A multiple group mold isn't really necessary, and it's a lot more work
to make multiple identical masters. It's better for consistency to make
up one master, and make a bunch of individual silicone molds from it.
For example, with the pickup bobbins that I use in my basses, I need 8
bobbin halves per instrument. I machined up an aluminum master, then
made up 8 silicone molds from it. Now I can mix up one little cup of
urethane and pour a full instrument set of bobbins in one shot. If and
when any of the molds get damaged, I'll make up new ones as I need
them. The important thing is that all of the molds came from the same
master, so that the parts will always be identical.
The hard urethane that I use is very easy on the silicone molds. I
generally use mold release spray for good practice, but it isn't
absolutely necessary with the urethane. Casting epoxy is another story.
Without a good coating of mold release, epoxy will pull chunks out of a
I don't remember having any urethane "go bad". It's a 50/50 mix and it
isn't that fussy. If you really screwed up the mix, it probably just
wouldn't harden. I use a pair of little stainless steel kitchen mixing
spoons, and count out so many spoonfuls of each part into two paper
cups. I visually check the levels of the two cups, side by side, pour
one into the other, stir for 15 seconds, and pour into the mold.
Urethane is great for all kinds of small parts. It sets up very
quickly, typically less than 10 minutes, but it takes about 30 minutes
before it fully hardens. After casting, it machines easily. One
drawback to hard urethane: it's naturally white, and you can use
pigments to make it any color, except black. Adding black pigment will
make it smoke grey, but it won't go black. I'm not sure about the
softer urethanes. I make my pickup bobbins smoke grey, and then
finished pickup coil goes into another mold and gets overcast with
Johnson's Extremely Strange Musical Instrument Co.