The influence and elimination of paper water absorption and contraction in color printing
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In recent years, China's color newspaper printing has developed rapidly. All large and medium-sized newspapers have regularly published color newspapers. Some newspapers have been colorized, and the printing quality of color newspapers has also been greatly improved. Due to the performance characteristics of newsprint, its water absorption is large, and the water absorption and contraction of paper during printing is a major factor affecting the accuracy of color printing. How to control and eliminate the impact of paper water absorption and expansion on the registration accuracy during printing, especially when the equipment and raw materials are not too good for newspaper printing.
First, the cause of paper water absorption and expansion
There is an index of the expansion ratio in the performance of the paper. The expansion ratio is expressed as a percentage of the original size in terms of the dimensional increment, which indicates the relative change of the web after wetting and drying. The news paper fiber is thicker, the density is small, the surface sizing amount is small, and the water absorption elongation is larger than that of the coated paper and the offset paper, which has a great influence on the overprinting precision when printing the color report.
The offset printing process commonly used in color printing is to first apply dampening solution on the plate and then add ink for printing. The main component of the dampening solution is water. The water is transferred from the printing plate to the blanket by pressure and then transferred to the paper. During the printing process, the stretching phenomenon of the paper due to water absorption and drying is inevitable.
The amount of paper stretched has a lot to do with paper performance. The better the paper performance, the smaller the water absorption, the better the dimensional stability after water absorption, and the small expansion ratio, which is very advantageous for color printing. Among various performance indexes of paper, sizing degree, surface strength, water content, tightness, smoothness, two-sidedness, water content, etc. all have a certain influence on the amount of expansion and contraction of the paper. As far as the news papers of different brands such as Jilin, Nanping, Guangzhou, Hansong and Owen are used, the expansion ratio is very different.
Second, the impact of paper stretching on registration accuracy
The water absorption and contraction of the paper has a great influence on the registration accuracy of the color newspaper. In the case that pre-press, imposition, etc. are well done, the water absorption and contraction of the paper cannot be solved well, and the overprinting accuracy is not guaranteed. Imprinting is not allowed to greatly affect the quality of the color report. A set of inaccurate color reports, regardless of the color of ink, dot, color and paper, even the best, the overall sensory effect will be greatly reduced.
The lateral water absorption and contraction of the paper has the most obvious influence on the registration, and the impact is also the most direct. The longitudinal shrinkage produced by the water absorbing and drying of the paper parallel to the running direction of the paper is generally eliminated by adjusting the registration adjustment roller between the units. In the direction of the horizontal direction of the newsprint, the diameter of a fiber can be increased by 30% when it is completely wet, and its length direction is only increased by 1% to 2%. Therefore, the longitudinal stretch of the wire is less in the printing process, which is about horizontal. 1/2 to 1/3 or so, has little effect on the accuracy of overprinting. The expansion and contraction of the transverse direction has a great influence on the overprinting accuracy of the newspaper, and the maximum change can reach about 0.15%. The maximum overprinting error can reach 0.3 mm or more without taking certain elimination measures. Generally, it is a version of the registration. There is no way to register the fourth edition, and this error is gradually expanded from one side. It cannot be adjusted simply by the printing machine. It greatly affects the printing quality of the color newspaper.
The structure of the equipment is different, and the influence of water absorption shrinkage on the registration accuracy is also very different. The printing process of newspapers is to print four colors in sequence on successive sheets of paper. The unit with Y-shaped printing structure generally completes four-color printing together by two units, and each unit prints two colors. If the unit is a tower structure, the paper path between the two units is short, and the shrinkage is not very large; if it is horizontally placed, especially if the drum arrangement is inverted Y-shaped, the paper path will be longer and the shrinkage will be Bigger. The satellite printing unit is very advantageous for the printing of color newspapers. The paper path between the satellite color groups is very short, and there is almost no drying time. The change of the amount of expansion and contraction of the paper after water absorption generally has no effect on the registration. Paper from the satellite printing unit is usually subjected to BB printing (usually black), and there is a certain amount of expansion and contraction of the paper due to a certain period of time.
