Abstract: A plastic pipe having spiral ridges on an outer surface and a continuous, cylindrical inner surface formed by a spiral wound plastic ribbon having at least one ridge-shaped hollow deformation extending longitudinally of the ribbon and spirally of the pipe. The outer radial surface of the hollow deformations are slit and sealed with a ribbon. Side portions extend from the opposite sides of the hollow deformations and are fused to each other to form the cylindrical inner surface.

Abstract: A method of manufacturing a plastic pipe having a spiral ridge on an outer surface and a flat inner surface, comprising the steps of: (a) supplying a plastic ribbon in a softened state onto a plurality of rotating forming rolls arranged circularly, so that the plastic ribbon is spirally wound around the forming rolls while partially overlapping to be fused with each other; (b) spirally winding an endless forming core belt around the forming rolls in such a manner that the endless forming core belt is sandwiched between the plastic ribbon already wound around the forming rolls and the plastic ribbon newly supplied, thereby providing the resulting plastic pipe with a spiral ridge; (c) cutting the spiral ridge after the plastic ribbon is fused with each other and sufficiently cooled, thereby providing the spiral ridge with a slit extending along it to remove the endless forming core belt from it; and (d) winding a second plastic ribbon in a softened state around the spiral ridge to fuse the second plastic ribbon