Analysis of Reasons for Folding and Wrinkling of Offset Offset Sheetfed Paper

In offset printing, when the paper enters the cylinder under pressure, it forms curved or irregular irregular folding marks on the surface of paper, which is called paper crease deformation. It is also called paper arching or discounting. The wrinkles appearing after drum embossing is a common process failure, which not only causes troubles for the process, but more importantly affects the accuracy of overprinting seriously and destroys the complete structure and appearance of the entire situation. In the upper grade product, if there is a wrinkle pattern on the printed sheet, it is an unqualified product.

Paper from the wrinkle distribution by state, can be divided into big fold, small fold, straight fold and branch fold and other types. The main reason is due to the mechanical and paper itself, especially when printing thin paper, wrinkles often occur.

First, the paper itself causes analysis

Mainly from the paper appears flounces and tight edges, they are caused by the paper wrinkles are not the same, the wrinkles caused by the lotus leaf around the paper, mainly to the tip position, the tight edge caused by the fold in the middle of the paper, Generally do not leave.

Second, the imprinting roller uneven teeth cause paper crease

If the impression roller uneven bite force, the paper imprinting process can not be balanced, resulting in partial forced stretch paper wrinkles, which corrugated with the elastic teeth, when the pressure on both sides of the impression cylinder clenched the middle loose or both sides loose The wrinkles in different positions will appear when the middle is tight or when one side is tight, that is, the position of the wrinkles corresponds to the side where the teeth are loosened.

In order to prevent paper creases, the flatness of the impression cylinder teeth is absolutely necessary. After calibration, the teeth of the drum teeth can easily be changed. Otherwise, not only is paper crease prone to appear, but also paper that cannot be bitten, paper breaks, etc. , sometimes it can cause other failures.

The above two reasons will also lead to the occurrence of wrinkles, which should be analyzed according to specific circumstances. The following points are discussed in detail.

1. Paper moisture distribution is not uniform. If the water content on the four sides is greater than the middle, ie, the “foliage” phenomenon occurs, the wrinkles on the trailing parts will be generated. If the water content on the four sides is smaller than the middle part, the phenomenon of “tight edges” of the paper will produce the wrinkles in the middle part.

2. The pressure on the transfer roller is uneven. The platen roller is a device that controls the leveling, constant speed, and positioning of the paper during conveyance. 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 bulge state at the front specified position. After the printing, the bulging cannot be achieved. Eliminated, causing wrinkles at the tip area.

3. The biting force of the impression cylinder bite paper teeth is not equal. The biting paper force of the roller bite teeth should be firm and strong, and the force of each bite paper should be equal, if the teeth are on both sides

If the bite force is insufficient, the paper will have a trailing crease. If the bite force of one side is insufficient, the corresponding dragging edge will produce a wrinkle. If the bite force in the middle is insufficient, the middle dragging pin will produce a wrinkle. If the middle is severely lacking (bite) and the speed difference between the printing cylinders is large, the trailing portion may be crushed and broken.

4. Paper is improperly handed over. If the length of the paper edge is changed (decreased) during the proximity of the paper before it is imprinted, it is prone to wrinkle or even chipping.

5. There is a big difference in the level of the tooth pad. When the paper is bitten, the edge of the paper is uneven (bite) but wavy, resulting in short crease deformation closer to the nip portion.

6. Knock paper marks too deep. Because the paper cannot be punched with force, the paper traces are deep and dense. As a result, paper wrinkles are generated on the creases of the knocked paper.

7. The edge of the paper is in the half tooth position. After the paper is positioned, the paper corner is bitten by the roller dentition half or less. As a result, the corners of the paper are often uneven and paper creases occur at the edges of the paper.

8. Excessive thick deposits on the impression cylinder. The ink on the surface of the embossing cylinder is too thick, so that the paper is imprinted unevenly, resulting in short-distance wrinkle deformation.

9. Paper fibers are aligned. When the paper enters the machine imprinting, the direction of the fibers is straight, and the paper tends to cause wrinkles.

10. Paper moisture content is too large, the paper itself fiber fat, in the pressure, fat fertilizer fiber to be narrow from the fat easily prone to bite the mouth stretched to the tip of the wrinkles wrinkle deformation.

The above is the main reason for paper creases. However, how to judge whether it is due to the paper itself or because of mechanical reasons is also critical for discount wrinkling. It determines the direction for us to resolve quality accidents.

In general, mechanical reasons are as follows:

1. Wrinkle paper due to improper mechanical adjustment. However, the mechanical causes of the wrinkles are generally the same. The wrinkles are in the middle or on the edge, or face outward. The position is fixed.

2. The frequency of wrinkles. Some wrinkles appear on the prints, some are open; sometimes there are few, some are not; some are larger, some are smaller; if the sheets are repeated, and the size of the wrinkles is basically the same, most are mechanical wrinkles. .

3. The wrinkled paper will be turned or turned over. No matter how you turn your head or turn over, the paper will not wrinkle. In the same area, it is usually a mechanical wrinkle.

4. Test with another paper of the same type, if it still wrinkles in the same area, it is basically a mechanical problem.

The wrinkles of the paper exclude the cause of the machine. Then it is the fault of the paper. The wrinkles appear on the paper itself. The main reason is that the paper itself is uneven and should be analyzed and resolved according to the specific circumstances.