Some Discussions on Several Problems in the Corner of Printing
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1. What is a horn?
The horns are also called the tails, which means that the printed sheets are slightly extended to the outside of each other to form a trapezoid-like shape, which causes the overprinting to be inaccurate or the image size to be deformed. This is a common printing failure in offset printing, especially in large format offset printing. The problem of having a corner of a print in the printing process is always a headache.
2. What are the characteristics of the horns on the printed matter?
1. Obvious when printing large format prints,
2. Serious when printing thin paper,
3. The problem of the corners appearing during the printing process is always gradually enlarged from the paper mouth to the graphic part of the paper in the printing direction.
4. If there is a problem with the corners on a multi-color press, the color gradually shrinks from the start of one color to the latter. Cause overprinting is not allowed.
3. What are the factors related to the occurrence of the horns?
It is mainly the deformation of the paper, which is caused by the water absorbing deformation of the paper and the deformation of the paper by the tensile compression.
This is followed by the structure of the press and adjustments during use.
4. How is the paper deformed during the printing process? Why is it that the deformation of the part is more serious than the part of the mouth?
1. We know that in offset printing, it is necessary to use water to complete the entire printing process. During the imprinting process, water passes through the printing blanket to the surface of the paper (in fact, we only want water in the printing to prevent the ink from being blank in the printing plate. Part of the adhesion and expansion is sufficient, but this transfer to the paper surface is something we can't avoid anyway).
During the printing process, the paper is dragged under the nip of the impression cylinder, enters the embossing area, begins to accept the embossing, and also begins to contact the moisture transmitted from the printing plate to the blanket, because the main component of the paper contains a large amount of hydrophilic The group's cellulose, and the surface of the paper is rich in capillary pores, water can be instantly adsorbed on the surface of the paper and spread around, while the pressure of the drum penetrates into the inside of the paper. However, this is a process in which the paper moves under high pressure by pressure, so the moisture that initially comes into contact with the paper does not completely penetrate into the inside of the paper, but accumulates in the opposite direction of the running direction toward the tail of the paper under the squeeze of the drum and the paper. Simply put, the more water is deposited on the surface of the paper, the more water will penetrate into the inside of the paper. Similarly, the moisture that has penetrated into the paper at the mouth of the paper does not immediately combine with the fiber. Some of the water also accumulates under the pressure to the tail portion of the paper, thereby causing the paper to absorb moisture. There is a tendency to gradually increase from the paper to the end of the paper, and we have come to the conclusion that the slight amount of water absorbed by the paper is higher than that of the paper.
Tests have shown that the fibers in the paper will expand rapidly after absorbing moisture, and will deform and expand by 20%-30% after absorbing enough water. It is easy to understand that there is a lot of water absorption at the end of the paper, and of course the deformation will be relatively large.
2. Pressure deformation is the deformation of the linear size and area caused by the paper being squeezed in the drum and the fibers being misaligned. In the printing process, due to the pressing action of the roller, the paper is fan-shaped from the mouth of the mouth, and the closer to the two sides of the tip, the greater the overprint error.
The paper is subjected to a positive pressure in the surface of the drum, and is also subjected to a pulling force opposite to the running direction along the tangent of the drum. The tensile deformation of the paper caused by the negative tensile force and the compression deformation of the paper due to the positive pressure of the paper Together, it accumulates on the paper at the end of the paper. The consequence of this is that the deformation at the end of the paper is of course much larger than the mouth.
Through the above analysis, we can get a conclusion: the paper is deformed by water absorption on the drum, and at the same time, due to the deformation caused by the force between the rollers, any deformation will have a tendency to become larger at the tail of the paper. When the amount reaches a certain value, there will be a corner that we don't want to see.
5. In the printing of multi-color machines, we will find that when the corners appear, the first color is always the most serious, that is to say, the first color is the largest, and then the color is reduced from one color to the back. Of course, it will not be accurate. Then why is this?
We know that the fibers in the paper will swell and deform after being absorbed. When the relatively dry paper is printed in the first color group, the amount of water absorbed is relatively large, so the water content of the paper increases greatly. When the printing of the second color is waited, the water content of the paper has been greatly improved, and the amount of water that the printing unit can provide does not change significantly. Although the water content of the paper is further increased, the relative increase is There has been no increase in the relative increase in the first color printing. That is to say, when the paper is printed in the second color, the dimensional change is smaller than the first color. Of course, the relative deformation of the paper after the third color is gradually reduced.
During the process of embossing, the deformation of the paper is similar to the process of water absorbing deformation. Paper deformation can be divided into elastic deformation and plastic deformation. When the first color is printed, the paper will be deformed. At this time, the plastic deformation is unrecoverable, and the elastic deformation of the paper is not enough time to recover due to the high-speed printing, so that the ink can only be printed in the second color. Transferred to the already deformed paper, and another plastic deformation occurs on the paper, but the amount of deformation at this time has been reduced due to the deformation of the first color. By the same token, the third color and the subsequent color group are the same, that is, the amount of deformation of the paper is gradually reduced.
6. What are the corner problems caused by the adjustment of the printing machine itself or the machine?
The manufacturing, construction and adjustment of the operation of the machine itself can also cause corner failures. The PZ4880-01A printing machine originally produced in China has much more horns. The reason is that the pad is sinking, the bite force is not uniform, and the inconsistency is easy to cause the horns in the printing process. The paper is subjected to the pulling force of the rolling force backward during the imprinting process. If there is not enough force to maintain the stability of the paper, the paper will easily slide relative to the impression cylinder during the imprinting, and the slippage It will also be accumulated and enlarged at the end of the paper, resulting in a horn.
7. For paper deformation and machine adjustment, is there any way to reduce the corner failure that has already occurred?
First of all, when we choose paper, we should choose the corresponding paper according to the requirements of printing. If the printing requires precise overprinting, then we should choose paper with low straw content, high surface sizing and high mechanical strength when selecting paper. Secondly, the paper should be conditioned before printing (the conditional unit is best to use the forced humidity control method to ensure the paper has a relatively high water content), so that the water content of the paper is stable, and the stability is improved during the printing process.
For the failure of the corner caused by the machine itself, we can adjust from the following aspects.
1. The teeth of the first color group are normally bitten, and the teeth at both ends of the 2-4 group of impression cylinders have a slightly smaller tooth force.
2. Place the pads of the 2-4 sets of blankets less (about 0.05mm or so). This can cause some convergence of the paper that has already had a corner, but this requires the operator to have a wealth of printing experience to master the scale.
3. In addition, the embossing roller can be raised at both ends of the embossing cylinder, so that the upper portion can be first contacted with the pressure, so that the paper converges in the two corners and does not spread.
8. What are the solutions used in the design of modern printing presses to solve the problem of corners?
China's modern printing press has been able to solve the problem of cornering in the printing process, such as:
1. The high-strength alloy steel structure of the fangs ensures smooth paper and avoids ghosting and corners.
2. The rear card is segmented to solve the "corner" phenomenon that occurs in printing.
3. A plurality of support screws are used on the support bar of the impression cylinder pad to prevent the corner failure caused by the sinking of the pad.
Several foreign printers also have different solutions. Such as:
1. In the process of machine design(the embossing roller can be raised at both ends of the embossing cylinder, so that the upper portion can be first contacted with the pressure, so that the paper converges in the two corners and does not spread.)
2. The shaft of the roller is designed to be bendable, and there is a pre-bending action before the paper is bitten, which can be adjusted as needed, so that the corner failure can be avoided.