There was a time when mold making and casting was just limited to plaster. Ask anyone how to make molds or casts and they would reply plaster or perhaps clay at best.
But today, man-made polyurethanes and silicone rainbow silicone rubber have almost replaced plaster as the medium of choice for making mold making. Indeed, silicone rubber has gained prominent popularity due to silicones’ almost magical properties in their flexibility and lightness. Silicone, or for that matter, polyurethane molds can capture the finest of details. They can be made very quickly and have long library lives. Silicone compounds are odor free and safe to use. The material has proved to be resistant to fire, water, heat and chemicals as well.
Many artists and mold makers now agree that these benefits of RTV silicone easily overcome the higher cost of silicone rubber compared to other mold making and casting materials.
But not many people may be aware that the most common silicone rubber used for mold making and casting is something known as RTV or ‘room temperature vulcanizing’ silicone. The RTV process makes the silicone rubber more durable and less sticky. In the past, vulcanizing, or toughening, required heating the rubber material in an oven to cure it. But not so with current formulations using RTV silicone, a two-part system, wherein a base silicone is mixed with a catalyst to induce curing from a liquid state to a solid one. The curing then takes place at room temperature instead of in an oven, making these formulations so much more friendlier to use in small studios and by hobbyists and professionals alike.
RTV silicone rubber is a very versatile material that can be used for most mold making applications. Today, its flexible and durable properties make it a material of choice for making molds and casts. Skin safe and food safe silicones have also been developed, further enhancing the utility of this compound.
The curing time is generally 18 to 24 hours, but certain special formulations allow a cure time in as little as 5-10 minutes depending on the catalyst being used. It has excellent release properties, too and in most case does not need a release agent at all.
But one point to keep in mind is that silicone rubber is quite viscous (thick) and needs to be de-aired properly in a vacuum chamber before using it for making a mold or cast. Also, silicone rubber does not have a long shelf life – 6 months to a year at the maximum, so you should buy the amount needed for a project as the effects of long-term storage will result in the silicone not curing.