Q: How can I change the amount of spreading?

David Roisum, Ph.D. Consulting Technical Editor . 920/725-7671 DRroisum@aol.com -- Converting Magazine, 1/1/2001

A: It is not unusual for a single spreader to adequately cover everything a machine might run in its lifetime. Sometimes the amount of spreading power should be varied, such as grade changes that include large changes in modulus. While aggressive spreading can be used on flexible materials, this same power trained on stiff materials can result in wrinkling. Similarly, spreading should be adjusted when grade changes include large changes in the flatness of the material. Aggressive spreading can and should be used when the material is profoundly troughed in the MD, but not when there is little excess width. Note that troughs at even a slight angle with the MD indicate something is crooked. The specific cause of the crookedness should be identified and corrected rather than treating it with spreading. Finally, spreading should be adjusted when changing the number of slits with an after slitter spreader, especially when winding on the same axis. More slits would require more spreading power.

Insufficient spreading is easy to diagnose: the web is not as flat as a pane of glass. However, this ideal can only be reached when the web is in good condition to begin with. It is unrealistic to expect flatness if the web is profoundly troubled, with baggy patches and lanes. Excess spreading can also be diagnosed. If the web is slit into multiple lanes, the separation at the center will be excessive, while the edges may be minimal and possibly even overlapped. The center of an unslit web may look good, while the edges are not flat. Indeed, you may often see chevron-patterned wrinkles at the edges of the web as it approaches the overly ambitious spreader.

It is ironic that people will often induce wrinkles on the edges of their web in their zealous spreading to treat baggy edges. Unfortunately, baggy edges can seldom be treated or even touched by spreaders. It is a common fallacy and misunderstanding that spreading can treat what is essentially a web-forming issue. Only when the web is reformed, such as hot film or nonwovens inside a tenter oven, can spreading moderate bagginess.

If large changes in spreading are needed, you may need to change the type of spreader. If the needs and budget are minimal, web flattening rather than spreading should be considered (more about that in next month's Web Works column). Modest spreaders include banding (taping the ends of the idlers), concave rollers and compliant cover spreaders. Moderate spreaders include bent pipes, D-bars and bowed rollers.

More powerful spreaders, for separating numerous slit lanes, are the dual spreaders that include the dual bowed roller, tandem bowed roller and Pos-Z. The most powerful spreaders are expander rollers (bands or slats), edge pull rollers, and tenters. Note that there is currently no evidence for spreading with either spiral grooved (see August 2000 Web Works column, "When should I use spiral grooved rollers?") or herringbone rollers.

If modest changes in spreading are all that is needed, you may be able to adjust the spreading power of some spreaders. These include the bow magnitude on a D-bar or variable-bow roller, the wrap of the dual spreaders, the cam angle or wrap of the expander roller, the angle of the edge pull roller and the separation of the tenter tracks. Note: you cannot adjust the power of a conventional bowed roller by aiming or orienting the bow. Interestingly, the only spreader that is profileable (selectable spreading power across the width) is the D-bar, found on newsprint winders.

It is very important to avoid the more-is-better approach to spreading. Indeed, aggressive spreading when it is not needed can get you in as much trouble as insufficient spreading. Even if the need is high, however, attempting to aggressively spread-and failing-will be more likely to cause wrinkles than to fix them. The classic example here is the ubiquitous overbowed spreader that has lost traction on the edges.

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