Method of manufacturing foamed thermoplastic resin profiles

A method of continuously extruding a thermoplastic body having a foamed inner portion and a dense surface layer from a thermoplastic resin selected from the group consisting of polyethylene, polypropylene, polystyrene and a mixture thereof comprising:

• a. a first step of melting and kneading in a screw extruder a mixture of a thermoplastic resin and a thermally decomposable foaming agent having a decomposition temperature higher than the softening temperature of the resin, at a temperature above the softening temperature of the resin and below the decomposition temperature of the foaming agent;
• b. a second step of heating the resulting mass of molten mixture, under pressure without substantial mixing action, to a temperature above the decomposition temperature of the foaming agent, by forcing the mass through a thermally decomposing means provided with external heating means and having an outlet of such smaller cross-section than its inlet such that a sufficient pressure is maintained on the mass to suppress substantial foaming in the mass;
• c. a third step of expanding the inner portion of the mass and simultaneously suppressing substantial foaming in the exterior surface of the mass under reduced pressure by forcing the mass through a foaming die having an outlet of larger cross-section than its inlet, said foaming die including a channel and walls, the walls of the channel being maintained at such low temperature, which is below the decomposition temperature of the foaming agent, but above the softening temperature of said resin, as to cool, and thereby suppress substantial foaming in the exterior surfaces of the mass which are in contact with those walls;
• and
• d. a fourth step of rapidly cooling the mass to a temperature below the softening temperature of the resin, by forcing the mass through a sizing die, which is a channel having walls maintained at a sufficiently low temperature such that the exterior surfaces of the mass, being in contact with said walls, are solidified within 10 seconds after the mass has been introduced into the channel.