expandable thermoplastic polymer beads are introduced into a tank and are agitated therein in a heated moisture - free condition . the beads are pressurized within the tank with a hot gaseous medium to soften them and are then subjected within the tank to a selected degree of vacuum to expand the beads or particles . the vacuum is removed and atmospheric pressure is re - established in the tank . the expanded hot moisture - free particles are introduced into a mold where they are fused into an integral mass , followed by cooling of the fused mass within the mold to form an outer skin on the mass . the fused mass is ejected from the mold .