Apparatus for welding sheets (3a,3b) of themoplastic material together along proximate edges including a supporting frame (25), a pair of joining rollers (11,13) mounted on the frame and forming a nip (12) with an axis parallel to the axes of the rollers, a motor (23) to drive at least one of the rollers, a horn (15) having a tip (17), and an ultrasonic transducer (39) to reciprocate the horn (15) and tip (17) in the direction of the longitudinal axis of the horn (15), the horn being so positioned in the nip (12) that the tip reciprocates in a direction transverse to the axis of the nip (12), so that proximate edges of the sheets (3a,3b) may move through the nip (12) with one edge on each side of the tip (17), be uniformly melted on their surfaces by frictional energy created by the reciprocal motion of the tip (17), and then be pressed and welded together in the nip (12). Fault detection circuitry stops the operation if the sheet material is not properly overlapping or if it runs out.