At present, of the more than one hundred unit-type flexo presses introduced in China, only a small part is printed on stickers, and most of the products printed by flexo presses are thin paper-based prints. The most typical there are three categories. The first type is a cigarette soft pack case, and the used paper is mostly 70-90 g/m2 single-sided coated paper. The second type is the beer standard, which mostly uses single-sided coated paper of 75 g/m2 or fire-resistant copperplate paper. The high-grade beer mark uses vacuum-coated aluminum plain or textured paper. The third type is printing of tipping paper, using about 30 g/m2 of tipping paper. Almost all users who have printed on paper have been reported to have encountered a series of problems that can be summed up in the following words: "wrinkle," "shrink," "warp," and "electrostatic." Below, we will discuss in detail the causes of these failures and their solutions.
One of the faults: WRINKLE
When flexo printing machines “walk†thin paper, the easiest and most problematic issue is that the paper produces “creases†along the direction of the paper in the machine. The wider the machine is (ie, the wider the paper is), the thinner the paper is. The easier it is to wrinkle. When this failure occurs, the first reason that should be considered is that the tension is too large. Generally, if a certain local tension of the machine is too large, wrinkles are generated in the area and affect the following parts. For example, when the unwinding tension is set too large, wrinkles are generated in the unwinding portion and the creases are more severe in the back, and even break the paper. Some users originally ordered a flexo printer that is thick paper type. Once used to print thin paper, it is easy to wrinkle. In this case, wrinkles generally occur in the middle and back part of a flexo press. Sometimes, this problem can be solved by lowering the tension setting of each part, but the most fundamental solution is to purchase a set of rollers suitable for printing thin paper, including the impression cylinder and the paper feeding and output paper rollers.
Another cause of the wrinkling of the thin paper is that the deflection angle of the correcting device is too large, so that the tension condition of the paper at the correcting device is tight on one side and loose on the other side, so wrinkles are easily generated. In the case of unwinding, “bark†wrinkles are found on the roll of paper feeding, of course, on the looser side of the paper. To solve this problem, let the corrective device “kick yourself†(by pressing the SERVO-CENTER key), walk the idle car, adjust the appropriate lateral position of the paper and stabilize it, stop it, and then adjust the corrective device’s eye so that it With the currently-monitored side edge position, the corrective system is switched to the "automatic" state. Doing so can minimize the deflection angle of the correcting device during "automatic" operation, minimizing the possibility of wrinkles. At this time, the horizontal position of the paper roll should be used as an obvious mark. When replacing a roll of paper, the position must be aligned.
When printing on thin paper, after the original paper roll is removed from the package, the paper near the surface of the outer package absorbs moisture from the air and wrinkles. When the outer layer of wrinkled paper comes into the machine, it will certainly cause problems. Of course, this situation is temporary, as long as the part of the paper is passed or cut off at a lower speed and then reconnected, the normal production can be started. As for how to avoid the above problems, we recommend that during the transportation and storage process, the paper roll should be strictly guaranteed to be well packaged, and the package should be removed when the machine is about to print. The time that the paper roll absorbs the moisture in the air should be minimized.
Other causes of wrinkling of thin paper include: foreign materials (such as large dry ink sticks) or dents on the surface of the impression roller or the paper guide roller; the parallelism of individual rollers is poor. To solve wrinkles, in addition to adjusting the tension and checking the machine, if necessary, two layers of electrical tape can be wrapped around the inner surface of the paper guide roller at a distance of approximately 15 mm from the two paper edges to prevent the paper from moving laterally inward. It is also possible to use a hedging guide roller to stretch the tissue outwards. However, when the stiffness of the tissue is too small and the tension is relatively large, it will wrinkle due to the rolling line. Therefore, specific problems must be analyzed.
The second failure: SHRINK
In order to increase the production efficiency, the current flexographic printing press pays great attention to improving the drying capacity, and generally uses natural wind, hot air (high, medium, and low grades) and infrared (adjustable and non-adjustable). The drying methods or their combination, some machines are also equipped with two sets of UV curing system. In this way, people will find a phenomenon in proofing or production, that is, a set of plates made of the same set of films, the horizontal dimensions will actually have deviations, the error hours less than 1 mm, when the error is large 2-3 mm. ......
As we all know, about 50% of the components in water-based ink are water, so for thin paper, if a large area of ​​water-based ink is printed, the surface of the paper is “printed wet†and expands and becomes wider. The larger the printing area, the anilox roll The lower the number of lines, the wider the phenomenon of widening. However, when the paper is dried after printing, the paper becomes narrow due to "shrinking." The larger the amount of heating and the more volatilization of water, the more the paper width becomes narrower. Most of the flexo printers' heating and drying systems (including UV curing systems) have a minimum speed start-up operating point, such as 15 m/min. For safety reasons, below this speed value, infrared and UV drying devices are not open, at or above this value will open. It can be imagined that if our starting speed is only slightly higher than this speed, the amount of paper heating is very large and the contraction is obvious. The farther away from the unit, the greater the horizontal overprint error. Therefore, in the printing production process, the starting speed should be increased as much as possible, and unnecessary hot air or infrared drying should be minimized during production so as to avoid lateral overprint errors. We used to print tipping paper on a 20-inch-wide flexo press to increase the speed to more than 80 meters per minute to ensure that no horizontal overprinting errors occur.
Of course, there is one exception. When we used the stencil vacuum aluminum-plated paper to print beer marks, there was a large deviation in horizontal overprinting. After careful inspection tests, the factors that caused the width change due to loss of moisture during drying were eliminated, and eventually we found that even if the machine emptied the paper, A paper width change may occur due to a change in the paper tension on the machine.
