Process for the production of tubes of a molecularly oriented plastic

A process for the production of a tube of a molecularly oriented plastic, wherein a section of a tube, heated to the temperature at which stretching induces a molecular orientation of the plastic, is placed in a mold and a portion of the section of the tube is clamped in a sleeve and the sleeve is caused to undergo a relative movement along the section of the tube while a fluid under pressure is introduced into the tube so as to cause radial expansion to an internal wall of the mold of portions of the tube at the outlet of the sleeve progressively along the section and the resulting oriented tube is finally cooled and removed from the mold, characterized in that the portions of the tube undergoing radial expansion slide over an annular surface which is located at the outlet of the sleeve and extends up to the internal wall of the mold, said annular surface providing a counter-balancing support to the fluid under pressure, thereby enabling the molecular orientation of the plastic tube to be controlled.