Abstract: A plastic closure device (1) with a lower part (2) is disclosed which can be fastened in a non-releasable and integrally bonded manner by means of ultrasonic welding to a plastic film or a plastic patch. A flange (20) of the lower part (2) is connected by an underside thereof to the plastic layer in an integrally bonded manner by means of introducing ultrasonic energy of a sonotrode into the flange (20). An energy-introducing arrangement, comprising a plurality of energy-introducing ribs (23), is used to introduce the ultrasonic energy into the flange (20), as a result of which the weld parts are welded under the action of a static joining force. Owing to the design according to the invention of the energy-introducing arrangement, a desired connection can be achieved in spite of a reduced static joining force, a reduced ultrasonic energy and, in general, with the use of a reduced welding power by comparison with prior art methods.