Abstract: A method of modifying compressive strains in formation of a pipe ring groove (16), in which a differential temperature profile is established across the pipe wall in a narrow bending locus (l3) corresponding to the ring groove position, and the material is bent to form the ring groove. The temperature differential creates a differential in elastic modulus between the inner and outer surfaces of the pipe material, modifying the strain induced in inner concave and outer convex surfaces of the ring groove during the bending step.

Abstract: A start-up sequence for a continuous process for producing oriented plastic tube is disclosed. A variable diameter extruder (112) is started and is set to extrude a start-up diameter tube (220) which is significantly over the diameter of a pre-expansion tube diameter of the continuous process. The start-up diameter tube (220) is passed to and engaged by a first haul-off tractor (122). The start-up diameter tube (220) proceeds through to and over an upstream plug (128). The large, start-up diameter of the extruded tube (220) at start-up facilitates passage of the leading end of the start-up diameter tube (220) over a deflated, expandable portion (212) or a non-expandable portion (210) of a partly expandable plug (210, 212), depending on the arrangement of the expandable portion (212) and non-expandable portion (210). The tube is then reduced in diameter for the continuous process of diametral expansion and circumferential orientation to occur.

Abstract: A method of modifying compressive strains in formation of a pipe ring groove (16), in which a differential temperature profile is established across the pipe wall in a narrow bending locus (l3) corresponding to the ring groove position, and the material is bent to form the ring groove. The temperature differential creates a differential in elastic modulus between the inner and outer surfaces of the pipe material, modifying the strain induced in inner concave and outer convex surfaces of the ring groove during the bending step.

Abstract: A method of modifying compressive strains in formation of a pipe ring groove (16), in which a differential temperature profile is established across the pipe wall in a narrow bending locus (l3) corresponding to the ring groove position, and the material is bent to form the ring groove. The temperature differential creates a differential in elastic modulus between the inner and outer surfaces of the pipe material, modifying the strain induced in inner concave and outer convex surfaces of the ring groove during the bending step.