(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
In the printing of color newspapers, the issue of overprinting is almost the most important criterion for evaluating the quality of newspapers. In the actual production, there are many factors that can affect the accuracy of the registration of color newspapers. It can be said that almost all the links in the production of color newspapers can affect the printing of the newspaper, such as: phototypesetting, printing, paper, fountain solution , Printing process, printing equipment, etc. In this article, some representative issues that affect the quality of overprinting will be discussed.
Prepress process:
(1) The phototypesetting process leads to:
After the pre-press processing and imposition process of the color graphics and text, the laser phototypesetting machine is finally exposed on the film. This type of exposure is performed on the four-color film by the imagesetter after color separation. However, due to the influence of mechanical transmission and other factors, there is no complete correspondence between the four films (in the sense of registration). This kind of error always exists and cannot disappear due to the improvement of the operation level.
(2) Error in film imposition
In many printing and printing manufacturers, because there is no direct phototypesetting machine, you can only use four or less film to make a pair of folios, and then print. However, since this process is a manual operation process, high requirements are placed on the operator's proficiency and detail. For the same imposition process, the results for operators of different levels will definitely not be the same. But even the most excellent imposition and printing operators will never achieve the elimination of this type of overprint error, which is always objective.
(3) Punching and bending error of printing plate
The printed plate must go through the operation of punching and bending before being put on the machine, but since this operation is still largely dependent on human operation, no matter how delicate special equipment is used, the existence of errors cannot be avoided. .
The above three points are all links that are easy to affect the printing overprint in the pre-press process. Although we found that the overprint error of each link is not very large, and the error of any single link is not enough to produce more overprint under normal circumstances. It is a serious impact, but we also mentioned that because these errors can only be reduced and cannot be avoided, the impact of the error stacking on printing overprint can not be ignored. It can be seen that improving the quality and sense of responsibility of the operators in the pre-press link is very important for the overprinting of color newspapers.
Printing process:
Factors that affect the stability of the tape tension (such as faults in the tension control system that make the tension of the tape unstable cause overprint errors) and factors that affect the precise transfer of graphics (such as the wear of the printing press roller bearings. Running eccentricity, etc. cause the roller The pressure is not normal, the axis of the cylinder is not parallel, and the positioning pin of the plate mouth is offset, etc., so that the printing plate graphics cannot be transferred accurately and produce overprint errors).
(1) Factors affecting the stability of the tape tension
Different types of high-speed wheel transfer newspaper machines have different methods and methods for controlling the tension of the paper tape, and the paper paper takes different paths between the printing units of the printing press. Judging from the current mainstream equipment used in domestic color newspaper printing, the following printed parts will have an impact on the color newspaper registration:
During the printing process, the tension of the paper tape is affected by the above components, and any component problems will affect the stability of the tape tension. In turn, it affects the accuracy of newspaper registration. The following is a detailed analysis of the impact of each component on the registration of the color newspaper.
â‘ Paper holder.
The brake pads of the press brake system of the paper roll of the printing press are worn. Changes in the gap between the brake pads and the brake disc, leakage of the brake seal airbag, etc., will have a great impact on the stability of the tape tension.
â‘¡ Floating roller.
The floating roller is an important component to ensure the stability of the printing tape tension in the newspaper printing process. Tension control generally refers to the process of tension control system composed of air path control elements, air cylinders and floating rollers, etc., to keep the tension stable during the printing process. The mechanical factors that affect the registration of the color newspaper mainly include the steel wire connecting the sliders at both ends of the floating roller, the sliding guide rail and the sliding bearing on the slider. In actual production, when the paper tape changes, such as changing paper, loading a new paper roll, etc., if the wire in this part is fastened. The sliding bearing and the guide rail have good working performance, and the floating roller is sensitive to the change of the paper tape, so the tension stability of the paper tape during printing is good; otherwise, if the steel wire is loose, the sliding bearing is dry and lacks oil, or paper dust is accumulated on the guide rail, making If the floating roller reacts slowly to the change of the paper belt, the tension stability of the paper belt is poor.
â‘¢Pre-tension unit, pull paper rollers in various parts.
In order to achieve the best printing effect, the different parts of the entire printing production line have different requirements for the tension of the paper tape. The pulling rollers with independent driving capabilities are installed in different parts of the machine to control the tension of the paper path section to meet each Partial tension requirements. The function of the pre-tension unit is similar to the paper passing roller. Its function is to further control the tension of the tape entering each printing unit during the printing process, compensate for the influence of the paper path on the tension of the tape, and stabilize the tape tension of the printing unit. To get better printing effect. However, the prerequisite for normal operation is that each paper roller, especially the paper rollers before and after each printing unit, must have a good transmission, and the speed overshoot value is set correctly. Otherwise, it will inevitably affect the tension of the paper belt of each printing unit, resulting in inaccurate overprint .
â‘£Overprint and cover cut paper storage roller.
For shaft-driven printing presses, the registration between the upper and lower units is mainly achieved by the paper storage roller. The overprinting and sleeve cutting paper storage roller is composed of a paper passing roller with a fixed axis and a cantilever roller. Under the control of the register motor, the cantilever roller can rotate horizontally around the axis of the fixed roller. Therefore, the horizontality of the cantilever roller and the stability of its position during the printing process will directly affect the color registration between the upper and lower units. At the same time, the stability of the tape tension will also be affected.
⑤ Paper guide rollers in various parts.
