Abstract: A secondary confinement pipe is assembled from identical segments of extruded polyvinylchloride with tenon and groove formations along longitudinal edges enabling the segments to be interfitted. The joints are sealed by flexible synthetic resin coextruded with one of the formations at respective edges.

Abstract: Highly uniform three-layer extrusions generally of flat siding or panels, preferably of PVC are extruded in an apparatus having a flow block in which lateral extruders supply the upper and lower layer streams to respective curved flow passages running to outlets of a flow block while a main extruder supplies the foam PVC to the central outlet. The outlets are aligned with orifices of a flow plate connected by respective channels to outlet orifices arranged within a common circle and communicating with a die. The outlet orifices include a pair of elongated kidney-shaped orifices encircling and straddling an ovoid central orifice.