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Quick Facts
Casting aluminum, zinc and magnesium in plaster molds provides advantages over
other casting processes. It is
 a refinement of sand casting in which
plaster replaces sand to give the finished casting a smoother surface and better
dimensional accuracy. Castings can be made with sections as thin as 0.060
in. and a surface finish of 80-125 rms. This capability to accurately
reproduce master patterns made from rapid prototyping technologies (i.e. stereolithography, LOM, etc.) has made the plaster process a popular rapid
prototyping casting process.
The Process
Plaster molds are made from a mixture of metal casting plaster, talc, or other
refactory materials and water. The slurry is poured from the mixer into a
flask containing the pattern. Plaster molds may be made in two or more
parts, with a parting line, as in ordinary sand molding. Molds are dried
in a continuously circulating air oven until there is no moisture present.
As a result, no gases or steam form when molten metal is poured into the mold.
Hollows and undercut surfaces in the casting are produced by cores made of
similar plaster mixtures. When the molds are poured and the castings
solidify, plaster is broken away and the cores of adhering plaster are removed
with a high pressure water stream.
Plaster Mold
Casting Advantages
There are a number of advantages to using the plaster mold process: 1)
Plaster molded castings have dimensional accuracy and surface finish appearance
practically identical to those of die castings and therefore, because of reduced
tooling costs, is an excellent quick turnaround, low cost alternative to
expensive die cast tooling; 2) Castings made in plaster molds are noted for
their uniform hardness and machinability; 3) Parts can be consistently produced
to close tolerances, which greatly reduces and often entirely eliminates costly
machining and other finishing operations; 4) Low quantity volume applications
where quantities do not justify die cast tooling.
Plaster Mold Casting Disadvantages
Plaster mold castings are higher in unit cost than sand castings, and the
plaster process usually is not considered to be competitive with the sand or
permanent mold casting processes. Rather, it is regarded as a process for
producing castings not attainable with other casting methods.
Alloys
Only non-ferrous metals may be poured with the plaster mold process.
Tooling
Both rigid and flexible patterns are used. Rigid patterns are usually
sealed wood, metal or plastic. Flexible patterns, made from silicone
rubber, are often used.
Design and Technical
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