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RPM Process
How Rubber Plaster Mold Castings are Made |
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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. |
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| 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. |
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| 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 |
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| 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. |
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| 5. FINAL CASTING |
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