Process for extruding a partially foamed thermoplastic product

Claim 1

In a process for the production of a partially foamed thermoplastic polymer product having a core and shell layered structure by means of the addition of a foaming agent during continuous extrusion of the polymer with a screw extruder, the improvement which comprises: first melting said thermoplastic polymer free of foaming agent and conveying the resulting melt under compression through a first screw zone of the extruder; dividing said melt free of foaming agent into at least two separate partial melt streams for individual conveyance from said first extrusion zone into a die shaping zone of the extruder; introducing and uniformly mixing the foaming agent into at least one of said separate partial melt streams in a second screw zone of the same extruder while maintaining at least one other of said separate partial melt streams free of said foaming agent during conveyance to said die shaping zone; and recombining the individual melt streams in said die shaping zone while expanding said foaming agent to form abutting foamed and non-foamed extruded layers of the core and shell structure.