The “warp†mentioned here refers to the warping of the paper in the direction perpendicular to the paper-feeding direction. At this time, looking in the direction of paper-feeding, people see that the paper or the finished product is curled up on both sides. As the speed of modern packaging machinery is getting faster and faster, the demand for product quality is bound to become higher and higher. Once warpage occurs in printed products, it will cause certain troubles for packaging line operations. Therefore, whether it is a tobacco factory or a brewery, the leveling requirements of the printed products are very strict. (To be continued)
One of the faults: WRINKLE
When flexo printing machines “walk†thin paper, the easiest and most problematic issue is that the paper produces “creases†along the direction of the paper in the machine. The wider the machine is (ie, the wider the paper is), the thinner the paper is. The easier it is to wrinkle. When this failure occurs, the first reason that should be considered is that the tension is too large. Generally, if a certain local tension of the machine is too large, wrinkles are generated in the area and affect the following parts. For example, when the unwinding tension is set too large, wrinkles are generated in the unwinding portion and the creases are more severe in the back, and even break the paper. Some users originally ordered a flexo printer that is thick paper type. Once used to print thin paper, it is easy to wrinkle. In this case, wrinkles generally occur in the middle and back part of a flexo press. Sometimes, this problem can be solved by lowering the tension setting of each part, but the most fundamental solution is to purchase a set of rollers suitable for printing thin paper, including the impression cylinder and the paper feeding and output paper rollers.
Another cause of the wrinkling of the thin paper is that the deflection angle of the correcting device is too large, so that the tension condition of the paper at the correcting device is tight on one side and loose on the other side, so wrinkles are easily generated. In the case of unwinding, “bark†wrinkles are found on the roll of paper feeding, of course, on the looser side of the paper. To solve this problem, let the corrective device “kick yourself†(by pressing the SERVO-CENTER key), walk the idle car, adjust the appropriate lateral position of the paper and stabilize it, stop it, and then adjust the corrective device’s eye so that it With the currently-monitored side edge position, the corrective system is switched to the "automatic" state. Doing so can minimize the deflection angle of the correcting device during "automatic" operation, minimizing the possibility of wrinkles. At this time, the horizontal position of the paper roll should be used as an obvious mark. When replacing a roll of paper, the position must be aligned.
When printing on thin paper, after the original paper roll is removed from the package, the paper near the surface of the outer package absorbs moisture from the air and wrinkles. When the outer layer of wrinkled paper comes into the machine, it will certainly cause problems. Of course, this situation is temporary, as long as the part of the paper is passed or cut off at a lower speed and then reconnected, the normal production can be started. As for how to avoid the above problems, we recommend that during the transportation and storage process, the paper roll should be strictly guaranteed to be well packaged, and the package should be removed when the machine is about to print. The time that the paper roll absorbs the moisture in the air should be minimized.
Other causes of wrinkling of thin paper include: foreign materials (such as large dry ink sticks) or dents on the surface of the impression roller or the paper guide roller; the parallelism of individual rollers is poor. To solve wrinkles, in addition to adjusting the tension and checking the machine, if necessary, two layers of electrical tape can be wrapped around the inner surface of the paper guide roller at a distance of approximately 15 mm from the two paper edges to prevent the paper from moving laterally inward. It is also possible to use a hedging guide roller to stretch the tissue outwards. However, when the stiffness of the tissue is too small and the tension is relatively large, it will wrinkle due to the rolling line. Therefore, specific problems must be analyzed.
The second failure: SHRINK
In order to increase the production efficiency, the current flexographic printing press pays great attention to improving the drying capacity, and generally uses natural wind, hot air (high, medium, and low grades) and infrared (adjustable and non-adjustable). The drying methods or their combination, some machines are also equipped with two sets of UV curing system. In this way, people will find a phenomenon in proofing or production, that is, a set of plates made of the same set of films, the horizontal dimensions will actually have deviations, the error hours less than 1 mm, when the error is large 2-3 mm. ......
As we all know, about 50% of the components in water-based ink are water, so for thin paper, if a large area of ​​water-based ink is printed, the surface of the paper is “printed wet†and expands and becomes wider. The larger the printing area, the anilox roll The lower the number of lines, the wider the phenomenon of widening. However, when the paper is dried after printing, the paper becomes narrow due to "shrinking." The larger the amount of heating and the more volatilization of water, the more the paper width becomes narrower. Most of the flexo printers' heating and drying systems (including UV curing systems) have a minimum speed start-up operating point, such as 15 m/min. For safety reasons, below this speed value, infrared and UV drying devices are not open, at or above this value will open. It can be imagined that if our starting speed is only slightly higher than this speed, the amount of paper heating is very large and the contraction is obvious. The farther away from the unit, the greater the horizontal overprint error. Therefore, in the printing production process, the starting speed should be increased as much as possible, and unnecessary hot air or infrared drying should be minimized during production so as to avoid lateral overprint errors. We used to print tipping paper on a 20-inch-wide flexo press to increase the speed to more than 80 meters per minute to ensure that no horizontal overprinting errors occur.
Of course, there is one exception. When we used the stencil vacuum aluminum-plated paper to print beer marks, there was a large deviation in horizontal overprinting. After careful inspection tests, the factors that caused the width change due to loss of moisture during drying were eliminated, and eventually we found that even if the machine emptied the paper, A paper width change may occur due to a change in the paper tension on the machine.
The “warp†mentioned here refers to the warping of the paper in the direction perpendicular to the paper-feeding direction. At this time, looking in the direction of paper-feeding, people see that the paper or the finished product is curled up on both sides. As the speed of modern packaging machinery is getting faster and faster, the demand for product quality is bound to become higher and higher. Once warpage occurs in printed products, it will cause certain troubles for packaging line operations. Therefore, whether it is a tobacco factory or a brewery, the leveling requirements of the printed products are very strict. (To be continued)
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