It’s discouraging to hear the same old misconceptions repeated.
I’m referring to Gerry Lewy’s comment that silicone rubber molds
shrink not at all and “vulcanizing rubber tends to have a shrinkage
factor of about 8-10%.”
There is rubber shrinkage and there is casting shrinkage and the two
are totally different.
Liquid silicone rubber compounds do indeed, as Mr. Lewy states, have
as shrinkage rate of 0%. But that’s for the rubber itself – less
than perfect casting techniques can easily generate shrinkage rates
of 8-10% in the final casting even with liquid silicone rubber molds.
Vulcanizing silicones (not all silicones are liquid RTVs and not all
liquid RTVs are silicones) and natural rubbers have varying rates of
shrinkage, from 2.3% for our Castaldo White Label to our Castaldo
Very Low Temperature (vulcanizing) silicone molding rubber, with a
shrinkage rate of 0.1%.
Once again, that’s the rubber shrinkage.
Length, thickness and placement of the injection sprue, injection
time,temperature and pressure, how soon after injection the wax was
cast and 100s of other factors create shrinkage (as well as
I hope all this was of help.