I’ve yet to find an RTV silicone rubber that doesn’t improve
with vacuuming. I’m most familier with the Dow Corning E-RTV.
For that, the process is as follows: Immediatly after mixing the
required amount of catalyst with the resin (do this in a greatly
oversized container. A large coffee can works nicely for the
amount of rubber needed for 3-4 molds. In essence, the can is
only about 1/4 full of rubber at most. Less is better. Now
vacuum, watching closely. The rubber will rise up and try to
overflow the can. Just before it does overflow, release the
vacuum valve, letting air into the bell jar. the rubber
collapses. Close the valve again. Just as it is about to
overflow, open the valve. Continue this for about 7 full minutes.
by the time you’re done with that, the rubber will still be
rising way up, but not quite as far, perhaps. It may still seem
like there must be a lot of air, but it will be almost gone, at
least in terms of bubbles big enough to see and interfer with
your waxes. Now pour the mold. The mold frame should also be
high enough to allow at least twice the volume that will actually
be required. Vacuum the poured molds (pour only enough to
barely cover the model) in the same manner as above, for another
five minutes. Be careful that the rubber does not start to seem
any thicker than when you first mixed it. Though it takes quite
a while to actually cure, it starts to begin to thicken in less
than 15 minutes, and you need to be done with the vacuuming and
pouring and revacuuming by that time. By the time you’ve
repeatedly vacuumed and released the vacuum on the poured mold,
you may have had a bit of overflow. that’s OK. Now top up the
mold to the level above the model that is desired, set aside and
rest assured that when you go to cut this mold, you will find no
traces of imbedded bubbles in the rubber. Other RTV rubbers may
require more or less vacuuming, but one key I’ve noticed is that
they seldom give you good instructions on how to do it. Took
some time and experimenting to figure out that it needed to be
risen and collapsed repeatedly, and for about 7 minutes, to get
the required results. (courtesy of AU enterprises and Linus
Drogs, who did that experimenting. I just took note of his
results, and relay them as I recall them. Hopefully I’ve
remembered them right.
By the way, an aside: Not all RTV rubbers are silicone. I
bought a black RTV rubber from Swest (forget the exact name)
It’s a polysulfone or some such name, if I recall. Looks and
handles like a slightly thick rubber similar to E-RTV, except
that it gave no bubbles with just a plain pour, no vacuum.
However, and this is important, It adhered strongly to the model,
which was gold plated over base metal. The instructions mention
casually that it will stick to copper based materials, but I
didn’t take it seriously enough. I was unable to seperate the
rubber from the model without (literally) shredding the mold.
The last of the rubber took vigorous abrasion and solvents to
remove from the model. Clearly a waste of time and money for no
useable mold. The instructions mention the need for a seperating
agent, but don’t say what. I’d assume that a thinned coating of
vaseline will work, but frankly, I’m not likely to use that crap
again. In addition to being a total waste for that mold, it is
no cheaper than the Dow product, and the finished rubber is so
soft that it’s almost laughable in terms of trying to keep mold
dimensions stable while injecting. I’d be willing to guess this
rubber was originally intended to be used in other ways, like
maybe a surface skin mold of a wax or plaster original, the
rubber skin then reinforced with another plaster outer mold, for
pouring was or plaster into. ie, a sculpture molding rubber,
instead of a jewelry molding one.
Peter Rowe