Patent ID: 7390455

Claim:
In a method for the continuous extrusion of a plastic tube with biaxial drawing, whereby a tubular blank (E) is produced by extrusion through a die, said tubular blank is brought to molecular orientation temperature, is made to undergo radial expansion by subjecting it to an internal fluid pressure applied between two axially distant obturators ( 7 , 11 ), and is sized ( 8 ) and cooled ( 9 ) while at the same time exerting axial tension on the exiting tube resulting in a tube having a larger diameter than the extruded blank, the improvement comprising applying a counterpressure against the exterior wall of the tubular blank in at least one region subjected to the radial expansion and only to the localized regions most susceptible to an uncontrolled increase in the absence of counterpressure, said counterpressure being lower than the internal pressure so as not to impede the expansion, but being high enough to ensure a controlled increase in the expansion, resulting in a substantially uniform peripheral thickness of the wall throughout the length of the resultant tube.