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Secondary Overprint Problems In Atlas Printing
- May 28, 2018 -

Multi-color atlas printing, as the name implies, refers to the printing method of printing more than CMYK four colors, and is mostly applied to the printing of large-scale thematic atlases. This is because thematic maps carry a large amount of information and are rich in color levels. It is impossible to meet the requirements only with CMYK four-color overlays. Therefore, craftsmen usually design several spot colors as supplements, such as steel gray, dark brown or red. Orange and so on. Many thematic atlases such as the “The People’s Republic of China Economic Atlas” and “The History Atlas of the People’s Republic of China” that the author has been responsible for are six-color printing.

Usually four-color offset printing machine printing this kind of multi-color atlas of the process is one side of India, namely, the printing of positive four colors → positive front color → back four colors → the back of the spot color, each side of the paper to be printed twice, this exists The problem of secondary overprinting, namely overprinting between four-color printing and spot color printing. For fine map products, the overprint error of the main part should be ≤ 0.2mm, and the overprint error of the minor part should be ≤ 0.5mm. So, how to guarantee such a high overlay accuracy? This is a challenge for both the craftsman and the machine.

The author believes that to solve this problem must control the stability of the paper deformation and paper feeding rules. There is no doubt that the requirements for these two elements are much more stringent than ordinary four-color printing. In ordinary four-color printing, one side of the paper is printed only once, and once formed, even if the paper is slightly deformed and the rules are a little small, the four colors are overprinted together and no serious quality problems will occur. The multi-color printing is not sloppy, as long as the first printing is completed, the paper has changed, the rules are beating, will be reflected in the second printing overprint, resulting in overprinting, dew, seriously affecting product quality, and even make The roads, rivers, and borders on the multi-color atlas are misplaced and mislead readers.

Control paper deformation

There are two main factors that cause paper deformation: the temperature and humidity in the workshop and the amount of fountain solution in the printing process.

1. Strictly control temperature and humidity in the workshop

Temperature and humidity control in the workshop is a commonplace topic, and it is crucial for multi-color atlas printing. As we all know, paper is a kind of variable material. It is a fiber-based porous material and has strong hydrophilicity. When its moisture content is lower than the humidity of the surrounding air, it absorbs the moisture in the air and the phenomenon of elongation occurs. On the contrary, it will release moisture and cause shrinkage. The elongation and shrinkage of the paper will cause the image overprinting to be inaccurate. It is necessary to control and adjust the temperature and humidity of the shop so that the water content of the paper and the surrounding air can reach a relative balance, so as to improve the dimensional stability of the paper and remove the hidden dangers of overprinting. Normally, the workshop environment is ideally maintained at a temperature of about 25°C and a relative humidity of about 60%.

The author had encountered several editions of the multicolored atlas. Because the temperature and humidity were not well controlled, the paper was severely deformed after the four-color printing. When the secondary printing was spotted, both sides “fighted” and depended on the top paper. Relying on the need to leave the paper, the captain was at a loss and he had no choice but to print a spot color version. Burning out half the contents and half printing half the time would not only be time-consuming, but also ensure the quality of the prints. Therefore, at the time of printing the multicolor atlas, each team should observe the temperature and humidity table before starting and during the production process. If it is not up to the standard, the humidifier, air conditioner or sprinkler can be turned on to improve the temperature and humidity of the workshop.

2. Control the amount of fountain solution

The amount of water in the printing should follow the principle of “the smaller the better the job is in the premise of not being dirty or dirty”. The use of water is small, and not only is the picture and text color full and bright, but the paper deformation is negligibly small, laying a good foundation for the second overprint. On the contrary, the use of water is large, and the four colors may be overprinted and inaccurate. How to determine the layout of the amount of water? Simple and easy way is to face the light from the side to observe the plate cylinder, if the layout is dark gray, indicating that the amount of water is just right; if the plate is bright and shiny, indicating a large amount of water, it is necessary to decline.

Keep paper rules in the printing process stable and consistent

1. Regulation components are adjusted to the best condition without sick work

According to the author's many years of experience, in the case of multi-color printing, the bottom rule has a smaller probability of problems, and it is more of a rule-out condition. Heidegger's four-color offset printing machine's edge rule works by pulling the ball up and down, and pulling and positioning the paper along with the pull tabs on the base. This determines that the edge rule is a problem-prone area and needs more. Notice (Heidelberg has two pull-sheet springs for each side of the offset press to accommodate different thicknesses of paper. The author recommends a ration of 157.

