How to adjust the optimal printing pressure of offset printing
What is the ideal pressure in printing? First let us know the role of printing pressure in printing. Printing pressure refers to the pressure required by the printing machine to transfer the ink on the printing plate to the substrate. If the pressure is not suitable, it will cause the printed graphics and text dots to be not full, unclear, ink color is light, the graphics and text are distorted, and the level is unclear; at the same time, the paper will be rough, the graphics will become larger or smaller, and the overprint will be incorrect; Make the printing plate with low printing resistance, shorten the life of the blanket and machine. The ideal printing pressure is to use the smallest and uniform printing pressure, so that the product dots are not expanded, the printed products are clear and full, the graphics are colorful, and the gradation is clear. The printing pressure is the amount of compression deformation between the two cylinders after being pressed together, that is, the degree of deformation of the paper after being pressed. In offset printing, we usually call the maximum deformation of the rubber blanket lined on the rubber cylinder under pressure as the printing pressure (compression).
After the center distance is determined, the size of the printing pressure is determined by the nature and thickness of the blanket of the rubber cylinder and the plate cylinder. Because the blanket and pads are elastomers, the offset press cannot rotate without friction between the two rollers. This is the key problem that the ideal pressure in offset printing is difficult to solve.
As can be seen from FIG. 1, under the combined pressure state of the rubber cylinder and the impression cylinder, the radius of the rubber cylinder is not equal to the radius before the uncompressed cylinder, and its value is within the range of greater and smaller than the radius of the unincorporated rubber cylinder. The radius R1 of the rubber roller at the point of maximum stress is different from the radius R2 of the point of minimum stress B. After the rubber roller is pressed, it rolls at C to form a convex hull. The radius of the highest point C of the convex hull is R3, B The radius of the point is R1 which is the size of three radii of the rubber cylinder after lining.
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Determine the ideal pressure by changing the center distance of the roller: use a feeler gauge or 0.5mm lead wire to flatten to accurately adjust the roller pillow clearance of the two rollers to make it meet the specified data. Adjusting the center distance of the drum is a method to obtain the ideal pressure.
Another way to adjust the pressure is to change the thickness of the drum lining. Since the cushion of the rubber cylinder is an elastomer and has a large thickness, the thickness of the cushion is generally changed on the rubber cylinder to obtain the ideal pressure for printing. But this method is only used when it is a last resort in actual work. Because the pad of the rubber roller increases or decreases, the diameter of the roller increases and decreases, so that the diameters of the three rollers are unequal, so that the circumferential speed is unequal, and the friction and sliding in the contact area are increased, which causes deformation of the text and dots. , The printed matter produces a "bar". It can be seen from Figure 1 that under the combined pressure state, the rubber cloth on the rubber roller is squeezed to a certain depth, and a convex hull is formed behind the rolling, which shows that the rubber roller is not reduced in diameter under the combined pressure state. Is increased. This increased part produces creep during the pressure rolling of the drum, and as a result, the creep stretches the blanket. In order to keep the diameter of the three rollers equal, without friction and sliding, the rubber roller should be lined with pads according to the instructions of the machine. It cannot be increased blindly. Generally, the thickness of the pads is 0.5 to 0.7 mm.
It can be seen from Figure 2 (a) that the liner of the figure printing plate is 0.1mm higher than the roller pillow, and the blanket is 0.3mm higher than the roller pillow. When closing, Al? B1 produces large convex hulls. Figure 2 (b) The pad of the printing plate is 0.2-0.3mm higher than the roller pillow, and the blanket is 0.05mm lower than the roller pillow. The convex hulls generated at A2 and B2 are small. Of course, there is less friction and sliding, and the plate is worn Less, there is no shaving and "bar" failure, the printing plate has high printing resistance, less dot expansion, clear layers, longer machine life, and less power consumption by the motor.
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It can be seen from the above explanation that printing within the ideal pressure range is the basis for ensuring the correct implementation of the printing process, the normal operation of the printing press, and obtaining high-quality prints. In actual work, the center distance of the cylinder and the thickness of the blanket of the rubber cylinder should be accurately adjusted, and the printing pressure should be kept within the ideal pressure range according to the conditions of paper, printing plate and work. This is the fundamental problem of offset printing One.
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