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Lithographic Overprint Problems
- May 29, 2018 -

The inconsistency of ink bars, ghosting, and overprinting in lithographic printing are the three major problems that plagued printing companies, which seriously affected the quality of printed matter. Among them, overprinting is not the norm, which is the most common, the most complicated, and the most widely involved printing failure. We know that in order to identify the quality of a printed matter, we must first look at it and then look at it. It can be seen how important the overlay accuracy of the product is. Here I would like to talk about my own understanding of overprinting in lithography and discuss it with everyone.

In the lithography process, there are many factors that affect overprinting. This article mainly talks about the impact of paper and mechanical factors.

First, the impact of paper on overlay accuracy

In lithography, the impact of paper on the overprint accuracy of the product is greater, especially non-coated paper, such as offset paper, the key factor is the change in the moisture content of the paper.

As the main constituent material of paper is fiber, no matter what kind of paper fiber, its commonality is that it is extremely sensitive to water and expands and syneresis. Therefore, the instability of moisture content in the printing process will inevitably cause the paper to expand and contract. Affect overprinting accuracy, especially the direction of the paper silk must pay attention. When printing with a multi-color offset printing press, the impact will be smaller because the manufacturing accuracy of the multi-color offset printing press is relatively high and the speed is faster, and the printing is wet and wet, a product is printed once, and the instantaneous change of the paper is small. , Overprint accuracy is higher, but its superimposition rate is not as good as a monochrome machine. When a multi-color printing machine is printed with a monochrome machine, the water content of the paper changes greatly due to the need to overprint once and once, and the printing difficulty will increase. Therefore, in order to obtain a more satisfactory print, offset producers must work hard to stabilize the moisture content of the paper. Pay attention to the following points when printing.

1. Freshly laid paper, the moisture content is generally very unstable and varies greatly. It cannot be used immediately. It must be stored in a warehouse or close to the temperature and humidity of the printing room for more than two weeks.

2 must implement the temperature and humidity control of the printing shop, the general temperature should be controlled at 22 °C -25 °C, relative humidity should be controlled at 55% to 65%.

3. The paper must be hung and air-dried before being placed on the machine. The ambient temperature and humidity of the paper machine should be the same as or similar to the temperature and humidity of the workshop to reduce the paper's sensitivity to the environment.

4. The minimum amount of water must be used in printing.

5. Semi-finished products must be stored in the printing shop or in a warehouse with a temperature and humidity equivalent to the workshop.

To obtain accurate overprint products, the above points must be noted. Many printers do not attach importance to the environment of the workshop, they cannot achieve constant temperature and humidity, and the humidity in the workshop changes greatly, which brings many difficulties to printing. This is one of the main reasons why the product quality does not go up.

Second, the impact of lining on overprint

The print length can be changed by changing the thickness of the lining of the plate cylinder. This has been widely recognized. That is, the radius of the plate cylinder is increased, and the printing length is shortened. Conversely, the radius of the plate cylinder decreases, and the printing length increases. This is because of the difference in linear velocity between two rollers with different radii in the rolling state. Therefore, we must accurately measure the pad lining, accurate calculations, so that the three rollers are basically the same radius, line speed is basically the same, in the adjustment to obtain the best pressure to ensure the accuracy of product overprinting.

Third, the impact of printing plate and film on overprint

Nowadays, the printing plates used by the printing factory are mostly aluminum-based PS plates. The plates are light and thin with a thickness of only 0.27mm. Not to mention whether the printing operation is standardized, the printing plate must be softened when the plate is baked in an oven at a temperature of 230°C to 250°C. Then, as the temperature returns to normal, the plate will harden again, causing the image size to change. If the oven temperature is not uniform or baking time is too long, the change of the printing plate is more obvious, so when printing high-grade fine products, it is best not to use the oven.

Film stretching, inaccurate collage, etc., can also result in overprinting. We will not elaborate here.

Fourth, mechanical impact on the accuracy of overprinting

Throughout the printing process, the paper has been effectively controlled by the various components on the machine from beginning to end. In the process of paper conveying, positioning, rolling, and transfer, the relative position must be stable to ensure accurate overprinting.

At present, the actual definition of overprinting is commonly used in the 80s of the 20th century, the FOGRA Institute of Germany and the German Association of India jointly developed the "Flat Sheet Press Technology Acceptance Manual." Among them, the tolerance for overprinting is as follows: the standard deviation of two-pass documents shall not exceed 20mm, and the standard deviation of the cross-overprints of four color groups shall not be greater than 10mm. For each additional color group, the deviation cannot increase more than 3mm. The entire standard deviation cannot exceed 20mm. The above data we can refer to, but the deviation of fine products can also be smaller.

There are two main mechanical factors that affect overprinting: the positioning of the paper and the transfer of the paper. Of course, the paper is peeled off from the blanket. If the printing pressure is too high or due to the factors of the ink paper itself, the enamel cannot overcome the peeling force, and the paper may also be displaced, resulting in inaccurate overprinting. The following mainly talks about the positioning and handover of paper.

