Details
Material hierarchy: Polymer - Thermoplastic - Styrenic
Polystyrene foams are shaped by extrusion and molding. In extrusion, a physical or chemical blowing agent is fed into the polymer melt near the die end of the extruder barrel; thus, the extrudate consists of the expanded polymer. Large sheets and boards are made in this way and are subsequently cut to size for heat insulation panels and sections.
Several molding processes are available for polystyrene foam. A more widely used process is expandable foam molding, in which the molding material usually consists of prefoamed polystyrene beads. The prefoamed beads are produced form pallets of solid polystyrene that have been impregnated with a physical blowing agent. Prefoaming is performed in a large tank by applying steam heat to partially expand the pallets, simultaneously agitating them to prevent fusion. Then, in the molding process, the prefoamed beads are fed into a mold cavity, where they are further expanded and fused together to form the molded product. Hot beverage cups of polystyrene foam are produced this way. In some processes, the prefoaming step is omitted, and the impregnated beads are fed directly into the mold cavity where they are heated, expanded, and fused. In other operations, the expandable foam is first formed into a flat sheet by the blown-film extrusion process and then shaped by thermoforming into packaging containers such as egg cartons.