Ultrasonic apparatus with non-rotating horn and rotating anvil for welding plastic parts

A method comprising the following steps:

• (a) intermittently moving first and second strips of thermoplastic material along a path that passes between an ultrasonic horn and a rotary anvil, the intermittent movement of said first and second strips being such that respective successive sections of said first and second strips pass between said horn and said anvil, successive intermittent movements being separated in time by respective dwell times during which said first and second strips are not moving along said path, wherein said first and second strips comprise respective interlockable profiles that are interlocked with each other;
• (b) intermittently rotating said anvil so that a contact portion of said anvil is at rest during at least a portion of each dwell time, said contact portion at rest being in contact with one of said first and second strips;
• and
• (c) activating said ultrasonic horn to transmit ultrasonic wave energy at least during the periods when said anvil is not rotating, the total amount of ultrasonic wave energy transmitted into adjacent portions of said first and second strips being sufficient to meld said adjacent portions together in regions that are spaced at regular intervals longitudinally along said first and second strips,
• wherein said horn does not rotate about a pivot point during activation, and said horn and anvil do not form any melded regions that are laterally displaced relative to said melded regions along said first and second strips.