(Received January 13, 1984)
Ultrasonic plastic welding with two longitudinal vibration systems is proposed and studied. The welding limit of this system is double that of a conventional ultrasonic welding method with one vibration system. In transmission welding, the vibration distribution along a welding specimen can be changed by controlling the vibration phase difference between upper and lower welding tips. The welding position can be set at a vibration stress maximum position by controlling the vibration phase difference. This method with two vibration systems has various useful features and is effective in plastic welding.