1-Types of RTV Silicone
There are two common classes of RTV silicones:
1.) Tin catalyzed or “condensation cure” silicones which require moisture to cure and;
2.) Platinum catalyzed or “addition cure” silicones.
They are typically of low viscosity (easily poured) and are not inhibited by many materials. In contrast, platinum cure silicones (often called “elastomers”) are inhibited by many naturally occurring materials, including sulfur, tin, and amines.
This makes them unsuitable for certain clays, which contain sulfur or latex gloves.
However, platinum-cure silicones have great chemical, microbial, and temperature resistance.
2-Proper Use of Catalysts
Fast catalysts are available for each type of silicone (tin or platinum), which can reduce cure times significantly–in some cases to less than 1-hour. With most silicones, there is some latitude allowed in the catalyst portion (adding more than the recommended amount will speed cure times, as will using a fast catalyst). However, there are limits to the amount that may be added and adding more than the recommended amount of catalyst, or using a fast catalyst, will shorten the life of the mold, making it more prone to tearing or becoming brittle over time. A catalyst-rich mold may last a year or so; one with less catalyst should last for many years.
3-Mixing the Silicone
Because of settling during storage, always stir the silicone in the original container before pouring into a mixing cup. Stir steadily with a circular motion rather beating it or using an up-and-down motion, to avoid trapping air bubbles, scraping the bottom of the can to loosen any settled material. Then let the silicone rest for a few minutes or more to allow air bubbles to rise to the surface (this small resting time will not cause any re-settling of the silicone itself). Tip:
Tongue depressors or craft sticks make good, cheap stirrers for small batches; paint stirrers make good stirrers for larger batches.
If a vacuum chamber is available, use it to remove trapped air from the mixture before pouring. When subject to a vacuum, the silicone mixture should well up as air pockets rise and burst. As soon as the material settles down, proceed to the pouring/application procedures described below. If a fast catalyst was used, you must work quickly to avoid having the silicone cure before it is applied.
For applications where a thick, paste like silicone is desired (such as molding an object on a vertical surface), “thixotropic” silicones or thickening catalysts are available.
Thinners or “diluetants” are available for thinning or decreasing the viscosity of some RTV silicones. However, they are expensive and do not seem very effective (one must add a large amount of diluetant to achieve a small effect on viscosity, and thorough mixing is difficult and time consuming). Diluents also weaken the cured silicone. Therefore you should purchase silicone in the desired viscosity rather than using diluetants.