Injection Moulding of Liquid Silicone Rubber Foams
For the first time it has become possible to foam liquid silicone by using physical blowing agents. This process permits production of components with entirely new properties.
When injection moulding liquid silicone rubber (LSR), the material is fed into the injection unit as a 2-component system via plunger-type metering pumps. Prior to entering the injection unit, the 2 separately stored components are mixed, so that a reactive mixture results. The Optifoam Process from Sulzer Chemtech permits incorporation of the physical blowing agent (carbon dioxide or nitrogen) into each of the 2 LSR components via porous sintered metal. Passage through a static mixer then ensures complete dissolution of the blowing agent in the material. The complete system, comprising 2 charging units and the mixing block, can be readily retrofitted to injection moulding machines equipped to process LSR.
Silastic LC 2004 FX from Dow Corning is a material with curing kinetics that are particularly well-suited to the requirements of the foaming process. As a consequence of the foaming, both the density and the Shore hardness are reduced by up to 50%. The degree of foaming, formation of an unfoamed skin layer and the appearance can be controlled via the mould temperature. The injection mould is vented, so that the blowing agent released does not cause defects in the moulded part. Both hot-runner systems and cold-runner systems with valve gates (needle shutoffs) have been successfully employed in preliminary trials.
There are many possible applications for physically foamed LSR, many of which can only be guessed at. Because of the high thermal stability of silicone rubbers, they are especially well-suited for use as gaskets in the automotive sector (Figure). In addition, household articles such as baking moulds, ice-cube trays or components in household electrical appliances represent applications with a great deal of potential. The low Shore hardness and uniform foam structure also make this material a candidate for soft-touch applications. The weight reduction achievable now makes LSR, with its exceptional properties, also attractive for applications previously served by classic elastomers and TPE.