once in order to increase locally the number of layers of meshes and thereby to obtain a localized super concentration of the welding energy as a matter of fact as shown in fig2 the heater mat 1 is folded over once and the two portions 1 a 1 b of the mat thus obtained are arranged so as to be partially staggered in relation to each other and in regard to the number of layers of meshes and the energy capacity to be supplied so as to obtain a first single mat portion 11 a double mat portion 12 and a second single mat portion 13 depending on the manner in which the two portions 1 a 1 b of the mat 1 are staggered the double mat portion 12 is relatively wide and relatively long fig3 shows a portion of a heater mat 1 of a device of the invention which differs from that of fig1 in that it has an uneven distribution of the meshes on the surface in question this uneven distribution of the meshes is obtained manually when the heater mat is positioned on the surface to be melted by concentrating the meshes over an area requiring a super concentration of energy and by.