Graphic printing overprinting failure processing

The overprint accuracy of plate prints is one of the important factors for measuring product quality. However, due to improper operation technology and technical measures, there are many phenomena in the printing process, such as pin position instability and overprint misalignment. Analyzing the failures, it is not difficult to find the reasons are many:

One, improper ink transfer, ink viscosity is too large, causing the pre-registration position of the printed sheet is not stable

Practice shows that maintaining moderate viscosity of ink is an important condition for printing and ensuring the quality of printed products. It can be said that the lack of a certain viscosity of the ink, can not print a solid color, fullness and good brightness of the ink layer. However, there is no standard for ink viscosity and it is impossible to unify a standard. Printing should be based on the characteristics of the printed material (substrate) and the layout of the printing layout, and the viscosity of the ink should be adjusted according to experience. If the viscosity is adjusted too low, the rubber roller will fail to slide. As a result, ink and ink will not be evenly distributed, and the print layout will appear as "oily". If the viscosity of the ink is too high, the printed sheets in the imprinting are liable to be pulled by the ink and slip with the drum, so that the predetermined position is not allowed to fail. DT400 type four open automatic platform printing machine, when printing a large area of ​​the field version, if the ink viscosity is too large, it will have a strong pull effect on the printed sheet on the drum, so that the printed sheet slides back from the bite of the paper teeth, the front position The error can reach about 3mm, and the consequences are very serious. In order to prevent this kind of trouble from happening, the printing plate product with larger ink-absorbing capacity can be added with an appropriate amount of de-bonding agent or varnish according to the viscosity of the ink, which can reduce the tension of the ink on the printed sheet and ensure the printing accuracy.

Second, printing pressure adjustment over the ambassador printed sheets

Slippage from the position of the paper teeth, overprinting is not allowed to print pressure is the role of factors to achieve a uniform transfer of the imprint, the ideal printing ink layer derived from moderately balanced printing pressure. In general, the substrate surface is rough, the printing format is large, and the printing pressure has to be increased accordingly. However, in actual operation, the printing pressure cannot be increased blindly to satisfy the visual effect of the printed ink layer. Once the printing pressure is adjusted too heavy, it will not only increase the wear of the machine and printing plate, but also cause overprinting misalignment. This is because on the one hand, the printing pressure is too large for the deformation of the printed sheets, which results in a phenomenon that the layout of the printed sheets is enlarged, resulting in inaccurate overprinting; on the other hand, the printing pressure is too high, and when printing, it is bound to exert pressure on the printed sheets on the rollers. The power causes the sheet to slip backwards and pulls from the gripper, which will make the pre-printed sheet unstable, resulting in a minor situation. To avoid this undesirable effect, the pressure adjustment of the plate printing should be available on the page. When the ink volume is normal, the pressure is gradually increased from light to heavy, so that the printing pressure can be accurately adjusted to the proper printing pressure to ensure the printing quality.

Third, the printing plate base is not solid, flatness, poor compressive strength caused by overprinting is not allowed

The hardness and precision of the plate support used in the plate printing (sticking plate base) affect the printing accuracy of the product to a great extent. If traditional plywood is used to serve as a large-format version of map printing, the plywood lacks flatness and fastness, and the compressive deformation of the plywood is prone to compression deformation. This will inevitably lead to pressure unevenness or undercuts in the printing process. Differently, it is very easy for the printed product to be deformed, resulting in a larger printed surface than the actual version. In addition, due to the drawback of easy bowing caused by the wooden tray, in this way, the imprint of the roller squeezing the teeth is inevitably printed due to slight vibration or movement of the plate, resulting in slippage and inaccurate overprinting of the product. For this reason, printing plates with large format and high pressure should use magnetic platen or aluminum bottom plate for sticking, which can ensure the accuracy of product overprinting.

Fourth, the good lining process technology to prevent the product from printing and deformation caused by overprinting is not allowed

The lining is a bridge that realizes transfer of layout printing and has a certain influence on the printing accuracy. Due to the differences in equipment structure, the requirements for the thickness of the lining are also different. Only in strict accordance with the printing machine specification parameters, control the machine's lining thickness, in order to ensure that the printing plate is not out of shape. If the thickness of the lining is not enough, the longitudinal dimension of the layout of the product is often smaller than the corresponding length of the printing plate; if the lining thickness exceeds the equipment specified parameters, the printed surface is larger than the actual size of the printing plate, which is a factor that affects the printing accuracy. In addition, if the lining is not sufficiently firm, loose and drum-like, and the outer lining is dug in a large area, and the upper pad only sticks to the side of the occlusal area, and the side of the tipping position is not affixed to let it flutter. , This will also produce a large deformation of the printing plate layout, causing print misalignment. In order to ensure the precision of the plate printing, it is better to use a neutral hard pack material and pack the packing material tightly.

Fifth, the roller paper bite misalignment caused by paper positioning failure

Printing the field version, if the pressure of the roller teeth is insufficient, then the paper in the embossing can easily slip out of the teeth. This is because the layout ink is sticky, which gives a certain degree of resistance to the printed sheet. Such as DT400 type four automatic platform printing press, printing a large area of ​​real map version of the product is prone to the above drawbacks and India is not allowed, the main reason is that the original machine drum teeth design and installation of spring steel sheet pressure is not enough, resulting in bite Tightly edged. In the past, in the practice of the machine, the author tried to increase the number of steel sheets by adding teeth to the teeth of the drum to prevent the appearance of slipping and slippage during printing and to ensure the printing accuracy. In addition, there is a flaw in the aircraft, affecting the accuracy of the printed sheet, which deserves attention. If the printed sheet enters the teeth of the roller and the jaw is closed by the teeth, the slight retreat will occur at one end of the printed sheet, and the printing will be skewed. This is mainly caused by the sloping side of the teeth corresponding to the accumulation of ink or a group of paper dust, when the teething closed movements, the raised teeth in the teeth just hit the top edge of the paper, there has been this failure. As long as the dust in the teeth is cleaned up, the fault can be eliminated.

(Author/ Kang Qilai)