While decurling can't prevent the curl discussed last month, it can reduce any direction, type or source of existing curl. Decurling yields or resets the material. One way to do this is to raising the moisture (paper), or temperature (metals or polymers) of the web, which can be thought of as an annealing or stress-relaxation process. This can also be done mechanically by reverse bending over decurling bars, sometimes enhanced by elevated moisture or temperatures.

Decurling bars are common equipment on (liner) board presses, envelope machines, sheeters and other paper-converting equipment. Decurling bars are either small-diameter rollers (thick webs) or bars with a radiused edge (thin webs).

The web is reverse-bent over one or more bars in an operation similar to curling a strip of paper by drawing it over the edge of a scissors or desk. The only difference is that web decurlers usually have a pair of bars that are Z-wrapped rather than just one edge.

The factors that increase the decurling effect of bars are:

• Material properties of the grade
• Web caliper increase
• Radius of curvature (roller or bar edge) decrease
• Tension increase
• Angle of wrap increase
• Increase in number of bars—only those which counter the predominant curl direction

The strongest factor is radius of curvature, at least until it gets down to a nearly square edge, when tension and angle then take over. Decurlers are usually adjustable on the fly by varying the wrap angle. They are also adjustable during a setup by changing the bar's radius.

It is important that both bars be independently adjustable because the first bar tends to undo the efforts of the second as they are bent in opposite directions. It is vital that the web be routed such that the last bar counters the predominant direction of curl. Finally, it is best not to run the web over small-diameter rolls in the predominant direction of curl after the decurler.

Countering roll set curl is a little trickier because the magnitude of curl increases as the unwinding roll radius decreases. Thus, the operator must make several adjustments every roll. However, modern equipment relieves the operator of some of this by adjusting the decurler on the fly to match the ever-decreasing roll radius. The operator needs only to select the starting and ending values of wrap. Automatic adjustments may be more consistent, but may not be as good as a manual adjustment in the hands of a truly skilled operator.

Adjustments should be moved as necessary to counter curl until all of the factors have been pushed to their respective limits. Obviously, the radius of the bar is limited to a sharp edge and wrap angle is limited to less than 180 degrees. Tension may also be limited on the high end by web breaks or the design range of the equipment.

There are also product limitations. For example, sliding over a sharp bar can scratch surfaces or shear the outer surface away. Also, brittle products can crack as they are bent. An example is the latex coating on coated food-grade board. Sometimes you can raise the threshold of cracking damage by increasing moisture/temperature, but this is quite involved and reduces the abrasion resistance of the surface.

Decurlers are simple and effective in the hands of a skilled operator in some situations. However, they can be frustrating if the operator does not understand how to adjust them or if the curl changes rapidly. Finally, there are limits to how much curl can be countered and on which products. While decurlers can do amazing feats of straightening, they can't perform miracles. It is up to the product/process developer to deliver products with reasonable curl challenges.

Adjustments should be moved as necessary to counter curl until all of the factors have been pushed to their respective limits.

David Roisum, Ph.D.

Consulting Technical Editor

920/725-7671

DRroisum@aol.com

www.roisum.com