I have a suggestion that might work. It is not tested, so other
members of this forum need to provide more detail or approve the
whole consept. Also... I'm assuming that water soluble wax can be
injected into a mold just like ordinary injection waxes. If that is
not possible this idea won't work grin
I'm assuming you have already made your pieces?
If so... Fill up the "hollow" area of your piece with wax that won't
melt when making a mold. Where the wax-core extends all the way out
to the surface of the piece, insert some "guidance rods" that extends
a few millimeters. When done make a mold of the whole thing.
Finalize mold and make a wax model of this shape. When done carve it
down so that you end up with the "hollow" space inside your piece.
Make a new mold of this shape. This shape is the "hollow core". The
material you have just removed represents the metal in you piece.
"Big mold shape minus hollow core shape equals final piece." Right?
The approach is to inject water soluble wax into the smaller/"hollow
core" mold. Once the mold is filled take out the wax shape and insert
into the bigger mold. The "guidance rods" makes sure it is positioned
correctly. Fill up with ordinary injection wax. The final wax piece
should now consist of a water soluble core with a layer of ordinary
injection wax on the outside.
Dump in water and you end up with the final master pattern.
Another approach might be to make the "hollow core" shape in a
material of your choosing. Make a mold. Then add on to the "hollow
core" shape some material that will represent the outer or bigger
shape. The material added represents the final piece.
Now.. this is theory only - and feedback or more elegant approaches
will be appreciated