The invention concerns a continuous manufacturing method which consists in producing
a blank (E) by extrusion, bringing said blank to a temperature for molecular orientation,
passing said blank around a radial expansion mandrel (12); gauging and cooling
while subjecting the blank to axial traction. The radial expansion produced on
the mandrel (12) is partial so that the internal diameter (B) of the blank
(E), when it leaves the mandrel, is less than the nominal internal diameter (D)
of the finished tube; performing an additional radial expansion of the blank, up
to its nominal diameter, by internal fluid pressure. In permanent operating conditions,
the inner surface of the blank (E) is no longer in contact with the mandrel (12).
