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Reasons For Overprinting And Solutions
- May 29, 2018 -

The overprint accuracy of letterpress prints is one of the important indicators for measuring print quality. Therefore, how to improve the precision of overprinting of prints is particularly important.

The main reasons that lead to overprinting of printed products are: the accuracy of the equipment, the improper adjustment of the machine, the warp and deformation of the paper, improper operation, etc.


First, the misalignment caused by poor accuracy of equipment

When the roller gears, plate racks, connecting rod bearings, delivery teeth, and cams of the printing machine, as well as the mechanical parts that interlock the front gauge and the side gauges, are worn out, the overprinting of the products can easily be misaligned. Identification of such failures can be carried out by an empty imprinting inspection method, that is, under the condition of normal tightness of the lining, continuously embossed several times at the normal printing speed to observe the accuracy of the lining of the liner lining, if the double-imprinting is serious , the machine's accuracy is poor, and parts that are related to the embossing movement are heavily worn. In addition, it is also possible to take several sheets of paper with a flatness and a good stiffness of 70 g/m2 or more, and use the same layout for continuous overprinting twice to observe the imprinting, if one side is overprinted and the other side is clearly double printed. It indicates that the accuracy of paper-feeding teeth, drum teeth, front gauge or paper-feeding mechanism is not enough, and corresponding repair and adjustment measures should be taken according to the actual situation.

In order to avoid overprinting due to equipment accuracy is not allowed, reasonable use of equipment, should not allow the machine to continue to operate 24 hours a day. Usually pay attention to the lubrication of the equipment to prevent the wear of key components due to poor lubrication.


Second, the misalignment due to improper adjustment of the machine is not allowed

In the printing process, if the amount of rubbing of the cavities is too small to hold the edges of the paper, slipping is likely to occur during stamping. Defective paper feeding, unbalanced opening and closing movements, insufficient pressure on the gripper, and misalignment of certain parts of the paper transport system can result in improper overfeeding or overfeeding of the paper feed. The press plate on the side gauge of TY401 four-press printing machine is adjusted too low. When printing thick paper, the phenomenon that the side gauge can not push the paper easily occurs; if the chain under the DT400 type four-open platform machine feeds paper, it will also accumulate paper dust. The sideways movement is not in place, making the overprinting of the printed product impossible. In addition, excessive printing pressure, blind lining thickening, loose packing and lining are also the reasons for the misprinted layout (larger layout) and overprinting.


Third, the paper is not allowed to register due to telescopic deformation

When the moisture content of the paper is abnormal and the edge of the paper is curled, overprinting may occur. For curled paper edges, it is generally achieved by tapping the paper to achieve the flatness required for normal printing. When knocking on the paper, be careful not to repeatedly press the edge of the paper to avoid forming “wavy” paper edges, resulting in inaccurate overprinting. In order to avoid paper deformation, the cut paper should be immediately transported to the printing shop, so that the moisture content of the paper is balanced with the temperature and humidity of the workshop. In addition, the printing shop must maintain constant temperature and humidity.


IV. Improper overprinting caused by improper operation process

1. The plate bottom is bad and the pressure is too great

Letterpress printing machine printing large area color printing products using multi-layer plywood bottom support, due to the material is not solid and the emergence of imprinting flowers, and the four-sided wooden tray is difficult to achieve right angle, straight edge process requirements, printing plate after the fast In the bow, it is often necessary to increase the pressure of printing in order to achieve a good transfer of the ink layer. Therefore, when printing large-format overprinting products with large ink-absorption capacity, metal magnetic platen and aluminum platen should be used to ensure accurate overprinting and good printing effect.

2 ink layer viscosity is too large

The ink viscosity of Toppan color printing products is relatively large. Therefore, when printing large-format background colors, “deep ink thin printing” should be adopted. That is, the hue of printing ink should be adjusted darker, and the printing ink layer should be thinner. This can satisfy the hue requirements of printing. , And can reduce the viscosity of the ink layer. Due to the low viscosity caused by the low temperature of the ink, additives can be used to adjust the ink flow to ensure accurate overprinting.

3 paper cutting is not standardized

Improper paper cutting can cause skewed or “wavy” edges on the cut paper, resulting in misprints during printing. In order to ensure accurate overprinting of printed products, the cut paper must reach right angles and straight edges.

In summary, there are many reasons for inaccurate overprinting of products. As long as we seriously sum up experience and effectively shut down good operating technology and technology, we can print qualified products and increase production efficiency.


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