Process for making in continuous production fibrous webs which are corrugated along the machine direction

Process for continuously producing ribbons of corrugated fibrous material the process comprising:

• placing at least a pair of first forming rollers (4,4′) having circumferential ribs alternating with circumferential grooves on lateral surfaces thereof so that the ribs of one roller of the pair of first forming rollers interpenetrate, at least partially, the grooves of the other roller of the pair of first forming rollers, thus defining a wavy gap between the two rollers;
• advancing a continuous ribbon (2) of fibrous material between said pair of first forming rollers (4,4′), said continuous ribbon (2) having a thickness less than a thickness of the wavy gap and a transverse extensibility of at least 8%, thereby causing the continuous ribbon to take on a corrugated shape corresponding to the wavy gap's shape, at the same time submitting the continuous ribbon (2) to transversal stretching in order to maintain the continuous ribbon's initial width despite the ribbon's greater transverse extension due to corrugation;
• passing the corrugated ribbon coming out of said pair of first forming rollers (4,4′) between a pair of second stabilizing rollers (8,8′) having circumferential interpenetrating ribs alternating with circumferential grooves on lateral surfaces thereof so that a wavy gap is defined between the circumferential interpenetrating ribs of one roller of the pair of the second stabilizing rollers and the grooves of the other roller of the pair of the second stabilizing rollers, and the wavy gap of the pair of second stabilizing rollers has a thickness less than the continuous ribbon's thickness so that the corrugated ribbon that passes between the second stabilizing rollers is subjected to a compression in a direction of the corrugated ribbon's thickness and to a subsequent stabilization of the corrugations thus obtained.