Rigid Foam (Silpak)Silpak SP 328 4 urethane foam consists of two parts (A and B) that, after mixing, form rigid foam products with a molded density of 5 8 lb ft. These rigid foams can be used for casting parts or filling voids; consider SP 328 4 foam for production of decorative objects, lightweight mold shells, production parts, models, patterns, fixtures, duplicate masters and general tooling use.