Wound rolls can have flared or belled ends for a variety of reasons. This may be more than cosmetic—the diametral bulge can stretch the web into bagginess there. Oscillation can sometimes be used to minimize the differential diameter buildup damage to the roll or web. Sometimes wound-roll diameter has to be limited to limit damage. Sometimes you can wind looser to get a bit of relief.

To find an effective solution, you need to know the specific mechanics that are operating in your specific case. The distribution of the flared ends is an important clue, especially when several narrower rolls are cut from a wider sheet. If only the outside ends of the two outer rolls (A & Z) are flared, then the cause must be a smile (or frown shaped) because that is the shape of the defect distribution. We could then eliminate roller alignment (taper) as a possible cause, but not roller deflection (smile).

If all edges of all rolls are flared, we might suspect something having to do with the creation of an edge. If some edges are flared and others are not, we might suspect variations in edge creation or an interaction with variations in manufacturing. Clues are also provided when some grades perform worse than others.

A classic problem with plastic film and some other materials is a 'raised edge' due to slitting. In fact, this nomenclature may not be strictly correct because the edge is probably stretched into bagginess rather than thicker. In either case, the damage from the slitter is originally quite narrow, less than an inch, but can ruin the outer several inches if winding is pushed to large or tight rolls. Super attention to slitting blade sharpness and setup can be one remedy, especially if the edge gets better when a blade is changed. Note that there are many degrees of sharpness. A scalpel, for example, is much sharper than a conventional razor blade, which is sharper than a box-cutter.

Often it is not original sharpness that is inadequate. Rather, it is not changing blades out soon enough. Sometimes changing the slitter type (razor, score, shear or waterjet) can make a difference. Some materials, however, seem to give people trouble no matter how they attempt to cut the web.

Another classic problem is due to caliper variations that happen to line up with a slit edge. It is quite common for dies to have thicker lanes, especially on the edges. Newer dies have internal deckling mechanisms that can help reduce the edge bead. Paper makers have even more sophisticated tools for profiling or correcting the edges of their sheet. They avoid 'hard ends' at all costs, as this can degrade runnability. In tentered materials, the clips restrain the material from stretching on the edges causing thicker and/or baggier edges. Taking more edge trim is a costly solution. Gage bands can also be formed in intermediate locations. When a slitter is dropped into any gage band, the newfound freedom allows the edge to flair more than it would have had it not been cut.

Finally, stretchy materials have a different mechanism that can make the edges thicker. Netting is easiest to see because it has 'grid lines' on the material. While the grids are evenly spaced to begin with, they bunch up more on the edges than in the center as the material progresses down the machine. The edges are under uniaxial tension because there is nothing outboard to pull in the CD. The center, on the other hand, is under biaxial tension. The CD restraint in the center acts like a spreader. This behavior begins in the first few inches after leaving a roller into a higher tension zone. Materials prone to this differential necking include nonwovens, netting and textiles. In addition to pulling less tension, edge pull spreaders are sometimes effective in applying corrective CD tension at the edges.

David Roisum, Ph.D.

Consulting Technical Editor

920/725-7671

DRroisum@aol.com

www.roisum.com