Silicon Molding

Step one is to build the tool for your liquid silicone rubber (LSR) part, typically of steel or aluminum. Gating and venting of the tool is critical to prevent flashing. Guidelines for the design of draft, parting lines and undercut are similar to those of injection molding. Gates should be placed on a surface that is not dimensionally or aesthetically important because they tend to leave a mark or blemish on the surface of the cured silicone part.

Two components, a base forming polymer and a catalyst, are mixed at the molding machine, usually via an automated system. A colorant may also be added at this time. The liquid mixture is injected under pressure into the heated mold and cures from the heat. After it has cured, the press ejects the finished part. LSR has good adhesion to plastics and metals. That makes it an excellent material for two-shot overmolding.

LSR’s low viscosity during molding can cause thin, unwanted material, or “flash,” to form at the mold’s parting line. It may be removed via manual, automated or cryogenic de-flashing, depending upon the volume of parts. In some cases, LSR parts may undergo a secondary post-cure to enhance or stabilize its properties. This is often done with parts used in medical and food-contact applications.