GB815173A - Improved method for moulding expandable thermoplastic resinous materials and mouldedarticles thereby obtained - Google Patents

Abstract

A method of moulding expandable granular thermoplastic resinous material to form a moulded foam structure comprises charging the granules into the injection chamber of an injection moulding apparatus where they are forced under pressure sequentially into and through a first cold zone to compact a portion of them into a solid granular mass below its foaming temperature, a second heated zone where a portion of the mass is heated to a flowable condition above the foaming temperature, and a discharge zone from which a portion of the heated mass is injected into a mould where it expands to the confining limits of the mould. The solid compacted portion of the mass in the first zone continuously maintains the heated portion of the mass in the second zone under pressure to restrain foaming therein throughout the intermittent moulding cycles. The effluence of the heated mass from the discharge zone is arrested, except during injection into the mould, either by means of a valve or by providing between the discharge zone and the mould a restraining passageway with a high pressure drop. The moulded foam structure may be given an unexpanded surface layer by injecting the material into a cooled mould with an enlargeable cavity, and, after the surface layer has been formed under pressure, relieving the pressure and enlarging the mould to permit the central uncooled heated mass to expand. Suitable resinous materials for use in this invention include styrene polymers and copolymers; vinylidene chloride copolymers; polymers and copolymers of methyl methacrylate, ethyl acrylate, and other derivatives of acrylic acid; polymers and copolymers of vinyl acetate and vinyl butyral; and cellulose derivatives, e.g. cellulose acetate and cellulose nitrate.