Winding nips, such as a layon roller, do many things depending on the circumstances. The primary functions are to meter air into the roll, improve stack, increase wound-roll tightness and to support the winding roll. Nips can also be destructive, however, by causing wrinkles, corrugations and other defects.

Nip rollers help keep some air out of the winding roll. This lets us reach higher speeds and/or wind tighter. The air-handling function is negligible on materials that are not smooth. On smooth film, however, a nip is usually necessary at speeds more than a few hundred ft/min. Without enough nip, the roll will be soft, the edges will be rough and the roll may buckle when the air weeps out.

However, if we exclude too much air, the roll will get too tight, causing gage bands to bloom and possibly causing the roll to block. Thus, metering rather than excluding air is the idea.

Unfortunately, there are many things that can spoil the metering function. Since the air layer going in is far less than one mil thick, geometrical errors such as layon roller deflection, alignment, wound-roll gage variations or vibration will simply hold the roll off too far to do any good. Also, it may take as much as 5 lb/in of nip load to keep most of the air out. Your winder may not be designed to load that high, or your product may not be able to take loads that high without wrinkling.

Nip rollers improve the stack merely by keeping the air out and winding tighter. They will improve the roll edges even more if the layon or nipped roller is wrapped. This causes a precision ground roller to draw and control the web's path rather than a much more geometrically crude winding roll. However, nip rollers can make the wound-roll edges rougher if the nip roller vibrates. Thus, nip rollers improve wound-roll edges and degrade wound edges depending on the circumstances.

Nip rollers increase the wound-roll tightness of "fuzzy" materials in a very different way. Nonwovens, textiles, tissue and some paper grades will be tightened by the interlayer slippage generated by the rolling nip. With these materials, the nip is usually much stronger in tightening the wound roll than is tension or torque differential. Unfortunately, interlayer slippage can also create defects such as corrugations, crepe wrinkles and a type of telescoping. Nip-induced slippage may be good if you want a tighter roll, but may be bad if non-uniform slippage causes a defect.

In any case, a nip is inevitable because a nip is always necessary to support the weight of the winding or unwinding roll. Many people would be surprised that their unwind has a nip, but it does. It is the pressure between the core and the inside of the roll. The internal nip on a core-supported winder/unwind has the same mechanics and can be just as destructive as the more commonly considered external nip.

We now see that nip is neither good nor bad. It is both at the same time. In fact, every knob in your plant is both good and bad at the same time. Exceptions are easy to spot: the knob is pegged or it does nothing. This is a hard concept for some people to accept. They think that a knob must have some sweet spot where the process will run well.

The realities are very different, however. A knob has sets of both good and bad things at both ends of its adjustment. Our job is to find a position where the knob does the least harm (rather than the most good). From this vantage, it's easy to understand why you can't always eliminate problems by adjusting a setting. You may have as many as a dozen winding defects that are tension- or nip-sensitive. You may not be able to solve a dozen problems with a single knob.