Paper spring with g/m2 or less, and paper spring with 157 g/m2 or more). The edge of the rule is to pull several times a day. After a long period of frequent contact and impact with the paper, the paper balls, pull gauges, and edge guards will wear out, and the dust on the paper will accumulate at the perimeter. Over time, pull the ball. If the rotation is blocked, the relevant parts will wear out due to the accumulation of paper powder, which will affect the pulling action and its accuracy. Therefore, in the daily production, we must do a good job in the daily maintenance of the edge rules. Before printing the fine multi-color atlas, check and test to see if the border work is normal, if any problems are solved in time, and no sick work is done.

The author once encountered a situation in which the crew appeared to print four colors, but the number of finished products was enough and did not care. Later, at the time of printing the multi-color atlas, when the first four-color printing operation, there was a phenomenon of running rules. When it was observed that it was a time when the edge was pulled, it was reported to the workshop director. The workshop director asked the captain to stop and adjust the rules before printing. After the captain had stopped, he adjusted the rules, but how to adjust them was useless. The problem persisted. He had to make the equipment branch to inspect it. The inspection was due to the blockage of the lubrication pipe at the pull regulation chute by the paper powder. The wear of the needle roller bearing is rusted, which affects the accurate operation of the pull gauge ball. By replacing the pull gauge arm and the needle roller bearing, the problem can be solved. Although the problem was solved, it delayed the man-hours and increased the consumption of raw materials. This event tells us that when the equipment is in a condition, it must be dealt with in a timely manner. The cleaning and cleaning should be replaced and replaced so that the mechanical parts are in the best working condition.

Of course, we must not ignore the rules when we are concerned about border rules. In order to stabilize the bottom gauge, the pressure of the paper feeder on the paperboard, the tension of the paper conveying belt, the pressure of the brush wheel, etc. must be moderate and uniform, so as to prevent the paper from appearing skewed, under, and over.

2. Diligent paper during the printing process, diligently check the rules

Printing is a dynamic production process, in which the dynamic variables must be monitored at any time, and the rules are one of the items to be monitored in the printing process. Because even if there is no problem with the paper on the edge, due to factors such as the speed, the distance from the edge of the paper, and the condition of paper feeding, the paper may not pull, pull, under, or over. Fluctuations occur. Therefore, when the captain prints a multi-color atlas, he must always pull the paper, diligently dig the paper, check whether the rules are in the same horizontal line, and check whether the edge line is riding on the edge. The rules are stable and consistent.

In addition, the assistant should assist the captain to close the rules and work hard at the details. For example, every time you print, you must draw a “10” character on the plate. The “—” line is used to check the bottom rule, and the “∣” line is used to check the rule. For ease of observation, the “—” line should be flat. Straight, ride on the edge of the line. The “Ten” character line should be drawn near the edge of the drawing paper (usually a small vertical line) so that it can accurately reflect the real situation. When the position is too far or too early, the rules may be fed back due to the paper hanging angle. Unable to pull or pull the information, mislead the lead. For another example, when the assistant picks up the paper, he or she must pull it up and down, and regulate the use of the side bar to adjust the paper. This way, when the paper is fed, the distance between the paper and the edge rule is kept at about 5 to 8 mm, which greatly reduces the paper edge irregularity. The risk of not pulling or pulling.

It should be pointed out that when the four colors are printed on the back of the first time, the back and forth rules must be accurately identified. Once the printing is started, the rules cannot be easily moved, and the top paper and the back paper must not be printed during the printing process. Many tricks and fewer tricks will change the rules even if the jog moves, which will affect the secondary overprint.

3. The speed should not be too fast

The speed is too fast, the positioning time is short, and the rules are unstable. In particular, when printing is in progress and the back is four colors, the speed is controlled at about 6500 to about 7,000/hour, and the rules are relatively stable.

In addition, the storage of semi-finished products is also a link that cannot be ignored. In order to ensure that the printed semi-finished products are not deformed by the external environment, they must be coated with plastic film. When operating, it is best that two people cooperate with each other, gradually winding up from the bottom of the paper table, and the package is tight without leaving gaps.

In short, the problem of secondary overprinting is not only the focal point but also the difficulty of multi-color atlas printing. It requires the captain and assistant to have a high degree of responsibility, regulate operations, and pay attention to observe various factors affecting the accuracy of overprinting. , will be able to print accurate overprint, quality products.

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