Dots in printing play a very important role. Dots can transfer the gradations, levels, and colors of the image. Any reflections or transmissions are made up of pigment particles or silver particles, which form a continuous blooming tone. This blooming tone and layer must be photographed or other methods to segment the image into large and small dots.

Dots are the most basic unit of graphics on the plate, and they are responsible for reproducing the tones of the original, the organizational hierarchy, and the outline of the image. According to the principle of the network, it can be divided into two types of amplitude-modulating network and frequency-modulating network. The former has been widely used. The use of various methods (glass screens, contact screens, and electrical separation technologies) can achieve this type of outlets, which can be widely used due to its simple, fast, and good imaging results. At the same time, people also clearly recognized its shortcomings, such as the absolute elimination of moire, so with the development of printing technology, people began to turn their attention to another type of network - FM network.

The frequency modulation screening theory has been produced for some time. In early 1993, the Agfa Division and Linotype-Hell Corporation officially launched the FM network screening technology. In August 1993, China Founder Beijing University also developed its own FM screening technology.

FM screening technology is the image is scanned and input, after processing, through the output device when the same size points are randomly distributed in different spaces, the number of points and density, that is, the frequency of appearance of the network point, in order to express the image Density and hierarchy of technical methods. Therefore, the fineness of the image it represents is no longer explained by the number of black and white lines in each centimeter, but defined by the size of the dots. The smaller the dot size, the higher the accuracy of the image that can be represented. The better.

The technology produces images that are closer to photographic effects. Due to the different exposure levels in the photographic process, there is a difference in the amount of photosensitive silver salt particles in the same area. After the development, there are few silver salt particles at the low density on the image, and there are many silver salt particles at the high density, and the density change is determined by the number of silver salt particles in different regions. This is similar to the FM network, and the amplitude of the AM sites is different. They are expressed by the dot size (in number), so they cannot achieve near-camera effect.

At present, companies have introduced different dot structures and given different names, such as Agfa's “Crystal Network”, Linotype-Hell’s “Diamond Network”, and Dainippon Screen’s “RANDOT”, all of which are different in algorithm. Therefore, the number and location of printing dots in each image area unit are different, but all belong to FM screening.

The network size of the FM screening depends on the output resolution and screening parameters. For example, if the output resolution is 2400 dpi and the screening parameter is 2×2 laser points as a network point, the sub-diameter of the screening point is 21 μm; the screening parameters are With 3×3 laser dots as a dot, the dot diameter is 32 μm.

In the whole process, the most crucial factor is the conversion and adjustment of the FM network. The core is to select the appropriate randomness and the laser beam diameter. The newly generated FM network points are smooth and solid, uniform in size, and have little isolated light spots, which is the key to control quality. The new typesetting machine satisfies this requirement. The laser beam diameter can be adjusted by the dot size. In addition, in order to cooperate with the next process, attention should also be paid to selecting Dot gain values, which can generally be controlled in the range of 5%. In addition to the above, the photographs of the photographs should be washed with attention to temperature, speed, and pH values, and they should be washed with high-speed hot stamps to ensure the quality of the ideas. In terms of printing, due to the fact that there is no limit to the screen angle in frequency modulation, the accuracy of equipment overprinting is rather low, and the color inks that are usually used are also acceptable.

Shaiban is an easily overlooked part, but it is also a link that has a greater impact on product quality. First of all, due to the small diameter of the FM network, no secondary exposure is required. Otherwise, the loss of the network will be a lot. It will sacrifice the cost of some or even all of the 2% of the network. Therefore, only one exposure is required. It should be performed strictly in accordance with regulations, and the time and light quantity must be controlled.

In general, FM networks have the following advantages over AM networks:

1. Randomly distributed by the outlets, there will be no moire after overprinting;

2. Due to the large size of the outlets, the intermediate adjustment will not cause the step change due to the large dot gain value during printing.

3. The detailed sharpness effect is good, some fine lines on the image will not be refracted or frizzy due to screening;

4. Can display delicate levels, suitable for high-precision printing needs;

5. The printing point of the printing plate is finely pinpointed, so the fountain solution is also finely dispersed, making the offset printing operation easy to stabilize;

6. The very small points are fully dispersed, making the sheets and blankets easy to separate, reducing the back of the printed sheets;

7. The smaller the idea, the richer the level of expression.

There are still some problems with FM screening:

1. After the image output can not be revised;

2. There are certain difficulties in copying;

3. Can not use the dot with very low resolution, otherwise it will produce aliasing;

4. Overprinting is very difficult when the image has no fit line, because the edge contour of the image is sometimes not clear;

5. Low print rate

To solve these problems, some people proposed a multi-screen image copying technology that uses both FM screening and AM screening. This kind of multiple screening is to solve the contradiction between traditional modulation and screening printing, which is easy to adapt to, but easy to appear, and the contradiction between emerging FM screening and the absence of “rhizome” but poor printability. This kind of screening method uses computer processing to use different screening methods in different areas of the same image. For example, in the middle tone, the "rhizome" area is screened by FM screening. In the highlight and dark tone areas, the amplitude modulation screening is used. Its long, shortcomings, this way of screening will also be promoted.

At present, in the field of flexographic printing, FM screening technology is widely used, such as printing some high-precision product packaging boxes, cigarette packets, etc., I believe that with the in-depth application of computer technology in the printing field, FM screening technology will also be In other areas of printing continue to promote.