Process and apparatus for molding foamed plastic article

Abstract From the Canadian patent

In the production of foamed plastic articles using a screw extruder, it is known to introduce a gaseous foaming agent into the plastic while the latter is passing through the extruder. It has been found that a foamed thermoplastic with pores or cells of uniform size and distribution can be produced by passing a gaseous foaming agent e.g. nitrogen gas, through a central passage extending substantially the entire length of the extruder screw, discharging the agent into the thermoplastic proximate the discharge end of the extruder and, mixing the foaming agent with the plastic immediately prior to discharge of the mixture into a mold.