The gold standard of slitting design is spread > slit > spread > wind. In this ideal, we use two different spreaders. The first is called the pre-slitter spreader, and the second the post-slitter spreader. Each has a separate and vital function for optimum converting health. The venerable paper industry has known this for decades, but the message has been slow to reach converters. Let me try to explain why spreading should (almost always) be paired with slitting.

The pre-slitter spreader has two functions. The first is to make the web dead flat going through the slitter section. Even the slightest of puckers, barely visible to the eye, will be enough to destroy tight width tolerances of the slit product. The reason is simple—the width of the web with a pucker is slightly wider than the distance between the slitter blades. This is not a problem with a constant pucker size, but this is also not usually the case.

The second function of a pre-slitter spreader is to provide a small amount of CD tension. This can assist the other stresses at the slitter blade(s) to boost cut quality. Beware, however, that there is a near epidemic of oversizing of almost all spreaders, resulting in web instability, wrinkles and loss of spreading.

The post-slitter spreader has just one function—to provide a tiny separation between the slit lanes. This keeps the individual strands from tangling each other. Some of this separation is the result of a possible slight tension increase at the slitting span with respect to the one just upstream.

Just as good slitting requires a rock-steady web in that area, so too does good edge trimming require a rock-steady trim. This can be done several ways. One is to use a trim pan at the slitter instead of a trim opening some distance away. If the opening is away from the cut point, the trim can be stabilized by running it over the next roller. Some pull their trim at a ZD angle instead of a CD angle, which is more benign to cut quality, but the above principles apply there as well.

We now see the reasons for the two spreaders. We also see these reasons are different. Thus, if we equip our slitters with two spreaders, they would be sized differently. Using a bowed roller on paper, for example, the pre-slitter bow may be about 0.125% and the post-slitter bow about 0.5%. The after-slitter bow has more (separation) work to do and the individual lanes are easier to move than increasing the width of an unslit web; thus two reasons for the difference in size.

How can I adjust tension to improve winding?

Every winder is equipped with an adjustable tension. Most winders can taper this tension automatically as the roll diameter builds. Some winders can even program a curve. Whether we have a simple setpoint, linear taper or arbitrary curve, we must choose how to set the tension. The best setting is determined the same way here as with any other adjustment: economics. Specifically, we choose a tension which avoids the most, but not necessarily all, defects.

We can group defects into the following categories: low tension, high tension, taper tension or independent of tension. Examples of low-tension defects are out-of-round rolls, some rough roll edges and loose cores. Examples of high-tension defects would include blocking, gage bands and some crushed cores. Some defects are independent of tension, such as wrong diameter rolls due to an operator setup error. A couple of defects, namely subsets of starring and tension, are very sensitive to a lack of taper.

If you had customer complaints of out-of-round rolls, you would look into roll handling practices as well as tighten up the tension. If you had loose cores, you would also consider increasing the tension. You also must check to make sure you don't have wet cores. Increasing tension may not be strong enough to counter shrinking cores.

If, on the other hand, you had gage bands you would look into manufacturing. Obviously, you would also decrease winding tension as much as possible. However, if the gage bands are severe enough you will not eliminate the problem solely by tension changes.

We use the same approach for the taper-sensitive defects—starring and telescoping. We make sure that a particular case is amenable to tension treatment. For example, starring on one side of a wound roll is not treatable by tension because it is probably caused by a gage variation. Gage variations may be unintentional, due to profile variations in manufacturing, or intentional such as printed patterns. If your case is amenable to tension, the appropriate amount of taper is maximum. You would start the roll at maximum, just short of breaking something. You would finish the roll at minimum, again just short of breaking something.

This tension strategy outlined here makes sense for several reasons. First, it is based solidly on economics rather than theory. Second, it adapts to any particular situation rather than conforming to a “one size fits all” guideline that probably fits very few very well. Third, anyone can get the answers if they are willing to spend the effort and ruin a few rolls finding the limits.

Notice that we have not promised complete relief from defects. Expecting any single knob to make the hurt go away completely is often unrealistic. Product/process design always offers alternative solutions, and they should also be considered. Sometimes design is so much more powerful that continuing to think about tension is distracting. Even so, tension is always the right place to start.

How do caliper variations affect winding?

Caliper is never truly level across the width of a web. However tiny these variations might be, they may still be large enough to cause the winder to complain. In fact, the winder is often the most fussy customer for gauge uniformity. If you can get the caliper-varying web through the winder without defect, the purchaser of your web may have no further cause for complaint in this regard.

Listen closely to the language the customer uses to complain. They do not usually talk about poor web caliper uniformity. Instead, they talk about poor roll quality. Not only is the winder the most fussy customer, the winder may also be the most sensitive measure for gauge uniformity. Wound roll variations may develop where the corresponding web variations cannot be picked up by conventional lab tests or online scanners. This situation is difficult for manufacturers because they can't tell for sure whether the web is good enough until after the roll is wound. Sometimes, such as with some films, it takes days for the air to escape, leaving ridges to bloom after the roll has been shipped to the customer.

First, which wound roll defects are commonly the result of winding a web with caliper variations?

• Baggy lanes
• Blocking
• Corrugations
• Crushed core
• Out of round
• Ridges
• Starring
• Telescoping
• Wrinkling on a roll

Second, how can you tell if caliper variations have an influence on the severity of a winding defect? Some of these, such as corrugations and ridges, are almost certainly the result of caliper profile problems. Others are often exaggerated by poor profile. One way to differentiate is if one of these defects favors a certain CD position. For instance, if a baggy lane coincides with a ridge in the wound roll, you can be pretty sure it was made worse by profile.

If the defect position moves around with time, you can be almost certain that profile had a role, though not necessarily caliper profile.

Caliper variations cause the window of defect-free operating tensions to be narrowed. However, tension sensitivity by itself only indicates winding is involved, not necessarily the winding of gauge-varying web.

Third, what do you do when you have one of these “winding” defects? The first thing to do is to reduce the tightness of the wind by lowering web tension and especially nip load. Tightness should be reduced to the point where it becomes obvious that going any farther would be counterproductive because loose defects would then outnumber the tight defects. After that, you may get relief by replacing the winder with a more tolerant arrangement, such as duplex with individual stations or duplex with differential shafts.

Ultimately, however, you will get the most benefit from eliminating the root cause, which is caliper variations. You must identify which specific manufacturing or converting element is responsible for a particular feature (there may be more than one source). Then you must identify what variation on the element is responsible.

For example, on an extruder it may be temperature or gap variations that cause a particular problem. Finally, you must change the design or maintenance of the offending element so that it is more uniform across the width than anything you've ever done or ever seen to date. If you want to know if it is good enough, don't ask extrusion, coating or QA. Ask the winder operator.