The embodiments of the invention relate to a method for creating a force for a welding jaw of a vertical tubular bag machine. The welding jaw is moved back and forth along a path to strike a second welding jaw moving in the opposite direction. A tubular film is compressed between the second jaw and at least one sealing surface of the welding jaws, to weld the tubular film by ultrasonic vibrations applied thereto via the sealing surface at an applied pressure under a sealing force (Fs) within a sealing time (ts). The sealing force (Fs) is predetermined by a control unit such that the sealing force (Fs) runs along a sealing force curve within the sealing time (ts). The sealing force curve has a first relative sealing force maximum (Fs1) and at least one second relative sealing force maximum (Fs2), wherein (Fs1) is lower than (Fs2).