flexible tubular film is helically cut into a flat strip by unwinding the tubular film in flattened form by a tumbling unwinder , advancing the flat film tube to a tube expansion zone in which the flat tube is expanded into a circular cylindrical tube and is passed axially over a hollow mandrel slightly lower in external diameter than the diameter of the expanded tube to be cut by knife helically while on the mandrel into a flat strip that is removed at an angle to the mandrel axis . the flat tube is longitudinally stretched by stretching rollers , which tumble with the tumbling unwinder . the apparatus further comprises stabilization means for stabilizing the orientation applied by the tumbling stretching rollers , applied to the helically cut film strip after it has been drawn from the mandrel .