Ceramic and Plaster Mold
This family of casting processes is unique in that ceramic and plaster are used as molding media. These processes offer a high degree of precision in regard to dimensions, as well as excellent surface finishes.
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, the 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 thoroughly dry 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.
The majority of investment castings weigh less than 5 lbs., but there is a trend to produce larger castings in the 10-30-lb. range. Castings weighing up to 800 lbs. have been poured in this process. Some of the advantages of investment casting include:
- excellent surface finishes;
- tight dimensional tolerances;
- reduced or eliminated machining requirements;
- ability to cast titanium as well as the other superalloys.
Generally, these processes employ a mixture of graded refractory fillers that are blended to a slurry consistency. Various refractory materials can be used as filler material. The slurry then is poured over a pattern that has been placed in a container.
First, a gel is formed in a pattern and stripped from the mold. The mold then is heated to a high temperature until it becomes rigid. After the molds cool, molten metal is poured into them, with or without preheating.
The ceramic molding processes have proven effective with smaller size castings in short- and medium-volume runs. At the same time, these processes offer castings with excellent surface finish and good dimensional tolerances.
Plaster molding is used to produce castings of the lower melting temperature metals, such as the aluminum alloys. In the process, a slurry containing calcium sulfate, sometimes called gypsum, is poured into a flask that contains the pattern. After the slurry has set, the pattern and flask are removed, and the drying cycle to remove the moisture from the mold begins.
After the mold has cooled, the cores and mold are assembled. After assembly, most molds are preheated before pouring. Because these molds have very poor permeability, vacuum-assistance or pressure usually is required during pouring.
The plaster mold processes are well-suited for short run and prototype work with the lower temperature alloys, particularly aluminum.