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,   Plaster of parris needs to be porous to let trapped gasses  escape
or they will be trapped in the casting  and make a terrible surface finish.
A high speed agitator on a drill motor can be used to entrain air in the
plaster which will let the hot gasses pass through.   Also the plaster must
be
very dry (baked for several hours)  otherwise the moisture will make steam,
spall off into the casting and possibly explode. It seems to help to
lightly
sand the plaster surface after baking.
        A simpler medium for a mould is green sand with an oil binder. To
get a fine surface finish  you would dust some fine grained sand on the
pattern with a kitchen flour sifter then tamp the rest of the coarser sand
over that.
     Aluminum generally will pour with a 4 inch tall pouring sprue with the
metal entering from channels under the part with vent holes out the top
and a riser near the thickest area to add metal as the casting
solidifies/shrinks.  Magnesium needs a much taller pouring sprue to get
enough pressure head to fill the mould quickly.

george grimes


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