A device for joining thermoplastic workpieces includes an ultrasonic welder including an ultrasonic transducer, a booster and a horn having a welding tip. A clamping mechanism attaches to the ultrasonic welder and includes an upper frame coupled to a boot. The welding tip of the horn of the ultrasonic welder projects into an aperture in the boot of the clamping mechanism. The clamping mechanism exerts a clamping force on the workpieces that differs from a welding force exerted on the workpieces by the ultrasonic welder when the ultrasonic welder is operating to join the thermoplastic workpieces.