LSR: Liquid Silicone Rubber
HCR: High Consistency Rubber
RTV: Room Temperature Vulcanizing
Silicone materials are available in two major forms; a low viscosity Liquid Silicone Rubber or LSR, used in Injection molding, and a higher molecular weight and higher viscosity material, HCR or High Consistency Rubber. HCR is used primarily in compression molding and transfer molding. There are other RTV or Room Temperature Vulcanizing materials, gels, adhesives, dispersions, silicone caulking compounds available, but they are not commonly used in molding applications.
For both forms of silicone materials, the cure mechanism is an important feature. They are also divided into two major forms; Peroxide cure and Platinum cure. Platinum cure gives us faster cure times and less volatiles, or chemical by products, in the material. Peroxide cure is a slower cure material which requires post baking in an oven after molding to remove any volatiles. The post baking is required for medical components.
The material hardness or Durometer is measured in the Shore A range. Materials are available from Shore 5A to 80A.
Silicone materials are then formulated to be either multipurpose or serve specific properties. Some of these purposes are high tear resistant, high temperature, electrically conductive, low compression set, low coefficient of friction, self-adhesive, and oil exuding or self-lubricating properties.
We will discuss further in our Molding Methods and Material Selection section regarding what to choose for your application.