Method for manufacturing plastic foam

A method for manufacturing thermoplastic resin foam having a predetermined shape, in which a polar thermoplastic resinous compound having a chemical foaming agent which produces a gas by a heat decomposition thereof is charged into a molding cavity which is defined by opposing cavity surfaces and said resinous compound is subjected to a dielectric heating, so that said resinous compound is molten and plasticized and the chemical foaming agent is decomposed, characterized in that the cavity surfaces define the shape of an automobile molding and that one of said cavity surfaces is movable to withdraw from and to move closer to the other cavity surface so as to vary the volume of the molding cavity, said resinous compound being charged into the molding cavity, so that said resinous compound is brought into surface contact with the cavity surfaces without an air gap between said resinous compound and the cavity surfaces, said movable cavity surface being moved, in response to the expansion of said resinous compound caused by the decomposition of the chemical foaming agent during heating, in order to maintain contact with and to accommodate the cubically expanding resinous compound, wherein the obtained thermoplastic resin foam contains a core portion having a comparatively low density and a shell portion having a comparatively high density, and the obtained foam is in the form of an automobile molding.