Polysulfide rubbers are two-component
systems (base plus curative; A+B) that have been the favorite
mold rubber of bronze foundries around the world (for casting
wax) for years. They are available for making molds that are
poured or brushed on.

Advantages polysulfide molds are very soft,
“stretchy” and long lasting (some molds still in production
are over 40 years old), and are good for making
molds with severe undercuts and/or very fine detail. Unlike
other mold rubbers, polysulfide rubber is not inhibited by sulfur
or water based modeling clays. Model preparation is
minimal. Once cured, polysulfide molds are good for casting
wax (lost wax process) and gypsum plasters.

Disadvantages the most common polysulfide rubbers
with lead curatives have an offensive odor. Newly
made polysulfide molds may stain plaster.
Polysulfides have poor abrasion resistance (not good for casting
concrete), and are not suitable for production casting of
resins. Polysulfides (A+B) must be mixed accurately by
weight (scale required) or they will not work. They are of moderate
cost; higher than latex and urethanes but lower than silicones.

Perma-Flex proudly distributes Smooth-On FMC Polysulfide Rubbers