Talking about FM Screening Technology
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The dot in printing plays a very important role. The dot can transmit the tone, layer and color of the image. Any reflective or transmissive manuscript is composed of the density of pigment particles or silver particles to form a continuous blooming color. The color tone and level of this smudge must be divided into large and small dots by means of photography or other methods.
The dot is the most basic unit of the graphic on the plate, and it plays the role of reproducing the original tone and organization level and image outline. According to the working principle of the network, it can be divided into two types: amplitude modulation network points and frequency modulation network points. The former has been widely used so far. This type of mesh can be obtained by various methods (glass screen, contact screen, and electric division technology), and it can be widely used due to its advantages of simple and fast screening process and good imaging effect. At the same time, people also clearly recognized its shortcomings, such as the absolute elimination of moiré, etc., so with the development of printing technology, people began to turn their eyes to another type of network - FM outlets.
The FM screening theory has been produced for some time. At the beginning of 1993, Agfa Branch and Linotype-Hell Company officially launched the FM screening technology. In August 1993, China Peking Founder also developed its own FM screening technology.
The FM screening technology is that after the image is scanned and input, it is processed through the output device, and the points of the same size are randomly distributed in different spaces, and the number of points and the density of the dots appear as the frequency of the dots, thereby expressing the image. Technical methods of density and hierarchy. Therefore, the fineness of the image it represents is no longer explained by how many black and white lines are in each centimeter, but by the size of the dots. The smaller the dot size, the higher the accuracy of the image. The better.
The image generated by this technique is closer to the photographic effect. Due to the difference in the amount of exposure during the photographic process, the number of photosensitive silver salt particles in the same area is somewhat different. After development, where the density of the image is low, there are few silver salt particles; where the density is high, there are many silver salt particles, and the density change is determined by the number of silver salt particles in different regions. This is similar to the FM network point, and the amplitude modulation dot is different, which is represented by the dot size (in number), so that the effect of close photography cannot be achieved.
At present, companies have different outlets and different names, such as Agfa, which is called “Crystal Net”, Linotype-Hell, “Diamond Net”, and Da Nippon Screen Company, “RANDOT”, 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 size of the network of the FM screening depends on the output resolution and the screening parameters. For example, the output resolution is 2400dpi, and the screening parameter is 2×2 laser points as a mesh point. The diameter of the screening point is 21μm; the screening parameter is 3 × 3 laser spots as a dot, the diameter of the dots is 32 μm.
In the whole process, the most critical factor is the conversion and adjustment of the FM network. The core is to select the appropriate random rate (randomdots) and the diameter of the laser beam. The newly generated FM network points are smooth and solid, uniform in size, and rarely isolated, which is the key to control quality. The new imagesetter is meeting this requirement and the laser beam diameter can be adjusted to the dot size. In addition, in order to cooperate with the next process, attention should also be paid to selecting the dot gain value (Dot Gain), generally can be controlled within the range of 5%. In addition, the rinsing of the photo film pays attention to the values of temperature, speed, PH value, etc., and uses fast high temperature punching to ensure the quality of the pip. In terms of printing, since the frequency modulation does not have the limitation of the screen angle, the precision of the overprinting of the device is not high, and the color ink generally used is also acceptable.
Printing is an easy to ignore link, but it is a link that has a greater impact on product quality. First of all, because the frequency of the frequency modulation dot is small, no double exposure is needed. Otherwise, the dot loss is a lot, and some or even all 2% dot will be sacrificed. Therefore, only one exposure should be performed strictly according to regulations, and the time and amount of light must be controlled.
In general, FM networks have the following advantages over AM networks:
1. Randomly distributed by the dots, no moiré will appear after overprinting;
2. Due to the large size of the outlets, the intermediate adjustment during printing will not cause the tone to jump due to the large expansion value of the outlets;
3. The fineness of the detail is good, and some thin lines on the image will not be refracted or crepe due to the screening;
4. Can express delicate level, suitable for high-definition printing needs;
5. The printing plate has a pinpoint point, so the dampening solution is also finely dispersed, which makes the offset printing work easy to stabilize;
6. The tiny dots are completely dispersed, which makes the sheets and blankets easy to separate, reducing the back of the sheets;
7. The smaller the idea, the richer the expression hierarchy.
There are still some problems with FM screening:
1. The image cannot be modified after the image is output;
2. There are certain difficulties in copying;
3. You cannot use a dot with a very low resolution, otherwise it will produce jaggedness;
4. When the image has no compliant line, overprinting is very difficult, because the edge contour of the image is sometimes not clear;
5. Low printing durability
In response to these problems, some people have proposed a multi-screen image replication technology that applies both FM screening and AM screening. This kind of multi-screening is to solve the contradiction between the traditional amplitude modulation and screen printing, but it is easy to appear "turtle", and the emerging FM screening does not appear "turtle" but the printability is difficult to grasp. A kind of screening method, using computer processing in different areas of the same image using different screening methods, such as the middle of the adjustment, the "turtle" area is used for FM screening, and the high-light and dark-adjusted areas are used for amplitude-modulation screening. The length of each, to avoid short, this screening method 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 cases, etc. It is believed that with the in-depth application of computer technology in the printing field, FM screening technology will also It is continuously promoted in other printing fields.