Influence and Elimination of Water Absorption of Paper in Printing of Color Newspapers

In recent years, China's color newspaper printing has developed rapidly. All major and medium-sized newspaper companies have regularly published color newspapers. Some newspapers have all been colored, and the printing quality of color newspapers has also been greatly improved. Due to the performance characteristics of newsprint, the amount of water absorption is large, and the water absorption and expansion of the paper in the printing process is a major factor that affects the accuracy of overprinting. How to control and eliminate the effect of paper suction and expansion on the accuracy of registration in the printing process, especially when the equipment and raw materials are not too good newspaper printing color newspaper, this issue is particularly important.

First, the cause of paper water absorption and expansion

There is an index of stretchability in the performance of the paper. The stretchability is expressed as a percentage of the original size in increments of size, which indicates the relative change of the web after wet and dry. Newsprint fibers are relatively coarse, have a low density, and have a small amount of surface sizing. The degree of water absorption elongation is greater than that of coated paper and offset paper, and has a greater impact on overlay accuracy in printing color newspapers.

The offset printing process commonly used in color newspaper printing is to first apply a fountain solution to the printing surface, and then add ink to print. The main component of the dampening solution is water. Water is transferred from the printing plate to the blanket and onto the paper. In the printing process, there is a phenomenon that the paper is stretched due to water absorption and drying.

The amount of paper stretched depends on the paper properties. The better the paper performance, the smaller the water absorption, the better the dimensional stability after water absorption, and the smaller the expansion and contraction rate, which is very advantageous for color newspaper printing. Among the various performance indexes of paper, the degree of sizing, surface strength, moisture content, tightness, smoothness, both sides, moisture content, etc. have a certain influence on the amount of stretching of the paper. As far as the newsprints of different brands such as Jilin, Nanping, Guangzhou, Hansong and Rufen that we currently use more are concerned, the telescopic rates vary greatly.

Second, the impact of paper stretching on the accuracy of registration

The water absorption and expansion of the paper has a great influence on the registration accuracy of the color newspaper. In the prepress, imposition, etc. are done well, the water absorption and expansion of the paper can not be well resolved, will not guarantee the accuracy of overprinting. Imprinting is not allowed to greatly affect the perceived quality of color newspapers. An inaccurate color newspaper, no matter how good the raw materials such as ink, outlets, colors, and paper, will be greatly diminished.

The transverse water absorption and contraction of paper has the most obvious effect on the registration, and the impact is also the most direct. The longitudinal shrinkage parallel to the direction of travel of the paper produced by the absorption of water by the paper is generally eliminated by adjusting the registration rollers between the units. In the direction of the cross-web of newsprint, when a fiber is completely wet, its diameter can be increased by 30%, and its length is increased by only 1% to 2%. Therefore, the longitudinal shrinkage of the silk thread is small during printing, and is approximately horizontal. About 1/2 to 1/3, the impact on overlay accuracy is not significant. The horizontal direction of the telescopic change has a greater impact on the overprinting accuracy of newspapers, and the maximum change is up to about 0.15%. The maximum overprinting error can reach 0.3 mm or more without taking certain elimination measures. Generally a version of the register, the fourth version there is no way to register, and this error is gradually expanded from the side, simply rely on the way the printing press can not be adjusted, greatly affect the print quality of the newspaper.

With different equipment structures, the effect of water absorption shrinkage on registration accuracy is also very different. The printing process of newspapers is to print four colors of sequential paper in sequence. The Y-printing unit generally prints four-color printing on-line by two units. Each unit prints two colors. If the unit is a tower structure, the paper path between the two units is short, the shrinkage is not very large; if it is placed horizontally, especially the drum arrangement structure is inverted Y type, the paper path will be longer, the shrinkage will also be Bigger. The satellite printing unit is very advantageous for color newspaper printing. The paper path between each satellite color group is very short, and there is almost no drying time. The change in the amount of stretching of the paper after absorbing water does not generally affect the registration. The paper from the satellite printing unit will generally have another BB print (usually black). Because there is a certain period of time, the paper also has a certain amount of expansion and contraction changes.

