Application of Rotary Offset Press
The home-made rotary offset press has a great relationship with the personal technical level of the operator in terms of overprinting, dot reproduction, and field density. However, as long as the machine is mastered well, satisfactory prints can be obtained.
1. Several factors affecting the quality of PJS4880-01 overprinting
In addition to plate-making, roller pressure, blankets, liners, ink-water balance and other factors, there are the following points that cannot be ignored.
(1) Current
The secondary tension of the PJS4880-01 machine of our factory has been used abnormally. In order to ensure accurate overprinting, the operator uses the method of manually controlling the current on the primary tension to ensure the consistency of the tape tension. Under normal circumstances, the current is controlled at 1.4A, but with the operation of the machine, the diameter of the paper roll is getting smaller and smaller, the speed of the paper roll is getting faster and faster, and the tension of the paper belt is getting bigger and bigger. Paper rolls with large outer diameters and paper rolls with small outer diameters are controlled with the same current. The tension of the tape is not the same, resulting in a large overprint error. The technician adopts the method of manually controlling the current. When the diameter of the paper roll is less than half, observe the position of the slider on the floating roller mechanism, and manually adjust the current so that the slider is always in the same position, ensuring the uniform balance of the tape tension.
(2) Continuously variable transmission
The adjustment of the continuously variable transmission has a great influence on the quality of overprinting. If the paper tape is too tight, it is easy to be broken, and if the paper tape is too loose, it is easy to fan up and down, which affects the quality of the overprint. Therefore, the maintenance of the CVT is particularly important, especially the tightness of the chain in the CVT has a great influence on the quality of the overprint. The chain is too loose, the paper feed fluctuates greatly, the paper will also jitter during the paper feed, the overprint change is large, and even appears Ghost failure.
(3) Clips on both sides of the blanket
After the machine has been installed for more than half a year, the printed products often have partial overprint failures. At first, the operator suspected that it was the problem of the machine itself. Use a dial indicator to check the roundness and straightness of the drum. The runout is within 0.02mm, which meets the requirements for use. The clearance of the roller bearings is also checked, which also meets the requirements for use. Later, it was found that the aluminum alloy clamp rods on both sides of the blanket were bent and wavy after production, and the maximum parallelism error of the clamp rods had reached 2mm. It turned out that the operator could not evenly apply force when hammering the blanket while clamping the blanket, which caused the local tension of the blanket after the machine was installed, resulting in partial overprinting and ghosting failure during the printing process.
(4) Bending machine
There is no diagonal plate pulling device on the plate cylinder of the PJS4880-01 machine. Therefore, it is very critical that the plate is bent on the bending machine. If the parallelism of the bending machine is not enough, the diagonal overprint will exceed the standard, causing the bending plate to be skewed, and this failure is difficult to adjust on the machine. After using the bending machine of PJS4880-01 for a period of time, some of the screws are loose. After checking the parallelism error of the guide rail of the bending machine platform, the maximum error can reach 0.3mm, and the bent printing plate cannot be corrected on the machine.
In order to ensure the accuracy of the bending plate, the technician specially made a special measurement and correction tool for correcting the parallel accuracy of the guide rail of the bending plate platform. When measuring and calibrating, put the special measuring and calibration tool on the platform of the bending machine. One side of the measuring tool rests on the side rail of the platform, and the dial indicator is installed on the other side. Loosen the adjusting screw under the bending machine. After the printing plate is installed, the overprint error of the diagonal line should be minimized or not corrected to ensure that the overprint error of the printed product is controlled within 0.1mm.
2. Elimination of PJS1880-01 machine's circumferential ghosting and virtual printing
In June 1999, our factory purchased a PJS1880-01 rotary printing machine. From the beginning of the installation and acceptance of the machine, there is a phenomenon that the printed product is false, and the printed product is becoming more and more serious, and there is a circumferential ghost again from 180 to 280mm from the mouth. The technicians carefully checked the machine and adjusted the pressure of the roller, water roller, and ink roller, but the problem could not be solved. To this end, the Beiren sent full-time maintenance personnel. After the repair and adjustment, the printed products have met the quality requirements, but two days later, there have been circumferential ghosting and virtual printing. In response to this situation, the technical staff specifically consulted the experts in the design of Beiren offset printing presses, and asked them why the nut will not be adjusted when the pressure of the cylinder is adjusted a few times. Finally, it is believed that the mouth is 180 ~ 280mm The circumferential ghosting at this location is related to the bearing clearance of the drum, while the ghosting failures at other locations are related to blankets, pads, water, ink, etc. Regarding the problems of circumferential ghosting and virtual printing of printed products, the maintenance personnel of Beiren adjusted the machine according to the technical requirements, dropped the bearings on both sides of the drum, and checked the bearing clearance.
In order to grasp the accurate data of the roller bearing clearance, we put the removed tapered roller bearing on the table of the grinding machine, with the small end facing up and the big end down, putting the inner and outer spacers, and placing the dial indicator on the grinding machine. On the workbench, use a dial indicator to measure the height of the inner and outer spacers on the tapered roller bearing. During the measurement, adjust the dial indicator to zero. Hold down the inner and outer spacers on the tapered roller bearing with both hands to translate the bearing and record the dial indicator. Measure the height data, change another bearing, put the inner and outer spacers, use the same method to measure the data of the inner and outer spacers, and compare the sum of the two data, the actual clearance of the bearing comes out, the inner ring and The data should be 0.02mm larger than the sum of the outer ring. In this method, all the roller bearing gaps were tested, and the maximum gap was found to be 0.3mm. After the test, the bearings with large gaps were processed to the required size on the surface grinder. After the machine is installed, the pressure of the cylinder is adjusted to the required data, the printing is turned on, and the printed product has no ghost images. The dots are clear, the density on the ground is good, and it meets the quality standards. Everything has been normal until now.
3. Trouble shooting for the gear and ink bar of JJ204 rotary offset press
The JJ204 rotary offset press was purchased in 1994. After more than two years of use, gears and ink sticks appeared on the printed product. After inspection, the drum gear is better. According to the experience of the flattening machine, there are two reasons for the appearance of the gear ink bar, one is the serious wear of the gear, and the other is the serious wear of the roller sleeve and the large gap.
The technician inspected the bearings on both sides of the drum and found that the bearings were severely worn and the gap was large. After replacing the roller bearing, adjust the bearing gap on both sides of the roller, and then adjust the pressure of the roller. After starting the printing, the gear and ink stick on the printed product are gone.
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