Abstract: A foamed polystyrene board useful as a thermal insulating material is produced by melting a polystyrene having a melt index of from 0.5 to 5 under heat in an extruder, mixing the molten mass with a blowing agent composed of methyl chloride and dichlorodifluoromethane, and extruding the resulting foamable molten gel from the extruder. As a novel essential feature, a flowability improver having a melting point higher than 70.degree. C., such as an aromatic sulfonamide, a brominated aromatic compound, a brominated alicyclic compound or a poly(.alpha.-methylstyrene), is mixed with the polystyrene within the extruder to impart superior extrusion formability. The foam has good dimensional stability both at room temperature and higher temperature, high mechanical strength and superior thermal insulation properties.

Abstract: An apparatus for producing a corrugated foamed plastic sheet includes an extruder and a die provided at the extrusion end of the extruder. The die has a plurality of grooves provided on one of its surfaces which make contact with the foamed sheet being produced. The grooves extending from the resin feed inlet toward the resin exit of the die.

Abstract: In a process for producing an olefinic resin foam which comprises the steps of forming a composition comprising an olefinic resin, a chemical blowing agent and a crosslinking agent into an unfoamed sheet-like or rod-like shaped article and foaming it by heating, the improvement wherein the heat-forming step is carried out in an oxygen-free atmosphere or in an atmosphere containing not more than 10% of oxygen.

Abstract: In a process for producing a foamed polyolefin article which comprises melting and mixing a polyolefin, a volatile organic blowing agent, a cell size-controlling agent, and an ester of a long-chain fatty acid and a polyhydric alcohol in an extruder comprising a feed zone, a compression zone and a metering zone, and extruding the resultant molten gel into an area having a lower pressure than the pressure of the inside of the extruder, the improvement wherein said ester is at least one ester of a long-chain fatty acid having 8 to 20 carbon atoms and a polyhydric alcohol having 3 to 6 hydroxyl groups, and said ester is fed in the molten state into the molten gel within the metering zone and mixed with it.