Silicone rubber has become the material of choice for making resin molds. Though the history of mold-making goes all the way back in time to ancient Egypt and China, using sand, wax, glue, animal fat, gypsum, alginate, metal, plastic, re-usable vinyl, and even gelatin, today’s resin mold-makers most often choose one of four different flexible rubber materials, latex, polysulfides, polyurethanes, or silicon rubbers, because of the traits they all exhibit:
1. they can reproduce exact detail;
2. flexibility makes demolding (removal from mold) easy;
3. their durability permits multiple castings;
Silicone rubber has several properties that make it the material of choice for most resin molds. Its inert nature gives it good release properties as well as good chemical resistance, and high temperature resistance. Its major drawback is its cost, particularly that of the platinum-cure which is the only formulation approved as Food Compliant by the FDA.
Resin crafters, especially those who make jewelry typically use silicone rubber for most of their molds. It is important to prepare for the project as much as possible ahead of time and to read all of the manufacturer’s instructions to make sure that you mix them in correct ratios and thoroughly, leaving enough time to form and if necessary pour your mold before the silicone begins to cure.
Tin-cure silicone (condensation-cure silicones) has two component materials which cure at room temperature (RTV) to flexible, high tear-strength rubbers. Tin-cured silicones can be poured onto models or they can be made brushable by adding a thixotropic additive. They are ideal for molds where easy release or high temperature resistance is required. Tin cured silicones can be used for casting polyurethane, epoxy and polyester resins, waxes, all gypsum products, and low-temperature metals. Though tin cure (condensation) silicone has a shorter shelf life than does its platinum cure (addition) counterpart and it tends to shrink somewhat, it is more cost effective and fine for most resin molds.
Platinum cure silicone rubber, also known as addition cure silicone, is two-component high tear strength, flexible mold compounds. They are recommended as a mold material for casting polyurethane, epoxy, and polyester resins, wax and a wide range of other casting materials. They exhibit extremely low shrinkage and high physical properties. These rubbers are chemically sensitive to latex, sulfur, and certain other materials. Platinum cure silicones can also be used to cast prosthetics for special effects makeup and medical purposes.