Weld seam, method and device for connecting plastic films (10) by thermal joining, wherein the plastic films (10) are connected between welding jaws (2, 4) along a weld seam (12). According to the invention, the device comprises a pair of unheated welding jaws (2, 4) between which the plastic films (10) are arranged parallel to each other for thermal joining. A device for impulse generation (30) applies an impact impulse to one of the welding jaws (2, 4) with a penetration time into the plastic films (10) of less than 10 ms with a first intensity (Fi1). The impact impulse acts on the plastic films (10), wherein a heating in the material caused by deformation is produced and the weld seam (12) is formed in an effective area of the welding jaws (2, 4); use of a blow-forging press.