a plastic closure device with a lower part 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 of the lower part 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 . an energy - introducing arrangement , comprising a plurality of energy - introducing ribs , is used to introduce the ultrasonic energy into the flange , 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 .