Causes and troubleshooting of paper wrinkling during offset printing
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During the offset printing process, the curved or dendritic irregular relief marks formed on the surface of the paper after pressing are called wrinkles or wrinkles. Wrinkling of paper is a very difficult fault, which not only causes trouble for production, but also affects the accuracy of overprinting, and the wrinkled printed sheet can become a defective product. The author discusses the causes and solutions of paper wrinkling from both the mechanical and paper aspects for your reference.
Mechanical cause
1. The pressure of the paper feed roller is uneven
The paper is subjected to an axial force during conveyance, causing the paper to bulge in the middle portion when the paper is in the front position, and wrinkles are formed after the embossing.
2. The pressure of the impression cylinder is different.
If the force on both sides of the squeegee cylinder is insufficient, the wrinkles at the tip of the paper are wrinkled; if the squeezing force on one side is insufficient, the corresponding trailing edge is wrinkled; if the middle force is insufficient, the middle portion of the paper Wrinkle.
3. Improper paper transfer
In the process of paper transfer before imprinting, if the mismatching is made, the length of the paper on the edge of the mouth is changed, and the tip portion is easy to wrinkle.
4. The pad is uneven
When the paper is caught, if the tooth pad is not flat, the paper edge will form a wave shape, and the mouth portion will be partially wrinkled.
5. The position of the paper edge and the tooth decay is incorrect.
After the paper is positioned, if the paper is caught by a tooth, the paper edge is uneven and wrinkles occur.
6. The ink adhered to the surface of the impression cylinder is too thick
The ink adhered to the surface of the impression cylinder is too thick to make the paper uneven when it is imprinted, and local short-distance wrinkles are also generated.
Paper cause
1. The paper marks are too deep and too dense
Due to the improper use of the paper, the paper-cutting marks are too deep and too dense, and wrinkles are likely to occur at the creases of the paper-cutting during printing.
2. Paper thread direction
When the paper enters the machine, if it is a straight thread, it is easy to cause the yarn to wrinkle.
3. Paper moisture content is too large
The water content of the paper is too large, and the fiber of the paper itself expands. During the imprinting process, the expanded fiber is narrowed by the hypertrophy, and the branch-like wrinkles in the direction of the tip of the mouth are easily generated.
4. Influence of ambient temperature and humidity
The temperature and humidity of the environment change, resulting in uneven water content of the paper. If the water content around the area is greater than the middle, the ruffles are generated, and the paper is easy to wrinkle during printing.
Method of exclusion
After the paper wrinkle failure occurs, measures should be taken in a targeted manner to ensure printing efficiency and quality. For the wrinkles caused by the above reasons, the following measures can be taken to overcome.
First, the sheet is appropriately subjected to a knocking treatment to improve the printing suitability of the paper, thereby achieving the purpose of eliminating the wrinkling of the next color printing.
Second, adjust the roller teeth to make the force uniform, so that the printed sheet is in the proper position of the tooth decay, and eliminate the wrinkle failure caused by the paper transfer misalignment.
Third, when embossing, ensure that the paper is flat and close to the impression cylinder.
In addition to mechanical reasons and the reasons for the paper itself, improper use of the liner can cause wrinkling of the paper. If the lining is too soft, there is a convex hull, a false real thing, or the lining is not tight, it will cause the paper to wrinkle. Therefore, attention should also be paid to the lining process to prevent wrinkles caused by the looseness of the lining.
There are many factors in the wrinkling of printed paper. Once the paper is found to be wrinkled, it should be carefully analyzed according to the specific conditions, find the cause, and prescribe the right medicine to eliminate the fault in time.