The Solution Process and Cause Analysis of Color Header Color Problem
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Most newspapers use color headers, the most common of which are red headers in magenta and yellow solid overlays. The newspapers printed by our company, such as "Qingdao Daily", "Qingdao Morning Post" and "Qingdao Evening News", all use red headers. During the printing process, a small amount of black, that is, cross-color, often appears on the red header, because the black is added, making the header look dirty. In this paper, the author introduces the solution process and cause analysis of this problem to the big characters for your reference.
Solution process
First of all, we communicated with the captain. According to the description of the captain and the observation of the scene, we found that after printing 30,000 to 40,000 newspapers, the red header will start to appear black, and the situation is lighter at first. As the printing continues, the cross-color will be aggravated, and the red header will appear more and more heavy black, which looks dirty. At this time, the captain's processing method is to replace the black version, so that it can be adhered to the printing.
At that time, the printing color sequence was green, yellow, black. At the end of the black, the head color magenta and yellow are in the second and third colors, respectively. The presses are all vertical, from bottom to top are cyan, magenta, yellow, and black. This color sequence, together with the vertical placement of the printing press, reminds us of whether the black ink has entered the ink fountain or rubber roller of magenta ink or yellow ink in various ways, so we carefully checked it. Printing press. As a result, it was found that all the ink fountains did not have a lid, and the ink in the yellow ink fountain was obviously not pure yellow, and the main pollution included the washing water in which the blanket of the black and yellow unit was dripped. So, it's no wonder that the color of the header will be black. After finding the cause of the problem, the shop immediately cleaned the yellow ink fountain and re-applied the magenta ink fountain and the yellow ink fountain.
I thought that this cross-color problem would be solved. But the actual situation is not as we think, the header still appears cross-color.
We also think that there is a problem with the plate? But we have been using the big brand plate, the quality is very stable, so we decided not to consider the issue of the plate, continue to check around the press. In further communication with the captain, it was found that a new fountain solution was being tested on the machine recently, so we took the acidity meter to the equipment for on-site measurement and found that the pH of the fountain solution in the tank was too high. We have also taken samples from the unsealed vats of fountain solution. After various concentrations, the buffering ability of the fountain solution is poor. The slight change of the ratio will cause the fountain solution to change greatly. The results of the buffering experiments of two different brands of dampening solution are shown in Fig. 1. This is also the reason why the pH of the dampening solution in the water tank is too high.
To this end, we immediately cleaned the water tank and replaced the latest used fountain solution with the stable quality fountain solution that we have been using. In this way, the problem is solved and the header no longer appears to be cross-colored.
Cause Analysis
In the end, what is the cross color? Why does the replacement of the dampening solution cause cross-color? What is the reason for the cross-color? Below, the author describes them one by one.
Cross-color refers to the text of a certain area composed of a specific color, but mixed with other colors. Then, why is the excess color printed on the paper? This is also from the principle of offset printing. The printing plate used for offset printing consists of a graphic part and a blank part. The graphic part is composed of a drug film, and the surface free energy is lower than the free energy of the ink surface, and is higher than the free energy of the surface of the dampening solution, so that the ink can be well adsorbed and the dampening solution is repelled. The blank portion is composed of porous alumina and belongs to a high free energy surface, which is higher than the surface free energy of the ink, so that water and ink can be spread on the surface thereof. At this time, in order to make the printing run smoothly, it is necessary to let the blank part firstly spread a layer of dampening solution, because the surface free energy of the dampening solution is smaller than the ink, and the blank part is occupied by the dampening solution, the ink is difficult to occupy the blank part again. . In this way, the printing plate is in contact with the ink, and only the image portion can absorb the ink, the blank portion remains clean, and the printing proceeds smoothly.
From the principle of offset printing, cross-color does not occur under normal conditions, but cross-coloring occurs frequently in offset printing. The intuitive reason is that the ink is contaminated. For example, in the above example, it is obvious that there is black in the yellow ink. Car wash water pollutants. For other reasons, we may wish to take a reverse approach. As you can see from the beginning of the article, in the case of cyan, magenta, yellow, and black color sequence printing, the header itself has been printed after the two prints of the magenta and yellow units, but the newspaper has so far only Three prints were finished and black was missing. Although there was no black component in the header, this did not prevent the paper from being stamped again in the black printing section. It was this embossing that made a series of colors.
When the paper enters the black printing section, it must be embossed with the rubber cylinder of the black unit. At this time, the color of the header (magenta + yellow) will be transferred to the black blanket according to a certain ratio, and then the blanket and the printing plate need to be pressed again. Printing, if the hydrophilic properties of the printing plate are reduced, or the ability to repel the ink is reduced, then the ink on the blanket may be transferred to the blank portion of the black printing plate. As the number of imprints increases, the ink transferred to the black unit plate forms a relatively stable oleophilic layer on the plate. When the oleophilic layer and the black unit are in contact with the rubber roller, they are infiltrated by the black ink, then transferred to the blanket and then transferred to the paper. At this point, it is found that the header begins to have a gray component, and if no interference is imposed, As it develops, it will appear dark gray. Thus, the cross color appears. At the same time, we found that the decrease in hydrophilicity of the blank portion of the printing plate is a condition for the occurrence of cross color.
Below we next analyze the reasons for the decline in hydrophilicity of the printing plate. The blank portion of the plate is hydrophilic because it is wetted by the ink before it can be repelled. The lack of hydrophilicity of the blank portion is the only cause of ink intrusion. The most direct cause is that the blank part of the printing plate is not cleaned and there is a residual film. This problem is caused by the plate making workshop. The method of inspection is to observe the development rinse control zone at the end of the plate. Each manufacturer's plate is equipped with different styles of control strips. For example, the control strip of Agfa's plate is a yin and yang line. If the width of the line is equal to the width of the line, it means that the development is suitable. In addition, the hydrophilic layer of the blank portion is continuously worn as the printing progresses, that is, the grinding plate. Under normal circumstances, the fountain solution can be continuously chemically reacted with the blank portion of the layout to form a new inorganic salt layer for replenishing the worn parts to ensure normal printing. However, if the concentration of the dampening solution is too low, or the quality of the dampening solution is poor, the blank part of the wear can not be replenished in time, and the hydrophilic property will continue to decrease. When it drops to a certain extent, it is easy to start lipophilic. The blank part of the layout forms a weak lipophilic base, then inks and cross colors.
So far, the cause of the cross-color problem has been analyzed. At first glance, the problem is quite complicated. After the analysis of the sputum, the cause is not very complicated. Therefore, in daily work, we must observe more and analyze more, and many problems will be solved.