How to Better Control the Overprint Deviation Problem of Four Color Printing

In some small and medium-sized printing companies that use single- and double-color offset presses, the problem of overprinting deviations in four-color printing is a headache for workers' masters. Usually only measures such as reducing the speed of the machine, reducing the amount of ink, adjusting the ink flowability, and dusting are also difficult to achieve.

In fact, the overprinting process can be divided into "dry overprinting" and "wet overprinting." “Wet overprinting” means that the inks used for overprinting on the substrate are wet inks, that is, the color of the previous color is printed immediately after the overprinting; and the “dry overprinting” is the same after the ink of the previous color is dried and then overprinted. .

1. Dry overprint

The advantage of this printing method is that it can ensure that the positioning of the paper is stable in the subsequent printing, the overprint position is basically fixed, and dry ink overprinting can obtain full, vivid, and correct color. However, due to the need to stop for a certain period of time, it is easy to cause the paper to expand and deform, resulting in the phenomenon of overprinting, and the working efficiency is not high. To make overprinting more accurate, it can only be solved by controlling the moisture in the layout and reducing the printing pressure and lining thickness.

2. Wet overprint

This printing method is relatively efficient, and the paper is accurately overprinted due to small deformation. However, it also has many problems. For example, when the ink volume is large or the printing area is large, the back stains are easily caused, the color of the printed product is dark, and the positioning of the paper in the subsequent printing is difficult, which affects the accuracy of overprinting. If you have an advanced four-color offset press, you can solve the problem, but usually only large companies have such conditions, and ordinary single and two-color machines can only be solved by experience and increased labor.

3. improvement measures

How to turn wet printing into dry printing with "time shrinking" is the goal of our exploration. The following improvements can be achieved:

1
Check the status of printed paper for printing. Do not use paper that is damp or air-dry. The new paper must be processed or stored for a certain period of time to ensure that it is in balance with the temperature and humidity of the printing shop. Cut the paper to ensure that its edges are neat and smooth to ensure the paper's stability.

2
In the ink, little or no added additives such as thinner and varnish are used, because it not only affects the color of the ink, but also causes the back of the printed sheet to become dirty. The ink drying speed of the ink is slow, causing the paper to be printed during the subsequent printing. Locating difficulties directly affects overprint accuracy.

3
Before printing, especially when using quick-drying ink for printing on copperplate paper, it must be ensured that the blanket is dry and clean, so as to prevent the surface of the blanket from sticking to the paper due to the presence of pre-colored prints, which will affect the stability of paper feeding.

4
Take measures to make the preceding semi-finished printing products dry quickly. Heat the arc-shaped paper holder outside the paper feed roller. Flexible use of powder spraying device, the first two colors do not need to spray powder, and keep the blowing pipe on the prints of the air drying effect, the size of the gas according to the paper thickness adjustment, the general advanced four color printing paper to reach more than 150g/m2, air volume can be appropriate To increase, the air volume can be adjusted to the maximum when the white paper is printed, so that the print can be fully air-dried. After the heating and air drying device is installed, the ink is almost dry when the printed product reaches the delivery table, which does not cause the back stains, and ensures the correct paper positioning during the subsequent printing. In this way, wet printing becomes dry printing without time intervals, the accuracy of the fitting is guaranteed, and the work efficiency is improved.

The above measures are all based on the conventional printing requirements. The printing operators must carefully and conscientiously engage in the printing production work, master the ink-and-wash balance, check the fitting situation at any time, find problems, and stop processing in time.

Source: Chaoyang Printing Factory