The investment casting process was one of the first processes used to produce metal castings. The process has been described as the lost wax process, precision casting and investment casting. The latter name generally has been accepted to distinguish the present industrial process from artistic, medical and jewelry applications.
In investment casting, patterns are produced in dies via injection molding. For the most part, the patterns are made of wax; however, some patterns are made of plastic or polystyrene. Because the tooling cost for individual wax patterns is high, investment casting normally is used when high volumes are required. When cores are required, they are made of soluble wax or ceramic materials.
The ceramic shell is built around a pattern/gating assembly by repeatedly dipping the "tree" into a thin refractory slurry. After dipping, a refractory aggregate, such as silica, zircon or aluminum silicate sand, is rained over the wet slurry coating. After each dipping and stuccoing is completed, the assembly is allowed to dry thoroughly before the next coating is applied. Thus, a shell is built up around the assembly. The required thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured. After the ceramic shell is complete, the entire assembly is placed into an autoclave oven to melt and remove a majority of the wax.
Some of the advantages of investment casting include excellent surface finishes, tight dimensional tolerances, reduced or eliminated machining requirements, and the ability to cast titanium and other superalloys.
Rapid Prototyping (RP) most commonly is used with investment casting to produce an actual cast part to test for form, fit and function, as well as to determine the approximate final properties of the cast parts. RP models for investment casting are created by converting a 3-D CAD model into an .STL file. The file then is printed three-dimensionally using either photopolymer, thermopolymer, polystyrene or other materials, depending on the RP method. The prototype models then can be attached to a gating system and processed through typical investment casting to produce cast prototype parts.