As multi-color rotary offset presses are more and more equipped with printing plants and play an important role in color printing in commerce, books and periodicals, the problem of inaccurate printing overprint is also placed in front of various printing manufacturers. Multi-color rotary offset presses overprint The reasons for the inaccuracy are not only complicated, but also in a variety of forms. The most serious and the most common is the inaccurate overprint between different colors, which is reflected in the printed product for the entire format. Two aspects), even partial overprinting in the printed products is not allowed, which makes a large number of products into defective products or waste products, causing great losses to enterprises. This article only analyzes and discusses the mastery and adjustment of color printing overprint accuracy of multi-color web offset printing presses.
Factors affecting overprinting accuracy
1. Pre-press aspects (1) Errors produced when making film.
(2) Errors in the production of printing plates.
2. Errors caused by paper moisture deformation
3. Various factors affecting overprinting caused by the operator's incorrect operation (1) Control of fountain solution (2) Installation of blanket (3) Installation of printing plate
4. Various factors affecting overprinting caused by mechanical aspects (1) Tension system (2) Gear wear of transmission part (3) Register adjustment mechanism
Master and adjust and pay attention to
1. before printing
(1) The collage process should first pay attention to the collage of pictures and text. The accuracy of the collage directly affects the accuracy of the overprint. Because the collage process is a manual operation, and it is an important part of production, in order to create conditions for the next process and improve the accuracy of overprinting, the collage precision must be absolutely accurate, and the collage error should be controlled below 0.02mm.
(2) The copy should pay attention to the following aspects:
â‘ Whether the regulation line is registered.
â‘¡Check whether the air intake of the copying machine is normal, and check whether the sealing strip leaks to ensure the flatness of the film.
â‘¢ Control the amount of film expansion and contraction, control the temperature of the developer and the temperature and humidity of the working environment, so as to improve the accuracy of overprinting.
â‘£When the printing plate is produced, because the multi-color printing takes four-color printing as an example, each type of printing needs four plates. During the printing process, due to insufficient air intake of the printing machine or leakage of the sealing strip The flatness of the plate and film is affected, and the accuracy of overprinting is reduced. Therefore, this problem cannot be ignored. Before printing, you should check whether the printing machine is intact and the flatness of the plate and film to improve the accuracy of overprinting. In addition, in order to improve the durability of the plate material, all manufacturers have adopted the baking process. Because the temperature and time of the drying plate are inaccurate, the change of the printing plate is too large, which affects the stability of the plate size, and causes the problem of inaccurate overprinting. Time to minimize the deformation of the plate.
2. Paper moisture deformation
The rotary offset press is printed on web paper. After unpacking the web, the humidity (water content) of the outside paper is greater than the water content of the paper. Both sides of the paper roll are stretched after absorbing moisture, and vice versa. (Moisture content) is lower than the moisture content of the paper, the two sides of the paper roll will distribute moisture, as a result, the edge of the paper shrinks and tightens due to dehumidification, resulting in a lotus leaf edge. In these two cases, the pressure of the printing paper will cause some parts of the paper to change and the overprint will be inaccurate. In order to avoid this situation, you should pay attention to the timing of unpacking. Machine printing and controlling the humidity of the environment between 60% and 65%.
3. The machine staff should master correctly
(1) Control of fountain solution. Under the premise of no dirty version, the minimum amount of water is used for printing to reduce the change of the paper due to water absorption to improve the accuracy of overprinting. Because the paper has the characteristics of hygroscopic deformation, the paper absorbs moisture (the fountain solution), which causes the fiber to wet and expand. Under the pressure of the printing cylinder, the paper changes and elongates and widens. The imprint does not form an overprint when printing. Not allowed. In the case where the registration agency cannot adjust, there are two forms of expression:
â‘ It occurs in the circumferential direction, that is, the circumferential overprinting is not accurate. You can reduce or increase the thickness of the plate cylinder cover, that is, change the diameter of the plate cylinder to change the size of the printing format to reduce overprinting errors. The specific thickness of the liner depends on the overprint error.
