Some Discussions on Several Issues about Angle Rejection in Printing

1. What is a dip?
The flick angle is also called loose tail, which refers to the two corners of the printed sheet extending to the outside of each side, forming a trapezoid-like shape, resulting in inaccurate overprint or deformation of the graphic size. This is a relatively common printing failure in offset printing, especially in large-format offset printing. In the printing process, the problem of flicking of the printed matter is always a headache.

2. What are the characteristics of the angle swing on the printed matter?
1. Obviously when printing large-format prints,
2. Serious when printing thin paper,
3. The flicker problem that occurs during the printing process is always more and more serious along the printing direction from the paper gripper to the paper's drag part.
4. On the multi-color printing machine, if the problem of flickering occurs, it gradually decreases from the beginning of one color to the back color. Causes overprinting to be inaccurate.

3. What factors are involved in the generation of flick angle?
It is mainly the deformation of the paper, which is caused by the water absorption deformation of the paper and the deformation of the paper under tension and compression.
Secondly, it is due to the structure of the printing press and the adjustment during use.
4. How did the paper deform during the printing process? Why is the deformation of the trailing part more serious than the mouth part?
1. We know that in offset printing, it is necessary to use water to complete the entire printing process. During the imprinting process, water is transferred to the surface of the paper through the printing plate blanket (in fact, we only hope that water can prevent the ink from printing on the plate blank during printing) Part of the adhesion and expansion is sufficient, but this transfer process to the paper surface can not be avoided anyway).
During the printing process, the paper is dragged by the teeth of the impression cylinder, and enters the impression area. At the same time, it begins to receive the impression and also contacts the moisture transmitted from the printing plate to the blanket. Because the main component of the paper is a large amount of hydrophilic The group's cellulose, and the surface of the paper is rich in capillary pores, water can be instantly absorbed on the surface of the paper and spread to the surrounding, while using the pressure of the roller to penetrate into the interior of the paper. But this is a process of high-speed movement of the paper under pressure, so the moisture that initially contacts the paper does not completely penetrate into the paper, but accumulates in the opposite direction of the paper to the tail of the paper under the pressure of the roller and the paper. To put it simply, the more water accumulated on the surface of the paper, the more water will penetrate into the paper. In the same way, moisture that has penetrated the paper at the mouth of the paper will not immediately be combined with the fiber. Some of this water will also accumulate under the pressure to the tail of the paper, which will cause the paper to absorb moisture. It must have a gradual increasing trend from the beginning to the end of the paper, from which we have come to a conclusion that the water absorption of the trailing part of the paper is higher than the mouth part 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 tail 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. During the printing process, due to the squeezing effect of the cylinder, the paper extends outward from the gripping portion in a fan-like manner. The closer to the sides of the trailing tip, the greater the overprint error.
In addition to the positive pressure on the surface of the roller, the paper will also be pulled along the tangent of the roller in the opposite direction of the running direction. The negative tensile force causes the paper to deform and the paper compression due to the positive pressure on the paper , Pile up on the paper towards the end of the paper, and the consequence is that the deformation at the end of the paper will of course be much larger than the mouth.

We obtained a conclusion through the above analysis: the paper is deformed by the water absorption on the roller, and at the same time due to the deformation caused by the force between the rollers, no matter which kind of deformation will have a tendency to become larger at the end of the paper, when this deformation When the amount reaches a certain value, a flick angle that we do not want to see will appear.

Fifth, in the printing of multi-color machines, we will find that the first color is always the most serious when flickering occurs, that is to say, the first color has the largest imprint, and then decreases from one color to the next color in turn. Such a printed matter Of course, overprinting will not be accurate. So why?
We know that the fibers in the paper will expand and deform after absorbing water. When the relatively dry paper is printed in the first color group, the amount of water absorbed is relatively large, so the moisture content of the paper will increase greatly. By the time of the second color printing, the moisture content of the paper has increased a lot, and the water content provided by the printing unit has not changed significantly. Although the moisture content of the paper will be further increased, this relative increase is There is no relative increase in the first color printing. That is to say, when the paper is printed in the second color, the amount of dimensional change is smaller than that of the first color. Of course, the relative deformation of the paper after the third color will gradually become smaller.
During the embossing process of paper, the deformation of force is also similar to that of water absorption. The deformation of paper can be divided into elastic deformation and plastic deformation. The paper will deform during the first color printing, and the plastic deformation cannot be recovered at this time, and the elastic deformation of the paper does not have enough time to recover due to high-speed printing, so that during the second color printing, the ink can only Transferred to the deformed paper, at the same time another plastic deformation will occur on the paper, but the amount of deformation at this time has been reduced due to the deformation of the first color in front. For the same reason, the third color and subsequent color groups are the same, that is, the amount of deformation of the paper is gradually reduced.

6. What are the swing angle problems caused by the printing machine itself or during the use of the machine?
The manufacturing, structure and adjustment of the operation of the printing machine can also cause the angle swing failure. The original PZ4880-01A printing machine produced in China has a much more obvious flick angle. The reason is that the tooth pad sinks, the bite force is uneven, and the inconsistency is easy to cause the flick angle in the printing process. The paper is pulled back by the calendering force during the stamping process. If there is not enough force to maintain the stability of the paper, then the paper will easily slip on the impression cylinder during stamping, and this slip It will also be accumulated and enlarged at the end of the paper, resulting in a flick angle.

7. In terms of paper deformation and machine adjustment, is there any way to reduce the flutter angle failure that has occurred?
First of all, when we choose paper, we must choose the corresponding paper according to the requirements of printing. If printing requires accurate registration, then we must choose paper with low content of straw pulp, high surface sizing degree and high mechanical strength. Secondly, the paper should be adjusted for humidity before printing (conditional units are best to use the forced humidity method to ensure that the paper has a relatively high moisture content) to stabilize the moisture content of the paper and improve the stability during the printing process.
For the flick angle failure caused by the machine itself, we can adjust it from the following aspects.
1. Make the teeth of the first color group bite the paper normally, and the teeth of both ends of the impression cylinder of groups 2-4, the tooth force is slightly smaller.
2. Padded the pads of 2-4 sets of blankets (about 0.05mm can be reduced). Doing so can make the paper with some flickering angles have some convergence, but this requires the operator to have a very rich printing experience to master the scale.
3. In addition, the tooth pads at both ends of the impression cylinder can be raised, so that the higher part first contacts the pressure, causing the paper to converge slightly in two corners and not spread. (Shown in Figure 1)


Figure 1

8. What schemes are used in the design of modern printing machines to solve the problem of flick angle?
China's modern printing machines have been able to solve the problem of flicker in the printing process, such as:
1. The gripper adopts the overall structure of high-strength alloy steel to ensure the stability of the paper and avoid ghosting and flicking.
2. The rear version of the card is segmented to solve the "swing angle" phenomenon that occurs in printing.
3. A plurality of supporting screws are used on the supporting rod of the impression cylinder dental pad to prevent the angle swing failure caused by the dental pad sinking.

Several printing presses abroad also have different solutions. Such as:
1. In the process of machine design, the impression cylinder's teeth are directly designed in the form of "Figure 1".
2. The tooth axis of the roller clenching teeth is designed to be flexible, and there is a pre-bending action before biting the paper, which can be adjusted according to the need, so as to avoid the occurrence of angle swing failure.

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