2.7 Printing pressure 2.7.1 The role of printing pressure 1. Make up the processing error of the machine. This is a very important role of printing pressure. If there is no printing pressure, due to the processing and installation errors of the machine, the surfaces of the two cylinders cannot be completely in contact, and the uniform transfer of ink cannot be guaranteed. There are unevenness errors on the blanket, printing plate surface, and paper. These errors must also be eliminated, so the printing pressure is also a reflection of the accuracy of the machine. 2. Transfer ink. The printing pressure is used to transfer the ink so that the printing can be completed. Through experimental analysis, the relationship between printing pressure and ink is approximately as follows: In order to make up for the comprehensive error of the machine, the ink in section BC enters the normal transfer stage. After point C, the pressure increases, and the transfer of ink not only does not rise, but decreases. There is basically a linear relationship between the transfer amount of the segment ink and the pressure. 3. Make paper and ink get good printability. The shape of the ink transferred to the paper has a very close relationship with the printing pressure. The pressure is large, the deformation of the ink is large, and it is easy to paste, but the permeability is good, the pressure is small, the ink is faint, and the permeability is poor. Paper changes or ink changes require re-selection of printing pressure. That is the best printing pressure. 2.7.2 Formation principle of printing pressure The printing pressure is reflected by the amount of compression on the contact surface. In general, the deformation of the rigid cylinder is negligible compared with the deformation of the liner, so the deformation of the liner is usually used to represent the printing pressure P: It can be seen from the above formula that the printing pressure has a formal relationship with the performance of the material itself and the two factors of deformation under compression. Therefore, with the same amount of deformation and different materials, the printing pressure is different, and paper has similar problems. Therefore, the pressure value measured by the fuse can only be used as a reference. How appropriate is it depends on the specific situation. There is an amount of deformation (ie, under printing pressure). There can be no line contact but only surface contact between the rollers. The calculation of the printing pressure distribution on the contact surface is roughly shown in Figure 2.10b. The centerline pressure of the contact surface is the largest, and the pressure on both sides of the centerline decreases in turn. This is because the amount of deformation on both sides is smaller than the middle, and the pressure in the middle of the drum is smaller than the pressure on both ends. as the picture shows. The smaller the bending, the more uniform the pressure. The size of the bend is inseparable from the performance of the material itself, the accuracy of processing and installation, and the printing pressure. To reduce bending, it can only increase the rigidity of the cylinder, improve the accuracy of machine processing and installation, and reduce the printing pressure. The distribution of the printing pressure is in a symmetrical state of the middle section, so it is better for the printed products to be synchronized with this symmetrical relationship, that is, symmetrically distributed with the same middle section. [next] 2.7.3 Adjustment of printing pressure There are two factors that affect the printing pressure, which have been analyzed above. In general, the lining material is unchanged (ie, E is unchanged), and changing the printing pressure becomes only the size of the lining compression (this is why the gap between the roller shoulder iron always represents the printing pressure size). There are two ways to change the compression of the lining: one is to keep the lining unchanged and change the size of the center distance; the other is to keep the center distance unchanged and change the thickness of the lining. Changing the center distance can be carried out by a special pressure regulating mechanism, changing the thickness of the liner, only opening the liner is very troublesome. From the perspective of gear transmission, it is best to mesh the gear in the position of the index circle. As soon as the center distance moves, this position will change, thus affecting the stability of the transmission. Therefore, ideally, the center distance is not changed, and the thickness of the lining is changed to adjust the printing pressure. However, although the printing paper changes frequently, the thickness difference is generally small, as long as the center distance is adjusted slightly. This change is also permitted by the involute gear, but the range of adjustment is limited. If printing thick paper, you can't just adjust the center distance, it needs to be combined with changing the thickness of the liner. In summary, it is impossible to fix the center distance, but it is best not to move. If the printing pressure is insufficient, it can be compensated by changing the thickness of the liner. The size of the