Analysis and solution of the reasons for offset and wrinkling of offset paper
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In the offset printing, after the paper enters the drum and is under pressure, a curved or dendritic irregular folding mark is formed on the paper surface, which is called paper wrinkle deformation, which is also called paper arching or folding. The wrinkles that appear after the roller is embossed are a common process failure, which not only causes troubles in the process, but more importantly, seriously affects the accuracy of the overprint and destroys the complete structure and appearance of the whole situation. For the top grade products, if the wrinkles appear on the printed sheets, they are unqualified products.
Paper wrinkles are distributed according to the state and can be divided into large folds, small folds, straight folds and branch folds. The main reason is caused by the mechanical and paper itself, especially when printing thin paper, the wrinkling of the paper often occurs.
First, the analysis of the reasons of the paper itself
It mainly comes from the presence of ruffles and tight edges on the paper. The wrinkles of the paper are different. The wrinkles caused by the ruffles are around the paper, and the wrinkles are mainly in the middle of the paper. Generally not out.
Second, the unevenness of the impression cylinder's biting force causes the paper to wrinkle
If the impression of the impression cylinder is uneven, the embossing process of the paper can not be balanced, resulting in partial forcible stretching of the paper. This kind of wrinkle is related to the tightness of the teeth. When the impression cylinder is loose on both sides, it is loose or loose on both sides. When the middle is tight or one side is tight, the wrinkles at different positions will appear, that is, the position of the wrinkles corresponds to the side of the loose teeth.
In order to prevent the paper from wrinkling, the flatness of the impression cylinder is absolutely necessary. When the cylinder is calibrated, the roller teeth are not allowed to change easily. Otherwise, the paper may be wrinkled, and the paper may not be able to bite the paper or break the paper. Sometimes it can cause other failures.
The above two reasons will also cause wrinkles to appear at the same time, and should be analyzed according to the specific situation.
1. The water content of the paper is unevenly distributed. If the water content of the four sides is greater than the middle, that is, the phenomenon of "running edge" occurs, the wrinkles of the tip portion will be generated. If the water content of the four sides is smaller than the middle portion, the "tight side" phenomenon of the paper will cause wrinkles in the middle portion.
2. The paper feed roller pressure is uneven. The platen roller is a device that controls the flatness, constant speed, and positioning when conveying paper. Due to the unequal pressure adjustment of each wheel, the paper is affected by the axial force during the conveying process, causing the paper to bulge in the middle of the conveying, and still maintain the middle bulging state in the pre-specified position. Eliminated, causing wrinkles in the tip.
3. The nip of the impression cylinder teeth is not equal. The bite force of the roller bite should be strong and strong. The force of each bite should be equal. If the bite force of the bite is insufficient, the paper will produce wrinkles. If the bite of one side is insufficient, the corresponding drag Wrinkles are formed at the tip edge, and if the middle bite force is insufficient, the middle tip is wrinkled. If the middle is seriously insufficient (bite force) and the speed difference between the printing cylinders is large, the tip portion may be crushed.
4. Paper is not properly handed over. If the length of the edge of the paper is changed (smaller) in the approaching process before the paper is embossed, it may cause wrinkles or even chipping.
5. The height of the dental pads is quite different. When the paper is bitten, the edge of the paper is uneven (bite) but wavy, resulting in a short wrinkle deformation close to the bite portion.
6. The paper marks are too deep and dense. Because the paper is not used properly, the paper-cutting marks are deep and dense, so that the paper-wrapped creases are generated on the creases of the paper.
7. The corner of the paper is at the half tooth position of the bite. After the paper is positioned, the paper corner is bitten half or less by the roller. Therefore, the paper corner tends to be uneven and the paper is wrinkled at the edge of the paper.
8. There is too much deposit on the impression cylinder. The ink on the surface of the impression cylinder is too thick, so that the paper is imprinted unevenly, resulting in short-distance wrinkles of the bite.
9. The orientation of the paper fibers. When the paper enters the machine, the fiber is arranged in a straight line, and the paper is liable to cause the yarn to wrinkle.
10. The water content of the paper is too large, and the paper itself is fattened. Under pressure, the fattening fiber should be narrowed by the fertilizer and easily cause the wrinkle deformation of the bite to the tip of the branch.
The above is the main reason for paper wrinkling. However, how to judge whether it is due to the paper itself or the wrinkling caused by mechanical reasons is also critical, which determines the direction for us to solve the quality accident.
In general, the mechanical reasons are as follows:
1. Paper wrinkles due to improper mechanical adjustment. However, the wrinkles of mechanical reasons are generally unchanged, and the wrinkles are in the middle or on the side, or outward, and the position is fixed.
2. The frequency at which wrinkles appear. Printing is now wrinkled, and some are Zhang Zhang; sometimes there are several, some are not; some are larger, some are smaller; if the sheets are repeated, and the position of the wrinkles is basically the same, most of them are mechanical wrinkles .
3. Turn the wrinkled paper over or turn over the test. No matter how you turn your head, how to turn over, the paper is still not wrinkled, and in the same part, it is usually mechanically wrinkled.
4. Tested with another similar paper, if it still wrinkles in the same part, it is basically a mechanical problem.
The wrinkles of the paper exclude the mechanical reason, then the paper is faulty, and the paper itself is wrinkled, mainly because the paper itself is not flat, and should be analyzed and solved according to the specific situation.