Printing pressure is one of the conditions of ink transfer, the printing plate surface can not be absolutely flat, the surface of the paper is also inevitable there are subtle convex and uneven thickness, if the printing pressure is less than or uneven, the printing ink layer is easy Uneven shades appear. The ideal printing process is based on the "three-ping", which requires the printing plate and the substrate and the envelope surface to achieve a relatively flat conditions, in order to thin the ink layer by balancing the role of printing pressure, so that printed on More uniform ink color. When the printing pressure is insufficient or uneven, the lining body and the printing plate surface there is bad contact, you need to increase the amount of ink to meet the layout inking the visual effects, but this will not only increase the ink consumption , But also easy to produce printing color and printed matter appear dirty quality problems. Therefore, to prevent the occurrence of printing color, we should grasp the basic conditions for printing "San-ping" and adopt uniform and constant pressure for printing. If it is a letterpress process (using a round flat or flat-type flat-type printing machine) a large area chart version of the product, it is best to use metal-based underlay material, so that its material flatness and robustness, printing pressure comparison Stable, help prevent color variations caused by pressure variation.