Expandable PS foams are produced in a two-stage process. The resin particles are first expanded to the density required in the finished item while remaining as discrete particles. In the second step, the foamed particles are placed in a vented mold of the final shape. Steam is forced into the mold, heating the particles by direct contact and causing them to expand and fuse together to form a void-free, dry foam part. Pressures in the process are usually below 50 psi, allowing the use of relatively low-cost aluminum tooling.
Unmodified polystyrene is rigid and brittle, has moderate strength, and is crystal clear.
Impact strength is increased significantly by blending the polymer with rubbers such as polybutadiene. Electrical properties are good at room temperature and are affected only slightly by higher temperatures and varying humidity conditions. Polystyrene is soluble in most aromatic and chlorinated solvents. It is insoluble in alcohols such as methanol, ethanol, normal beptane, and acetone. Styrene copolymers set up rapidly and provide fast molding cycles.