Control of color printing overprint accuracy of th

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Control of the overprint accuracy of multi-color rotary offset printing machine

as multi-color rotary offset printing machines are increasingly equipped with printing plants and play an important role in color printing such as commerce, books and periodicals, the problem of inaccurate overprint has also been placed in front of printing manufacturers. The reasons for inaccurate overprint of multi-color rotary offset printing machines are not only complex, but also in various forms, Among them, the most serious and common is the inaccurate overprinting between colors, which is reflected in the inaccurate overprinting between colors of the whole format (divided into axial and circumferential aspects), and even in the printing of local overprinting, which makes a large number of products become defective or scrap, and causes great losses to the enterprise. In this paper, we only analyze and discuss how to master and adjust the overprint accuracy of multicolor rotary offset press

factors affecting overprint accuracy

1 Prepress

(1) errors in film production

(2) errors in making printing plates

2. Error caused by moisture absorption deformation of paper

3 Various factors affecting overprint caused by incorrect operation of operators

(1) control of fountain solution

(2) installation of blanket

(3) installation of printing plate

4 Various factors affecting overprint caused by mechanical aspects

(1) tension system

(2) gear wear of transmission part

(3) registration adjustment mechanism

mastering and adjusting and aspects needing attention

1 Prepress

(1) the collage process should first pay attention to the collage of pictures and texts. The collage accuracy directly affects the accuracy of overprint. Because the collage process is a manual operation, and it is an important part of the production. In order to create conditions for the next process and improve the overprint accuracy, the collage accuracy should be absolutely accurate, and the collage error should be controlled below 0.02mm

(2) pay attention to the following aspects when copying:

① whether the gauge line is aligned

② check whether the air intake of the copying machine is normal and whether the sealing strip is leaking to ensure the flatness of the film

③ control the expansion and contraction of the film, control the temperature of the developer and the temperature and humidity of the working environment, so as to improve the accuracy of overprint

④ when making printing plates, because multicolor printing takes four-color prints as an example, each print requires four plates, and in the process of printing, the insufficient air suction of the plate printer or the leakage of the sealing strip will affect the flatness of the plate and film, resulting in the decline of overprint accuracy, so this problem cannot be ignored. Before printing, check whether the plate printer is intact and the flatness of the plate and film to improve the overprint accuracy. In addition, in order to improve the printing resistance of plates, manufacturers have adopted the baking process. Due to the inaccurate grasp of the drying temperature and time, the printing plate changes too much, affecting the dimensional stability of the plate, resulting in the problem of inaccurate overprint. Therefore, the drying temperature and drying time should be accurately adjusted according to the requirements of the print, so as to minimize the deformation of the plate

2. Moisture absorption deformation of paper

the rotary offset press is printed with web paper. After the web paper is unpacked, because the external humidity (water content) is greater than the water content of the paper, the edges of both sides of the paper roll will stretch after moisture absorption. On the contrary, if the external humidity (water content) is lower than the water content of the paper, the edges of both sides of the paper roll will emit water, resulting in the shrinkage and tightening of the edges of the paper due to dehumidification, resulting in ruff edges. In these two cases, the pressure of the printing cylinder on the printing paper will cause some parts of the paper to change and overprint inaccuracy. In order to avoid this situation as far as possible, we should pay attention to the timing of unpacking, and it is best to print on the machine as soon as it is disassembled and control the humidity of the environment between 60% and 65%

3. Machine personnel should correctly master the control of

(1) fountain solution. On the premise of no dirty version, print with the minimum amount of water to reduce the changes caused by water absorption of the paper, so as to improve the overprint accuracy. Because the paper has the characteristics of moisture absorption and deformation, the paper absorbs water (fountain solution), which causes the fiber to wet and expand, and changes, elongates and widens the paper under the pressure of the printing drum, resulting in inaccurate overprint when the imprint is not formed at one point during printing. When the registration mechanism cannot be adjusted, it has two forms:

① it occurs in the circumferential direction, that is, the circumferential overprint is inaccurate. You can reduce or increase the thickness of the plate cylinder lining, that is, change the diameter of the plate cylinder to change the size of the printing format, so as to reduce the overprint error. The specific lining thickness is determined according to the overprint error

