Offset press: there is no adhesion between the upper and lower printed sheets
4.4 There is no adhesion between the upper and lower printed sheets Due to the friction on the surface of the paper during the printing process, static electricity will be generated. To eliminate static electricity, in addition to ensuring proper temperature and humidity, another one is to install a special static elimination device at the delivery location. The cohesive force of the ink between printed sheets must be avoided during the paper collection process. The main method to prevent sticking is to spray powder, which prevents the back side of the upper printed sheet and the front side of the lower printed sheet from contacting each other. Powder spraying and ink drying are mutually shielded. This must be taken into consideration when choosing the amount of powder sprayed. Of course, there are other factors that need to be considered. The following is a brief analysis of these precautions. 1. The thinner the powder, the better. It is best if it is just not sticky and has minimal effect on drying. 2. The powder should not be too fine. If it is too thin, the stronger its hiding power on the surface of the printed matter, the slower the ink will dry. 3. The powder cannot be too coarse. The thicker it is, the worse the uniformity is. The place with powder particles is separated up and down, and the place without powder particles, the upper and lower paper may stick together. 4. The most effective use of powder. The paper-spraying area is equipped with a control device for the powder-spraying area, even if it is sprayed on the effective width of printing, non-printing parts are not sprayed. The length of spraying time can be adjusted steplessly according to the size of the paper (this adjustment device is generally installed on the sprocket above the stack), and the adjustment of the left and right spraying positions can independently control the spraying of each nozzle, as shown in Figure 4.6 As shown. 5. Impact on human health. Although the powder is non-toxic, it is also uncomfortable to fly into people's eyes or nostrils, so it should be as far away from the operator as possible. Some machine dusting devices are installed near the take-up roller, and some machines are installed near the brake roller. The former is far away from people, and in the protective cover of the paper collection part, it has the smallest impact on people, but the powder is sprayed after printing, which is not conducive to the drying of the ink. The latter is closer to people and can easily fly to people, which has a great impact on people. However, since the printing is started only after the printed sheet falls on the paper pile, the effect on the drying of the ink is minimal. 6. Impact on equipment. Due to the flow of air, the powder particles may fly to the chains on both sides, or other relatively moving parts. After the powder falls on the top, the mutual friction between the contact surfaces is increased, which causes wear and damages the contact accuracy and the stability of operation. Therefore, from this point of view, the powder should be prevented from leaving the surface of the printed product as much as possible. 7. The effect on the equipment and printing quality during flip printing. When reverse printing, the powder particles on the printed matter may drift to various parts of the machine, which will have a greater impact on the equipment. At the same time, the powder on the surface of the printed product will be transferred to the blanket and affect the transfer of ink, and the quality of the printed product cannot be guaranteed. 8. On the ground, the amount of powder sprayed is large; on the flat screen, the amount of powder sprayed is small. 9. The amount of powder should be strictly controlled when registering, and can be relaxed when not registering. If the amount of powder is not strictly controlled during color registration, there may be too much powder when printing, and the subsequent ink cannot be transferred to the surface of the printed product. In summary, the control principle of the amount of powder sprayed is: in the effective part of printing, the upper and lower printed sheets are just not sticky, and the powder volume used at this time is the optimal powder volume. The purpose of powder spraying is to prevent the upper and lower printed sheets from contacting. The main reason is that the ink on the surface of the printed product is not completely dry. If the ink has dried, powder spraying is meaningless. Therefore, some devices have been equipped with a drying device to accelerate the drying of ink, which can reduce or even eliminate powder spraying. However, when the paper is damp and then heated, it is easy to cause deformation of the surface of the paper. Once it is deformed, it is often unable to recover, which has a great impact on the color registration, so it must be used with caution. Theoretically speaking, powder spraying is only needed in places with pictures and texts, and powder spraying is not needed in places without pictures, but this is very difficult to control, but people are working hard in this direction. Multifunctional Children's Study Table Chair Set Myopia in children is mostly caused by bad habits in the learning process, such as improper sitting posture and poor lighting. The learning environment created by a good learning table is still helpful for preventing myopia, so buy anti-myopia learning desks for children. There is still a certain effect, let children develop good study habits, which will accompany them for a lifetime.
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Figure 4.6 The shaded part is the text
