Author: Liu Yusheng
Q: What are the differences, advantages and disadvantages of AM and FM screening?
Answer: The dot is the basic way to reproduce the color continuous tone original. The dot size is responsible for the thickness of the ink layer, the change of the tone of the tone and the organization of the color of the reproduced image. There are currently two screening methods for color manuscript copying, namely amplitude modulation (AM) screening and frequency modulation (FM) screening. AM screening is a periodic screening method and is a common process in the four major printings; FM screening It is a non-periodic screen screening method, and it is a high-fidelity color reproduction new technology proposed by Agfa and Lino-Haier in 1993.
1. Amplitude modulation screening is a screening method that allows continuous adjustment of original manuscripts to form an orderly arrangement of small squares on the negative through screening. Each small square represents a basic unit, called a mesh, and in each mesh there is a dot exposed by the laser, distributed at the center of the grid. Because the small square is very small, the human eye cannot distinguish the size of the dots in the mesh within a certain viewing distance, and can only see the depth of the tone level. AM screening is to use the change of the dot area in each small square to express the original layer.
2. FM dots are also called random dots (diamond dots). They are laser dots with a very small pixel unit of 7 ~ 35μm randomly distributed in a specific unit area. The dot size is the same, which is based on the density of different dots. Expressing the shades of the image, the smaller the dots, the more layers and colors can be reproduced. Its advantage is that since the dots are randomly distributed, moire will not occur after overprinting. Imagesetters with high definition, uniform reproduction and low resolution can use FM technology to output high-quality images. It is beneficial to multi-color reproduction and improves production efficiency.
3. The difference between the two screening technologies: The traditional amplitude modulation screening parameters include the number of screening lines, screening angle, tone range, and dot shape. FM screening has only one parameter, namely the dot size, and FM screening will not produce moire. The difficulty of amplitude modulation and screening is directly related to the moire pattern, especially textile patterns, wood grain patterns, fine arts, Chinese paintings, etc. Once printed moire products will be scrapped. The moire is a product of the interaction of periodic structures, and the FM screening has no screen lines and screen angles. The dot distribution is random and irregular, and several printing plates are overprinted together, even if the printed matter or textiles are copied. Moire will appear. In terms of clarity, no matter how good the AM / screening technique is, it can only avoid the large tortoise pattern, and the small specks (snowflake, star flower, rose spot) cannot be eliminated. The speckles copied by amplitude modulation and screening are the result of light diffraction and cannot be avoided. These speckles are the root cause of the rough image. The FM screening has no speckles, which makes the image more fine and clear, strong three-dimensional sense, and extremely rich levels.
4. The level reproduction of FM dots is evenly reproduced. Because there is no dot structure, there will be no density jump when adjacent layers increase. The level reproduction transition is naturally harmonious and the layers are beautiful. The printed products are sufficiently close to the original to achieve a high-fidelity effect, which is beneficial to multi-color printing. . When printing from 5 to 7 colors or more, amplitude modulation screening will increase the difficulty of the technical arrangement of the screening angle. The use of FM screening technology fundamentally solves the generation of moire, provides conditions for multi-color printing, meets some special reproduction requirements, expands the color reproduction range of printed products, and meets high-fidelity printing requirements.
Q: Can screen printing use FM screening process?
Answer: At present, there are still some problems in the use of FM screening technology in screen printing. FM screening technology is to achieve FM screening technology through the computer's ability to rotate, but in practical applications, there are still some difficulties in pre-press, screen printing process equipment and materials and related technologies.
1. The FM screening dot is too small, 7 ~ 35μm, and the 20μm dot is only equivalent to 2% dots of 100-line printed matter or 5% dots of 200-line printed matter. Because the dots are too small, the ideas can't be exposed in the printing, the level is lost, and it is difficult to reproduce the dark tone level.
2. The expansion value of FM plus network outlets is high. Compared with general conventional outlets, the expansion value of FM outlets is 20% ~ 40% higher.
3. There are certain difficulties in revision and copying. The size of the conventional film dots can be processed by chemical modification such as thickening and thinning, and the FM film dots are too small to handle, and the printing ink dots are too small to copy and fax.
4. At present, the number of screen lines in the screen printing process is mostly 60 to 80 lines / inch, and the tone range of the dots is 10% to 85%. With the missing printing process, small ideas are not easy to print.
5. The screen printing of FM screening is low and the printing quality is not stable.
FM screening technology is in the development stage of experiment and application. For example, the combination of two different sizes of FM dots or the combination of AM and FM dots has been gradually applied in the four major printing. The appearance of FM screening technology has improved and promoted the color reproduction technology. Some domestic flexographic printing companies have adopted FM screening technology locally. At present, it is still difficult to adopt FM screening technology from material equipment to screening technology in screen printing. Therefore, in the reproduction of color screen-adjusted screen printing, in addition to adopting FM negative film to enlarge and copy, FM screening cannot replace the traditional AM screen technology.
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