Production of partially foamed extruded plastic products

Claim 1

In a method of making an extruded plastic product having a foamed core layer and a substantially unfoamed skin layer contiguous and bonded thereto, by forcing contiguous concurrent flows of a resin in a fluent extrudable state to and through a coextrusion die at a predetermined temperature for respectively forming said core and skin layers, wherein the core-forming flow comprises a resin containing a blowing agent and a nucleating agent in proportions effective to cause foaming of the core layer and the skin-forming flow comprises said resin containing said blowing agent but at last substantially free of any nucleating agent, and wherein said blowing agent, said nucleating agent, and said temperature are mutually selected such that foaming occurs in the core layer and the skin layer remains at least substantially unfoamed downstream of the die, the improvement which comprises the combination of steps of (a) establishing an advancing stream of the resin in fluent state having the blowing agent incorporated therein in a proportion as aforesaid, and (b) dividing the advancing stream into separate streams and separately conducting the separate streams to the coextrusion die to respectively constitute the core-forming flow and the skin-forming flow therein, while (c) selectively adding the nucleating agent, in a proportion as aforesaid, only to that one of the separate streams which constitutes the core-forming flow.