Rotary ultrasonic sealing

1. An apparatus for forming an ultrasonic weld at least partially across, and between, two or more webs of sheet material as they pass through a sealing gap in the apparatus, the apparatus comprising:

a sonotrode comprising an ultrasonic actuator coupled to an elongate sealing end face for delivering ultrasonic energy to the webs in the sealing gap; and

a rotating cylindrical anvil having at least one raised portion extending longitudinally along an outer surface thereof, said raised portion having a radially outer surface, whereby said sealing gap is defined intermittently between the end face of the sonotrode and the radially outer surface of the raised portion of the anvil as the raised portion rotates past the end face of the sonotrode;

wherein the elongate sealing end face of the sonotrode has a substantially helical surface, whereby the sealing gap travels helically along said surface as the raised portion of the anvil cylinder rotates past the end face of the sonotrode.

2. The apparatus of claim 1 , further comprising a web feeding mechanism for feeding the webs to be sealed through the sealing gap at a speed such that the helical movement of the sealing gap generates a transverse seal across the moving web that is substantially linear and perpendicular to the direction of motion of the web.

3. The apparatus of claim 1 , wherein the width of the end face of the sonotrode and/or of the raised region of the anvil cylinder is not constant along its length, whereby seals having varying widths along their length can be produced.

4. The apparatus of claim 1 , wherein the sonotrode end face and/or the anvil raised portion are configured such that the sealing gap is tapered in the direction of travel of the webs such that the gap is wider at an entry point of the webs into the gap and narrower at an exit point of the webs.

5. The apparatus of claim 1 , wherein the height of the end face of the sonotrode and/or of the raised portion of the rotating anvil, measured from the axis of rotation of the anvil, is not constant along its length, whereby seals can be formed in webs having varying thicknesses along their width.

6. The apparatus of claim 1 , wherein the pitch of the helical sealing end face of the sonotrode is such that a first end of the sealing end face is displaced circumferentially from a second end of the sealing end face by a distance of from about 1 to about 5 times the circumferential width of the sealing end face.

7. The apparatus of claim 6 , wherein a plurality of said raised sealing regions are circumferentially spaced around the rotating anvil.

8. The apparatus of claim 1 , wherein the apparatus further comprises one or more setting rollers, configured to apply a compression to the sealed web after it has exited the sonotrode gap in order to aid fusion between the web layers during cooling.

9. A method of welding a first web of sheet material to a second web of sheet material comprising the steps of:

providing an ultrasonic welding apparatus according to claim 1 ;

rotating the anvil while feeding the webs to be sealed through the gap between the anvil and the sonotrode sealing end face in a direction substantially perpendicular to the axis of rotation of the anvil; and

actuating the sonotrode to form seals in the webs substantially transverse to the direction of motion of the web,

wherein the speed of rotation of the anvil and the speed of feeding of the webs are selected to produce said transverse seals at predetermined intervals in the web.

10. The method according to claim 9 , wherein the webs to be sealed are fed through the sealing gap at a speed such that the helical movement of the sealing gap generates a transverse seal across the moving web that is substantially linear and perpendicular to the direction of motion of the web.