FM screening fault handling

Dots are the most basic units of a graphic on a printing plate, and they are responsible for reproducing the tones, levels, and contours of the original. According to the principle of the network, it can be divided into two types of amplitude modulation network and frequency modulation network. The former can be widely used due to its advantages such as simple, fast, and good imaging effect. At the same time, people also clearly recognized its deficiencies, such as the absolute elimination of moire, and 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, Agfa Corporation and Linotype-Hell Corporation officially launched FM screening technology. In August 1993, China Founder Founder Corporation also developed its own FM screening technology.

FM screening technology is the image is scanned by input, after processing, through the output device when the same size points are randomly distributed in different spaces, with the number of points and the degree of density, that is, the frequency of appearance of the network to represent the image density and Hierarchical 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. The amplitude of the network is different. It is represented by the dot size (in number), so it cannot approach the effect of photography.

At present, the company's launch of the network structure is different, and the name of the network is also different, such as Agfa company called "Crystal Network", Linotype-Hell company called "Diamond Network", Dainippon Screen Company called "RANDOT", etc., are algorithmically Different, so the number and location of printing dots within each unit image area 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 entire process, the most critical factor is the conversion and adjustment of the FM network. The core is to select the appropriate randomness (random dots) 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 the dot gain, which can generally be controlled within the range of 5%. In addition to this, the photographs of the film should be paid attention to temperature, speed, pH and other values, with high-speed rapid film, in order to ensure the quality of the idea. 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, because of the small diameter of the litter network, no secondary exposure is required. Otherwise, the network will suffer a lot of losses, at the expense of some or even all of the 2% outlets. Therefore, only one exposure is needed, and the time and light quantity must be controlled.

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

1) Because the dots are randomly distributed, there will be no moiré 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 details are good, some fine lines of the image will not be refracted or burrs 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 printing operation easy to stabilize;
6) The full dispersion of extremely small dots makes it easy to separate the printed sheets and blankets, and reduces the back-scratching of printed sheets.

Everything has its advantages and disadvantages. There are still some problems with FM screening:

1) No revision can be performed after image output;
2) There are certain difficulties when copying;
3) Do not use a dot with a very low resolution, otherwise aliasing will occur.
4) Overprinting is very difficult when images have no fit lines, so the edges of the image are sometimes not clear;
5) Low print rate.

In response to these problems, a multi-screen image copying technique using both FM screening and AM screening has been proposed. 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, each avoiding its short.

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.

Source: "Printing World" Author: Huang Wei