Silicone rubber is made up primarily of sand or silicon dioxide. Not beach sand of course but a specialty type of sand. This is not to be confused with other types of rubber. Liquid silicone rubber or LSR as it is sometimes referred to is a specialty type of compound. It is often used for plugs, seals and other products where flexibility and toughness are important. Of course, resistance to heat is often a critical reason why LSR molding is used.
The liquid silicone rubber molding process is very exact and a very specialized area. It is similar to injection molding but not exactly the same. The silicone is formed rather than extruded.
In many ways, it has good characteristics of both thermoplastic and thermoset materials. It is more flexible than regular PVC but not as expensive as other rubber compounds. Tooling is also simplified with LSR molding as cool silicone is injected into a hot mold. The compounds are purchased as Part A and Part b and then mixed and added to the mold.