Many silicone rubbers are thick and most have a high viscosity. These silicones do not de-air themselves as readily as urethane rubbers and need to be vacuumed.

Consequently, if you mix and pour high-viscosity silicone rubber that has not been vacuumed over a model, air may be trapped in the mixture as it turns from liquid to solid and these bubbles may show up on the working surface of the mold.