Control of tubular film size

Control apparatus for maintaining a desired circumference of a longitudinally extending thin-walled expanded plastic tube extruded from an annular die, comprising:

• internal cooling air supply means connected to the interior of said tube for supplying a continuous flow of cooling air to said interior;
• exhaust means for exhausting warmed air from said interior;
• deviation sensing means for sensing the deviation in position of two or more circumferentially-spaced portions of the wall of said tube, said deviation sensing means including
• two or more energy-beam sensors positioned so as not to make physical contact with said wall and
• means for providing a variable electrical output for each said wall portion that is directly dependent upon the deviation in position of said portion, over a range of positions, from a predetermined set point, said output having a multiplicity of different output levels, each said level corresponding to a different said deviation in position;
• control means for processing said electrical outputs to compute therefrom a difference signal representative of the change of the circumference of the tube from said desired circumference;
• and
• said deviation sensing means and said control means including means for making said difference signal insensitive to transverse sway of said tube, said sway leaving the tube circumference unchanged,
• valve means responsive to said difference signal for adjusting the open position of said valve means for controlling said continuous flow of air to the interior of said tube to reduce said difference signal, thereby to restore the circumference of the tube toward the desired value,
• whereby the size of the tube can be precisely controlled without physical contact with the tube and in the presence of the tube sway.