In the offset printing process, the curved or dendritic irregular marks formed on the surface of the paper after being pressed are called wrinkling or wrinkling. Wrinkling of paper is a very troublesome fault, which not only brings trouble to production and affects the accuracy of overprinting, but also wrinkled printed sheets will become unqualified products. The author discusses the causes and solutions of paper wrinkling from two aspects of machinery and paper itself, for your reference.
Mechanical cause
1. The pressure of the pressure roller of the feeding table is uneven
The paper is subjected to the axial force during the conveying process, causing the paper to bulge in the middle when the paper is in the previous position, forming wrinkles after stamping.
2. The gripping force of the gripping cylinder of the impression cylinder varies
If the gripping force on both sides of the platen of the impression cylinder is insufficient, the trailing edge of the paper is wrinkled; if the gripping force on one side is insufficient, the corresponding trailing edge is wrinkled; if the gripping force in the middle is insufficient, the middle of the paper Wrinkle.
3. Improper handover of paper
In the process of handing over the paper before imprinting, if the fit is improper, causing the length of the paper at the edge of the grip to change, the trailing part is likely to wrinkle.
4. The unevenness of the dental pad
When the paper is gripped, if the dental pad is not flat, the edge of the paper will be wavy, and the gripper will be partially wrinkled.
5. The position of the paper edge and gripper is incorrect
After the paper is positioned, if the paper is gripped by the teeth a little, the edges of the paper are uneven and wrinkled.
6. The ink sticking on the surface of the impression cylinder is too thick
The thickness of the ink adhered on the surface of the impression cylinder is too thick, which makes the paper uneven when it is imprinted, and also produces local short-distance wrinkle deformation.
Paper cause
1. The trace of paper knocking is too deep and too dense
Due to improper knocking on the paper, the traces of knocking paper are too deep and too dense, and wrinkles are easily generated in the folds of knocking paper during printing.
2. Paper thread direction
When the paper enters the machine for imprinting, if it is a straight thread, it is easy to cause wrinkles.
3. Paper moisture content is too large
If the moisture content of the paper is too large, the fiber of the paper itself expands. During the imprinting process, the expanded fiber narrows from hypertrophy, and it is easy to produce branch-like wrinkles that extend from the mouth to the trailing direction.
4. The influence of environmental temperature and humidity
The temperature and humidity of the environment change, resulting in uneven water content of the paper. If the water content in the surrounding is greater than the middle, the lotus leaf edge is generated, and the paper is easily wrinkled during printing.
Method of exclusion
After the paper wrinkle failure occurs, it is necessary to take targeted measures to ensure printing efficiency and quality. For the wrinkles caused by the above reasons, the following measures can be taken to overcome them.
First, the printed sheets are appropriately knocked to improve the printability of the paper, so as to eliminate the wrinkling of the next color printing.
Second, adjust the roller gripper to make gripping force even, so that the printed sheet is in the proper position for gripping, and eliminate the wrinkle failure caused by the misalignment of the paper.
Third, when imprinting, make sure that the paper is uniform and tightly attached to the impression cylinder.
In addition to mechanical reasons and the paper itself, improper use of the liner can also cause paper wrinkling. If the liner is too soft, convex or false, or the liner is not tight, it will cause the paper to wrinkle. Therefore, attention should also be paid to the lining process to prevent wrinkling caused by loose lining.
There are many factors for the wrinkling of printed paper. Once wrinkling of paper is found, it should be carefully analyzed according to the specific situation, looking for the cause, and taking the right medicine to eliminate the fault in time.
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