Plaster Mold Casting

Plaster mold casting is a manufacturing process having a similar technique to sand casting. Plaster of Paris is used to form the mold for the casting, instead of sand. In industry parts such as valves, tooling, gears, and lock components may be manufactured by plaster mold casting.

The Process

Initially plaster of Paris is mixed with water just like in the first step of the formation of any plaster part. In the next step of the manufacture of a plaster casting mold, the plaster of Paris and water are then mixed with various additives such as talc and silica flour. The additives serve to control the setting time of the plaster and improve its strength.

The plaster of Paris mixture is then poured over the casting pattern. The slurry must sit for about 20 minutes before it sets enough to remove the pattern. The pattern used for this type of metal casting manufacture should be made from plastic or metal. Since it will experience prolonged exposure to water from the plaster mix, wood casting patterns have a tendency to warp. After striping the pattern, the mold must be baked for several hours, to remove the moisture and become hard enough to pour the metal casting. The two halves of the mold are then assembled for the casting process.

Properties and Considerations of Manufacturing by Plaster Mold Casting

When baking the casting mold just the right amount of water should be left in the mold material. Too much moisture in the mold can cause metal casting defects, but if the mold is too dehydrated, it will lack adequate strength.

The fluid plaster slurry flows readily over the pattern, making an impression of great detail and surface finish. Also due to the low thermal conductivity of the mold material the casting will solidify slowly creating more uniform grain structure and mitigating casting warping. The qualities of the plaster mold enable the process to manufacture parts with excellent surface finish, thin sections, and produces high geometric accuracy.

Figure:28

Castings of high detail and section thickness as low as .04 - .1 inch,
(2.5 - 1 mm), are possible when manufacturing by plaster mold casting

There is a limit to the casting materials that may be used for this type of manufacturing process, due to the fact that a plaster mold will not withstand temperature above 2200F (1200C). Higher melting point metals can not be cast in plaster. This process is typically used in industry to manufacture castings made from aluminum, magnesium, zinc, and copper based alloys.

Manufacturing production rates for this type of metal casting process are relatively slow, due to the long preparation time of the mold.

The plaster mold is not permeable, which severely limits the escape of gases from the casting.

Solving the Permeability Problem

When manufacturing a metal casting by the plaster mold casting process one of the biggest problems facing a foundry man is the lack of permeability of the plaster mold. Different techniques may be used in order to overcome this problem. The metal casting may be poured in a vacuum, or pressure may be used to evacuate the mold cavity just before pouring. Another technique would be to produce permeability in the mold material by aerating the plaster slurry before forming the mold for the casting. This "foamed plaster" will allow for the much easier escape of gases from the casting. Sometimes in manufacturing industry a special technique called the Antioch Process may be used to make a permeable plaster metal casting mold.

The Antioch Process

In the Antioch Process 50% plaster of Paris and 50% sand is mixed with water. The mixture is poured over the casting pattern and let set. After the pattern is removed, the mold is autoclaved in steam, (placed in an oven that uses hot steam under high pressure), and then let set in air. The resulting mold will easily allow the escape of gases from the casting.

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