With multi-color rotary offset presses increasingly equipped with printing plants and playing an important role in the color printing of commercials, books and magazines, the problem of inaccurate printing overprinting is also placed in front of the printing manufacturers, multi-color rotary offset printing The reason for the inaccuracy is not only intricate, but also manifests itself in a variety of forms. The most serious and common is the misregistration between colors, which is reflected in the printed product, which is misaligned across the entire range of colors (divided into axial and circumferential directions). In terms of two aspects, even partial overprinting of printed products is inaccurate, causing a large number of products to become defective products or waste products, causing enterprises to suffer great losses. In this paper, we only analyze and discuss the mastering and adjustment of the color printing overprinting accuracy of multi-color rotary offset presses.
Factors affecting the accuracy of overprinting
1. Prepress (1) Errors in film production.
(2) Errors produced when making plates.
2. Inaccuracy caused by paper moisture deformation
3. Various factors caused by incorrect operation of the operator. Overprinting factors (1) Control of the fountain solution. (2) Installation of the blanket. (3) Installation of the printing plate
4. Various factors caused by mechanical overprinting factors (1) Tension system (2) Gear wear of the transmission part (3) Registration adjustment mechanism
Mastering and adjusting and paying attention to
(1) The collage process should first pay attention to the collage of images and texts, and the accuracy of the collage will directly affect the accuracy of overprinting. Because the collage process is a manual operation, and it is an important part of the production, in order to create the conditions for the next process, improve the overlay accuracy, the accuracy of the collage must be absolutely accurate, and the collage error should be controlled below 0.02mm.
(2) Copying should pay attention to the following aspects:
Is the 1 line registered?
2 Check whether the suction volume of the duplicator is normal and check whether the sealing strip leaks to ensure the flatness of the film.
3 control the amount of expansion and contraction of the film, control the temperature of the developer and the temperature and humidity of the working environment, in order to help improve the accuracy of overprinting.
4 In the production of printing plates, because multi-color printing uses four-color printing products as an example, four printing plates are required for each type of printing product. During the printing process, the amount of air drawn by the printing machine is insufficient or the sealing tape leaks. Affecting the flatness of media and film, resulting in a decline in the accuracy of overprinting, so can not ignore the problem in this regard, before the printing work should be checked whether the plate is in good condition and the flatness of the plate and film to improve overprint accuracy. In addition, in order to improve the printing material's resistance to printing, all manufacturers have adopted a baking process. Because mastering the temperature and time of the baking plate caused by the inaccurate change of the printing plate greatly affected the dimensional stability of the plate layout and caused overprinting problems. Therefore, the temperature and the baking plate of the baking plate must be accurately adjusted according to the requirements of the printing plate. Time, minimizing the deformation of the plate.
2. Hygroscopic deformation of paper
The web offset press is printed on a web. After the web is unpacked, the humidity (water content) of the web is greater than the water content of the web. The two edges of the web are stretched after absorbing moisture, conversely if the external humidity The (moisture content) is lower than the moisture content of the paper, and moisture is emitted from both sides of the paper roll. As a result, the edges of the paper shrink and tighten due to moisture removal, resulting in a flounced edge. In both cases, the pressure of the printing paper by the printing cylinder will cause certain parts of the paper to change and overprinting will occur. To avoid this situation as much as possible, it is necessary to pay attention to the timing of unpacking, and it is best to follow the instructions. The machine prints and controls the humidity of the environment between 60% and 65%.
3. Machine personnel must master correctly
(1) Control of fountain solution. Under the premise of not contaminating the plate, use the smallest amount of water to print, reduce the change caused by the paper due to water absorption, in order to improve the overlay accuracy. Because the paper has the characteristics of moisture absorption and deformation, the paper absorbs moisture (fouding solution), which causes the fiber to wet and expand. Under the pressure of the printing roller, the paper is changed to elongate and widen, causing the print to not print at one point during printing. Not allowed. In the case where the registering institution cannot be adjusted, there are two manifestations:
1 Occurs in the circumferential direction, that is, overprinting is not allowed in the circumferential direction. The thickness of the plate cylinder liner can be reduced or increased, that is, the diameter of the printing plate cylinder can be changed to change the size of the printing web to reduce overprinting errors. The specific lining thickness dimension depends on the overlay error.
