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In all printing methods, gravure printing exerts the greatest printing pressure. The uniformity and size of the printing pressure directly affect the change of the paper tension, and thus affect overprinting.
1. Uniformity of printing pressure
According to many years of practice, the author believes that for products with less color number (within 4 colors), the uniformity of the printing pressure will not affect the overprint as long as the printing pressure reaches the requirements of the printed product; but for printing When the number of colors is 4-6 colors or even more, the pressure roller of each printing unit must be adjusted so that the pressure roller of each unit remains flat and consistent, and the pressures of the front and rear units are the same, otherwise the unit There may be a “crepe†phenomenon between the unit and the paper, causing the paper to become slack, or the paper wrapping around the gravure plate cylinder and damaging the squeegee blade. In addition, in the daily work, it is necessary to keep the surface of the imprint rubber roller clean, because some abnormal shutdown (such as paper break, sudden stop, etc.) will make the ink tank and small ink tank overflow the ink, and the imprinting roller will inevitably stick. On the other hand, the ink forms a solid ink on the rubber roller after drying, which increases the local circumference of the rubber roller, thus changing the uniformity of the entire rubber roller and accelerating the aging of the rubber roller.
2. The size of the printing pressure
The size of the printing pressure is also more critical. Under normal circumstances, the print pressure can be verified by normal shutdown. If the machine is normally stopped, the pressure roller of each unit can just lift automatically, which means that the printing pressure is right. Of course, at this time, for some color blocks (such as gold, big red bottom, etc.), false printing may occur, that is, the phenomenon of “defectiveness†may occur. Therefore, according to actual printing conditions, a certain unit can be appropriately added on the basis of maintaining pressure uniformity. The printing pressure.
The gravure plate cylinder is divided into a shaft plate cylinder and a shaftless plate cylinder. For a shaftless version, a plate shaft is required to support each plate change. After the plate shaft is used many times, the shaft head will wear more and more thinly, and the outer metal sleeve will move up and down, so that the eccentric motion will occur when the plate cylinder rotates, and overprinting will be affected. For shafted cylinders, due to the higher platemaking costs, the printing manufacturer will use the plate cylinder for multiple plate-out operations. During this process (more than 8 million rotations), the shaft head will become finer and thinner. The above result. If you want to continue to use this version of the drum, it is recommended to relatively reduce the inner diameter size of the shaft head metal sleeve, because the production cost of a gravure plate roller with a shaft is higher, and the processing price of the shaft head metal sleeve is relatively cheaper.
In the change of the plate, if the gravure tight plug is loose, it will cause the plate cylinder to move axially under the action of the embossing rubber roller, and there will be failures in the horizontal and horizontal overprinting. Therefore, at the end of the plate change work, before the product overprinting, it is necessary to carefully check whether the solid plugs of all gravure printing units are loose.
The wear of the internal bearings of the fastening plugs will also have an effect on overprinting. If the wear is severe, there will be a failure of the shaft head metal sleeve buckling and gravure jumping. In order to prevent the wear of the bearing, it is necessary to apply a certain amount of butter to the inner bearing of the fastening plug at each change of plate.
The photoelectric scanning head is the key part of detecting gravure overprinting, and the scanned signal is directly transmitted to the main console, so whether the signal is stable or not plays an important role in overprinting. The following is a brief description of the use and maintenance of the photoelectric scanning head.
The detection angle of the photoelectric scanning head is closely related to the printed color. In general, darker colors such as red, black, gold, and green, because the signal is strong, the scanning head can be directly detected; for yellow (transparent yellow, medium yellow, light yellow), light brown, gold and silver For white printed on cardboard, etc., because the signal is weak, the scanning head must be rotated at a certain angle for detection, and the fine adjustment knob on the scanning head must be gently adjusted until the signal displayed on the main console is stable. When arranging a gravure process and making a gravure plate, a darker color is generally used as the reference color for overprinting. The advantage of this is that it is convenient for overprinting.
In order to prevent the scanning head from providing the wrong signal, the photoelectric scanning head should be kept clean during the daily work. The main cover personnel should regularly check the scanning head so that the detection head is transparent and clear, preventing dust, paper hair, paper dust and ink particles. Other debris enters the photocell and interferes with the scanning signal.
Source "Printing Technology"
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