How to control the overprint deviation of offset printing
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When printing multi-color prints with a single-color offset press, the four-color overprinting deviation is often a headache for the worker's master. The first two colors are generally ok, but the latter two colors are difficult to control. Usually only reduce the speed of the machine, reduce the amount of ink, adjust the fluidity of the ink and dusting, but the effect is not great, and the above measures will affect the work efficiency and product quality, and at the same time bring about a multiple increase in labor intensity.
In fact, we can divide the printing into two types: dry and wet. The so-called "wet printing" means that the two overprints on the substrate are wet and wet, that is, the first color is printed after the printing is finished; and the "dry printing" is printed after the one color is dried, and then the color is printed. Monochrome machine prints four-color products.
1, dry printing
The advantage of this printing method is that the positioning of the paper feed in the subsequent printing is stable, and the printing position is basically fixed. In addition, dry ink overprinting can achieve full color, bright and accurate color. However, due to a certain amount of downtime, it is easy to cause paper deformation and inaccurate overprinting, and the efficiency is not high, and it cannot be completed in case of urgency.
2, wet printing
This printing method saves time and the paper is accurately overprinted due to small deformation. However, it has many problems, such as easy to cause backside smudging, gray color of printed products, large amount of ink or large printing area, and it is difficult to position the printed paper after printing, which affects the accuracy of overprinting. If there is a problem with a four-color offset press, the single and two-color offset press can only be solved by experience. How to reduce the time of wet printing is the goal we explore.
Method 1. Check the status of the printed paper to be printed. The paper with high humidity or air-drying deformation cannot be used. The newly-introduced paper should be air-laid or hoarded for a certain period of time to ensure its balance with the temperature and humidity of the printing workshop. Cut the paper to ensure that its edges are neat and smooth, and ensure the paper stability of the paper;
Method 2: Do not put or put less additives such as thinner and varnish in the ink, because it affects the color and easily causes the back surface to be dirty, and the printed ink is not easy to dry quickly, resulting in the post-printing Paper positioning is difficult, directly affecting the accuracy of overprinting;
Method 3: Before printing, especially before printing on coated paper with fast-drying ink, it is necessary to ensure that the blanket is dry and clean, so as to prevent the blanket from sticking to the paper due to the semi-dry state of the front print, which affects the stability of the paper feed. The blanket should also be cleaned during the middle stop;
Method 4: Take measures to accelerate the drying of the printed semi-finished products. Here, the arc-shaped paper towel holder outside the paper feed roller can be converted into a oil-proof electric sheet for heat treatment. In addition, the dust spray device should be used flexibly. The first two colors do not use powder spraying to maintain the air drying effect of the blowing tube on the printed product. The volume of the air volume is adjusted according to the thickness of the paper. Generally, the air volume of the advanced four-color printing paper can be appropriately increased, and the air volume can be adjusted to the maximum when printing on the whiteboard paper. In order to obtain sufficient air drying effect of the printed product. After the hot air drying device is installed, the ink is basically dry before the printing to the delivery table, which does not cause the back surface to be dirty, and ensures the correct paper positioning of the paper during the post-color printing. Thus, the wet printing seems to have become Dry printing without time interval not only ensures the accuracy of overprinting, but also solves problems such as dirty printing and poor drying.
The above measures are based on the requirements of conventional printing. The printing operator must operate conscientiously, master the balance of ink and water, control the amount of ink, check the overprinting situation at any time, find problems, and stop the operation in time.