2 The misalignment occurs in the axial direction. A bracket can be installed between the printing units to raise the middle of the paper, that is, to raise a point on the paper. Once the middle of the tape rises, the two sides of the paper will tend to shrink toward each other. This will cause the width of the print to become wider after a certain color printing due to the change of the paper. A change before the next printing is narrowing to improve the overlay accuracy. The method can reduce the axial overprint error by 50%. The height of the specific bracket should be based on the actual situation.
(2) The correct installation of the blanket. Because the blanket needs to be tight on the cylinder, if the tension is not uniform, due to the uneven surface tension, the stress in some areas will be stretched tightly, and the stress in some areas will be loose. Thus, the blanket has elongation when printed. Large and small, the size of the picture and text will also change, so it will lead to the overprinting of the partial picture and text. Therefore, when installing the blanket, it must be as uniform as possible.
(3) The correct installation of the plate. Due to the fact that the rotary cylinders of rotary offset presses are mostly of a socket type (or a hanging type), the plate needs to be bent before the installation. Although the current bending machine has a positioning device, the plate material has different degrees of deformation (unevenness) after baking. If the plate does not pay attention to the degree of parallelism between the plate and the graphic, the formation of a twist angle will affect the overlay accuracy. Because web offset presses currently do not have diagonal adjustment mechanisms (except for commercial web offset presses), they should be as accurate as possible in order to improve overlay accuracy.
4. Reasons for overprinting due to mechanical reasons
(1) The first thing to understand is the web transfer mechanism of a web offset press. Its main component is a tension control system, which controls the flatness and non-movement of the paper web, so that the graphic overlay is accurate if the tension system is unstable or Insufficient tension can cause overprinting errors. Therefore, in order to accurately control the tension and ensure the stability of the tension system, it is necessary to perform serious maintenance and maintenance on each component that will affect the tension change. The paper feeding mechanism of web offset press mainly has the following parts:
1 In the paper holder section, the wear of the brake pads of the paper roll tension control system, the gap between the brake pads and the brake disc, and the leakage of the brake sealed airbag will all result in a decrease in the accuracy of the overprinting.
2 The floating roller is an important part of the tension stability. The tension control system composed of pneumatic control elements to control the cylinder and the floating roller keeps the tension stable in the printing process. If the pneumatic control part of the tension control system and the air cylinder have a leak or the operation is not smooth, it will directly affect the stability of the tension and cause the overlay accuracy to decrease.
3 The bearing of each part of the paper-pulling roller and the paper-storing roller must be rotated flexibly, otherwise it will inevitably affect the tension of the paper-tape, leading to a decrease in overlay accuracy.
(2) The roller bearing is the support point for the printing roller to provide printing pressure. As the production process always maintains high-speed operation, wear over time, bearing accuracy decreases, and the operation is not smooth during the operation, causing the rollers to be out of synchronization and overprinting is not allowed. In order to avoid the occurrence of similar problems, it is necessary to do a good job of checking the lubrication system and always maintain the roller bearings working in a good working environment.
(3) The transmission gear part and the roller bearing have the same function, because the gear will cause wear during the high-speed operation process, and the gear gap will be too large. When some pressure is applied to the work, it will cause the printing roller to stop momentarily, that is not Synchronization, resulting in inaccurate overprinting, must ensure that the gear transmission is stable, with particular attention to the lubrication system.
(4) Registering agencies should always give inspections. Since the register adjusting mechanism is based on the self-locking of the worm wheel, the worm screw, the screw rod, and the nut, the printing cylinder is stabilized and adjusted. If they wear out, a gap will be formed and the displacement of the printing cylinder will cause the overlay accuracy to decrease.
In summary, it can be known that the overprint accuracy of multi-color rotary offset press is affected and restricted by many aspects. In the actual production process, it must be combined with the actual situation to carry out scientific analysis to find out the real reason for overprinting and use scientific methods to solve it. .
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