Process for the production of sheets of foamed thermoplastics synthetic resins

A process for continuously producing an endless sheet product of a foamed thermoplastic synthetic resin comprising feeding a thermoplastic synthetic resin into an extruder, introducing at least about 35% based on the volume of the resultant admixture of a low-boiling volatile organic liquid expanding agent into the synthetic resin in said extruder under high pressure, said liquid expanding agent being capable of dissolving the thermoplastic resin at temperatures above 100.degree.C and under pressures greater than one atmosphere, admixing the resin and expanding agent to provide a fluid admixture, heating said fluid admixture to a temperature substantially above the boiling point of the liquid expanding agent under normal pressure and subjecting the fluid admixture to high pressures, extruding the admixture through a die of the screw extruder having a plurality of small extrusion nozzles whereby the admixture suddenly expands and forms a plurality of filaments of synthetic resin foam, aligning the foam filaments exiting from the nozzles by an air stream, placing the foam filaments into an oscillating motion, loosely layering the oscillating foam filaments into a continuous web on a sieve-like conveyor belt traveling offset by about 90.degree. with respect to the oscillating motion of the foam filaments, continuously pressing the thus-layered web of synthetic resin foam into a crude sheet between two sieve-like conveyor belts, tempering the crude sheet so formed by passing hot air therethrough whereby substantial removal of the expanding agent is effected, and thereafter forming said crude sheet into said endless sheet product.