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Rubber Plaster Mold Castings

General Foundry started the Rubber Plaster Mold department in the mid to late 80’s. Castings from this process will give you very similar characteristics to die castings: 63 RMS surface finish, thin (down to .080”), and complex/deep draw geometries. Beyond more design freedom, plaster offers customers significantly lower tooling costs and significantly faster lead times then die casting. General Foundry uses the plaster process for prototyping, proving the design and low volume production (or bridging to high volume). The rubber positive tooling is poured against master negatives, allowing quick and economical rev. changes after the prototyping phase. Once the design is solid some customer’s choose to build a die for higher volume production (>3,000 pieces EAU), as they have limited their risk through a couple plaster iterations. Sometimes in these cases General Foundry bridges to production or until the die is steady state. However, sometimes lower volumes do not justify the costly investment of die cast tooling and the parts remain in the plaster process. Plaster parts accommodate among the highest design freedom (breaking many of the die rules), and any hollow features or undercuts are made with plaster cores (instead of sand). Rubber plaster mold yields a 63 RMS surface finish.


The rubber plaster mold process is as follows:

- create master negatives
- pour rubber tools against master negatives & coreboxes
- pour plaster slurry against rubber positives & coreboxes
- plaster solidifies (~20 mins.), creating plaster mold halves & cores
- bake molds & cores
- pin molds together & pour metal
- shake out part from plaster molds