Curl can be defined as either a curled condition of paper or a tendency of paper to adopt a non-flat shape when exposed to changes in humidity or temperature during normal use. Curl problems can cause jamming in various kinds of converting and printing equipment, interfere with the stacking of paper and board products, and make the appearance of the product unacceptable.
There are many factors that affect the curl properties of paper. Many such problems can be traced back to non-uniformities in either the structure of processing of the paper. Structural differences may be inherent in the way the paper was produced, as in the case of Fourdrinier sheets that tend to have fine material washed out of the wire side of the sheet.
Some practical measures to correct curl problems include (a) adjusting the relative temperatures of top and bottom dryer cans (see later), (b) reducing the refining level, (c) increasing the short-fiber content of the sheet to reduce the degree of fiber alignment, and (d) taking a variety of steps to minimize the structural or chemical nature of the two sides of the paper.
Diagonal curl, a problem that often has the most serious consequences, it often due to a tendency of fibers to be aligned at a skewed angle relative to the machine direction. Such problems can arise due to excessive flow through a headbox or a poorly adjusted headbox and jet impingement. A related approach is to minimize jet-to-wire speed difference and produce a relatively "square" sheet having a minimum alignment of fibers in general. The down-side is that such a sheet may be more streaky or mottled, compared to a sheet formed with a moderate level of rush or drag.
Curl problems tend to be amplified as the inter-fiber bonding is increased. By contrast, a weakly bonded sheet, composed of relatively straight fibers with small bonded areas between them, is expected to show much reduced curl tendencies. Based on these principles there may be a slight advantage to decreasing the level of refining and compensating for strength effects by increasing the dosage or effectiveness of dry-strength additives such as cationic starch.
Adjustments of the temperatures of top and bottom dryer cans can be used to control the final curl condition of paper in the machine direction. Paper tends to curl most in the direction of the side or surface from which moisture migrates last.
Curl problems during use can be expected if the moisture content of the paper does not correspond to the equilibrium humidity condition that will be present when the paper is converted or printed. Since many of these operations have poor or non-existent air conditioning, and since paper is shipped to different locations, there is no way to completely prevent such issues from occurring. On the other hand, the moisture content of paper used in copy machines and laser printers should be about 4.2 to 4.8 (if basis weight is 75 gsm or less). Higher moisture contents can result in curl away from the printed side. In offset printing, excessive one side wetting can produce curl away from the printed side initially and toward the print after drying.
Green, C., "Solving Curl Problems: The Basics," Solutions! 2001 (11): 40 (2001).
More information is available at Charles Green's website: http://www.PaperCurl.com
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (email@example.com). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.