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Vacuuming
Silicone & Urethane Rubbers
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© Smooth-On, Inc. (2005)
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FAQ: I
measured and mixed my silicone rubber as directed, but
did not
vacuum the material prior to pouring. My cured rubber
mold
had many bubbles. Is vacuuming the rubber that important?
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Answer:
Silicone rubbers are thick and most have a high viscosity.
Silicones do not de-air themselves as readily as urethane
rubbers and need to be vacuumed to remove entrapped air.
Consequently, if you mix and
pour silicone rubber that has not been vacuumed, air may be
trapped in the mixture as it turns from liquid to solid and
these bubbles may show up on the working surface of the
mold.
Remedy: If you
are going to be making silicone molds on a regular basis,
invest in a vacuum pump and vacuum chamber – approximate
costs: $800 - $1,000. (You can also choose to use a urethane
rubber instead of silicone rubber. Be prepared to use a
sealing agent and release agent to prepare your model as
directed.
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FAQ: Is there a way around investing in a vacuum pump
and
vacuum chamber?
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Answer: Although a vacuum pump and chamber will make your life easier
if you are making silicone molds on a regular basis, it is a
lot of money to spend on equipment.
Some of our mold
makers who do not own vacuum equipment claim that the
following method works well to minimize bubbles in their
finished mold;
After their
silicone rubber is mixed, they will brush a thin coat over
their model and then pour the balance of the mixture behind it
(off model) over their model.
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FAQ: I
recently obtained a vacuum chamber and pump to get the air out
of my silicone rubber. When I vacuum the material (after
mixing Parts A + B), my silicone takes forever to de-air and I
still end up with some bubbles in my silicone rubber. Is this
normal?
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Answer:
It sounds like your vacuum pump is not pulling enough of a
vacuum to quickly and efficiently get the air out of your
silicone mix.
After your
mixing container is placed in the vacuum chamber and the
vacuum pump is turned on, the rubber should rise, break and
fall in less than 5 minutes.
The
minimum level of vacuum to be pulled is 29 inches of mercury
(also measured as “-1 Bar”.
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FAQ: I have a vacuum pump and
chamber, but mostly use urethane rubber that I do not vacuum.
Is there any advantage to vacuuming my urethane rubber before
pouring?
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Answer:
although urethane rubbers are low in viscosity and generally
do not need to be vacuumed, they do contain air and you will
benefit by vacuuming the urethane rubber.
General Rule: If you are
fortunate enough to have a vacuum pump and chamber, use it for
best results.
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Smooth-On