Silicone sponge, or closed cell silicone sponge, is derived from a thin sheet of uncured silicone gum. The silicone gum compound is mixed with pigment and other ingredients during a rigorous mixing process. The still uncured silicone is then rolled into thin sheets and heat cured in a high temperature press or similar procedure.

At this point in the process, the silicone expands. The product is then often submitted to an additional four hour post-curing process at 480°F in a hot-air, circulating oven to enhance the compression set properties and reduce the incidence of out-gassing in the final assembly.

Typically, the silicone is cured on a Teflon-coated, fiberglass bleeder cloth. Doing this allows gasses to escape as the sponge expands during the curing process. Without the bleeder cloth, large air pockets would form that would affect the sealing properties of the gasket material. This is also what produces the fine, fabric surface finish on the cured sponge, characteristic of all closed cell silicone sponges.

‘Closed cell’ means that the cells are non-interconnecting, subsequently not allowing water or air to leak through the sponge at low pressures. Generally, the superior mechanical properties of closed cell silicone sponge provide better weatherproofing than open cell silicones. It provides excellent sealing properties for products subject to rain and harsh outdoor conditions. One downside of closed cell silicone sponge products is the inability to achieve flame ratings beyond UL94HB.

Silicone Foam

Silicone foam is normally considered to be an open cell, lightweight product cast from low-viscosity, liquid silicone ingredients. These ingredients are catalyzed, rapidly mixed, and cast onto a smooth, plastic liner.

The cast foam is dispensed in a controlled thickness onto the plastic liner. It is then passed through a curing oven, and post-cured to drive out residual moisture or vapors. The finished product typically has a smoother surface.

The lower density grades of cast silicone foams are typically not as successful in harsh-environment testing, such as wind-driven rains. Air and moisture can pass through low density open cell silicone foam unless the design allows for extensive deflection of the gasket.

Higher density silicone foams have been developed in recent years that permit good weather sealing with medium and firm density silicone foam products. These higher density silicone foam products are actually composed of many closed cells. A major benefit of the liquid silicone foam product is the fact that it is flame-retardant, with many products meeting the UL94V0 rating.