To The Group,
 An entirely new question. When making a male plug for mold
 casting, what is the mixture and ratio. I have already tried a mix
 of Plaster of Paris and Baking Soda as well as a mix of Plaster
 of Paris and Corn Starch, each a mixture of 50/50. Neither of
 these has worked. Any suggestions!
 Frank Clementi

 What material are you casting?

 Paul Lamar


Frank,

Martin Hollmann sez in his book "How to Build Composite Aircraft" on page 47
"The most common method [used in industry for making plugs] is to use a plaster
of paris male mold made up over plywood formers and a wire mesh.  The plaster
is inexpensive, it is easy to work with, it can take the temperature when post
curing and the shrinkage is minimal.  Another method for making a plug is to use
the Dow blue styrofoam that we talked about before….Using full-sized templates
which have been generated using the method outlined in my book "Modern
Aircraft Drafting" styrofoam sections are cut out and glued together with resin.
The outside is sanded to the final shape and a single fiberglass layer using
epoxy is layed up over the outside.  Bondo is used to fabricate a smooth outside
finish.  Sand, prime, and paint.  When dry, apply a paste wax to the outside and
spray on several coats of PVA.  Brown Paste Wax is made by Trewax Co., Santa
Fe Springs, CA 90670 can be purchased in any automotive store.  The PVA can
be purchased from Alexander Aeroplane Company.  Let dry for several hours.
Next we apply a RAM gel coat to the outside of the plug.  The gel coat provides
a smooth hard surface for the inside of the mold.  Ram tooling gel coat can be
purchased from Polyester Chemical Co. [818] 896-5208."

John Morena, in his book "Advanced Composite Mold Making" sez on page
132 "Some companies prefer a plaster combination composed of a rough coat,
with or without plaster/bats, with a superfine coat above.  Others prefer mixed
plaster with plastic, achieved by blending mixed plaster with an epoxy resin
system that contains a hardener (hydrophobic) which cures in the presence
of moisture.  Still others have applied plaster and splined with plastic.  Finally
others prefer the plastic splined over plastic [sic]…..the finished surface [if
plaster] should be coated with several coats of sanding sealer or lacquer
sealer."

I have also seen male plugs made of other materials also, like sheet metal.
Hope this helps.

Gene Kahn


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