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 gage 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 gage uniformity. Wound roll variations may develop where the corresponding web variations can not 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 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 gage-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 winderman.