in a process and a device for carrying out a welding process on thermoplastic material webs , the value of the pressing force of a pair of pressure rollers is determined as a measured value representing the weld seam thickness and is used as the actual value of the controlled variable of a welding process regulating unit , and it is compared with a corresponding set value . the feed speed of the welding device is used as the correcting variable of this regulating unit by increasing the speed in the case of decreasing weld seam thickness and decreasing the speed in the case of increasing weld seam thickness to compensate a possibly existing deviation .