1. Introduction In the process of image duplication, traditional screening methods are used to represent the shades of images. After more than a century of unremitting efforts, printing technicians have achieved a very high level of success. However, in this screening method, many regular rosette patterns appear in color overprinting, and even moiré sometimes appears. At the same time, due to the decomposition of the dots, with the decrease of the number of lines, the sharpness of the image is drastically reduced and the characteristics of the original are lost. In recent years, with the research of high-fidelity duplication technology, there have been superfine dot printings of more than 120 L/cm, which can accurately reproduce fine-grained and continuously tuned image effects. However, regardless of the degree of fineness, the characteristics of its outlets are the regular distribution of positive or negative points of different sizes in the image part, and there are always its fatal weaknesses. The structure of the rose pattern, that is, the pattern phenomenon caused by the screen angle It is independent of the number of network lines. At the same time, if the number of screen lines exceeds a certain limit, it will not only bring more difficulties to the copying, but also the tone value, especially the high-light part, will be more serious and the resolution will be limited. This is the biggest drawback of the traditional screening method.
2. The characteristics of FM outlets In the 1970s, foreign countries began to study an FM screening method. Unlike traditional screening, printing outlets are calculated based on the statistics of the tones and levels of adjacent parts of the image. , randomly distributed in the image area, and the same size (mostly 7-40μm), to show the shade of the image. Due to the limitation of the technical conditions at the time, it was unable to satisfy a large number of computational needs. Therefore, it was postponed to the 1980s. With the development of the electronic computer, the computing capacity has greatly increased, and this screening method was realized. However, FM screening at this stage is only a random random screening of first-order functions. The dot sizes are the same. Dot pitch variation and photo image are formed by different size and distances of halogen particles (0.5-5 μm) to form a tone or color. Still different. Therefore, people continue to study that if the second order function is used for random screening, the distance and area of ​​the point can be randomly changed. Then, the second-order function screening and the original photograph are used to express the mechanism of the tone. Very similar, the replica will be more realistic and completely close to the effect of photo images. Of course, the computational complexity of stochastic screening of second-order functions is even greater and more complex, but it is believed that this will be achieved in the near future.
In fact, at the present stage, people have adopted a first-order function FM screening to show unparalleled advantages over the traditional screening methods, in order to show that the use of computer algorithms to control the arrangement of points can actually eliminate unsightly pattern periodicity. The structure is more suitable for image reproduction above the three primary colors. Because of the subtle fine tuning of the FM screen, if a 15 μm diameter dot is used, there is no doubt that a 1% -99% tone value can be reproduced. Therefore, the replica has a high resolution, a soft image tone, a rich gradation, and a realistic image. There is no Moll effect, this is why the FM screening method can get the charm of printing experts.
3. Minimum dot size of engraving In the FM screening technology, the only key parameter is the size of the dot, which is much less than the traditional art printing parameters of the screening. It does not have the dot of the dot, the number of lines, the shape of the dot problem. The size of the FM spot is only related to the recording accuracy of the exposure device. Theoretically speaking, a 1200-dpi photo-recording spot size is a square point with a side length of 1/1200 in. However, the spot of light actually exposed by laser on a photographic film is a circle with a slightly smaller area than the square. 21μm, which is equivalent to a 1.2% dot of 60L/cm, or a 1.5% square point, see Table 1.
From Table 1, it can be seen that as the recording accuracy increases, the light spot becomes smaller. From the point of view of the output precision of the current image-setter, the size of the FM dot network that meets the printing requirements is completely guaranteed, and the problem depends on the dot size that can be printed in the printing. Therefore, it is not so much the computer hardware as FM printing and screening. technology. Therefore, the minimum printable size is a key factor in printing, but its value varies depending on the printing equipment, materials, process and technical level. According to reports, assuming a certain resolution, take a light spot as the ink-jet point, and foreign countries perform high-quality printing on glossy paper. The smallest printing dot can reach 8 μm, and 20-inch prints can also be made on rough paper. 40μm. However, in China, it is quite difficult to print less than 20μm dots. It is clear that there is a large gap.