The paper guide roller is the guide roller of the printing paper path. The reasonable setting of the paper passing rollers in various parts of the printing machine can optimize the paper feeding method and also facilitate the control of the tape tension. Therefore, it is necessary to ensure that the level of each paper passing roller is consistent, especially the level of the paper passing rollers of the upper and lower printing units, which is an important factor affecting the registration between the upper and lower units.
â‘¥ In addition, the rotating bearings of the above types of rollers must be regularly inspected. If necessary, new bearings should be replaced to prevent the bearing from running into the inner and outer rings caused by wear, oil shortage, and seizure, to ensure that the bearings are in good working condition. Avoid inaccurate overprinting of color newspapers due to bearing problems.
(2) Factors affecting the precise transfer of graphics
The printing unit is a direct printing and processing component that realizes the precise transfer of graphics from the printing plate to the paper. Therefore, any mechanical deviation of the printing unit will affect the precise transfer of newspaper graphics. According to experience, the mechanical factors that affect the registration of color newspapers in the printing unit are as follows:
â‘ Version pin. The plate pin is a positioning component that enables the accurate positioning of the printing plate on the plate cylinder. If the plate pin of a certain color plate is worn, loose, detached or improperly installed, it will inevitably cause the overprint of the corresponding color image of the plate.
â‘¡Roller bearing. The cylinder bearing is the support body of the printing cylinder, and is also the main mechanism for providing printing pressure for each printing cylinder and achieving parallel cylinder axis. If the roller bearing is worn and the eccentric position of the bearing is deviated, the printing pressure of the non-parallel roller will be changed, which will make the overprint of the color newspaper inaccurate and flowering.
â‘¢ Clutch pressure connecting rod. The clutch pressure connecting rod is a control operating mechanism that makes the printing cylinder clutch pressure, stabilizes bearing eccentricity, avoids damping vibration, and ensures that the axes of each printing cylinder are parallel. The precision of the gap between the connecting rod support and the copper sleeve and bearing is very high. If the connecting rod support and the copper sleeve are seriously worn or the bearing is loose, it will definitely affect the registration accuracy of the newspaper.
â‘£ Register adjustment mechanism. The registration mechanism is an adjustment mechanism that realizes accurate registration of various colors and graphics. During the printing process, the registration mechanism is to achieve axial and circumferential registration by pulling the printing plate or rubber cylinder axially, and to keep the corresponding axial position of the cylinder unchanged of. The bearing of the registration mechanism rotates together with the roller at high speed, which is prone to the failure of the outer ring running out of oil, which causes the bearing end surface to contact the roller to wear or form a gap. When the roller is pulled in the circumferential direction, there is a backlash or the axial position of the roller is uncertain , Resulting in inaccurate overprint of color newspapers.
⑤ Edition mouth. Paper breaks, roll blankets, high-speed emergency stops and other events often cause deviations in the position of a plate cylinder in the printing process. For shaft-driven high-speed wheel transfer newspapers, the axial and circumferential registration The adjustment range is limited, so when there is a deviation in the position of the plate opening, the circumferential overprint of the corresponding plate will be affected, and even the front and back of the overprint will be inaccurate.
â‘¥ The main motor is also one of the mechanical factors that affect the registration of the color newspaper. Because there is a gap between the printing drum gears, when the motor drive voltage fluctuates or malfunctions, such as the speed motor failure causes the main motor speed to be unstable, the cylinder gears will have forward and backward strokes, which will cause the overprinting of the color newspaper. Accurate, and even ghosting and flowering.
printing material:
The printed materials mentioned here mainly refer to newsprint, fountain solution, etc.
(1) Newsprint is affected by environmental temperature and humidity.
In the printing workshop, the change of paper moisture content must not exceed ± 0.1%, otherwise it will affect the accuracy of overprinting. When the humidity in the printing workshop is too high, the newsprint easily absorbs moisture and expands and contracts. It appears as a "lotus leaf edge" on both sides of the tape. When the humidity in the workshop is lower than the newsprint, the edge of the paper roll will appear tight. Of course, this kind of paper cannot guarantee normal overprinting when it is printed on the machine, and it is easy to cause other problems such as broken tape or wrinkles.
Workshop temperature and workshop humidity are closely related to the moisture content of the paper. Experiments have shown that when the relative humidity of the workshop changes by 10%, the moisture content of the paper will change by 1%. If the relative humidity of the workshop exceeds 80%, when the humidity changes by 10%, the moisture content of the paper will change accordingly. 1.5 ～ 2%; when the temperature changes, the humidity of the workshop will change accordingly. Every time the workshop temperature changes by 5 ℃, the moisture content of the paper will change by 0.15%
The moisture content of the paper is closely related to the surface form stability of the paper. On the premise that the form stability of the paper cannot be guaranteed, the stability of the overprinting can of course be guaranteed. Therefore, it is very important to ensure the stability and standardization of the temperature and humidity of the workshop for the overprint accuracy of printed color newspapers. It is generally recommended that the temperature in the printing workshop should be maintained at 18 to 24 ° C, the temperature change should not exceed ± 3 ° C, and the relative humidity should be controlled within the range of 55 to 65%.
(2) The effect of fountain solution on overprinting.
Similar to the effect of environmental temperature and humidity mentioned above on the overprint of color newspapers, fountain solution will also affect the size of the surface area of ​​the paper by changing the humidity inside the paper, thus having a certain impact on the overprint during the printing process. We can't avoid this kind of influence. In actual work, we always require the minimum amount of water to be used under the premise of ensuring the balance of water and ink. Minimize the resulting overprint changes.
In summary, in color newspaper printing, to ensure the standard operation before printing; maintain the stability of the machine work; fully understand the printability of the printed materials; these factors can help us to further ensure the quality of overprint printing.
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