attaches to tube 40 for maximum cooling efficiency and minimum disruption of tube stability the combined air flow from both the upstream and downstream annuli are now briefly allowed to accelerate in the region denoted as 39 and then to converge between tube 40 and chimney 37 an adjustable iris assembly 49 is also provided to further shape the bubble in the region above the chimney if desired it has been found generally useful to adjust iris leaves 48 to a position near tube 40 such that the gap between the inside diameter of iris and tube 40 is approximately twice the minimum gap between the inside of chimney 37 and tube 40 the following examples of blown film extrusion using the air ring described in this application were observed in all cases the same 10 inch single layer die were used along with an 80 mm 24 1 l d helical grooved extruder equipped with a barrier screw the adjustable iris was brought to within approximately 1 inch from the tube wall the blower was adjusted for maximum flow output as limited by tube stability tube inflation was adjusted for a layflat width of the collapsed tubing of 42 34 equivalent to an approximate blow up ratio of 2 7 film thickness was controlled by adjustment of the nip roll speed a resin blend consisting of 15 by weight of a high pressure low density.