â‘¡The overprint that occurs in the axial direction is not accurate. You can install a bracket between the printing units to lift the paper in the middle, which is to raise a certain point of the paper tape. Once the paper tape rises in the middle, it will inevitably cause the two sides of the paper to shrink towards the middle. This will cause the print to be widened due to paper changes after printing through a certain color. There will be a change before the next printing, which is narrowing to improve the accuracy of overprinting. The method can reduce the axial overprint error by 50%. The height of the specific bracket should be determined according to the actual situation.
(2) The correct installation of the blanket. Because the blanket needs to be tightened on the roller, if the degree of tension is different, due to uneven surface tension, the stress in some areas is stretched tightly, and in some areas the stress is stretched small, so the blanket elongation rate is Large and small, the size of the picture and text also changes, so it will lead to inaccurate overprinting of some pictures and text, so when installing the blanket, you must try to make the tension level consistent.
(3) Correct installation of printing plate. Since the plate cylinders of the current rotary offset printing presses mostly use a socket type (or called a plate-hanging type), the printing plate needs a bending process before installation. Although the current bending machine has a positioning device, the plate has different degrees of deformation (unevenness) after baking. If you do not pay attention to the parallelism of the plate and the graphic, forming a twist angle will affect the accuracy of overprinting. Because the current rotary offset press does not have a diagonal adjustment mechanism (except for commercial rotary offset presses), it should be as accurate as possible when bending to improve the accuracy of overprinting.
4. Reasons for inaccurate overprinting in mechanical
(1) First of all, we must understand the paper feeding mechanism of the rotary offset press. Its main component is a tension control system, which controls the smoothness of the paper belt and does not move left and right, so that the graphic overprint is accurate. If the tension system is unstable or Insufficient tension will cause overprint error, so in order to accurately control the tension and ensure the stability of the tension system, we must carefully maintain and maintain the components that will affect the tension change. The paper feeding mechanism of the rotary offset printing press mainly has the following parts:
â‘ The part of the paper frame, the brake pads of the paper roll tension control system are worn, the gap between the brake pads and the brake disc, and the leakage of the brake seal airbag will cause the accuracy of the printing overprint to decrease.
â‘¡The floating roller is an important part of tension stabilization. The tension control system, which is composed of pneumatic control elements to control cylinders and floating rollers, keeps the tension stable during the printing process. If the pneumatic control part of the tension control system and the cylinder have air leakage or poor movement, it will directly affect the stability of the tension and cause the overprint accuracy to decrease.
â‘¢ The bearings of the paper-pulling roller and the paper-storing roller in each part must rotate flexibly, otherwise it will inevitably affect the tension of the paper belt, resulting in a decrease in the accuracy of overprinting.
(2) The cylinder bearing is the supporting point for the printing cylinder to provide printing pressure. Since the high-speed operation is always maintained during the production process, the wear will be caused over time, the bearing accuracy will be reduced, and the instantaneous operation will not be smooth during the operation, which will cause the rollers to be out of synchronization and cause overprinting. In order to avoid the occurrence of similar problems, it is necessary to carefully check the lubrication system, and always keep the roller bearings in a good working environment.
(3) The transmission gear part and the roller bearing have the same effect. Because the gear will cause wear during high-speed operation, the gear gap will be too large, and when the pressure is applied during the work, it will cause a momentary pause in the printing cylinder, that is, not Synchronization results in inaccurate overprinting, so the gear transmission must be stable, and special attention must be paid to the lubrication system.
(4) The registration agency should conduct regular inspections. Because the registration adjustment mechanism relies on the self-locking of the worm gear, worm, screw, and nut to stabilize and adjust the printing cylinder, if they are worn out, a gap will be formed and the displacement of the printing cylinder will be caused, resulting in a decrease in the accuracy of the printing.
In summary, it can be seen that the overprinting accuracy of the multi-color web offset printing press is affected and restricted by various aspects. In the actual production process, it must be combined with reality, scientific analysis should be carried out to find out the true cause of the overprinting inaccuracy, and use scientific methods to solve it
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