Causes and treatment of paper wrinkles in offset printing
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Wrinkles in printed matter can be divided into large folds, small folds, straight folds, branch folds, and other types. These defects are mainly caused by mechanical factors and paper-related factors. Wrinkling is especially likely to occur when printing thin paper.
1. Uneven paper surface
If the moisture content of the air is lower than that of the paper, moisture at the edges of the paper will evaporate, resulting in a "tight edge" phenomenon (as shown in the figure). During printing, wrinkles will then appear in the middle of the paper.
When the moisture content of the air is higher than that of the paper, the paper absorbs moisture, causing excessive moisture at the edges while the center remains relatively dry. This leads to a "leaf edge" phenomenon, and wrinkles tend to appear at the paper edges.
If the moisture content on the front and back sides of the paper is uneven, the paper will curl (especially single-sided coated paper), which can also cause wrinkling during printing.
In addition, if the paper surface has severe ridges or if the paper is handled improperly, deep and dense paper marks may form. Wrinkles are then likely to occur along these creases or at knock marks during printing.
2. Uneven pressure on the platen pressure roller
If the pressure adjustment of each roller is uneven, the paper will be subjected to axial force during conveyance, causing it to bulge in the middle. This deformation may remain before printing and cannot be eliminated after passing through the impression cylinder, resulting in wrinkles at the paper edge or tip.
3. Uneven gripper movement
To prevent wrinkling during printing, the flatness of the impression cylinder must be properly adjusted. The calibrated gripper pad should not be changed arbitrarily. Otherwise, wrinkling may occur, and the paper may not be held securely, leading to paper breaks or other malfunctions.
If the gripping force on both sides is insufficient, wrinkles will appear at the paper tip. If the force on one side is insufficient, wrinkles will occur on the corresponding side. If the gripping force in the middle is insufficient, wrinkles will appear in the center. In severe cases, large speed differences between printing cylinders may cause the paper tip to be crushed or torn.
4. Improper paper transfer
During the paper transfer process before printing, if the paper edge length is too short, the leading edge is prone to wrinkling or even tearing.
5. Uneven printing pressure
If the blanket lining thickness is severely uneven or if there are excessive deposits on the impression cylinder, the paper will be printed unevenly. This can result in short-distance wrinkle deformation or creasing.
Determining the cause of wrinkles
It is critical to determine whether paper wrinkling is caused by mechanical factors or by the paper itself. This can generally be judged using the following methods:
Mechanical causes
Wrinkles caused by improper mechanical adjustment usually appear in fixed positions-such as the center, one side, or the outer edge. If the wrinkled sheet is turned over or replaced with similar paper for testing and the wrinkles still occur in the same position, the cause is mechanical.
Paper-related causes
If the wrinkle position is not fixed and appears irregularly or at regular intervals, and if turning the paper over or testing with similar paper results in wrinkles appearing in different positions, the cause is usually the paper itself.