Process

• The process of casting rubber plaster molds revolves around the production of positive and negative molds, from which a final metal part is cast. To begin the process, a positive mold is made of the entire assembly to be produced; from this model, a negative silicon rubber mold is created of the entire assembly. A model of a specific area of the negative mold is created, using a different form of silicon rubber; this forms a positive mold of a small region of the same dimensions as the initial positive mold created. Around this positive mold, a negative casting is created from plaster, into which a metal is poured to create the final rubber plaster molding.
  
  

Costs

• The use rubber plaster moldings provides a large number of cost-saving opportunities over the other casting techniques rubber plaster can replace, according to General Foundry Service. These cost-saving opportunities include the reduction in finishing and machining processes that can be needed with sand molding. The finish created using rubber plaster is often smoother and more accurate than castings created from the other casting types.
  
  

Plaster

• The plaster molds used as negative molds are created from gypsum and water; once mixed and formed into the mold shape, the plaster molds are dried. Once dry, the plaster molds are baked in an oven to remove any water remaining in the mold, negative molds are often made in halves. The halves of the plaster molds are clamped together with any required inserts positioned within the mold, General Foundry Service reports. Metal is poured into the negative plaster mold and allowed to dry before the negative molds are broken open; the final part is machined in any required way.
  
  

Quantities

• Because of the extensive process of creating rubber plaster molds, the process is not used to produce large numbers of parts, or in the mass production of parts. The number of parts made from rubber plaster molds is usually limited to short runs of parts. The use of rubber plaster molds is also used to produce small quantities of parts used in marketing equipment that are mass produced in the future.