A method and apparatus for extruding thermoplastic plastics material foamed by steam

摘要

Thermoplastics material and a pore regulating agent are fed (at 6) into an extruder (5), are fused in a zone (1). the fused material pressure is augmented in a zone (1a) and the material then flows past, or through the pores of an obstruction (9) to form thin layers or strands. Saturated or super heated steam, and a propellant are sequentially injected into the fused material in respective zones (12; 13), the injected steam temperature being not lower than that of the material, The pressure of the material upstream of the obstruction (9) is higher than the steam vapour pressure downstream of the obstruction. The propellant is injected into material which is at a pressure greater than the steam condensation pressure. The mixture is then cooled and discharged in a zone (4) whereupon it foams.