a method and an apparatus for ultrasonically welding plastic parts together . the ultrasonic welding apparatus includes : an air cylinder having a piston rod ; an ultrasonic horn substantially rigidly coupled to an end of the piston rod and having a distal end ; a rotary anvil having an axis of rotation generally perpendicular to an axis of the piston rod and comprising a periphery ; a transducer for converting electric potential differences into ultrasound wave energy , the horn being acoustically coupled to the transducer ; and drive means for causing the anvil to rotate . the distal end of the horn confronts the periphery of the anvil throughout rotation of the anvil . on one application , interlocked zipper strips are passed through a gap between the distal end of the horn and the periphery of the anvil . the anvil periphery has a plateau that presses the zipper against the distal end of the horn . the transducer is energized in synchronism with the plateau being opposed to the distal end of the horn .