How to eliminate tissue printing folds

The easiest and most problematic problem for flexo printing machines when printing thin paper is that the paper produces wrinkles along the direction of the paper in the machine. The wider the machine (ie, the wider the paper), the thinner the paper, the easier it is to wrinkle. When this failure occurs, the first reason that should be thought is that the tension is too great. Under normal circumstances, if a certain local tension of the machine is too large, wrinkles will occur in the area and affect the following parts. For example, if the unwinding tension is too large, wrinkles may occur in the unwinding portion, and the more serious it is, the more severe it may be, and even if it is broken. Some users originally ordered a flexo printer that was thick paper type, and it was very easy to wrinkle once it was used to print thin paper. Wrinkles generally occur in the mid-to-back part of the flexo press. Usually, this problem can be solved by reducing the tension setting of each part. But the fundamental solution is to purchase an additional set of rolls suitable for thin paper printing, including embossing cylinders, paper, and paper rolls.

Another cause of wrinkling of thin paper is the deflection angle of the correcting device. The deflection angle causes the paper tension at the correcting device to be tight on one side and loose on the other side, so wrinkles are easily generated. If it is the unwinding part, the phenomenon of "bark" wrinkle is found on the paper feeding roller, of course, on the looser side of the paper. The solution to this problem is to let the corrective device "kick yourself", walk a section of paper, adjust the proper lateral position of the paper and stabilize it. After stopping, adjust the electric eyes of the corrective device to align with the position of the paper edge that needs to be monitored. At this time, the correction system will be switched to work in the “automatic” state. In this way, the deflection angle of the correcting device during the "automatic" operation can be minimized and the possibility of wrinkling can be minimized. At this time, the lateral position of the paper roll should be marked clearly, and the position of the roll paper should be aligned when it is replaced. 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 in the air and wrinkles. When the outer wrinkled paper comes into the machine, it will certainly cause problems. Of course, this situation is temporary, as long as the paper is passed at a relatively low speed or is simply cut off and reconnected, normal production can be started. As for how to avoid the above problems, the author suggests that during the transportation and storage process, it should be strictly ensured that the paper roll is well packaged, and when it is about to be printed on the machine, the package is opened again to minimize the time that the paper roll absorbs the moisture in the air.

Other causes of wrinkling of thin paper: Foreign materials (such as large dry ink sticks) on the surface of the impression cylinder or the paper guide roller, and the parallelism of the individual rollers are very poor. To solve wrinkles, in addition to adjusting the tension and checking the machine, if necessary, two transparent adhesive tapes may be wrapped around the inner surface of the paper guide roller at a distance of about 15 mm from the two paper edges to prevent lateral movement of the paper inward. It is also possible to use a paper guide roller with a helix to stretch the tissue outwards.

In order to increase the production efficiency, the current flexographic printing press pays great attention to improving the drying ability. Generally used natural wind, hot air (high, medium, low) and infrared (with stepless adjustable and not adjustable) and other drying methods. Some of their combined machines are also equipped with two sets of UV curing systems. In this way, in the proofing or production will find a phenomenon that a set of films made of the same set of films, the horizontal dimensions will actually have deviations, the error hours less than 1 mm; error is 2 mm -3 mm. In the actual production process, if there is no problem with the film or the printing plate, the deviation in the lateral dimension is caused by the two opposite processes of printing and drying.

About 50% of water-based ink composition is water, then for the thin paper, if a large area of ​​printing water-based ink, the surface of the paper will be printed and wet expansion will become wider, the larger the printing area The number of anilox roller line is lower, wider The more obvious the phenomenon is. When drying is performed after printing, the paper is narrowed due to shrinkage. The greater the amount of heating and the more volatile water, the more narrow the width of the paper becomes. Most of the flexo printers' heating and drying systems (including UV curing systems) have a minimum speed startup operating point, such as 20 meters/minute. For safety reasons, below this speed, the IR and UV drying devices are not open; reaching or exceeding this value will open. If the starting speed is slightly higher than this speed value, the heating amount of the paper is large and the shrinkage is obvious. The farther away from the unit, the greater the horizontal overprint error. Therefore, in the printing production process, the speed of the printing press should be increased as much as possible, and unnecessary hot air and infrared drying should be minimized so as to avoid the lateral overprint error. In addition, overprinting errors can also occur when the speed of the machine is appropriately increased during a single printing process. Tension, mechanical speed, and temperature have a constant value, often 1:1.2 (1.3):1.

Author / (Chen Quandong)