② if the overprint in the axial direction is inaccurate, a bracket can be installed between the printing units to lift the middle of the paper, that is, to raise a certain point of the paper tape. The innovation center will seize the great opportunity of Gongyi City to build a 100 billion aluminum industrial base. Once the middle of the paper tape is raised, it is bound to shrink both sides of the paper to the middle, which makes the impression wider due to the change of the paper after printing through a certain color. Before the next printing, there is a change, that is, narrowing, to improve the overprint accuracy. This method can reduce the axial overprint error by 50%. The specific height of the bracket shall be determined according to the actual situation

(2) correct installation of rubber blanket. As the blanket needs to be tightened on the drum, if the degree of tightening is different, due to uneven surface tension, some areas are tight with high stress and some areas are loose with low stress. In this way, the elongation of the blanket varies during printing, and the size of pictures and texts also changes, which will lead to inaccurate overprint of local pictures and texts. Therefore, when installing the blanket, we must try to make the degree of tightening consistent

(3) correct installation of printing plate. At present, the plate cylinder of rotary offset press mostly adopts socket type (or hanging type), so the plate needs to be bent before installation. Although the current bending machine has a positioning device, the plate has varying degrees of deformation (unevenness) after baking. If you don't pay attention to the parallelism between the plate and the picture and text, forming a twist angle will affect the overprint accuracy. At present, the rotary offset press does not have a diagonal adjustment mechanism (except for the commercial rotary offset press), so LG, SK and meimeimeike all purchased the 3-yuan material patent authorization from 3m to improve the overprint accuracy when bending

4. Mechanical reasons for inaccurate overprint

(1) first of all, we need to understand the paper feeding mechanism of the rotary offset press. Its main component is a tension control system, which controls the flatness of the paper tape and does not move left and right, so as to make the graphic overprint accurate. If the tension system is unstable or insufficient, please click to cause overprint error, so in order to accurately control the tension and ensure the stability of the tension system, The components that will affect the tension change must be carefully maintained. The paper feeding mechanism of the rotary offset press mainly includes the following parts:

① the paper frame, the wear of the brake pad of the paper roll tension control system, the gap between the brake pad and the brake disc, and the leakage of the brake seal air bag will all cause the decline of the printing overprint accuracy

② the floating roller is an important part of tension stability. The tension control system composed of air circuit control elements, control cylinders, floating rollers and other components keeps the tension stable during the printing process. If the pneumatic control part of the tension control system and the cylinder leak or the action is not smooth, it will directly affect the stability of the tension and cause the overprint accuracy to decline

③ the bearings of the paper drawing roller and the paper storage roller at each part must rotate flexibly, otherwise the tension of the paper tape will be affected, resulting in the decline of overprint accuracy

(2) the roller bearing is the support point for the printing roller to provide printing pressure. As the high-speed operation is always maintained in the production process, wear and tear are caused over time, the bearing accuracy is reduced, and the instantaneous operation is not smooth during the operation process, which causes the drum to be out of sync, resulting in inaccurate overprint. In order to avoid similar problems, it is necessary to carefully check the lubrication system and always keep the roller bearing working in a good working environment

(3) the transmission gear part and the roller bearing have the same effect. Because the gear will cause wear in the process of high-speed operation, the gear clearance is too large, and when it is under some pressure in the work, it will cause the instantaneous pause of the printing cylinder, that is, asynchronous, resulting in inaccurate overprinting. Therefore, it is necessary to ensure the smooth transmission of the gear, especially pay attention to the lubrication system

(4) the registration mechanism should be checked frequently. Because the registration adjustment mechanism relies on the self-locking of worm gear, worm, screw rod and nut to stabilize and adjust the printing cylinder, if they are worn, they will form a gap and cause the displacement of the printing cylinder, resulting in the decline of overprint accuracy

to sum up, it can be seen that the overprint accuracy of multi-color rotary offset press is affected and restricted by many aspects. In the actual production process, we must combine reality, conduct scientific analysis, find out the real cause of inaccurate overprint, and solve it with scientific methods

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