What is silicone rubber?

Article courtesy of Smooth-On, Inc.

Silicone rubbers  are two-component systems (base plus curative; A+B) available in a hardness range of very soft to medium. Silicones can be cured with either a platinum catalyst (addition cure) or a tin catalyst (condensation cure). They are available for making molds that are poured, brushed or sprayed on to a model and have performance characteristics that no other mold rubber has.


  • 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. 


  • Silicones are generally high in cost - especially platinum-cure.
  • Silicones 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. [Related: Viscosity Scale Guide]
  • Some silicone systems (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 as long of a library life compared to platinum-cure silicone or urethane rubber. 

Tin Cure Silicone:

Also called condensation-cure silicone, these are high-tear silicone rubbers that are known as the “work horse” of the industry because of their performance and economy. Tin cured silicone molds are used for casting wax, gypsum, polymer modified gypsum, etc. but are best known for standing up to production casting of urethane, epoxy and polyester resins. They are also good for casting low temperature melt metal alloys.

See Smooth-On's Tin Cure Silicone Rubber Selection ›

Platinum Cure Silicone:

Also known as addition-cure silicone, platinums exhibit the lowest long-term shrinkage and have the longest library life of all mold rubbers (with a price tag to match). While good for making molds for casting a variety of materials, platinums also have application and physical properties that other rubbers do not. As a result, they are used for other applications. For example, some are certified “skin safe” and can be applied directly to human skin. They are also used for making prosthetic and orthotic devices, skin safe appliances and effects, etc. Some are certified “food safe” for culinary applications.

See Smooth-On's Platinum Cure Silicone Rubber Selection › 


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.