were made regarding the “precision” casting process. These claims could have lead many engineers to believe that they could safely discard their “old fashioned” machine tools. Many technical articles purported to establish the dimensional tolerances for production castings to limits as extreme as 0.0001 in. per inch. In reality, however, liner tolerances of 0.002-0.005 in. per inch where difficult to obtain. As a result, castings were incorrectly considered competitive (with respect to accuracy and economy) with modern machine tools.
Properly designed investment castings will afford economic advantages over alternate product designs intended for the same function. The gain occasionally is reflected in the initial cost. More often, it is the result of an increase of endurance or efficiency, which is not readily apparent. Many times, the first and final costs are less where the optimum design and alloy selection practices have been observed. If these factors are not applicable, then the use of the investment casting may not be warranted.
Of the casting methods in use- precision or conventional- the investment technique is the most flexible. It is competitive with all other casting processes where the size of the products is within a mutually castable range. Investment castings also compete with powder metal products, forgings, stampings, spinnings, coined parts, weldments, solderments, brasements and innumerable assembled parts held together with rivets, pins, bolts or other fasteners.
The significance of investment castings as an economical substitute for alternate hot or cold forming technique lies in that the process gives designers a major new technique to solve difficult production problems.