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.

Abstract: A siding die utilizes a lip defining the orifice and provided with a pair of lip members with mating formations at their opposite ends so that the lip members are forced together when end screws bridging the lip members are tightened. The formations include inclined surfaces of respective noses which transform force applied longitudinally to the lip members to transverse sealing forces holding sealing surfaces of the lip members together on each end of the orifice.

Abstract: A coextrusion block for siding communicates through respective separate circular inlets with two extruders at the inlet side of the coextrusion block while, at the outlet side, a curved kidney-shaped outlet is disposed about an inlet of acorn shape so that the outlets have boundaries lying along the common circle which corresponds to the circular inlet of the broad-mouth die.