Before the welding procedure starts, the temperature of the heating element that is required for welding has to be controlled. This is carried out e.g. by means of a temperature measuring device with a fast display used for surface measurements. The control measurement has to be carried out within the area of the heating element that corresponds with the semi-finished product. In order to create a thermal balance, the heating element must have had the desired temperature for at least 10 minutes before it may be used.