printing pressure can be calculated, as shown in Figure 2.11. The printing pressure is expressed by the compression of the lining, that is, L = R1 + R2-A; A increases, that is, the center distance increases, and L decreases; A decreases, that is, the center distance decreases, and L increases. R2 and R1 increase, that is, the thickness of the lining increases, L increases; R2, R1 decrease, that is, the thickness of the lining decreases, L decreases. To change the thickness of the lining, it is usually to add or reduce paper. The above is to find L by the values ​​of R1, R2 and A, and also to find R1, R2 and A by L, that is: What is often used in the actual process is not the specific values ​​of R1, R2 and A, but the thickness of the paper (Tz), the thickness of the rubber pad (Tbc), the thickness of the rubber (Tb), the thickness of the printing plate (Tp ), The thickness of the plate pad (Tpc), let Ri, Rp, Rb denote the diameter of each cylinder, Aib, Abp denote the center distance between the impression cylinder and the rubber cylinder, and the center of the plate cylinder and the rubber cylinder respectively Distance, Lbi represents the amount of compression between the blanket cylinder and the impression cylinder, Lpi represents the amount of compression between the plate cylinder and blanket cylinder, so there are: 1. In general. ①For the equipment that does not take the shoulder iron: The factors affecting Lpb are: Tpc, Tp, Tbc, Tb, Abp; the factors affecting Lbi are: Tbc, Tb, Tz, Abi. ②For the equipment that takes the shoulder iron (walking between the printing plate and the rubber cylinder, not between the rubber cylinder and the impression cylinder): The factors affecting Lpb are: Tpc, Tp, Tbc, Tb; the factors affecting Lbi are: Tbc, Tb, Tz, Abi. 2. Assume that the thickness of the blanket (Tb) and the thickness of the printing plate (Tp) are constant. ①For the equipment that does not take the shoulder iron: The factors affecting Lpb are: Tpc, Tbc, Abp; the factors affecting Lbi are: Tbc, Tz, Abi. ②For the equipment that takes the shoulder iron: The factors affecting Lpb are: Tpc, Tbc; the factors affecting Lbi are: Tbc, Tz, Abi. 3. If the gear is completely in the standard meshing state, the other conditions are the same as 2. ①For the equipment that does not take the shoulder iron: The factors affecting Lpb are: Tpc, Tbc; the factors affecting Lbi are: Tbc, Tz. ②For the equipment that takes the shoulder iron: The factors affecting Lpb are: Tpc, Tbc; the factors affecting Lbi are: Tbc, Tz. 4. Assuming that the rubber pad and the plate cylinder pad are unchanged, the other conditions are the same as 2. ①For the equipment that does not take the shoulder iron: The factors affecting Lpb are: Abp; the factors affecting Lbi are: Tz, Abi. ②For the equipment that takes the shoulder iron: The factors affecting Lbi are: Tz, Abi; Lpb is a fixed value. The influential factors in 1 of the above four cases are the most comprehensive, but in practice the adjustable factors are not always so many. This situation can only be used when overhauling or commissioning the machine. The factor affecting the pressure in 3 is to exclude the influence of the center distance. Even if the relative relationship of the gears remains unchanged, it is very beneficial to the smooth operation of the machine, but this situation is also difficult to achieve in the actual process. Relatively speaking, two or four sets of formulas are used the most. [next] From the above formula, we can see that there is a linear relationship between Lpb or Lbi and their influencing factors, that is, how much the influencing factors increase or decrease, how much Lpb or Lbi changes accordingly. As can be seen from the analysis in 1, the thickness of the paper (Tz), the center distance between the rubber cylinder and the impression cylinder (Abi) only affect the pressure between the rubber cylinder and the impression cylinder; the thickness of the printing plate (Tp), The thickness of the plate pad (Tpc) and the center distance between the plate cylinder and the blanket cylinder (Abp) only affect the pressure between the blanket cylinder and the impression cylinder; and the thickness of the blanket (Tb) and its pad Thickness (Tbc) affects the pressure at both places. This has guiding significance for adjusting printing pressure. When you only need to change the pressure between the impression cylinder and the rubber cylinder, you can adjust the center distance (Abi) or replace the paper. It is actually impossible to replace the paper, so the most common is to change the center distance of the cylinder, but the center distance The change should consider the effect of paper thickness. When it is only necessary to change the pressure between the rubber cylinder and the plate cylinder, adjust the center distance (Apb), or replace the printing plate, or replace the printing plate, or replace the thickness of the plate pad, encountered in the actual process of replacing the plate There are very few, and the center distance is not used much, the main reason is that there is only ink between the rubber and the printing plate, and the ink is basically the same. So it is relatively common to change the thickness of the liner under the printing