Manufacturing process for a flexible tube, in which a tubular blank is fitted around a punch so that an end portion of the tubular blank protrudes by a height H from the side wall of the punch; a die is brought up to the punch and the end portion is imprisoned in the cavity thus formed; plastic is injected into the cavity until it comes into contact with the end portion so that it is welded to the skirt, without any additional contribution of heat or matter. The impression of the die used is at a depth greater than the protruding height H of the end portion, and has a ring-shaped boss obstructing the flow of the plastic, the outer edge of the ring-shaped boss and the inner edge of the end portion being distant from each other by a value less than the average thickness e of the transverse wall.