Design News for Mechanical and Design Engineers

The current market for liquid injection-molded silicones (LIMS) is about 25 million lb in the Americas, up from 15 million lb in 2003. According to materials’ producer Shin-Etsu, the biggest market is health care at 9 million lb, followed by automotive at 7.5 million lb.

“The inherent properties of silicones make them an obvious choice for a number of applications,” says Mary A. Krenceski, a chemist who works at silicone specialist Extreme Molding in Watervliet, NY. Silicones possess thermal stability over a broad temperature range, are resistant to solvents, have high gas permeability and excellent biocompatibility.

Growing applications in health care include wound drain bulbs, masks, valves, diaphragms and urological devices. They’re used in cars to make connectors, anti-drainback valves, grommets, spark plug boots and mold-in-place gaskets.

New technical developments make them particularly interesting to design engineers. One of the more interesting is compound flexibility. For example, by varying core reactants and the level of silica or other reinforcing filler used, it’s possible to design compounds with varying degrees of modulus, hardness and tear strength.

One of the strengths of silicones is their high degree of gas permeability which accounts for their use in applications such as membranes for gas separation. “However, the permeability can also be detrimental in applications where the molded part needs to provide a barrier to oxygen, air or other gases,” says Krenceski. Copolymers, such as silicone urethane, can be used as a barrier to oxygen in some sealing applications.

In another example, a new family of polyurethane-silicone materials can overcome some of silicone’s problems when used as implants. The urethane segments boost abrasion resistance and toughness.

New grades of silicones that reduce assembly costs through use of two-shot or insert overmolding processes is another new development. New Select-Hesive LIMS materials bond to thermoplastic substrates without primers or pre-treatment while having only a weak interaction with mold steel. Grade KE2090 self- bonds to polycarbonate, PBT-type polyester and polyphenylene oxide (PPO). Care must be taken in designing the tools for LIMS. Precise gating, venting and shut-offs are all essential.

Krenceski spoke at Molding 2008 in San Francisco in February. Liquid silicone rubber processing goes back to 1944 when GE and Dow Corning developed synthetic rubber.