Curls aren’t always cute: answers to a few frequently asked questions about the frustrating problem of paper curl
Noted paper scientist Douglas Wahren once remarked about paper curl that “the lack of information is all-pervasive … every link in the production and logistics chain can make a difference.”
We would add: though many articles make it appear that solving curl problems should be easy, analysis and solutions are not at all simple. Each problem is usually a unique combination of paper properties and end-use conditions. As a papermaker, curl can be particularly frustrating because a sheet that appears to be flat “as manufactured” may curl quite badly in the customer’s converting operation.
The following responses to frequently asked questions are intended to provide some useful information about curl and curl problems.
Q WHY CAN IT BE DIFFICULT TO DEFINE AND SOLVE CURL PROBLEMS?
There are many variables and situations involved. There are different kinds of printers and printing methods. Other converting and end uses have their effect. Examples are one side wetting, one side heating, bending and the like.
Q HOW CAN CURL BE DEFINED IN A CONSISTENT MANNER?
Unfortunately, there is no universal consensus on how to define curl. Some mills define it by the axis of the curl cylinder (MD or CD) and whether it is toward the top or bottom of the sheet. Alternately, curl is sometimes defined by the trough the curvature forms (MD or CD). The method used should be indicated with the measurements or description. For example, in Figure 1, MD (axis) top side, or CD trough, top side would define the curl.
The resulting curl shown in Figure 1 is the result of the top side contracting more in the CD than the bottom side (or the bottom side expanding more).
Q WHAT CAUSES CURL DURING MANUFACTURING?
1. Forming and drainage of the sheet
2. Surface size and/or coating
4. End-use interaction with the above three items.
[FIGURE 1 OMITTED]
Q IS THERE A SIMPLE “CURL TEST” THAT CAN BE USED TO DETERMINE CURL PROPERTIES?
There is no standard TAPPI test for curl. Depending on the grade of paper or board, many mills usually have their own test. Common tests use high and/or low humidity cabinets to condition the sheet, after which curl is measured. Other mills might use an end-use test, such as a printer to test curl.
When setting up a new curl test for multi-use fine paper, an inexpensive and relatively easy option is a laser printer, such as the Brother 2040 or HP LaserJet 1018, but many others would also work. One thing to consider is how hard it is to clear jammed paper, which commonly occurs with paper testing.
You will want to set up your own procedures, but here is a starting point. After running five sheets to warm up the printer, run five sheets top-side printed and five sheets bottom-side printed (mark top side of each sheet) MD feed.
Repeat the test with paper samples fed CD. If 8.5-in. x 11-in. grain short is not available, cut the grain long paper to 8.5-in. x 8.5-in., marking the MD, and feed CD.
Measure curl on each set of sheets about 30 seconds after they are run. A template with various degrees of curl is useful. If curl is unbalanced between top and bottom, re-wet and free-dry (no restraints) some paper, and repeat the measurements. If curl becomes balanced, a drying curl problem is suggested.
Q IF THE CURL TEST INDICATES AN UNBALANCED STRUCTURE, WHAT ACTION CAN BE TAKEN?
The first step is to determine whether the unbalanced curl is from drying or from the wet end. If adjustment in basic sheet structure is indicated, the papermaker can make changes to one or more of the following: (1) jet to wire speed, (2) jet impingement angle and (3) positioning of the forming board. Depending on the wet-end configuration, changes to the Dandy roll or top-wire former might give some benefit. The overall approach is to make the top and the bottom of the sheet as similar as possible from the standpoint of fiber structure and fiber orientation. This is not an easy task on a conventional fourdrinier.
[FIGURE 2 OMITTED]
Q ARE THERE OTHER SHEET PROPERTIES THAT CAN BE UNBALANCED IN LASER PRINTING?
One property is the ability to diffuse moisture evenly. High concentration of fines on one side of the sheet, for example, could retard diffusion. Retarded diffusion from the non-printed side will increase the tendency for away-from-print curl (75gsm paper). Conversely, retarded diffusion on the printed side would tend to increase toward-print-curl. A difference in coating applied on one side vs. the other is a common case.
