Learn the causes of paper curl and methods to minimize it

Paper curl is critical for many grades of paper, especially copy machine paper, papers that get coated and printed on, and multi-layer paperboard grades.

What causes curl?

Curl occurs when in-plane layers within a sheet expand or contract at different rates. Different expansion/contraction rates are caused by differences in fiber orientation, difference in composition (fines, fillers, coatings), draws, drying conditions and differences in sheet moistures.

Fiber orientation difference is the most common source of interlayer curl followed by layer Coefficient of Moisture Expansion (CME) differences. The fiber alignment of the wire side is mostly influenced by the speed difference between the jet and the wire. The greater the speed difference is, the more the wire side fibers will be aligned in the machine direction. This can create a machine direction wire side curl (MDWS) as fibers will shrink more in the cross direction than the machine direction.

Reducing curl

There are several things that can be done to minimize paper curl. First, select a fiber supply with a low CME in multilayer grades.

Next, experiment with Rush – Drag in 10-20 fpm increments and measure curl across the machine. Try using Rush-Drag instead of Rush/Drag ratio for machines that run high speed ranges, as fiber alignment is dependent on relative speed difference no matter what speed the machine runs at.

The edges may be different than the middle of the web, so headbox edge flow or slice deflection adjustments may be necessary.

Drying of the web can impact curl. Typically, curl will be toward the side that was dried last. Some machines split the steam controls of the top and bottom dryers in the last dryer section to independently control top and bottom drying rates.

If the paper is passed through a film size press, curl can be affected by size pickup on each side.

For C1S coated grades, it is necessary to coat the opposite side with an equivalent amount of binder material, normally a starch solution, to control curl. On C2S coated grades, coat weight splits can be adjusted to control curl.

For more information on controlling paper curl, contact your Valmet representative.