The assembly is intended for application to manufacturing of tubes, bottles, boxes. Characterized by improved quality of the weld and low reject percentage. It has a mandrel (1), on which there is a head (3) and a tube body (2), put over in a typical manner, as well as a cooling element, coaxial with the mandrel (1), and a nozzle D, coaxial with the mandrel. Said nozzle contains two parts—internal 7 and external (8)—which form channel (9) for feeding heating gas HG. The assembly consists of only one work station. The cooling element is also forming element OFE and consists of cooling metal ring 4, located in forming metal ring (6). There is heat-insulating ring (5) between the latter and the cooling metal ring (4). In the position of heating, said heat-insulating ring (5) is positioned around the end of the tube body (2). The heat-insulating ring (5) is of solid-state material or gas of low thermal conduction.