flange 7 fig2 exaggerates the height of the flange in comparison with the width of the mask is provided around the central welding spots at the shorter sides with notches and decoupling slots to give the mask flange areas a relatively great freedom of movement around the welding spots to combine maximum stability with maximum elasticity the notches 10 and decoupling slots 11 are designed as shown in fig3 to this end notches are provided at both sides of the welding spots sp3 and sp8 and additional decoupling slots are provided at some distance from the sides of the notches 10 that face away from the welding spot both the notches 10 and the decoupling slots 11 are preferably shaped as arches however it is also possible to use any desired shapes that taper in their width with an increasing depth i e at an increasing distance from the outer mask edge the depth h2 h3 of the notches 10 is of particular importance in attaining the necessary stability and elasticity in an especially satisfactory manner it has been found that a ratio of 3 1 between the height h2 of the mask flange and the remaining height h3 of the mask flange at the point where the depth of notches 10 is maximum meets the conflicting demands in a particularly satisfactory manner further improvements will be achieved when.