All mold making rubbers may be subject to cure inhibition by contaminants on the surface of a model being used for mold making. Silicone rubber is sensitive to sulfur that can be found on different surfaces and are present in some types of modeling clay. Platinum-cure silicone rubbers are critically sensitive to sulfur and will not cure under any circumstances when exposed. Tin-cure silicone can cure over models containing sulfur if the model surface is first thoroughly sealed with an acrylic spray.
A major difference between these two systems is shrinkage of the cured rubber over the short and long term. Tin-cure silicone rubber molds generally exhibit higher shrinkage over time depending on the type of mold rubber being used, material being cast into the rubber mold, mold configuration, and other variables.
Tin-cure silicone rubber molds have a limited library life (Related FAQ: What is "library life"?) Depending on the specific tin-cure silicone, the library life of a rubber mold can be as little as 12 months, which may be acceptable if your project is short term. Smooth-On's Mold Max™ Series tin-cure silicone rubbers have a relatively long library life at a minimum of 5 years. At the end of a rubber mold's library life, the mold will tear easily and become unusable. Platinum-cure silicone (e.g. Smooth-Sil™ and Mold Star™) has a relatively long library life that is measured in decades. Museums choose platinum-cure silicone rubbers to make molds of valuable artifacts and works of art.
Which silicone rubber chemistry (tin vs. platinum) will give you the longest life for production casting of wax, gypsum, concrete, resin, etc.? It depends on different variables and is usually application specific (Related FAQ: What is "production life"?) To determine which rubber mold material is right for your project, contact your local Smooth-On distributor or the Smooth-On technical help line.