respective openings 9 which are formed in the bottom of the collection chamber 7 and are adapted to form respective substantially concave portions of the stringer 2 during the molding operation in particular the two molding elements 8 can move along a sliding direction which is substantially parallel to the direction of translational motion of the upper mold part 4 and are provided with respective heads 8 a which are for example rounded which protrude at least partially from the respective openings 9 toward the upper mold part 4 means for actuating the two molding elements 8 not shown in the figures for the sake of greater clarity in illustration can be actuated by the upper mold part for the translational motion in cooperation of the upper mold part 4 along the direction of translational motion and of the heads 8 along such sliding direction in particular the expression u201c translational motion in cooperation u201d is understood to designate the translational motion of the upper mold part 4 from the inactive position to the active position and the simultaneous sliding of the molding elements 8 from a position which protrudes within the collection chamber 7 to a position which is retracted within the chamber both positions being preset advantageously the machine 1 comprises means for introducing a first hot fluid such as steam into the collection chamber 7 which comprise a plurality of ports 10 a for dispensing steam which are formed at the outer surface of the molding elements 8 at the internal surface of the collection chamber 7 and at the lower surface of the upper mold part 4 with particular reference to the molding elements 8 the dispensing outlets 10 a are formed within the heads 8 a.