pipe made from thermoplastic polymeric material and including a transversely corrugated outer wall and a smooth inner wall of constant diameter fusion welded to the inner crests of the corrugations of the outer wall is produced by an improved method involving simultaneous extrusion of two tubes , blow molding of the outer wall and fusion of the two walls while the inner wall is supported by an internal shaping and cooling mandrel . even when the polymeric material is polyvinyl chloride , good fusion welds and lack of brittleness are achieved by the manner in which extrusion is carried out , control of the location at which the fusion welds are established , and the extrusion formulation . the finished pipe is characterized by the fact that , when the fusion welded areas have been cut for inspection , no line of demarcation between the materials of the two walls can be observed .