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 blown-film 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 long-term 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 lay-flat 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.
https://www.flexpackmag.com/articles/86906-nip-rollers-in-th...


A nip is the pressure between two rollers that are forced together. vA nip impression is a measure of the
uniformity of that pressure. The pressure profile is sometimes referred to as a footprint. It can be measured
in many ways. The simplest is to make sure that there is pressure everywhere across the width. This can
be done by shining a light through the nip. Light escaping through a gap between the rollers tells us that
there is no nip there. However, even if there is contact everywhere, the pressure may still be quite uneven.
A better approach is to measure the width of the footprint. If the width is uneven, the pressure profile must
also be uneven. However, measuring nip widths accurately can be difficult if the width is narrow. Also, as
we will see, nip width variations are not a very sensitive measure of nip uniformity. The very best measure
of the nip is to measure the pressure profile directly. Both nip widths and nip pressures can be measured
with paper thin products often referred to as nip impression paper, even though some are these products are
made of plastic rather than paper. Nip impression paper behaves like carbon or carbonless paper which
turns color when a sufficient pressure has been applied such as by typing or writing with a pen. Nip
impression paper is an indispensable tool for machine PM or process troubleshooting.
https://www.tappi.org/content/enewsletters/eplace/2004/05-4R...