Silicone Rubber
Silicone rubber is a synthetic, two-component, elastomeric polymer that is made from silicone elastomers that can be cured at room temperature into a solid elastomer for use in molding; however, it is usually molded from a liquid form. Supplied in two parts, the liquid form requires a catalyst, which is typically platinum. Silicone rubber differs from other polymers in two major ways: first, silicone rubber does not have a carbon backbone, but a Si-O-Si backbone; and second, although it is a polymer, silicone rubber acts like an elastomer. As a very flexible polymer, silicone rubber can come in many different grades and forms that are defined by characteristics such as steam resistant, electrically conductive, flame-retardant, metal detectable and chemically-resistant. Silicone rubber naturally offers properties such as high electrical insulation, extreme temperature resistance, highly inert and ability to withstand harsh environments. Able to be molded as a liquid and a solid, silicone rubber moldings have a long list of applications such as medical devices,
gaskets, lab instruments, automotive heating hoses, shaft sealing rings, door seals,
o-rings, spark plug caps and
grommets. As a result, a wide variety of industries find silicone rubber beneficial including aerospace, medical, automotive, construction, electronics, appliance, architecture, petrochemical and pharmaceutical.
Silicone rubber can be molded using five different molding processes: cast molding, injection molding, compression molding, transfer molding and dip molding. The cast molding process involves pouring silicon rubber in its liquid form into an open mold. The open mold contains a hollow cavity of the desired shape. The silicone rubber is then allowed to solidify in the mold. During the plastic injection process, a silicon rubber resin is fed into a hopper, usually in pellet form. These pellets are heated with the aid of injection screws and brought to their melting point. Once in liquid form, the silicone rubber is poured into a metal mold that is the inverse of the desired final shape. The silicone rubber is then allowed to cool in the mold. In compression molding the silicon rubber is preheated and then squeezed into a preheated mold cavity. Inside the mold, pressure is applied to force the silicone rubber into contact with every mold surface, while heat and pressure are maintained until the silicone rubber has cured. The transfer molding process involves a plunger-like device that is built into the mold. First, the silicone rubber is squirted into the mold's cavity through small holes. Then, a piece of uncured silicone rubber is placed into a chamber, known as a pot. The mold is closed and hydraulic pressure is used to force the silicone rubber through a small hole, called the gate, into the cavity. The mold is held closed while the silicone rubber cures. Lastly, in dip molding the silicone rubber is kept in a liquid state while heated and primed aluminum or steel molds are dipped into the liquid silicone rubber from an overhanging rack.