plate. When the pressure in both places needs to be adjusted, the thickness of the blanket or the thickness of the pad underneath can be changed. When the pressure in both places needs to be adjusted, the thickness of the blanket or the thickness of the pad underneath can be changed. Changing the thickness of the blanket is rarely encountered in practice. It is common to change the thickness of the underlying pad. Summarize the above analysis: ①When only adjusting the pressure between the rubber cylinder and the impression cylinder, the most commonly used is to adjust the center distance between the two cylinders. ②When only adjusting the pressure between the plate cylinder and the rubber cylinder, the most commonly used is to change the thickness of the pad under the plate. ③When the pressure in both places needs to be adjusted, the most commonly used is to change the thickness of the pad under the rubber roller. 2.7.4 The best printing pressure In the previous section, the general method of forming and adjusting the printing pressure was analyzed in detail, but in the actual process, the printing pressure must be able to ensure a good transfer of ink, so there is a problem of choosing the best printing pressure. The best printing pressure should be: complete imprinting, smooth and smooth, distinct layers, bright colors, minimal distortion. To obtain the best printing pressure, it is necessary to make the eight factors of paper, ink, fountain solution, printing plate, printing equipment, printing process, printing environment, and liner out of the best adaptive state. From the perspective of paper, the value of Lbi is different for different papers. The Lbi required for coated paper is small, and the Lbi required for offset paper is large. Because the surface roughness of offset paper is large, some places may not be printed without increasing Lbi, and the roughness of the surface of coated paper is small, Lbi is large, and the deformation of the blot increases faster, which is easy to paste. From the perspective of water-ink balance, water-ink balance is a guarantee of print quality. The greater the printing pressure, the greater the possibility of spreading the print, and more water is required to block the expansion of the ink. Therefore, when the printing pressure changes, the water-ink balance also changes accordingly. Considering the ink alone, it can be seen from the pressure curve that the pressure must be in the BC section, and the transfer of the ink is the most ideal, and it is also the most sensitive to control. From the perspective of printing equipment, the smaller the printing pressure, the greater the periodic impact and vibration. Considering only the printing equipment, the printing pressure should be zero, but in fact this is not possible. It is best to put it in the state described in section 2.7.3 3. From the perspective of printing technology, the transfer effect of the first color ink is the best, and the transfer effect of the subsequent colors is more and more difficult. According to the separation rule of the ink, the value of Lbi should be greater from the back. From the perspective of the ink, the more viscous the ink, the greater the pressure; the thinner the ink, the smaller the pressure. From the point of view of the printing plate, the surface should have good hydrophilicity, and the place of the picture and text should have good ink affinity and water repellency, and the other is to hinder the deformation of the imprint. From this point of view, flat gravure is better than flat relief. From the perspective of printing environment, when the temperature and humidity in the environment change, paper, ink and fountain solution will be affected, resulting in changes in pressure. In summary, the optimal printing pressure adjustment method is to make the gears in a standard meshed state and to achieve the best printing suitability by changing the thickness of the pad. When printing thick paper, the best adjustment method for printing pressure is: ①Reduce the thickness of the pad under the rubber roller;
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(Figure 2.10a)
P = E · L (E is the elastic modulus and L is the compression)
(Figure 2.10b)
(Figure 2.11)
R1 = L + A-R2; R2 = L + A-R1; A = R1 + R2-L.
Lpb = Rp + Tpc + Tp + Rb + Tbc + Tb-Abp
= Tpc + Tbc + Tb-Abp + Rp + Rb
Lbi = Rb + Tbc + Tb + Ri + Tz-Abi
= Tbc + Tb + Tz-Abi + Rb + Ri
Lpb = Tpc + Tp + Tbc + Tb-Abp + C11 C11 = Rp + Rb
Lbi = Tb + Tbc + Tz-Abi + C21 C21 = Rb + Ri (1-1)
Lpb = Tpc + Tp + Tbc + Tb + D11 D11 = Rp + Rb-Abp
Lbi = Tbc + Tb + Tz-Abi + D21 D21 = Rb + Ri (2-1)
Lpb = Tpc + Tbc-Abp + C12
C12 = C11 + Tp + Tb
Lbi = Tbc + Tz-Abi + C22
C22 = C21 + Tb (1-2)
Lpb = Tpc + Tbc + D12 D12 = D11 + Tp + TbL
Lbi = Tbc + Tz-Abi + D22 D2 = D21 + Tb (2-2)
Lpb = Tpc + Tbc + C13 C13 = C12-Abp
Lbi = Tbc + Tz + C23 C23 = C22-Abi (1-3)
Lpb = Tpc + Tbc + D13 D13 = D12
Lbi = Tbc + Tz + D23 D23 = D22-Abi (2-3)
Lpb = C14-Abp C14 = C12 + Tpc + Tbc
Lbi = C24 + Tz-Abi C24 = C22 + Tbc (1-4)
Lpb = D14 D14 = D12 + Tpc + Tbc
Lbi = D24 + Tz-Abi D24 = D22 + Tbc (2-4)
â‘¡ Increase the thickness of the pad under the plate cylinder;
â‘¢ If there is a slight difference, it can be compensated by the change of the center distance. 
