A Better Understanding of Silicone Rubbers

Silicones are commonly used in the liquid molding process to make molds and parts. Understanding the differences between the different types of silicone can be helpful before deciding what to buy for your application.

Silicone Molecule

There are three basic types of what are called RTV (Room Temperature Vulcanizing) silicones. The simplest are called RTV-1 silicones which are commonly used for sealing or calking. All materials in the RTV-1 group are one component, condensation curing materials. This means that they only need to be exposed to the moisture in the air to cure. This type of silicone is not used to make molds or parts but can be useful if sealing a mold box or assembling a prototype.


Tin and Platinum based systems are both RTV-2(Two component) silicones. Tin based systems are condensation-cure and Platinum based systems are addition-cure. They are both composed of two components, designated A and B.

Condensation(Tin) cure silicone rubbers are excellent for mold making and prototype applications.  They are generally easier to process and they will cure at room temperature over almost any surface with minimal shrinkage.

Hapsil 360 Group

Platinum based RTV rubbers are more expensive than tin based materials. They provide two major advantages for mold-makers:

1.They give a longer mold life for production items.

2. They have superior heat resistance.

Whatever the application, it is always a good idea to talk with a customer service rep from the silicone manufacturer before you make a purchase. There are also a host of forums online that focus on casting and mold-making, where discussions with other members can help you find the right silicone. Finally, no matter what the circumstances, always test a small amount of your casting resin with a cured sample of the silicone to make sure they are compatible.

Fun Fact:  Vulcanization is named after Vulcan, Roman god of fire.