Pressure adjustment of the printing press when the paper thickness changes
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There are two major changes in the printing press when the paper thickness changes:
First, the pressure between the blanket cylinder and the impression cylinder changes, and the original pressure state cannot be maintained. When the paper is thickened from thinner, the pressure between the blanket cylinder and the impression cylinder is increased; when the paper is thinned by thickness, the pressure between the blanket cylinder and the impression cylinder is reduced.
Second, the size of the graphic printed on the substrate (the length of the print) has changed. When the paper is thickened from thinner, the length of the image obtained by the impression cylinder is relatively elongated; when the paper is thinned by thickness, the length of the image obtained by the impression cylinder is relatively reduced.
In order to have a more accurate adjustment of the printing machine when the paper thickness changes, let us first understand the adjustment of the printing pressure and the relationship between the radius of the two rollers and the size of the graphic.
1. Roller pressure
In the lithographic offset printing process, the ink on the printing plate is first transferred to the blanket, and then transferred to the substrate by the rubber. During this transfer process, the surface of the plate cylinder must be in full contact with the blanket cylinder, the blanket cylinder and the impression cylinder (substrate), that is, there must be a certain contact pressure between the contact surfaces to allow the ink to be fully transferred between them. This contact pressure is the printing pressure. Roller pressure: The interaction force between the embossing bodies during printing.
Printing pressure is the primary condition for lithographic offset printing. The ideal pressure between the rollers is the key to ensuring print quality. The ideal printing pressure means that the pressure can ensure the good transfer of the ink (that is, to ensure that the printed matter is strong in the print, the dots are full but not spread, and the tone level is clear and rich), and the printing pressure between the rollers is reduced as much as possible. Whether or not the pressure can be achieved (that is, the use of ideal pressure) is one of the important indicators for measuring the level of technology, and is also an important condition for ensuring high quality and high yield.
Excessive or too small printing pressure has a great impact on product quality. Excessive pressure can easily cause dot deformation, paste, product color, and even image distortion. Excessive pressure will accelerate plate wear and machine wear. If the pressure is too small, the ink transfer amount will be insufficient or unstable, resulting in an empty print. The ink color is light, the outline of the image is unclear, and the depth of the ink is inconsistent.
The method of adjusting the printing pressure in actual production:
1.1 adjust the printing pressure by adjusting the center distance of the drum;
1.2 Adjust the printing pressure by adjusting the thickness of the lining (plate cylinder, blanket lining).
Note: The size of the center distance is related to the meshing condition of the drum gear. It should be adjusted according to the actual needs of the printing process. It is not suitable for frequent adjustment, and must be adjusted within the range indicated by the machine operation, and the pressure on both sides must be consistent.
2. The relationship between the radius of the three rollers and the size of the graphic
Generally, the transmission of the three-roller of the offset printing machine is driven by three gears with the same number of teeth, modulus and spiral angle. Therefore, regardless of whether the radii of the two rollers are equal, the angular velocities of the three rollers are completely equal.
Ideally, the radius of the drum after the lining should be equal to the linear speed of the surface of the drum during the printing process, and the rolling of the roller should be kept without slipping, so as to ensure the accurate coincidence of each dot between the printing surfaces after each operation. Deformation or ghosting. If the three rollers are rigid bodies, as long as the radii of the three rollers are equal, the line speeds of the three rollers can be equal.
However, the materials covered by the three rollers are different, and the radius of the three rollers after the lining is different. In order to maintain pure rolling, no slip state, the radius of the blanket cylinder should be less than the radius of the plate cylinder and the impression cylinder, in principle, should be equal to the pitch circle radius.
In order to facilitate the study of the relationship between the radius of the three rollers and the size of the graphic, we regard the three rollers as rigid bodies. Assume that after the plate is installed, the graphic area accounts for 180o. Now let's study the following:
2.1 If the operating diameters of the plate cylinder and the impression cylinder are equal, and the running diameter of the blanket cylinder is relatively small (or large), since the angular velocities are equal, when the plate is rotated through l80o, the other two rollers are also rotated through 180o. That is, the image area occupies 180o on the impression cylinder. Since the operation diameters of the blanket cylinder and the impression cylinder are equal, the length of the imprinted cylinder surface and the surface of the impression cylinder are equal. ).
Conclusion: When neglecting the deformation caused by the elastic deformation of the blanket cylinder, the running diameter of the blanket cylinder does not affect the length of the graphic imprint, but it will affect the printing quality.
2.2 If the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, when the plate cylinder rotates 180o, the other two rollers also rotate through l80o, that is, the image area occupies 180o on the impression cylinder, due to the pressure. The diameter of the printing cylinder is smaller than the diameter of the printing cylinder, and the actual length of the printing cylinder is relatively reduced (the image is shortened).
Conclusion: When the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, the graphic is shortened.
2.3 If the running diameter of the impression cylinder is larger than the diameter of the plate cylinder, the image area occupies the position of l80o on the impression cylinder. Since the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the actual length of the impression cylinder is relatively Elongation (text is stretched).
Conclusion: When the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the image is elongated.
3. Pressure adjustment of the printing machine when the paper thickness changes
Taking paper thickness from thin to thick as an example
3.1 If the paper changes little from thin to thick, the elongation of the graphic does not exceed the range of qualified products.
Adjustment method: The center distance between the blanket cylinder and the impression cylinder can be directly pulled apart. The extent of the pull can be obtained by measuring the change in the thickness of the paper.
3.2 If the paper changes greatly from thin to thick, the elongation of the graphic has exceeded the range of the qualified product, but the influence of the adjustment center distance on the meshing condition of the drum gear is still acceptable.
Adjustment method: correspondingly increase the amount of the lining of the plate cylinder (the increase range can be obtained according to the change of the thickness of the paper), and at the same time pull the center distance between the plate cylinder and the blanket cylinder, between the blanket cylinder and the impression cylinder, and Readjust the ink roller pressure.
3.3 If the paper changes greatly from thin to thick, not only the elongation of the graphic has exceeded the scope of the qualified product, but also the adjustment of the center distance has affected the meshing condition of the drum gear.
Adjustment method: correspondingly reduce the amount of lining of the impression cylinder, increase the amount of lining of the plate cylinder, and re-adjust the pressure of the ink roller.
Note: The first adjustment method is applied the most, the adjustment is the simplest; the second adjustment method is more reasonable, the image size can be kept basically unchanged; the third adjustment method can make the pressure in the normal range, and can also make the map The size of the paper remains basically the same, but due to this adjustment method, the running diameter of the blanket cylinder is much different from the running diameter of the plate cylinder and the impression cylinder, which causes a slip phenomenon between the rollers, which affects the printing quality.
4. Inspection of printing pressure
The most common and intuitive method of printing pressure testing is the bar method. The specific operation is as follows: lift the water roller, drop the ink roller, run the machine at low speed and press it together, and wait until the surface of the three rollers is evenly inked, then stop. Each roller has a ink bar at the parking place, such as the width of the ink bar is uniform and uniform within the numerical value given by the machine, and the ink bar width of the impression cylinder and the blanket cylinder is larger than the ink bar width of the plate cylinder and the blanket cylinder. , indicating that the pressure adjustment is even and appropriate.
If the width of the pressure bar is narrow at one end and the width is narrow, the maximum distance between the two sides of the roller may be inconsistent.