Apparatus for forming transversal welds on a composite web material 100, comprising at least two superimposed sheets of material, suitable for use in the production of sanitary articles 10, comprising:- a feeding unit 180 including first and second feeding means 181, 182 configured to advance the composite web 100 along a main straight direction X at a speed V1.- an ultrasonic welding unit 80 arranged between the first and second feeding means 181 and 182, comprising at least one ultrasound source 900 coupled to a sonotrode 914 having a flat welding surface 915 and an anvil unit 850, 1850 having at least one anvil element 855, 855' movable along a closed path 815, which has a straight portion parallel to the main direction X.During the production step of the welds, the anvil element 855, 855', which has a width 853 equal to or greater than the width L of the transverse weld 13, is pressed with a predetermined force (F) against the flat welding surface 915 of the ultrasound source 900 with the interposition of the web material (100).
