Method and apparatus for producing double-walled thermoplastic pipe

Abstract From the Canadian patent

A double-walled pipe of thermoplastic material is produced, constituted of an inner pipe and an outer pipe concentric therewith, in which the outer pipe is formed into the shape of a helical thread. The pipes are extruded the, outer over the inner, and prior to the setting of the outer pipe it is subjected to forming into a helical thread by a rotating mold. The mold includes vacuum forming rollers spaced along a helical path about the outer pipe the circumferential, edges of the rollers being spaced at different distances from the longitudinal axis of the pipes.