The main goal of the film blowing process is to manufacture a stable film with good physical and optical properties at a maximum production rate. Moreover, constant bubble diameter (affecting unform lay-flat width and thickness of the film) is necessary otherwise thickness-variation instability may appear. Is this case, the bubble shape is changed in the area between the die exit and freezeline height which leads to reduced product properties, line failures, and large amounts of film scrap. Such instabilities decrease significantly the window of stable processing condition for blown film production.