spaced seams in a disposable garment web containing a plastic component and moving rapidly along a direction of manufacture is formed by bringing an ultrasonic welder into pressurized contact with the machine web for a predetermined period of time , heating the web along the seam for a first segment of the predetermined period of time and then maintaining pressurization with the ultrasonic welder during a second segment of the predetermined time period without heat sufficient to permit the plastic material to resolidify .