typically , injection molded expandable thermoplastics exhibit imperfect surfaces in the form of mottled areas , visible flow lines , and pin holes ; these imperfections are caused by the quick chilling of the surface of the expanded thermoplastic as it contacts the mold surface . this invention is a method of injection molding expanded thermoplastics free of these defects by injecting a short - shot of the expandable thermoplastic into an elastic membrane positioned within a cooled mold cavity , containing the short - shot within the membrane until a skin forms on the thermoplastic in contact with the membrane , and then permitting expansion of the thermoplastic in the cavity to form a smooth skin or surface on a substantial portion of the article .