Abstract A TRAVELLING WEB OF MATERIAL IS FORMED INTO A TUBE WITH AT LEAST ONE OF THE LONGITUDINAL EDGES OF THE WEB PASSING THROUGH A HEATING MEANS SUPPLIED WITH HEATED AIR. THE TWO EDGES OF THE WEB ARE PRESSED TOGETHER TO FORM A WELDED SEAM AFTER PASSING THROUGH THE HEATING MEANS. THE HEATING MEANS COMPRISES A HEATING CHAMBER HAVING AN INTERNAL LINING OF A NON-ADHERING MATERIAL. AN AIR PIPE OPENS INTO THE CHAMBER WITH MEANS FOR SUPPLYING HEATED AIR. THE LINING IS ATTACHED TO THE EXTERIOR SURFACE OF THE HEATING CHAMBER AND IS PASSED AROUND THE AIR PIPE AND HAS A VERTICAL SLIT-SHAPED OPENING THROUGH WHICH ONE EDGE OF THE TRAVELLING WEB CAN PROJECT INTO THE INTERIOR OF THE HEATING CHAMBER.