Third, the absorption and control of water absorption and expansion
Of course, before we talk about the effect of water absorption and telescopic on the accuracy of overprinting, we must solve the other problems of prepress image processing, film output, spelling printing, printing registration, etc. Otherwise, the elimination of water absorption and expansion will be overwhelmed. Among other factors.
The elimination and control of paper water absorption and contraction are mainly carried out from the following aspects:
1. When the paper path between the units is long and the equipment without the fan-shaped expansion device prints the color report, the water absorption and contraction of the paper has a significant influence on the registration accuracy. In this case, according to the amount of expansion and contraction of the paper in the printing, it is possible to eliminate the influence of the water absorption and contraction of the paper on the overprinting precision by performing the imposition and borrowing of the film on the side of the fourth edition. Take the four-page big newspaper as an example. For example, the printing structure of the printing machine is Y-type, the first unit prints C color and M color, and the second unit prints Y color and K color. We found that in the case of accurate imposition and complete registration of the first edition, the overprint error between the leftmost C, M and Y, K of the fourth edition is about 0.2 mm, C and M are left, and Y and K are right. Then, at the time of imposition, we will slap the CM two-color film to the right by 0.2mm, that is, move 0.2mm in the direction of the middle slit to offset the shrinkage caused by the paper drying and then drying.
We noticed that when the film is assembled according to the process requirements, the film requires the film to face up, that is, the film is reversed, and the left and right directions are opposite. Therefore, it is convenient to use the middle seam as the reference line to determine the direction of borrowing; preferably on the layout paper. Draw a line so that the amount of each loan is the same; the paper brand level is different, and the amount of expansion and contraction is different. Different borrowing should be determined for different papers.
2. Control the ink-and-water balance relationship during the printing process. The amount of water in the first unit at the time of printing is the main factor that determines the effect of the amount of expansion and contraction of the paper on the overprint. The amount of dampening water should be uniform and uniform during printing. It should not be too big or small. Especially when using poor quality paper, the water absorption is large and the expansion ratio is relatively large. This problem is more prominent. For equipment with a fan-shaped expansion mechanism such as an eight-color tower unit, it is not necessary to borrow it during the imposition, and it is required to control the amount of water.
3. At present, all factories should pay attention to ordering new equipment. Whether it is a double-frame machine or a single-frame machine, a fan-shaped expansion roller must be added to improve the control of water absorption and contraction of the paper. Due to the different conditions of the paper used in each factory, there is still a certain gap between the quality of domestic paper and the imported paper. It is necessary to control the water absorption and contraction of the paper from the equipment.
4. Try to use good quality paper, not only the amount of water absorption and expansion is small, but also good for printing processes such as dot reproduction, color reproduction, and ink balance. To produce good quality newspapers, the performance of paper is a very important condition.
5. The printing process of the printing press is preferably a tower type, so that the paper path between the four-color printing of the paper is short, which is advantageous for controlling the water absorption and expansion of the paper and improving the overprinting precision. The satellite structure is basically continuous printing between the satellite color groups, which has great advantages for overprinting accuracy, and the reproduction quality of the dots is good, but the cost is high, and the newly ordered machines are relatively rare. At present, we can see a ten-trum tower machine, which is a new type of satellite printing machine with good process structure.
6. The paper used for printing the color report should not be too dry, otherwise it will not only be easy to break the paper when printing, but also has a large water absorption and a high expansion ratio. The storage environment of the paper should meet the requirements of the specification, and the time should not be too long.
7. The outer packaging of the paper should be removed about 1 hour before printing on the machine. If the paper is not packaged for a long time, there will be a certain degree of humidity difference between the two sides of the paper. The humidity on the cross-section of the paper is different, and the amount of water absorption and expansion will be different, which not only affects the overprinting accuracy, but also wrinkles during printing.
Under the premise of careful analysis of the water absorption and expansion of the paper, through proper process adjustment, we can effectively control the influence of paper water absorption and expansion on the printing quality. At the same time, when selecting raw materials and ordering new equipment, we must also consider the control of water absorption and expansion. Through the joint efforts of various aspects such as process, materials and equipment, the overprinting precision of the color newspaper can be effectively improved, thereby ensuring the printing quality of the color newspaper.