Abstract: Pre-foamed particles are produced by dispersing resin particles, which contain nucleocells, in a dispersing medium in a closed vessel, heating the contents to a temperature of the softening point of the particles or higher and holding the contents at the temperature for a predetermined period of time while pressurizing the contents with an inorganic gas, and then opening the vessel at one end thereof so as to release the particles and medium into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby causing the resin particles to foam. The above production process provides good pre-foamed particles with ease and without failure. It is also possible to avoid deleterious effects due to the swelling action of foaming agents.

Abstract: An apparatus is provided for the heat treatment of polyolefin resin particles. The apparatus is equipped with an autoclave in which the particles are heated and agitated in the presence of an aqueous medium and dispersing agent. The autoclave has a tilted bottom wall portion, over which an impeller, preferably, a turbine-type impeller is provided.