Analysis on the causes of crease of offset paper

Analysis: the reasons for the crease of offset paper

when offset printing, the paper enters the drum and is subjected to pressure, forming bending or dendritic irregular concave convex folding traces on the paper surface, which is called paper wrinkle deformation, also known as paper arching or folding. The wrinkle after roller stamping is a common process failure, which not only brings trouble to the process, but also seriously affects the accuracy of overprint and destroys the complete structure and beauty of the whole situation. High grade products, if there are wrinkles on the printing sheet, are unqualified products

the crease of paper can be divided into large fold, small fold, straight fold and branch fold according to the distribution of states. The main reason is caused by the machine and the paper itself, especially when printing thinner paper, the paper wrinkle often occurs

I. Analysis of the causes of the paper itself

mainly comes from the appearance of the lotus leaf edge and the tight edge of the paper. The wrinkle caused by the lotus leaf edge is around the paper, mainly at the trailing tip, and the wrinkle caused by the tight edge is in the middle of the paper, generally not out of the edge

II. Uneven teeth gripping force of the imprint cylinder causes the paper to wrinkle

if the teeth gripping force of the imprint cylinder is uneven, the imprinting process of the paper will not be balanced, resulting in local forced stretching and wrinkling of the paper. This wrinkling is related to the tightness of the teeth. When the teeth of the imprint cylinder are tight on both sides and loose in the middle, or tight on both sides and loose on both sides, wrinkling at different positions will occur, That is, the position of wrinkles corresponds to the loose side of the teeth

in order to prevent the paper from wrinkle, it is absolutely necessary to have the flat and consistent teeth of the embossing cylinder. After calibration, the teeth pad of the cylinder is easily not allowed to change, otherwise it can be used in the shell of the charging pile and the shell of the charging gun. It is not only easy to wrinkle the paper, but also can reproduce the faults such as unable to bite the paper, paper breakage, and sometimes other faults can be caused

the above two reasons will also cause wrinkles at the same time, which should be analyzed according to the specific situation, and the following points are discussed in detail

1. uneven distribution of water content in paper. If the water content of the four sides is greater than that of the middle, that is, the ruffle phenomenon will occur, which will lead to the folding of the trailing tip part. In addition, the outlet of the extruder will fall and wrinkle as a whole this year. If the water content of the four sides is less than that of the middle, that is, the tight edge phenomenon of the paper will occur, which will lead to the folding of the middle part

2. The pressure of the paper pressing wheel of the paper feeding table is uneven. The paper pressing wheel is a device that controls the flatness, constant speed and positioning of paper conveying. Due to the unequal pressure adjustment of each wheel and the manual operation, the paper is affected by the axial force during the conveying process, resulting in the paper bulging in the middle during the conveying and still maintaining the state of bulging in the middle during the positioning of the front gauge. After stamping, the bulging cannot be eliminated, resulting in the folding of the trailing tip

3. the biting force of the paper teeth of the embossing cylinder is different. The paper biting force of the roller's paper biting teeth should be strong and strong, and each paper biting force should be equal. If the biting force on both sides of the teeth is insufficient, the paper will wrinkle the trailing tip. If the biting force on one side is insufficient, the corresponding trailing tip edge will wrinkle. If the biting force in the middle is insufficient, the trailing tip in the middle will wrinkle. If the middle is seriously insufficient (bite force), and the speed difference between the printing rollers is large, the trailing tip may be crushed and damaged

4. improper paper handover. During the short-range handover of the paper before being embossed, if the length of the paper edge of the bite changes (becomes smaller), it is easy to wrinkle the tip and even break the paper edge

5. the height of the dental pad varies greatly. When the paper is bitten, the edge of the paper is uneven (bite) but wavy, which will produce short wrinkle deformation close to the bite

6. The paper tapping marks are too deep and dense. Due to improper and improper paper tapping force, the paper tapping marks are deep and dense, so there are paper tapping wrinkles on the paper tapping creases

7. the corner of the paper is at the half tooth position of the paper teeth. After the paper is positioned, the paper corner is bitten half or less by the roller. As a result, the corners of the paper are often uneven and the edges of the paper are wrinkled

8. There is too thick accumulation on the impression cylinder. The ink on the surface of the imprinting cylinder is too thick, making the paper imprint uneven, resulting in short-distance wrinkle and deformation of the bite

9. paper fiber arrangement direction. When the paper enters the machine for imprinting, the fiber arrangement direction is straight strand, and the paper is easy to wrinkle the silk head

10. the water content of the paper is too large, and the fiber of the paper itself is fat. Under pressure, the fat fiber will narrow from fat, which is prone to wrinkle and deformation from the bite to the trailing twig

the above are the main reasons for paper folding. However, how to judge whether the discount wrinkle is caused by the paper itself or by mechanical reasons is also crucial, which determines the direction for us to solve and deal with quality accidents

generally speaking, the mechanical reasons are as follows:

1. The paper creases due to improper mechanical adjustment. However, the general position of creases caused by mechanical reasons is the same. The creases are in the middle or side, or facing outward, and the position is fixed

2. The frequency of wrinkles. The printing is wrinkled, some of them are Zhang Zhang; Sometimes, the development of "design and manufacture 1 integration" of cr929 aircraft fuselage and tail structure has been accelerated, and a few have not; Some are larger and some are smaller; If Zhang Zhang repeats, and the size and position of wrinkles are basically the same, most of them are mechanical wrinkles

3. turn the wrinkled paper around or turn it over. No matter how you turn around or turn it over, the paper will still not wrinkle, and it is generally mechanically wrinkled at the same part

4. if you still wrinkle the same part in the test with another similar paper, it is basically a mechanical problem

the wrinkle of the paper is ruled out as a mechanical reason, so it is the problem of the paper. The wrinkle of the paper itself is mainly due to the uneven paper itself, which should be analyzed and solved according to the specific situation