ALJ Advantage
RPM Process
About ALJ
Contact Us

Send Us E-mail
Free Casting Design Kit
RPM Process
How Rubber Plaster Mold Castings are Made
Molten metal is gravity fed into a permeable plaster mold consisting of two or more segments that had been formed in individual rubber molds, then dried and assembled. When the casting has cooled, the plaster mold is easily crumbled away.

1. Tooling usually starts with a pattern. The most accurate patterns are machined and can consist of several components in positive or negative form. A.L. Johnson's Rubber Plaster Mold process is also an excellent process using rapid prototype methods such as S.L.A and F.D.M. in creating quick turn patterns for tooling.  
2. A MASTER MOLD of tooling epoxy, that had been either cast against a positive pattern or integrated with a negative pattern The production gating system is a permanent part of the master tooling and is therefore accurately reproduced by the rubber mold for each production cycle. These necessary positive-negative reversals within the tooling cycle provide the valuable option of constructing the pattern or portions of the pattern either in positive or negative form. Thus most box type castings are tooled with a positive exterior pattern and a separate negative interior pattern. Using this technique allows for relatively simple and inexpensive tooling reworks.  
3. A RUBBER MOLD which is always positive and therefore has been cast either against a master mold or against a core pattern. The rubber mold, which is used repeatedly for making the production plaster molds, can be easily duplicated for replacement or accelerated production. The use of rubber offers several advantages. It permits easy ejection of plaster molds without draft in nearly all instances. Absence of draft simplifies machining the pattern, saving tooling time and  

4. The PLASTER used for the production molds is an extremely fine-grained material of low heat conductivity, enabling production of castings with smooth surfaces, accurate dimensions, thin section and uniform density.