Molding method for producing expanded polyolefin foam

A method of making a low density polyolefin resin foam utilizing a mold having a molding cavity of fixed volume determined by a fixed cross-sectional area and a defined substantially uniform thickness which is open at its top throughout said area and is adapted to be closed by means of an openable platen overlapping around its margins with the ends of the side walls of said cavity, which method comprises the steps of forming into a solid resilient pre-form a molding composition consisting essentially of a polyolefin resin selected from the group consisting of polyethylene, co-polyethylene/vinyl acetate, and mixtures thereof, and homogeneously distributed through said resin finely divided particles of a solid blowing agent in the amount of 10-15 parts per 100 parts resin in a peroxy-cross-linking agent for said resin in an amount at least substantially sufficient to cross-link the same, said blowing agent undergoing decomposition and releasing a gas when heated above its decomposition temperature and said cross-linking agent undergoing a cross-linking reaction with said polyolefin resin when heated above an initiating temperature, each said temperature being within the range of about 225.degree. F.-400.degree. F. and higher than the softening temperature of said resin, said pre-form having a cross-sectional area fitting within the mold cavity area, a thickness in excess of said defined mold thickness, and a solid volume exceeding the fixed mold cavity volume by 0.1-1.0 percent;

• placing said solid pre-form in said mold cavity while said platen is open and then closing said platen to close said cavity and thereby compress the solid pre-form to the thickness of said cavity, the excess volume of said pre-form extruding laterally between the ends of the cavity side walls and platen margins around said cavity as a flashing;
• after closure of the mold applying an external pressure to said platen in the range of about 8,500-12,000 pounds per square inch of the mold cross-sectional area while heating said pre-form to a temperature higher than said decomposition and initiation temperatures for a time sufficient to soften said resin and essentially decompose said blowing agent and generate incipient gas bubbles in said softened resin and at least initiate said cross-linking reaction while said pre-form remains in said closed mold;
• without releasing said pressure cooling the closed mold to below the softening temperature of said resin;
• opening the closed mold and removing the cooled molded product therefrom;
• and finally heating the removed product to a temperature to expand the incipient gas bubbles therein and complete the cross-linking reaction thereof, to give a foamed product with fine substantially uniform closed cells distributed homogeneously therethrough.