Water and ink balance in color newspaper printing

With the advancement of science and technology and the development of the market economy, newspapers in many newspapers have changed to color printing, but domestic newspapers have adopted color printing in large quantities for nearly two or three years. At present, most of the machines used by newspapers for color printing are imported machines. There are mainly Gaussian, Mann Rolland and Swiss Victory Machines. Among them, Germany Man Roland series machines have more users in China. These imported equipments are similar in terms of printing performance and principle, and require higher quality of raw materials and operators. This article takes the German MAN Roland press machine as an example to talk about ink and ink balance in the printing process of a newspaper.

First, the physical characteristics of offset printing plate
A layer of pre-coated photosensitive layer is applied on the surface of the PS printing plate for offset printing. After plate making, exposure, development, fixing, rinsing, and drying, an on-machine plate can be made. The printing plate is divided into a graphic part and a blank part. The graphic part is The hardened photosensitive resin layer has a lipophilic and water repellent property; the surface of the blank portion is a layer of a hydrophilic base layer and has a hydrophilic and oil-repellent property. Since the wet behavior of water and ink in the printing plate and the blank part is exactly the opposite, the balance between water and ink on the same surface of the printing plate can be realized.
Second, the printing press structure and color distribution
The overall structure diagram of the host part of the GEOMAN printing machine is shown in Fig. 1. Its color sequence is blue, red, yellow and black from bottom to top. FIG. 2 is a schematic diagram of a blanket cylinder, a plate cylinder, and a rubber roller of one of the printing units. FIG. The pressure between each roller is shown in Fig. 2. It must be adjusted according to the regulation during the adjustment. If the pressure is too large or too small, it will not only affect the transmission of ink, but also have a great influence on the printing capacity of the printing plate.

Third, the state and effect of ink and water balance
In the printing process of color newspapers, the overall control of ink-ink balance between blue ink, red ink, yellow ink, and black ink will directly affect the print quality of color newspapers. Newsprint is thicker and more absorbent than coated paper. Under normal circumstances, the ink-water balance ratio in newspaper printing is greater than the ink-water balance in sheet-fed offset printing. In the newspaper printing process, as the printing speed continues to increase, the contact time between the paper and the blanket cylinder is getting shorter and shorter, and the amount of water absorbed by the paper is getting smaller and smaller. At this time, the water supply is mainly used to balance the ink on the printing plate. Therefore, the overall water supply will continue to decrease as the speed increases, eventually reaching a critical value. In addition, since the color newspaper is generally only a picture or a little title with color, most of the text and the title is still black, so in the printing process black ink printing unit water supply is always larger than the color printing unit, but the water volume can not be too large, should No lateral deformation of the paper will prevail. Newspapers printed when water is too heavy have wet hands. Therefore, we must strictly control the ink balance in the printing process. The following theoretical analysis of the impact of ink balance on newspaper printing quality.
The so-called ink-water balance means that in order to ensure the normal printing quality and production of newspapers, under a certain printing speed and printing pressure, adjust the amount of fountain solution so that the emulsified ink contains less than 26% fountain solution. The formation of a slight W/O emulsified ink is counterbalanced by the minimum amount of liquid supplied and the ink on the printing plate.
In the production process, when the prepared PS plate is printed on the machine, the graphic part of the ink is transferred directly to the blanket cylinder, and the graphic is transferred to the paper by the two blanket rollers. At the same time, the blank of printing is printed. Part of the water to prevent the ink from adhering to the blank surface. It can be seen that there is a balance between water supply and ink supply on the printing surface. If the ink supply to the layout exceeds the balance (ink > water), the ink will expand toward the blank after being strongly squeezed, invade the blank part, and cause dirtiness; otherwise, if the water supply to the layout exceeds balance (water) Ink), will leave more water on the surface of the printing plate graphic, through the strong force of the water roller and the ink roller, so that the ink layer attached to the graphic part of the deep emulsification, resulting in pale and dull imprint.
In a certain sense, only when the water film on the blank surface and the ink film on the surface of the picture have a very strict demarcation line, that is, it is objectively seen that the water blank area and the inked graphic area do not invade each other and do not mix with each other. It is considered that the ink and water balance is achieved, but in fact, absolute mixing is impossible, and the emulsification of the ink is inevitable. Therefore, ink-water balance is only a relative concept.
It is worth noting that when the printing press maintains high-speed stable operation for a period of time, the lower the supply of water, the better. In actual production, due to the high speed of imported rotary presses, a large amount of heat will be generated in the high-speed printing process, and water will be used for cooling and cooling. Therefore, the effect of the fountain solution is not only on the ink and ink balance, but also on the reduction of printing plates and The blanket temperature plays an important role.
In addition, ink emulsification is not entirely a bad thing. In fact, a slight W/O emulsification is advantageous for printing, which lowers the viscosity of the ink, improves the ink transfer performance, and at the same time prevents the ink from sticking and pulling excess paper powder back onto the roller. In view of the above reasons, the actual supply of water is slightly higher than the critical level.