Patent ID: 7217379

Claim:
An improved method for producing a biaxially oriented tube from thermoplastic material, wherein a tubular preform is extruded from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, wherein the preform is subjected to a temperature conditioning of the preform, so that a tempered preform is obtained having an orientation temperature which is suitable for the thermoplastic material of said preform, and wherein the tempered preform is forced over a dimensionally stable mandrel, which mandrel comprises an expansion part having an outer surface which substantially corresponds to the surface of a truncated cone, which mandrel brings about expansion of the tempered preform in the circumferential direction of the tempered preform forced over said mandrel, in such a manner that said preform is transformed into a biaxially oriented tube with thermoplastic material which is oriented in axial direction and in circumferential direction of the tube, wherein said biaxially oriented tube is cooled, the method comprising the use of a preform speed-control means which acts on the preform upstream of the mandrel and of a drawing device which is arranged downstream of the mandrel and acts on the tube, the improvement including the measure that the outer surface of the expansion part of the mandrel is provided, at a plurality of locations around the circumference of the expansion part, with elongate grooves and/or ribs which extend in the axial direction, said grooves and/or ribs causing a reduction in the wall thickness variation of the biaxially oriented tube.