Why offset printing needs water supply

In essence, the offset printing process is the process by which the printing plate selectively adsorbs the ink and transfers the ink to the substrate. Unlike the gravure plate and the gravure plate, the offset plate has a height difference of only a few micrometers between the graphic part and the blank part. It can be considered to be on the same plane, and the graphic part can absorb the ink, and the blank part can also absorb the ink and have no selectivity. . Therefore, we must use the fountain solution as a medium, according to the printing plate blank part adsorption dampening solution and the graphics part of the principle of exclusion of fountain solution, the first supply of water to the printing plate, so that the high energy surface of the blank part of the printing plate fountain solution low energy surface Instead, ink is then supplied to the plate. Since the graphic part of the printing plate is not wetted by the fountain solution, the ink can still be adsorbed, and the blank part is covered by the fountain solution. Although the surface tension of the fountain solution is still higher than the surface tension of the ink, if the ink spreads on the surface of the fountain solution, the surface of the fountain solution will be replaced by the ink surface, and the interface of the ink and the fountain solution will also be generated. The surface tension of the fountain solution is lower than the sum of the surface tension of the ink and the interfacial tension between the ink and the fountain solution, so the ink does not adsorb and spread on the surface of the fountain solution. At the same time, the ink and the fountain solution are insoluble and the ink is not dissolved in the fountain solution, so the ink can only be selectively adsorbed on the graphic basis of the printing plate, so that the need for transferring the image can be satisfied. Now.

If the printing plate is supplied with ink without supplying the printing plate first, since both the graphic portion and the blank portion of the printing plate can absorb the ink, the need for transferring the image cannot be accomplished. When the full-print version is printed on the ground, it is this feature of the printing plate. In the case that the water is not supplied to the printing plate but only to the printing plate, the printing plate left by the previous printing can be used to print the full version of the plate. Without the need to print plates in full version on the ground.