The first cut at diagnosing the reason for a specific roll-edge feature requires you to determine if the wrap is offset or has a width change. You can do this simply by comparing both sides of the roll by unwinding or slabbing down to that layer(s). If there is an 'insie' on one side and an 'outsie' of the same size on the other side of the same wrap, then the wrap is offset. If there is an insie on both sides, then the wrap is narrow and vice versa for an outsie. Obviously, all wraps are a combination of offset and width variations. However, you only need to look at the largest factor to begin with.

You must do this bit of homework because, as we will see, there is virtually nothing in common between the causes of offset and width variations. In other words, two identically appearing outsies on the same side of the roll could have different causes because one is associated with an offset and the other with width.

Width changes typically come from only a few sources. One would be a tiny pucker that comes and goes in the slitter section. You would be quite surprised how small a pucker can be and still put you out of width spec. One common trouble here is improper pulling of the trim. If the trim is pulled at even a tiny angle with respect to the machine direction, or is not pulled with the same tension, a shear pucker will form just ahead of the blade. In any case, our expectations of a good slitter area is a web that is as flat and stable as a table.

Another source of width variation is a variation of tension at the slitter section, going into the windup or as wound-in-tension in the roll. A momentarily higher tension makes a momentarily narrower web and vice versa due to the Poisson effect. A good place to see this phenomenon is to change the speed of the machine and see the mark made on the side of the wound roll. This blemish is incorrectly called an acceleration offset. It is not the acceleration itself which made it, but rather less-than-perfect tension control during the acceleration.

Probably the most demanding test you can make of a drive is to be able to change speeds without changing the edge of the roll. Vibration of the web run, machine or wound roll also causes width variation that is so rapid that it might make the edge of the roll fuzzy or sometimes gives it a corduroy appearance.

There are other sources of width variation as well that we will simply list without explanation: any change in the raw material, especially temperature (film) or moisture (paper) could do it; movement of the slitter blades, holders or framework; bottom slitter-blade wobble, for example, makes a bull's-eye appearance; air entrained in wound rolls of film and other smooth webs can pull edges in.

Offsets can be broken up into two subcategories: in the web run and in the wound roll. Web paths stray for any number of reasons, which will simply be recorded on the wound roll edges. A pair of edge sensors just upstream of the windup can separate web-path offsets (difference) from web-width variations (sum). Movement of the wound roll has several possibilities. Any axial movement of layers with respect to layers (such as telescoping), core to chucks or shafts, core arms or slides, framework and so on can cause the roll to shift during winding—and the web will thus enter in a new place on the roll.

A final clue to causes is the diametral distribution of a feature. If it favors the bottom, certain middle radii or the outside of the roll, it is most probably winder related. If it is randomly distributed throughout the roll diameter, across the rolls in a set or between sets, the source might be the winder but could also be forming or process variation.

The edges of the roll can be read like the rings of seasonal growth in the cross section of a tree trunk. Some seasons were good; some seasons were not so good.

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