In recent years, the printing of color newspapers in China has developed rapidly. Major and medium-sized newspapers have regularly published color newspapers, and some newspapers have been fully colored. 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 expansion of paper during printing is a major factor affecting the accuracy of color newsletter printing. How to control and eliminate the effect of paper water absorption and expansion on the accuracy of registration during the printing process, especially when the newspaper and equipment are not too good for printing color newspapers, this problem is particularly important.
First, the cause of paper absorption and expansion
There is an index of stretch rate in the performance of paper. The stretch rate is expressed as a percentage of the size increment to the original size, which indicates the relative change of the paper after wetting and drying. Newsprint fibers are thicker, less dense, have a smaller amount of sizing on the surface, and have a greater degree of water absorption than coated paper and offset paper. They have a greater impact on the accuracy of overprinting when printing color newspapers.
The offset printing process commonly used in color newspaper printing is to apply fountain solution to the plate surface first, and then add ink to print. The main component of the fountain solution is water. The water will be transferred from the printing plate to the blanket and then transferred to the paper under pressure. There must be expansion and contraction of the paper due to water absorption and drying during the printing process.
The amount of paper expansion and contraction has a great relationship with paper performance. The better the performance of the paper, the smaller the amount of water absorption, the better the dimensional stability after water absorption, and the smaller the expansion and contraction rate, which is very beneficial to the printing of color newspapers. Among the various performance indicators of paper, sizing degree, surface strength, moisture content, tightness, smoothness, double-sidedness, water content, etc. all have a certain effect on the amount of paper expansion and contraction. Judging from the newsprint we currently use in different brands such as Jilin, Nanping, Guangzhou, Hansong, Russia and Finland, the expansion rate varies greatly.
Second, the effect of paper expansion and contraction on registration accuracy
The water absorption and expansion of the paper has a great influence on the registration accuracy of the color newspaper. In the case of good pre-press, imposition, etc., the paper's water absorption and expansion can not be solved well, so that the accuracy of overprinting cannot be guaranteed. Inaccurate overprinting will greatly affect the perceived quality of color newspapers. Even if the raw materials such as ink color, dot, color and paper are good, the overall aesthetic effect will be greatly reduced.
The lateral water absorption and expansion of paper has the most obvious and direct impact on the registration. The longitudinal shrinkage parallel to the paper running direction caused by the water absorption and drying of the paper is generally eliminated by adjusting the registration adjustment roller between the units. In the transverse thread direction of newsprint, the diameter of a fiber can increase by 30% when it is completely wet, and its length direction only increases by 1% to 2%. Therefore, the longitudinal expansion and contraction of the linear thread in the printing process is small, about horizontal About 1/2 ~ 1/3, has little effect on the accuracy of overprinting. The expansion and contraction in the lateral direction has a great influence on the overprint accuracy of the newspaper, and its maximum change can reach about 0.15%. Without certain elimination measures, the maximum overprint error can reach more than 0.3mm. Generally, it is a one-page registration. There is no way to register a four-page registration. This error is gradually expanded from one side. It cannot be adjusted simply by the printing machine's cylinder method, which greatly affects the printing quality of the color newspaper.
The structure of the equipment is different, and the effect of water absorption shrinkage on registration accuracy is also very different. The printing process of newspapers is to print four colors on successive sheets of paper in sequence. For units with Y-shaped printing structure, two units are generally online to complete four-color printing, 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 placed horizontally, especially the roller arrangement is inverted Y, the paper path will be longer and the shrinkage will also be Bigger. The satellite printing unit is very beneficial to the printing of color newspapers. The paper path between the satellite color groups is very short, there is almost no drying time, and the change in the amount of expansion and contraction after the paper absorbs water generally has no effect on the registration. The paper after coming out of the satellite printing unit usually needs to be BB printed again (usually black). Because there is a certain period of time, the paper also has a certain amount of expansion and contraction.
3. Elimination and control of water absorption and expansion
Of course, before we talk about the effect of water absorption and shrinkage on the accuracy of overprinting, we must solve other process problems such as prepress image processing, film output, imposition, printing registration, etc., otherwise, the elimination of water absorption and shrinkage will drown in Among other factors.
The elimination and control of paper water absorption and expansion are mainly carried out from the following aspects:
1. When the paper path between the units is long and there is no fan-shaped expansion device for printing color newspapers, the paper's water absorption and expansion have a significant effect on the registration accuracy. In this case, according to the amount of paper expansion and contraction during printing, the effect of paper water absorption and expansion on the accuracy of overprinting can be eliminated by imposition and borrowing the film on the side of the fourth edition. Take the folio four-page newspaper as an example. For example, if the printing structure of the printing press is Y-type, the first unit prints C and M colors, and the second unit prints Y and K colors. We found that in the case of accurate imposition and full registration of one edition, the overprint error between the leftmost C, M and Y, K of the fourth edition is about 0.2mm, C and M are to the left, and Y and K are to the right. Then, in the imposition, we put the CM two-color film to the right by 0.2mm, that is, move 0.2mm in the direction of the middle seam, to offset the shrinkage caused by the paper after drying and then drying.
We noticed that the film requires the film to face upwards when it is assembled according to the process requirements, that is, the film is reversed, and the left and right directions are opposite. Draw a line to make the amount of borrowing consistent each time; the paper brand level is different, and the amount of expansion and contraction is also different. Different borrowing amounts should be determined for different papers.
2. Control the water-ink balance in the printing process. The amount of water in the first unit during printing is the main factor that determines the effect of paper expansion and contraction on overprinting after water absorption. When printing, the amount of dampening water should be uniform and should not be large or small. Especially when using poor quality paper, the water absorption is large and the expansion and contraction rate is relatively large. This problem is more prominent. For the equipment with fan-shaped expansion mechanism such as eight-color tower unit, it is not necessary to borrow the spelling, but it is also required to control the amount of water.
3. At present, each factory 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 paper water absorption and expansion. Due to the different conditions of paper used by various factories, the quality of domestic paper still has a certain gap with imported paper. It is necessary to control the water absorption and expansion of paper from the equipment.
4. Use good quality paper as much as possible, not only the small amount of water absorption and expansion, but also the printing process such as dot reproduction, color reproduction, and ink balance. To produce good quality color papers, the performance of the paper is a very important condition.
5. The process structure of the printing press for printing color newspapers is preferably tower type, so that the paper path between the four-color printing of the paper is shorter, which is beneficial to control the amount of paper expansion and contraction and improve the accuracy of overprinting. The satellite structure is basically continuously printed between the satellite color groups, which has a great advantage in the accuracy of overprinting and the quality of the dot reproduction, but the cost is higher, and the newly ordered machines are relatively rare. At present we can see a ten-cylinder tower machine, which is a new type of satellite printing machine with a better process structure.
6. The paper used for printing the color newsletter should not be too dry, otherwise it will not only break the paper easily during printing, but also have a large water absorption and high expansion and contraction rate. The paper storage environment must 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. The paper will not be packaged for a long time, which will cause a certain degree of humidity difference between the sides and the inside of the paper. The moisture on the cross-section of the paper is different, and the amount of water absorption and expansion will also be different, which not only affects the accuracy of the overprint, but also tends to wrinkle during printing.
Under the premise of careful analysis of the paper's water absorption and expansion, and through appropriate process adjustments, we can effectively control the impact 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 technology, materials, equipment and other aspects, the overprinting accuracy of the color newspaper can be effectively improved, thereby ensuring the printing quality of the color newspaper.
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