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Any web process is certain to have nip rollers. Typically, one roller is hard and the other is soft. They are commonly opened and pressed together with one or more air cylinders whose air pressure is controlled through an air-pressure regulator. In a blownfilm process, the primary nip rolls at the top of the tower perform three functions. They must open to allow a startup knot through them, while also opening the tops of the collapsing frame below. They must draw the film from the extrusion die consistently with no speed variations, and must also trap air inside the bubble, especially on a conventional non-IBC air ring cooling system.
Running at a consistent speed is more challenging. This has a direct effect on film quality and profitability. Any short- or longterm speed variation in primary nip rolls will affect the average film gauge, assuming that the extrusion system is producing a consistent output. Short-term speed variations will have additional consequences including bubble instability and layflat width variations. It should also be understood that a primary nip's speed can be increased or overhauled by web tension from another downstream nip, even with a right-angle gear reducer driving the primary nip. That is why a regenerative DC or AC drive with dynamic braking are commonly used, only on the primary nip rollers. Such a drive has the ability to hold the nip at the proper continuous speed even if downstream web tension tries to increase its speed. This can be dramatically demonstrated if a wire supplying the primary nip drive is removed during production and you observe that the line continues to run.
In addition, it is not uncommon to see zero or negative amps on a primary nip drive during this condition where a secondary nip is actually trying to pull the primary nip at a faster web speed. In...