Device and method for forming ultrasonic welds on web materials in continuous movement

An apparatus for ultrasonic transverse welding of a composite web comprising at least two superimposed sheet materials, the apparatus comprising:

• a feeding unit including first and second feeding devices configured to advance said composite web along a main direction at a speed;
• and
• an ultrasonic welding unit arranged between said first and second feeding devices and comprising:
• at least one ultrasonic generator coupled to a sonotrode having a flat welding surface parallel to the plane of movement of the web and fixed in said main direction;
• and
• an anvil unit having at least one anvil element movable along a closed path having a straight portion parallel to said main direction, said at least one anvil element moving at said speed along said straight portion of said closed path, wherein:
• during use, said at least one anvil element is pressed against said flat welding surface with a predetermined force and with of said composite web material interposed therebetween,
• said at least one anvil element and said flat welding surface have an extension in a transverse direction to said main direction that is equal to or greater than a length of said transverse weld, and
• said flat welding surface has a total length parallel to said main direction between 0.1 and 1.0 meters.