Q WHY DO WE SEE MD CURL TO ONE SIDE AND CD CURL TO THE OTHER SIDE OF A SHEET?
This is a normal effect of fiber orientation, when more fibers are oriented in the MD on one side compared with the other. If the side with more MD orientation is wet, then the sheet curls around the MD axis. If the sheet is wet on the other side, the sheet may curl around the CD axis. This is a common phenomenon on fourdrinier machines making board grades because there is little activity on the top of the sheet during the drainage process.
Q IF THE CURL IS BALANCED, WHAT DO WE DO?
Compare the base sheet without surface treatment, to the sheet with surface treatment. A much lower base sheet curl suggests the surface sizing or coating needs to be adjusted.
Q HOW CAN WE CORRECT CURL IN THE DRYER SECTION?
Papermakers know that the sheet curls away from the hotter dryer cans. So if there is MD top-side curl, it can be corrected by increasing the temperature on the bottom dryers in comparison to the top dryers in the last dryer section. Since the bottom dryers impinge on the top of the sheet, the top will become drier and the bottom of the sheet somewhat wetter. This differential will move the curl toward the bottom side of the sheet.
However, you must be careful with this approach because you have created uneven stresses in the top and bottom of the sheet. In the converting operation, curl may reappear. This is one cause of differences between “as-made curl” and “end-use curl.”
Q WHAT IS THE EFFECT OF COATING (CIS, C2S) AND SURFACE SIZE?
This is a complicated issue. Anytime you add a surface material to one side of the sheet and no (or different) material to the other, there is a potential curl problem. Remember the previous discussion about retarded diffusion and unequal stresses in the sheet. The curl tester can be a guide to adjustments, but this is always a balancing act between the base sheet and the surface applications. Each type of paper and each end-use needs to be investigated separately.
Q WHAT ABOUT TYPES OF FIBERS?
What is really of concern is the coefficient of moisture expansivity or CME (coefficient of moisture expansion)–that is, how reactive the fibers are to changes in humidity. Generally, softwood fibers have a higher CME than hardwood fibers, but not always. CME can be measured for a given fiber, but an integrated mill has very little choice over fiber supply.
Q IS THERE AN EFFECT OF REFINING? FILLER?
Refining opens up the fibers and allows them to swell more easily, so reducing refining should be better in that overall CME would be less. Fillers do not react to moisture, so more filler would lower CME, therefore less curl. However, you would need a high level of filler to make a significant difference.
Q WHAT CAUSES DIAGONAL CURL?
Diagonal curl is caused by fiber alignment that is not in the machine direction (positive or negative TSO angles). To illustrate this effect, sheets of paper are cut at an angle and glued together. Diagonal or twist curl is the result (Figure 2). Diagonal curl is often a problem in converting, even if the overall level is not greater than conventional MD curl.
Q WHAT ARE IMPORTANT VARIABLES AFFECTING MULTI-USE PAPER CURL?
* Final moisture content: for 60-75gsm paper with low (below 4.5%) reel moisture, expect curl toward the printed side. For paper with high reel moisture, expect curl away from the printed side.
* Copy machine or printer design (how paper is bent in fusers, paper paths).
* Sheet structure, i.e. differences between top and bottom layers (fiber orientation, coatings, filler and fines).
Q WHAT EFFECT DOES PREPRINTING OFFSET HAVE ON LASER PRINTING?
When paper is printed by offset, moisture is added by the fountain solution. This re-wetting as well as exposure to high RH during processing can increase sheet moisture. If the moisture is 5.5% or higher, laser printing curl will tend to be away from the printed side.
Q HOW CAN THE CURL IN OFFSET PRINTING BE EXPLAINED?
Initial wetting curls the sheet away from the printed side, because moisture expands that surface. On drying, the wet surface shrinks to be shorter than its original length because internal strains are released. These have been put in the sheet by drying.
1. C. Green, J. Atkins, “Solving Curl Problems,” Solutions Magazine p.40-43 Nov 2001.
2. A. Hellstrom, “The two sides of fiber orientation,” 2005 TAPPI Papermakers Conference.
Charles Green and Jim Atkins are consultants to the paper industry. Contact them at email@example.com and firstname.lastname@example.org.
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