We often sense the confusion over the term foam versus sponge. I thought I’d weigh in on this:
Closed Cell Silicone Sponge is heat cured from a mixed silicone gum rubber base. The silicone gum compound is mixed on a mill with pigment, catalyst and blowing agent added during the mixing process. A thin calendered sheet of uncured silicone is either placed in a sheet mold or run through a continuous rotocure to be heat cured at approximately 350°F. A secondary post-curing operation for 4 hours @ 480°F in a hot air circulating oven is often required to enhance the compression set properties and reduce the incidence of out-gassing in the final assembly.
The silicone is normally cured on a Teflon coated fiberglass bleeder cloth to allow gasses to escape as the sponge expands during the curing process. Without the bleeder cloth, the surface would be pocked with large air pockets that would affect the sealing properties of the gasket material. Instead, you see the fine fabric surface finish on the cured sponge that is characteristic of closed cell silicone sponge. The finished product is closed cell silicone sponge, which generally provides excellent sealing properties for enclosures subject to rain and harsh outdoor conditions.
Open Cell and Closed Cell Silicone Foam is cast from low viscosity liquid silicone which is catalyzed, rapidly mixed and cast onto a smooth plastic release liner. The cast foam is dispensed onto the release liner in a controlled thickness, passed through a curing oven then post-cured to drive out residual moisture or vapors. The finished product is typically a continuous roll with a smooth surface. However, the lower density grades of cast silicone foams normally do not pass rigorous environmental sealing tests – such as wind-driven rain conditions.
How do we distinguish the two materials? We describe the gum based product as Silicone Sponge and the liquid based product as Silicone Foam.