Hi Dave, For metal models, i have developed this trick. Make a normal
Depending on the thickness of the master, take a rigid PVC sheet.
Typically, this ranges between 0.5 to 1.5 mm.Cut the top 2 to 3 mm of
a cheap tapered plastic ballpoint pen. Draw the outline of the master
with the pen touching so you get a border which is 2 to 3 mm bigger
than the periphery of the master. If the master is deep, then you may
have to draw another outline, still more enlarged, around it. Cut the
plastic sheet on this marking.
You now need only the cavity side. Place the cut sheet on top of the
cavity. Put this in the same metal frame the same way as the
original. Add sheets of unvulcanized rubber to make a new punch/male
side, with the plastic sheet in the middle.
If you have not used sparting powder or spray beyond the plastic,
cut the mold to expose the sheet. Remove the plastic sheet. You will
get a mold that you can get a thin wax piece from. Some trimming of
the wax will be necessary from the sides.
You can get a thicker article too with this process if the original
was too thin, by choosing the proper sheet.
You can try variations with a cloth or some flexible sheet for RTV
This process can be useful for using masters made from 2 1/2 D CAD