Pros and cons of RTV (room temperature vulcanized) verses HV (heat
vulcanized) rubber molds
I like and use silicons RTV’s a LOT (mostly) and I primarily use Tin
Catalyzed silicon because of costs and lower "contamination"
problems verses platinum catalyzed silicon. Platinum silicon does
have a near unlimited mold life while tin silicons usually are
stated to have a 5 to 10 year life. Either of these have good shelf
life for the rubber base but the catalyzers can go be bad after a
few years but new cat. can be purchased if needed. There are a
number of other “rubber type” RTV’s each having pros and cons
themselves and too much to go into here, but there are many other
RTV mold materials besides silicon. Silicon molds release wax very
well compared to rubber heat vulcanized molds. Releases do eliminate
sticking waxes in heat vulc. molds but it is one more thing that can
cause wax surface “problems” and one more think that HAS to be done
when injecting waxes.
Heat Vulcanizing rubber now is available in rubber (many different
ones) as well as silicon. Heat vulcanized molds (rubber) have GREAT
mold life (decades if stored out of sunlight and heat) silicons I do
not have much experience with. Heat vulcanized molds have been a
mainstay in the jewelry industry for decades while RTV materials are
being used by some but it seems that the “old fashioned” heat
vulcanized molds still hold a major place by many.
I got into mold making over 20 years ago making molds for my wife’s
(ultimately) bronze sculptures. Her masters were done in an oil clay
(non hardening) and there was often a lot of fine detail (she was a
goldsmith and master model maker for 30+ years before we met, I was
an organic apple farmer and fruitstand operator). When I started
making molds for her sculptural pieces, I/we did use a number of
different RTV materials. Some were very cheap (compared to silicons)
but some had short mold life or poor mold stability, or lack of good
detail and wax finish and/or other
What I found as a problem with HV (heat vulcanizing materials) is
one HAD to have a “mold master” (to me this is different that the
actual “master”) in a somewhat high temp material, usually metal, so
as to withstand the heat of making the heat vulcanized mold. This
would require and master made in, say wax, to be cast to metal
before making the mold. Two problems here, at least to me, 1) being
you could loose your wax master in the casting process if you had a
problem, and 2) you had to make the wax oversized to account for the
loss is size that occurs in casting it into metal.
There is another a loss is size when making the mold (except
silicons have almost NO shrinkage when making the mold but this is
not true with other RTV’s or heat vulcanized rubber (not including
HV silicon here, it does not shrink) AND another loss when making
the wax and again a loss when casting.
So if one is making a wax master that is ultimately going to be
molded with a heat vulcanized rubber (not silicon), it has to be “up
sized” to account initial casting loss, detail loss during cleanup
of the casting master, mold making loss, wax loss and casting loss.
Additionally, there WILL BE a detail loss every step of the way.
If one uses an RTV material to make the mold, the mold can be made
directly from the wax master. Now there is going to be potential
size and detail loss occurring only at the mold making, wax
injection and casting steps, only occurring 3 times with an RTV
verses 5 “losses” in the sequence for a heat vulcanized mold.
Additionally, with an RTV mold, one STILL has the master wax in it’s
original state so additional molds can be made if needed from the
original wax master. To me, this fact of having the “original wax
master” after the mold is made, makes RTV molds very advantageous in
You do not need a heat vulcanizer and mold making plates for RTV
materials either, just one more thing to consider when thinking
about RTV verses HV mold making. We have both but we haven’t used
the vulcanizer for many years, but Cynthia has not been making
Just my thoughts, maybe some will help and maybe they won’t.