Description is made of an open cell polystyrene resin foam having a
content of organic volatile matters of less than 100 ppm. The open cell
foam is suitable as a core material for a sound insulating material or a
vacuum heat insulating material.The open cell foam can be produced by
melt-kneading a polystyrene resin having a content of organic volatile
matters of 100 ppm or higher together with water in an extruder, sucking
out the organic volatile matters as an azeotrope with water through a
vent, injecting a blowing agent and kneading the injected mixture, and
extruding the mixture into a lower pressure area to cause the resin to
foam.