A plastic closure device (1) must be welded by means of ultrasound onto a tubular bag (2) consisting of a multilayer plastic film (20). For this purpose, an energy introduction arrangement (182) on the underside (180) of a flange (18) is proposed, in which at least one energy conduction rib (183) concentric with the outer edge (181) of the flange is present, as well as a plurality of outwardly-directed energy conduction ribs (184). These energy conduction ribs can intersect in the region of intersection (185) or they can be interrupted in the region of intersection.