Hi Gus, You might try Silastic J RTV mold rubber. It’s a two
part silicone based product. There’s the white base material
which is mixed 10 parts to one part green catalyst. When the
stuff is mixed to a homogenous green, it’s ready to pour. You
need to buy or make frames. I use about 14 gauge brass strips
that are about 3/4 inches (can use other widths) wide by 12
inches long. I bend them into a flat bottomed “U” frame that can
stand up by itself. Then I solder a sprue base to the inside
bottom of the “U” frame. I make sure that the sides are
absolutely flat and then I attach my sprued model or item to the
sprue base with wax or glue. Then I rubberband two pieces of 1/8"
plexiglass sheet, one on each side. I carefully pour the super
viscous gunk into the frame. Then I vacuum it to debubble it. I
have to periodically release the vacuum to let the bubbles pop
and have the gunk keep from overflowing. This goes on for at
least 4 minutes. Then I set the frame on paper or in a pan just
in case there’s a leak, and I leave it to cure for 12 to 15
hours. If the frame is flush and the rubberbands or clamps are
tight, you shouldn’t have leaks.
After the mold is cured, take the frame apart and cut as you
would any other rubber mold. The big difference is that this
stuff cuts like butter so you have to be very careful. You can’t
pull hard and cut as you would with regular rubber.It doesn’t
offer the same kind of resistance so be careful that you don’t
cut toward yourself. Since it’s opaque, you need to remember how
you oriented the model in the frame. I mark one plexiglass plate
as front and as soon as I disassemble it, I felt pen mark that
side of the mold. Also, this stuff doesn’t have the same tear
strength as rubber so you can’t cut corkscrew pullouts. They will
When you’re cutting the mold, anything fragile used as a model
will probably be destroyed by cutting, bending or cracking. The
big advantage of this stuff is that you can mold things that
can’t take heat, like a wax model you don’t want to cast first.
Remember, though, that if you’re not confident in cutting it
right the first time, your wax, especially an original carving,
Vent and shoot the mold using two mold plates. This mold
self-releases, giving a slick surface, and there’s very little
shrinkage. If you need to vent more air, a trick that sometimes
works is to carefully powder your venting cuts, not the mold
cavity, with a fine brush.
Hope this helps you out.