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At present, multi-color machines at home and abroad, including double-sided printing machines, can be said to adopt the unit-type structure form without exception. The unit-type multi-color offset press represents the development direction of today's offset printing machine technology. Improving the resistance to deflection and deformation of the impression cylinder during printing and reducing the bending rate of thick paper. Multi-color machines also use double-diameter impression and paper transfer cylinders. Unit-type offset presses are different from general monochrome machines, satellite two-color machines and BB double-sided machines. The biggest difference between them is that the paper transfer path is long and the roll paper transfer times are large, so the transfer accuracy requirements must be very high. However, due to the short history of the development of domestic machine-type offset presses, people have a superficial understanding of machine-set offset presses, especially with a poor understanding of the accuracy of paper transfer. Handover, the line speed of the roller surface is equal during handover, which can ensure the accuracy of paper transfer, so the design and manufacture of the roller is often taken lightly. In fact, the line error of the unit-type double-diameter cylinder offset printing machine is often unexpected and fails to meet the standard requirements. The most typical, tricky, and most common printing failure is that the printed product has a line-by-line deviation. Because the imprinting and paper transfer cylinders are double-diameter, it is simply inferred that the symmetry of the axis hole of the double-diameter cylinder is not Meet the requirements, large symmetry error. Therefore, the practice of continuously improving the symmetry requirements of the double-diameter roller tooth axis hole is everywhere. The higher the requirement for the double-diameter roller tooth axis hole symmetry, the final effect is not satisfactory, and the effect is very small. The real problem is Where? Where is the solution to the problem? People began to think calmly and analyze carefully.
The large symmetry error of the cylinder hole of the double-diameter cylinder is not the root cause of the inaccuracy of the two sets of front and back overprints, that is, the AB line error. From the analysis of the source of paper transfer, we can know that the paper is handed over to the front transfer cylinder through the swinging transfer arm. Assuming that the manufacture of the swinger and the front transfer cylinder fully meets the requirements, it will be handed over to the one-color impression cylinder at a fixed position because the front transfer cylinder is Double the diameter, so in theory its transfer position is unchanged. There are two possibilities for manufacturing the symmetry of the axis hole of the one-color imprint cylinder: one is the symmetrical distribution that meets the requirements; the other is that the deviation of the symmetry does not meet the requirements. The axis hole divides the cylinder into a large semi-arc and a small semi-arc. There is no objection to the handover in the former case, and the size of the paper bite does not change. During the latter handover, due to the asymmetry of the impression cylinder, the timing of the arbor hole running to the paper receiving point is sequential, resulting in two differences: the time of the synchronous handover (distance) with the front roller changes, or the position of the closed tooth. Assuming that the symmetry error is â–³ mm, the difference in synchronous handover (travel) time is negligible, generally â–³ is within 0.05, and the change in the position of the closed tooth is reflected as the difference in the size of the paper bite. The position of the cam that is transferred to the one-color embossing and the front pass does not change after assembly and adjustment. When the axis of the tooth is asymmetrical, there may be two cases: the actual line and the dashed line. The tooth axis is in the solid line position, and the imprint needs to take an additional â–³ mm distance. The tooth is closed late, and in the dotted line position, the tooth is closed early. Closed teeth will be sent more than â–³ mm arc length lately, the mouth of the paper is large, and the mouth of the teeth is small. When the shaft hole of the double-diameter roller is not symmetrical, only the size of the bite is changed alternately by â–³ mm, but as long as the synchronous handover time is sufficient, the paper running speed will not change. The relative position of the particle and the impression cylinder will not change. In the same way, one-color imprinting and one-two-color imprinting are the same reason. To ensure that there is no difference between M cross line and N cross line, the core problem is that the running speed of the drum must be constant. The gear is in the state of no backlash, and the gripper is stable and reliable, and there is no sliding. There is no microscopic change, and it does not depend on the symmetry of the AB axis of the tooth axis.
Excluding the possibility of paper position change at the junction, why is there still AB line error, at this time we also need to look at the shape and position accuracy of the roller. When the diameter of the double-diameter cylinder jumps seriously, the gripper shaft separates the impression cylinder into two arcs of size, and the cross line on the rubber cylinder makes one revolution every time. The length of the radius arc is long, and the long and short distances are just one piece at a time. The AB line error is formed. Because the drum runout is a processing and assembly and comprehensive error, the position is also uncertain, and it is generally difficult to exclude through adjustment, so exclude the AB line. The error is very difficult, and its appearance is associated with the double-diameter roller. In essence, it is caused by the beating of the double-diameter roller. People all blame the crime on the symmetry of the tooth axis hole. This symmetry is impossible. What's changed is that the AB sets of wires are left unattended. In serious cases, manufacturers often solve the problem by changing the rollers. In fact, changing the roller is to change the roller with good symmetry, but also to change the roller without a jump. This is the right way, people are more convinced that the symmetry of the tooth axis is not in line with the requirements is the culprit causing AB sets of line errors. Of course, although the roller with good symmetry is sometimes replaced, the problem cannot be solved because of the excessive runout, and it also causes the elimination of the AB line or the overprint failure of the four-color dots, which makes people feel the mystery of the AB line. .
For a single-diameter roller, even if the roller jumps, because each paper meets the crosshair of the rubber roller at the same position at the same speed, the particles overlap, so no AB line error will occur. If irregular crosshair ghosting occurs, Or overprinting is not accurate, it is mainly due to low paper transfer accuracy, such as weak gripper, dead teeth, sparse arrangement of grippers, large gear backlash, poor rigidity of the tooth shaft, etc., will not be generated regularly one by one. error. If the AB axial ghost image is generated at the slightly end of the paper tail, it is also due to the weak displacement of the gripper at the left and right ends of the roller and the angular displacement of the paper. When the stiffness is insufficient, the jump of the single-diameter cylinder is proportional to the printing speed. The higher the rotation speed, the more severe the overprinting and ghosting phenomenon.
Based on the above analysis, the beat of the double-diameter cylinder and the poor quality of the gripper teeth are the main reasons for the overprinting. The multi-color machine must control the output accuracy of the roller diameter jump assembly within 5μm, because the allowable error of dot printing is generally 15μm, which is equivalent to π times of 5μm diameter jump.
The multi-color machine is in wet press wet printing mode, as long as the imprints do not overlap, the previous color ink will be printed on the back color rubber or the impression cylinder to produce ghost images, so the multi-color machine's roller movement accuracy, paper Transfer accuracy is the core precision index of the offset press that guarantees the quality of printing.
In fact, some well-known foreign brand offset presses do not have high requirements on the accuracy of the tooth axis symmetry, some even adopt the form of shaft seat assembly, and the beating tolerance of the drum is very strict, generally controlled within 3μm ~ 5μm.
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