These are tin (or condensation) cured elastomers and are generally the best for those mold making applications where surface inhibition might be a problem. SR-1621 is a relatively soft, (shore A:23) tough material and is what we consider to be our "flagship" silicone. Easy to use in its ten to one ratio (by weight) this elastomer can be modified by the addition of Viscosil for those situations where one might need a brushable compound, and it can also be accelerated by the addition of FC-200 for those of you who might be in a hurry to make a mold.

SR-1644 and SR-1654 are harder polymers (shore A 40 and 50 respectively) but are not as tough as the SR-1621 and are used where one might need a mold that need not be flexed as much, or more accurate "keying" of multiple part molds is necessary. The newest addition to this family of elastomers is the SR-1618 which has a Shore A hardness of 15 and is great for those molds where maximum flexibility is needed. The 1600 series silicones are best used with waxes, cements, polyurethanes and polyester resins. Not recommended for casting epoxy resins or metal alloys.