Aluminum & Zinc Aluminum Castings - Patternmaking - Sand Casting - No Bake (Air Set) Sand Casting - Permanent Mold - Rubber Plaster Mold (RPM) - CNC Machining
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PATTERNMAKING       PRECISION SAND CASTING       NO BAKE SAND CASTING       RUBBER PLASTER MOLD       PERMANENT MOLD       CNC MACHINING

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Rubber Plaster Molding (Plaster Castings or RPM Castings)

If you need die-cast prototypes without the high cost of tooling (typically needed for Die-Casting), General Foundry Service has the solution for you.

Rubber Plaster Molding (RPM) is similar to the sand casting process in which plaster replaces the sand to give the finished casting a smoother surface and better dimensional accuracy. Because metal shrinkage can be controlled better in plaster molds, warpage and distortion of castings is less than with the Precision Sand process. A significant feature of the RPM process is that parts can be consistently produced to close tolerances, which greatly reduces and often entirely eliminates costly machining and other finishing operations. RPM castings are higher in unit cost than sand castings; however, it is frequently used as a process for producing castings that are not obtainable with other casting methods.

RPM castings frequently have improved metal properties and are noted for their uniform hardness and machinability. This process is particularly suitable for castings with extremely thin sections. RPM castings are frequently used as an economical answer to prototype and pre-production marketing quantities while Die-Cast tooling is being produced.







Foundries use various methods to create tooling for the RPM process. We utilize epoxy materials "poured" against the pattern to make the tooling halves that will then be utilized to produce the Rubber tooling halves that will, in turn, produce the gypsum "plaster" mold halves.



The sprue, runner system, and gates produced in the rubber tooling that will form connecting channels in the resulting plaster for entry of the molten metal into the casting cavities.



Hollows and undercut surfaces in the casting are produced by cores, also made from plaster, that are placed in position before the mold is closed. An undercut surface is one from which the pattern cannot be withdrawn in a straight line, so must be formed by a core in the mold. The core(s) are held in place by the core-prints, formed in the plaster by pattern projections.






A mixture of water and gypsum is poured against the tooling to form the "negative" mold halves. After the material solidifies, the mold can be removed to a drying rack to be placed in the oven for baking.

After removing the water in the baking process, the mold halves are clamped together (with any inserted cores) and metal is poured into the gypsum "plaster" molds.



When the metal has solidified, the plaster mold halves are broken, leaving the finished casting(s) ready to be sawed from the gates and runners.







Please contact General Foundry Service to discuss Rubber Plaster Moldings and our design recommendations.

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