Gage uniformity has a profound affect on wound-roll quality. In fact, gage variation can sometimes be so overwhelming that nothing can be done at the winder to effectively deal with it. This includes adjustment of any of the TNT's (Tension, Nip or Torque) of winding. Not even changing to a different type of winder may do it.

But we get ahead of ourselves. First, let's define the subject. The word "profile" is shorthand for "variation of something across the width of the web." Here, we will confine our discussion to the effects of gage variation across the width and its effect on winding and wound-roll quality.

This is not to say other types of profile are not problematic. Variations of bagginess, for example, may be cause for wrinkling, especially with a nip.

I recently had a customer complain that one side of their rolls was loose, while the other side was tight. They asked what could be done. "Fix the gage variation or throw away the wound rolls," was my reply. Either or, nothing more. Winding tighter to fix the loose side will aggravate the already tight side, and vice versa. It does not matter which of the TNT's you choose, because the relationship will remain.

A similar situation exists for some cases where a roll is starred on one side, but not on the other. While a uniform case of starring is very amenable to roll structure (e.g., taper tension), starring on one side of a roll is usually not correctable at the winder.

The same is usually true if one roll of a set is starred, but its neighbors are not. The same can be true for other tension-related defects, such as blocking or gage bands. A defect occurring on only some rolls of a set is often a symptom of a profile problem. What many people do not appreciate is that winding tightness variation is more often determined by gage variation than it is by the winder settings themselves—especially for dense materials like film, paper and foil.

The only way we can accommodate gage variations or wind one roll tighter or looser than its neighbors is to wind each roll separately, such as on full duplex, differential or slip-core winding (Web Works, July 2003).

Another area of problems with gage varying material is wrinkles. The area on either side of a gage band is at great risk for diagonal wrinkles, which in themselves indicate that something is "crooked." The classic defect here is the corrugation—also called chain marks or ropes, and often improperly called tin-canning (the true tin-can is annular ridges that align directly in the MD).

However, wrinkles can also occur immediately under the nip. Here, we can reduce the problem by winder settings. In this case, the nip should be reduced to absolute minimum. What does minimum mean? So low that the material or machine will not let you go further, because some even more serious problem develops. With layon rolls, it is often possible to completely remove them. In fact, if you have gage variations, the air-handling function of the nip is destroyed anyway because the roller is held off from complete contact by the high spots in the wound roll.

This is not to say that all diagonal wrinkles in winding are caused by gage. Sometimes it is the machine that is "crooked" such as by roller misalignment or nip-load bias.

As we've seen, many defects are directly caused by winding gage-varying material. Gage variation is a root cause, is an ingredient and is in the mechanics of defect generation. However, you will also find that many other defects, while not directly caused by gage variation, are affected by it. Simply put, the wider the gage variation, the narrower the window of defect-free operation. If we want to troubleshoot these problems, we need to be able to measure the thing that is causing them, i.e., gage variation. Stay tuned: next month we'll discuss some surprising aspects of gage measurement.