Overprint Errors of Prints (4)

Fourth, positioning marks

In the printing process, the reasons for inaccurate overprinting are complex and the forms of performance are different. For example, overprinting between image lines is inaccurate; misprints between various inks are not allowed; overprinting of solid parts is not allowed; overprinting of halftones or thin lines is inaccurate; positive and negative front and back surfaces due to image displacement Overprinting is inaccurate, and gaps between front and back sheets are not allowed. In order to determine the accuracy of overprinting, the “positioning mark method” is often used to detect changes that occur at the time of overprinting, so that the printing operator can take timely measures to improve overprint accuracy.

In the offset printing process, the “cross-marking method” is generally used to position the cross-marker. The cross-marker is calibrated on the four corners and the middle part of the printing plate at the time of plate making, as shown in Figure 8-3. After the printing, it is cut along with the paper edge. Drop it. In multi-color printing, it is possible to check overprinting in the direction of printing up/down, left/right, and oblique lines from the degree of misalignment or skewness of the cross mark of various colors.

Figure 8-3 cross mark and the name of each part of the press sheet

When printing, the paper is sandwiched by grippers. One end of the paper gripper paper is called "mouthwash" and the other end is called "paper tail" or "drag." Gripper grips the paper with a large force, so it will leave obvious traces on the paper, especially on the coated paper. With this trace, the direction of printing can be identified. In general, most of the overprints do not occur at the two corners of the paper, as shown in the right figure of Figure 8-4. In other words, except for special circumstances, the image of the mouth part will not be overprinted, as shown in the left figure of Figure 8-4.

The positioning of the paper on the printing press is determined by the front-end position in the up-down direction, and the position of the left-right direction is determined by the side stopper.

If overprinting is not allowed in the left and right directions, the serious side must be the opposite of the side fence.

Figure 8-4 Overprint status

When using the cross mark to determine the accuracy of overprinting, it is necessary to understand the sequence of printing colors, that is, the order of printing colors. If you do not understand the printing sequence, you cannot analyze the reason for overprinting. For example, monochrome printing presses may use black, ephedra, magenta, yellow (transparent yellow ink) printing color sequence; may also use yellow (opaque yellow ink), cyan, magenta, black printing color sequence. When black and cyan overprinting occurs inaccurately, if two colors are aligned at the end of the mouth, two colors are not registered at the end of the paper, and black moves to the inside of the cyan, that is, the black cross mark is off the tail. Farther away, then, in the case of black and blue printing, it was indicated that overprinting was not caused by shrinkage of the paper. If the printing sequence is cyan or black, it means that overprinting is not caused by the elongation of the paper. The so-called shrinkage or elongation of the paper is based on the relative comparison between the size of the printing plate and the size of the paper. In general, relative expansion and contraction are indicated by the displacement between the cross marks.




Author: FENG Rui dry Source: "Principles and printing process"