RTV Silicone is a type of silicone rubber

Silicone rubber

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations...

 made from a two-component system (base plus curative; A+B) available in a hardness range of very soft to medium - usually from 15 Shore

Shore durometer

Durometer is one of several measures of the hardness of a material. Hardness may be defined as a material's resistance to permanent indentation. The durometer scale was defined by Albert F. Shore, who developed a measurement device called a durometer in the 1920s. The term durometer is often used...

 A to 40 Shore. RTV Silicones can be cured with either a platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 catalyst or a tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

 catalyst. Applications include making molds

Molding (process)

Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....

for reproducing.

RTV silicone rubber applications

RTV Silicone Rubber will reproduce the finest detail and is suitable for a variety of industrial and art related applications including prototypes, furniture, sculpture and architectural elements. RTV Silicone Rubber can also be used to cast a variety of materials including wax, gypsum, low melt alloys/metals and urethane, epoxy or polyester resins (without using a release agent).

Features of RTV silicone rubber

1. Good characteristics of easy-operation
2. Light viscosity and good flowability
3. Low shrinkage
4. Favorable tension
5. No deformation
6. Favorable hardness
7. High temperature resistance, acid and alkali-resistance and ageing resistance

Correct mixing ratio

Component A is silicone rubber, and component B is curing agent or vulcanizing agent. Usually the mixing ratio will be 2-3%, e.g. to 100g silicone rubber should be added 2g catalyst and then mix them thoroughly. (Note: the weight of the silicone and catalyst should be weighed correctly). The following table gives suggested proportions at various temperatures:

Temperature °C	Mixing ratio %	Notes
-5	-	no reaction
10	5	-
25	2	-
38-45	1	-

Preparation of the original molds

Because RTV silicone reproduces the surface texture of the original model exactly, the surface of the original to be molded must be finished (i.e. all defaults removed, and sanded or polished to the desired smoothness). If a smooth finish on the final casting is desired, the original must be polished. likewise, for a matte finish, polishing is not necessary.

Vacuum-pumping process

To mix the silicone and catalyst with a standard mixing ratio and then do the process of Vacuum-Pumping. Bubbles will come out during the mixing of the silicone and hardener, if without Vacuum-Pumping, the bubbles cannot be released from the mixture, as a result, there will be many bubbles in the finished silicone mold. Then the mold has a less tensile strength which will affect the reproduction times. Such an unsmoothed silicone mold will affect the quality of the production directly.

Attention for making a mold by RTV silicone rubber

1. RTV Silicone Rubber is white flowing liquid with general hardness of 22-40A° after curing.
2. If the mold makers want to make large products and large size molds, the mold should be made of silicone rubber with high hardness. On the contrary, if the products with fine and precision patterns, it should be made of silicon with low hardness.
3. The amount of the curing agent depends on the actual requirements of the customer. If molds requiring fast drying and demould, more curing agent is needed; Contrarily, if molds requiring slower drying and demould, less curing agent is needed.
4. it doesn't better to add silicone oil into silicone rubber. If a special soft silicon mold or fine detailed and complex patterns products are required, 5%-10% silicone oil can be added to soften the mold for easy demould. )

Advantages

RTV Silicone Rubber has the best release properties of all the mold rubbers, which is especially an advantage when doing production casting of resins (polyurethanes, polyesters and epoxy). No release agent is required, so there is no post-production cleanup. Silicones also exhibit very good chemical resistance and high temperature resistance (400°F / 205°C and higher). High temperature resistance makes silicone the only mold rubber suitable for casting low melt metal alloys (i.e. tin, pewter, lead). The combination of good release properties, chemical resistance and heat resistance makes silicone the best choice for production casting of resins.

Disadvantages

RTV Silicone Rubber are generally high in cost - especially platinum-cure. They are also sensitive to substances (sulfur clay for example) that may prevent the silicone from curing (referred to as cure inhibition). Silicones are usually very thick (high viscosity), and must be vacuum degassed prior to pouring to minimize bubble entrapment. If making a brush-on rubber mold, the time factor between coats is long (longer than urethanes or polysulfides, shorter than latex). Silicone components (A+B) must be mixed accurately by weight (scale required) or they will not work. Tin catalyst silicones will shrink somewhat and do not have a long library life.