Third, the elimination of water absorption and control

Of course, before we talk about the effect of water absorption and expansion on the accuracy of overprinting, we must properly solve other processing problems such as pre-press image processing, film output, copy-printing, printing registration, etc. Otherwise, the elimination of water-absorbing shrinkage will be drowned out. Among other factors.

The elimination and control of the water absorption and shrinkage of the paper mainly proceed from the following aspects:

1. When the paper path between the units is long, and there is no fan-shaped expansion device equipment printing color report, the water absorption and expansion of the paper has a significant impact on the accuracy of the registration. In this case, according to the size of the paper expansion and contraction in the printing, the influence of the paper suction and expansion on the overprint accuracy can be eliminated by forcing the line on the fourth printing side. Take the fourth edition of the folio four as an example. If the printing structure of the printing press is a Y-shaped pattern, the first printing unit prints C and M colors, and the second printing unit prints Y and K colors. We found that the overprint error between the leftmost C, M, Y, and K of the fourth edition is about 0.2 mm when the imposition is accurate and the version is fully registered. C and M are leftward, and Y and K are rightward. In the case of imposition, we will use the CM two-color film to the right to fight 0.2mm, that is, to move to 0.2mm in the direction of the middle seam, in order to offset the contraction of the paper after drying after water absorption.

We have noticed that the film requires the film to face upwards when it is applied according to the process requirements. That is, the film is reversed, and its right and left directions are the opposite. Therefore, it is more convenient to determine the borrowing direction by using the midline as the reference line; it is better to use the layout paper. Draw a line so that the amount of each loan is the same; different levels of paper brands, the amount of stretching is also different, should determine different borrowing for different paper.

2. Control the ink-water balance in the printing process. The amount of water in the first unit at the time of printing is the main factor that determines the influence of the amount of retraction of the paper upon overprinting. The amount of dampening water should be uniform when printing, and must not be changed suddenly. In particular, when paper with poor quality is used, its water absorption is large, and the expansion and contraction rate is relatively large. This problem is even more prominent. For eight-color tower-type units and other equipment that have fan-shaped expansion mechanisms, spelling can be used without spells, and it is required to control the amount of water.

3. At present, when ordering new equipment, each plant should pay attention to whether it is a double-width machine or a single-width machine. It is necessary to add additional fan-shaped expansion rollers to improve the control of water absorption and contraction of the paper. Because of the different conditions of the paper used in each factory, there is still a certain gap between domestic paper quality and imported paper. It is necessary to control the water absorption and expansion of the paper from the equipment.

4. Use paper with good quality as much as possible, not only has a small amount of water absorption and expansion, but also has advantages for printing processes such as dot reproduction, color reproduction, and ink balance. To produce good quality newspapers, paper performance is a very important condition.

5. The technical structure of the printing press printing machine is preferably a tower type, so that the paper path between the paper four-color printing is shorter, which is conducive to controlling the amount of water absorption and contraction of the paper and improving the overlay accuracy. The satellite structure is basically continuously printed among the satellite color groups, which has a great advantage over overlay accuracy, and the reproduction quality of the dot is also good, but the cost is high, and the newly ordered machines are relatively rare. At present, we can see that there is a ten-roller tower crane, which is a new type of satellite-type printing press and has a good process structure.

6. The paper used for printing color newspapers should not be too dry, otherwise it is not only easy to break paper when printed, but also has a large amount of water absorption and a high expansion rate. The paper storage environment must meet the specification requirements, the time can not be too long.

7. The outer package of the paper should be removed only about one 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 on both sides of the paper. The humidity on the cross-section of the paper is different, and the amount of water absorption and expansion will also be different. This will not only affect the overlay accuracy, but also make it easy to wrinkle when printing.

Under the premise of careful analysis of the paper's water absorption and expansion, we can effectively control the influence of the paper's water absorption and contraction on the printing quality through proper process adjustment. 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 processes, materials, and equipment, it is possible to effectively improve the overlay accuracy of color newspapers, and thus ensure the printing quality of color newspapers.

Source: Bison