Method for producing a double wall pipe

The method of producing a double wall PVC pipe comprising the continuous steps of

• extruding concentric outer and inner tubes of PVS from respective inner and outer die orifices, said inner die orifice spaced from said outer die orifice in the extrusion direction by a distance form 0 to 2.5 inches,
• inflating by pressurized gas the outer PVC tube while in a state of thermal fusion against a surrounding annular transversely corrugated mold surface which is traveling in the extrusion direction to form a corrugated outer tube having inner crests,
• expanding by pressurized gas the inner PVC tube to fuse it to said outer tube crests while advancing said inner PVC tube onto a coaxial shaping and cooling mandrel which commences at an upstream tip spaced from the inner die orifice and tapers smoothly outwardly to a point of larger transverse dimension substantially equal to an inner diameter of the PVC pipe to be produced, said point being spaced from the outer die orifice in the extrusion direction by a distance which is less than the PVC pipe diameter such that the temperature of both the inner and outer PVC tubes at said point is at least equal to the fusion temperature of the PVC and
• cooling the resulting double wall PVC pipe to a solid state such that scanning electron micrographs of an enlargement at 20 times exhibits no line of demarcation at the fused joint between the outer and inner PVC tubes.