1. To make the paper accurately positioned, the paper must first be balanced and reliable, and the paper must run straight. There should be no fast, slow side or too fast or slow paper feed. Generally, the paper comes before the rules, leaving enough time for adjustment. The height of the front gauge is generally 3 times the paper thickness or 150g/m2 paper plus 0.2mm space. There are four pre-registration machines in general, of which only two are for positioning. The other two are for paper and ensure correct positioning of the paper. The alignment of the front gauge must be parallel to the axis of the drum to ensure that the overlay is accurate.

2. The lateral cardboard must be at right angles to the front stopper. It is necessary to make the side guide paperboard block all the paper edges, the tension of the side gauge is adjusted by the compression spring, the pull force is moderate, the pull force is too large, the paper will bend, too small will pull the paper out of position. The height of the platen is the same as that of the front gauge, and it is also a paper with a thickness of 3 times or 150 g/m2 or more and 0.2 mm in space. Pay attention to the relationship between the lifting time of the side profile and the paper feed teeth, generally when the paper feed teeth close the paper, and the maneuver time is 77°.

3. Feeding paper teeth When the paper table closes the paper, it must be at zero speed. At this time, the rollers must leave the cam 0.03-0.05mm to ensure that the delivery paper is absolutely stable when it is creped, otherwise it is impossible to register. At this time, the forces on both sides of the mountain should be the same. Tooth pads should be in a straight line. The distance from the stage is generally 0.3-0.5mm. The force of the grippers must be the same. The distance between the grippers is generally 4-5mm. To prevent the axial turbulence of the shaft and the inconsistency of the left and right eccentricity, to prevent the wear of the open tooth bearing and the cam, so that the transfer tooth and the pressure roller must have a 2 to 3 mm distance to ensure the accuracy of the transfer. Transfer.

4. The impression roller has a closing time of 0°. It must be ensured that the rollers and shafts are free from turbulence. The pressure of the pressure springs on both sides of the caliber shaft should be sufficient. Each gingival chip has no wear, and the drop-off force is even, usually about 8 kg. The pad has no wear and is flattened into a straight line. If the pad is uneven, it will inevitably make the edge of the paper wavy, causing overprinting, ghosting, and peeling. Therefore, all bushings, open teeth bearings, cams, needle roller bearings and other components must be intact.

5. Multi-color machine due to the color group, the number of paper transfer, speed, so it put forward higher requirements for overprinting.

In general, the fewer the number of times of multi-color printed paper transfer, the better, such as Heidelberg 102V four-color machine in the early 1990s, the number of paper transfer is 16 times, and now the CD1 02 Speedmaster or CP2000 Heidelberg four-color machine transfer The number is reduced to 10 times. Now the world's major offset printer manufacturers are working hard to reduce the number of paper transfers. The use of double-diameter impression cylinders and transfer cylinders, double-diameter impression cylinders, and triple-diameter air-pass transfer cylinders has become more and more popular, providing a guarantee for accurate overprinting of products.

The paper transfer between the various color groups of the multi-color machine is accomplished through the impression teeth on the impression cylinder or the transfer cylinder. In this process, as long as one of the cylinders does not meet the requirements, it will affect the subsequent colors. The paper positioning of the group results in overprinting. In order to prevent these problems, we must pay attention to the following points:

(1) The multi-color offset press has a high speed. Because the impact force of the paper before reaching the front gauge is large and it is not easy to position accurately, it uses multi-speed feed paper. When the paper comes down from the paper feed table to the front specified position, the paper feed speed should be adjusted to The slowest to ensure accurate paper positioning.

(2) It should be noted that no roller in any of the rollers can be tamped. The axial movement of the drum must be controlled within 0.05mm.

(3) The printing plate registration mechanism cannot be loosened either in the circumferential direction or in the axial direction.

(4) Due to the fast speed and large peeling force, the pressure of each roller, especially the impression roller, is generally maintained at more than 16kg, and the tooth chip and the pad must always be kept clean, open and close freely, and the force is uniform. In a good condition without wear, the spring force of the torsion spring in the shaft is greater than the sum of the force of the small springs in each tooth.

(5) The position where the rollers cross each other should be at the intersection of the center lines of the two rollers. The paper feed time of the previous drum should be 2-3 mm slower than the pick-up time of the next drum, so that the paper has a total licking time.

(6) In order to achieve the purpose of accurate overprinting of sheet-fed multicolor offset printing presses, the lining amount of the printing plate cylinder can be reduced color by color. Generally, the first color to the second color can be reduced by 0.05mm, and the second and third colors are Between the fourth color and the third color, 0.05mm or less.

(7) The partial overprint is not allowed. Except for the factors of the original or film, the flatness and the printing pressure of the surface of the printing plate and the plate cylinder can be checked.

In addition, when printing, avoid water, large pressure, otherwise it will cause the paper drag overprint "swing angle" phenomenon.

In the lithography process, there are many factors that affect overprinting, whether it is a monochrome offset press or a multi-color offset press. If the operation can meet the above requirements, the overprint accuracy can be improved and the quality of the product can be guaranteed.

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