In the printing process, dots are generally used as the basic unit. Because the dots are very small, the dot is always changing through screening and printing to printing on paper. Causes changes in the hue of the entire picture and affects the quality of the print. The main change is the increase in dot. Dot gain includes optical increase and physical increase. The optical enlargement, that is, the reflection and diffusion of light, causes the visual area that enters the human eye to reach the retina to be larger than the actual area of ​​the dot. The physical increase includes the increase of printing pressure, the lack of printing exposure, and the dot gain caused by ink absorption and diffusion caused by the characteristics of paper and ink. Physical increase is the main form of dot gain and can be reduced by changing the printing conditions.

According to the geometric dot gain, the edge portion uniformly spreads outward, and the percentage increase of the dot points in the radial direction is consistent. For example, a 10% dot gain (a virtual edge) is 10 μm, and a 90% dot gain value is also 10 μm. However, in the actual print transmission, because the lengths of different network sides are not the same, the ratio of the coverage area increased by the dot is different. The area covered by the 50% square dot increase is the largest. When the dot area is between 10% and 50%, the increase is at the edge of the dot; while 60% of the dot is increased within the black circle, that is, the white dot is reduced.

Dot gains often result in darkening of the image and darkening of the midtones, and darkening of parts of the hierarchy. If you do not deal with this problem, it will not be useful to make more adjustments to the image. Therefore, it is necessary to fully consider the influence of various factors in the process of plate-making on the dot gain, and set a compensation amount in advance to compensate for dot gain in printing.

First, the factors that affect the increase of outlets

1. Influence of printing conditions on dot gain

The area of ​​the dots during the printing process will vary depending on the printing conditions. In the case of normal printing conditions, in order to maintain the proper contrast of the picture, there must be enough ink and proper pressure, so normal dot gain is unavoidable. However, excessive ink volume and pressure will increase the number of outlets too much, resulting in the parallelization of the screen hierarchy.

2. Effect of ink layer thickness on dot gain

When the ink dot passes from the plate to the blanket, and then it is transferred to the paper, the ink layer has a certain thickness and is a soft body, and the dot must increase after being pressed. The degree of increase depends on the amount of ink to be supplied, so the amount of ink must be strictly controlled. If too much or too little ink is used on the press, halftone dots may be filled with ink due to dot gain, or they may be completely "exposed". The same phenomenon can occur if the ink balance is out of balance. Since the ink is sticky and wet, printing many inks onto the paper may cause wrinkling, sticking to the ink film, and may cause various troubles. Therefore, in image processing software such as Photoshop, all have the function of removing UCR and gray components instead of GUR, which can delete the colors of various inks and reduce the overall percentage of ink to a reasonable value (usually 240%). And between 360%), the specific value depends on the type of printer, paper status and so on.

3. Influence of printing pressure and process conditions on dot gain

In general, the greater the printing pressure, the more serious the dot gain. Offset press produces certain deformation of the blanket, which can also cause dot gain. Any problem on the same press can affect the tone value of the image. For example, if the printing offset is too loose, DotDoubling is usually generated. It looks like a serious dot gain problem, or there are too many scratches on the paper and the paper slips slightly relative to the impression cylinder, which usually causes DotSlur problem. This dot gain will vary depending on the printability of the press, the type and status of the press, and the ink brand. Therefore, according to the different printing conditions to trim the state of the level of the curve, will be helpful to the level of printing quality performance. Generally, according to the increase of printed dots, the appropriate reduction in the middle will enhance the printability of prepress production.

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