In order to make the thickness and strength values uniform in an extruded foil tubing made of high density polyethylene (HDPE), in the form of a long neck, an air nozzle ring is provided at the level of the end of the long neck in addition to an outer blow ring surrounding the ring nozzle of the blow head. The rate of air flow through the nozzles of the air nozzle ring is controlled dependent on a rotating measuring device for measuring the thickness of the foil which is located above the expansion portion of the foil tubing. The cooling effect of the outer blow ring is preferably twenty times greater than that of the air nozzle ring. To stabilize the foil bubble, an inner pin is provided at the level of the end of the long neck.