Multi-color web offset printing machine control of color printing overprint precision
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As multi-color web offset presses are increasingly equipped with printing plants and play an important role in color printing such as commercials, books and magazines, the problem of inaccurate printing overprinting is also placed in front of various printing manufacturers, multi-color web offset press overprinting. The reasons for the inaccuracy are not only complicated, but also their manifestations are varied. The most serious and most common is the misregistration of the colors, which is reflected in the printing of the entire format of the color inaccurate (divided into axial, circumferential Two aspects), even in the printing of partial overprinting is not allowed, so that a large number of products become defective or scrap, causing great losses to the enterprise. Here, we only analyze and discuss the mastery and adjustment of the color printing overprinting of multi-color web offset presses.
Factors affecting overprint accuracy
1.Prepress aspect
(1) Errors caused when making film.
(2) Errors caused when making plates.
2. Error caused by hygroscopic deformation of paper
3. Various factors affecting overprinting caused by incorrect operation of the operator
(1) Control of dampening solution
(2) Installation of blanket
(3) Installation of printing plates
4. Various factors affecting overprinting caused by mechanical aspects
(1) Tension system
(2) Gear wear in the transmission part
(3) Register adjustment mechanism
Mastery and adjustment and aspects to be aware of
Prepress
(1) The collage process must first pay attention to the collage of graphics and texts. The accuracy of the collage directly affects the accuracy of overprinting. Because the collage process is manual operation, and it is an important part of the production process, in order to create conditions for the next process and improve the overprint precision, the accuracy of the collage should be absolutely accurate, and the tile error should be controlled below 0.02 mm.
(2) Copying should pay attention to the following aspects:
1 Whether the line is registered.
2 Check if the air intake of the copy machine is normal, and check whether the seal 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 to improve the accuracy of the overprint.
4 printing plates are produced. Because multi-color printing uses four-color printing products as an example, each printing product requires four plates. In the process of printing, the insufficiency of the printing machine or the leakage of the sealing strip will be Affect the flatness of the plate and film, and the overprinting accuracy is reduced, so this problem cannot be ignored. Before the printing work, check whether the printing machine is in good condition and the flatness of the plate and film to improve the overprinting accuracy. In addition, in order to improve the printing durability of the plate, each manufacturer has adopted a baking process. Due to the inaccurate temperature and time of the baking plate, the change of the printing plate is too large, which affects the dimensional stability of the printing plate, which causes the problem of inaccurate overprinting. Therefore, the temperature and baking of the baking plate should be accurately adjusted according to the requirements of the printing product. Time to minimize distortion of the media.
2. Paper hygroscopic deformation
The web offset printing machine is printed on a web. After the web is unpacked, the external humidity (water content) is greater than the water content of the paper, and the two sides of the paper roll absorb moisture and then elongate. (Water content) is lower than the water content of the paper, and the edges of the paper roll emit moisture, and as a result, the edge of the paper shrinks due to dewetting, resulting in a ruffle. In these two cases, the pressure of the printing paper by the printing cylinder will cause some parts of the paper to change and the overprinting may occur. In order to avoid this situation, it is necessary to pay attention to the timing of unpacking. Machine printing and control the humidity of the environment between 60% and 65%.
3. Machine personnel must master correctly
(1) Control of dampening solution. Under the premise of not being dirty, the printing is carried out with a minimum amount of water to reduce the change of the paper due to water absorption, so as to improve the overprinting precision. Because the paper has hygroscopic deformation characteristics, the paper absorbs moisture (dampening solution), causing the fiber to wet and expand. Under the pressure of the printing cylinder, the paper is changed and elongated and widened, so that the printing does not form an overprint at one point during printing. Not allowed. In the case that the registration mechanism cannot be adjusted, there are two manifestations:
1 occurs in the circumferential direction, that is, circumferential overprinting is not allowed. The thickness of the printing cylinder can be changed by reducing or increasing the thickness of the plate cylinder lining to change the printing cylinder to reduce the overprinting error. The specific thickness of the lining depends on the overprinting error.
2 In the axial direction, the overprinting is not allowed. A bracket can be installed 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 strip is raised in the middle, the two sides of the paper will shrink toward the middle, which makes the print become wider after printing through a certain color, and there is a change before the next printing, that is, narrowing to improve the overprint precision. The method can reduce the axial overprinting error by 50%. The height of the specific bracket should be determined according to the actual situation.
(2) Correct installation of the blanket. Since the blanket needs to be stretched on the drum, if the degree of tension is different, due to uneven surface tension, the stress in some areas is too tight, and the stress in some areas is small and loose, so that the elongation of the blanket during printing is Large or small, the size of the graphic will also change, so it will result in inaccurate overprinting of the partial graphic, so it is necessary to make the tension as uniform as possible when installing the blanket.
(3) Correct installation of the plate. Since the current rotary offset printing plate cylinders are mostly socket type (or hanging type), the printing plate needs to be bent before installation. Although the current bending machine has a positioning device, the plate material has different degrees of deformation (unevenness) after baking, and if the parallelism of the plate material and the graphic is not paid attention to, the twist angle is formed, which will affect the overprinting precision. Because the current rotary offset printing machine does not have a diagonal adjustment mechanism (except for commercial rotary offset printing machines), it should be as accurate as possible to improve the overprinting accuracy.
4. Reasons for inaccurate overprinting caused by mechanical aspects
(1) First of all, we must understand the paper feeding mechanism of the web offset press. Its main component is a tension control system, which controls the flatness of the paper strip and does not move left and right, so that the graphic overprinting is accurate, if the tension system is unstable or Insufficient tension can cause overprinting errors, so in order to accurately control the tension and ensure the stability of the tension system, careful maintenance and maintenance of the components that will affect the tension must be performed. The paper feeding mechanism of the web offset press mainly has the following parts:
1 The paper frame part, the brake pad of the paper roll tension control system wears, the gap between the brake pad and the brake disk, the brake seal air bag leakage will cause the printing overprint precision to decrease.
2 The floating roller is an important part of the tension stability. The tension control system consisting of the pneumatic control element controlling the cylinder and the floating roller 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 overprinting accuracy to decrease.
3 The bearings of the paper-feeding roller and paper-feeding roller of each part must be flexible, otherwise the tension of the paper tape will be affected, resulting in a decrease in overprinting accuracy.
(2) The roller bearing is a support point for the printing cylinder to provide printing pressure. Since the high-speed operation is always maintained in the production process, the wear is caused over a long period of time, the bearing precision is lowered, and the instantaneous operation is not smooth during the operation, thereby causing the cylinder to be out of synchronization and causing misregistration. In order to avoid the occurrence of similar problems, we must carefully check the lubrication system and always keep 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 high-speed operation, the gear gap is too large. When some pressure is applied during the work, the printing cylinder will be stopped temporarily, that is, no Synchronization, resulting in inaccurate overprinting, so we must ensure smooth gear transmission, pay special attention to the lubrication system.
(4) Registration agencies should always be inspected. Since the register adjustment mechanism relies on the self-locking of the worm wheel, the worm, the screw rod and the nut to stabilize and adjust the printing cylinder, if they wear, a gap is formed and the displacement of the printing cylinder causes the overprinting precision to decrease.
In summary, it can be seen that the overprinting accuracy of multi-color web offset presses is affected and restricted by many aspects. In the actual production process, scientific analysis must be carried out in combination with the actual situation to find out the real reason for inaccurate overprinting and solve it with scientific methods.