apparatus for the production of plastic film by the extrusion of a tube thereof includes a first air ring disposed at the die exit and delivering an annular stream of cooling air to the surface of the emerging tube , which is in a stable expansion phase as it escapes the die internal pressure . preferably the stream is directed axially of the tube path . the apparatus includes the combination of a sizing cage and a second air ring that is mounted for longitudinal movement along the tube . the tube is established with the air rings close together and they are moved as soon as possible as far apart as possible . the second air ring is located just below where the tube begins to expand and delivers an annular stream of air , preferably in a known manner away from the tube to form a vacuum which stabilizes the tube . the attached sizing cage supports the tube and holds it concentric with the second air ring against turbulence and the possibility of oscillation . an additional mechanical tube stabilizer is provided below the second air ring and attached thereto and consists of a stabilizing cylindrical shield surrounding the tube , or a roller cage of a plurality of rings of rollers of low friction coefficient material .