The first step in producing a prototype plaster casting is to construct a physical model or pattern using 3D CAD data. The most commonly used pattern is an SLA. This process uses a Stereo Lithography Apparatus (SLA) machine to build a plastic part from the 3D file. A laser beam is fired into a resin bath that solidifies the resin in layers producing an SLA pattern. For high-tolerance prototypes or larger order quantities, an aluminum master can be machined from stock, creating a more stable and durable pattern.

One of the most critical steps in the plaster casting process is the tooling design. Our expert engineering staff can quickly analyze parts to determine potential problem areas and casting difficulties. This allows us to effectively design tools that ensure even the most complex part turns into a high quality casting.

The first step in the casting process is to "tool-up" the SLA pattern. This is accomplished by surrounding the lower half of the SLA in clay and pouring liquid rubber onto the exposed portion. After the rubber has cured, the entire assembly is inverted. The clay is then removed and the SLA is revealed within the cured rubber. Rubber is then poured onto the exposed half of the SLA and left to cure. After the rubber has cured the two halves are separated and the SLA is removed, revealing an exact negative of the SLA geometry. It is during this process that the gates and risers required for metal casting are built into the tooling.

From these negatives, a similar process is used to produce plastic patterns. The plastic patterns are positive images of the part, and cast a negative image into a plaster mold material. The plaster molds are then assembled and transferred to one of several ovens to bake. Depending on size, plaster molds require anywhere from 1-3 days to dry completely.

The same construction principals are employed to produce a casting with complex geometry and cores.

After the plaster mold is dry and has reached temperature, it is removed from the oven and filled with molten aluminum, magnesium, or zinc. When it has cooled, the plaster mold is broken open to reveal the part.

This process has many similarities to the classic Rubber Plaster Mold (RPM) technique. Our refinement of this casting process allows us to create parts with much thinner walls, better feature replication, and tighter tolerances. Prototype Casting, Inc.’s unique process also works extremely well with magnesium. Many foundries consider it difficult or dangerous to produce magnesium prototypes utilizing the RPM casting system. We can also generate tooling for cast parts by utilizing a variety of pattern types, such as wood or wax.