1. Field of the Invention
The present invention relates in general to an extruded film processing system, and in particular to control systems utilized in extruded film processing systems.
2. Description of the Prior Art
Blown film extrusion lines are used to manufacture plastic bags and plastic sheets. A molten tube of plastic is extruded from an annular die, and then stretched and expanded to a larger diameter and a reduced radial thickness by the action of overhead nip rollers and internal air pressure. Typically, ambient air is entrained by one or more blowers. The ambient air provides a cooling medium, which absorbs heat from the molten material. This speeds up the change in state from a molten material back to a solid material.
Additionally, the ambient air entrained by the blowers is used to provide air pressure, which is utilized to control the size and thickness of the film tube. One type of blown film extrusion line utilizes air flow on the exterior surface of the film tube in order to absorb heat. A different, and more modern, type of blown film extrusion line utilizes both an external flow of cooling air and an internal flow of cooling air in order to cool and size the film tube. Whether the blown film tube is cooled from the interior surface, the exterior surface, or some combination of the two, one common problem in blown film extrusion lines is that of obtaining precise control over the diameter of the extruded film tube. Tight control over the diameter ensures uniform product dimensions, which includes the size of the extruded product, as well as the thickness of the plastic material.
Acoustic sensors may be utilized to gauge the diameter of the product. When such acoustic sensors are utilized, a feedback loop is established to alter dynamically one or more controllable variable of the process, such as blower speed, and/or temperature control over the cooling air stream.