Injection-expansion molded, thermoplastic resin product and production process thereof

A process for the production of the injection-expansion molded, thermoplastic resin product comprising an expanded portion as an inner layer and an unexpanded portion as a surface layer, said expanded portion having an average cell diameter of from 0.01 to 50 .mu.m and an average cell population of from 10.sup.8 to 10.sup.16 cells/cm.sup.3, and said unexpanded portion having a thickness of from 10 to 1,000 .mu.m and integrated with said expanded portion, said process comprising:

• (I) as a gas dissolving step, melting 100 parts by weight of a thermoplastic resin at 100 to 450.degree. C. in a continuous plasticator equipped with a feed line for a blowing agent, adding supercritical carbon dioxide and/or nitrogen in a proportion of 0.1 to 30 parts by weight per 100 parts by weight of said thermoplastic resin, and forming a molten resin composition in which said thermoplastic resin and said supercritical carbon dioxide and/or nitrogen are in mutually-dissolved state;
• (II) as a cooling step, cooling, within said continuous plasticator, said molten resin composition to a temperature of from 50 to 300.degree. C. while maintaining a pressure equal to or higher than a critical pressure of said carbon dioxide and/or nitrogen;
• (III) as a metering and injection step, metering the thus-cooled molten resin composition by an injector, which is connected to said continuous plasticator and equipped with an injection plunger, and filling the same in a mold;
• and
• (IV) as an expansion controlling step, lowering an internal pressure of said mold to a pressure lower than said critical pressure of said carbon dioxide and/or nitrogen to produce cell nuclei, whereby said cell diameter is controlled;
• wherein said carbon dioxide and/or nitrogen are maintained in a supercritical state up to said metering and injection step, and said gas dissolving step and cooling step by said continuous plasticator and said metering and injection step by said injector are conducted independently.