4. The production of the suitable printing dots of the FM network from the negative film to the printing material must go through many procedures. The change in each conversion directly affects the color reproduction of the image. This is the result of any screening technology. . However, due to the differences in structure, finer mesh points, and the regularity of change and traditional screening, the control conditions may be more stringent. According to the actual conditions, assuming that the recording accuracy is 1200dpi, that is, the dot is 20μm. To discuss the requirements for the printability of the FM network.
4.1 Image Density of Output Film Generally, the laser point for FM screening is often a zigzag dot. The smaller the dots, the less the halogen silver particles deposited on the film and the lower the density. According to the requirements of the printing plate standardization operation, the density must be greater than 2.30. At the same time, there must not be more than two bright spots with a diameter of 6 μm inside the outlet. Otherwise, they do not meet the requirements.
Therefore, the film output from the laser recording apparatus preferably uses a special hard film that matches the apparatus. The outlets required are strong, dense, and transparent. Because the dots are small, it is not allowed to use etching methods to eliminate spots or correct dots in the film process, and to avoid repeated copies. Therefore, in addition to the film processing conditions must be strictly standardized, standard operation, it is best to have suitable for the frequency modulation and screen processing liquid and dust-proof clean environmental conditions.
4.2 Printing requirements The basis of the printing dots is the sand spot on the surface of the spot. The finer the sand, the more the number of sand spots supporting the dots, the stronger the adsorption force, and the smoother the edges of the dots, so for a size of only 20 μm. The FM network, in order to maintain its expected printing rate is not to be worn, should use sand grain fine, high resolution printing plate. According to international standards, the ideal average grain size is 2.5 μm and the depth is 3 μm. Assuming that each grain is considered to be a cone, there are 64 grain supports for each 20 μm frequency modulation point, and the total contact area is about 817 μm. ×μm. However, at present, the average thickness of the PS plate in China is more than 5 μm. Obviously, only 16 sand supports are required for a single FM gauze, and the total contact area is 490 μm×μm. Whether it is the number of sand grains supported or the total contact area is relatively small, so the adsorption The force is greatly reduced, which is one of the reasons for the dot-outs. At the same time, the particles are coarse and the amount of the photosensitive resin to be coated is increased, and the thicker the film layer is, the network dots are easily increased due to the light penetration effect. Of course, it is not the finer the better, otherwise it will affect the hydrophilicity of the printing plate. From this point of view, the printing plate of the FM spot is best to increase the side area of ​​the particles or change the nature of the plate to obtain the best Hydrophilic.
The key to the printing operation is to ensure that the dots are not lost or deformed. Therefore, the printing plate is required to be in close contact with the negative film, and the point light source is used for exposure to reduce the distortion of the dot caused by the photo-bleeding effect, and the developer concentration, temperature and the printing plate are strictly controlled. Environmental conditions, any dust or glass stains, scratches can affect the quality of the printing plate.
4.3 Selection of printing materials Due to the finer frequency and different structure of the FM network, printing materials such as ink, paper, and blanket have higher requirements.
a. Ink: The FM network is almost completely isolated and randomly distributed in the image area. Even in dark places, calculation methods can be used to make it possible to avoid duplicate dots as much as possible (at most only some of the dots may be close to each other). When the tone of a dot with a conventional screen is more than 50%, the dots start to be connected or networked. different. Under a small 20μm ink point, the ink must exhibit good viscoelasticity on the printing plate, blanket, and transfer to the paper. When the external force is intermittent or completely stopped, the ink-spot can immediately restore the original structure and remain isolated. Features not lodging, so the ink thixotropic, yield value and viscosity are larger.
Due to the fine dot network, the ink layer cannot be very thick, and the density will be affected. For this reason, the coloring power, color saturation, fine grain size, and strong water resistance of the ink are required. Considering the overall effect, inks that are suitable for printing with FM dot printing should be developed.
b, paper. Paper and ink are two inseparable aspects of the printing effect. In order to ensure that the FM network is not lost, in addition to the general requirements of paper whiteness, surface tensile strength, the smoothness of the paper, thickness uniformity is particularly important, according to foreign reports, advanced coating paper can be printed out 8μm size of the network, and The rough surface of the paper is only about 30μm, so it should be chosen as smooth as possible coated paper.
c. Blanket. The rubber blanket is an intermediate carrier in offset printing and plays a role in the transfer of ink. Therefore, the blanket must not only have good ink adhesion and water resistance, but also have a good ability to transfer ink. In FM network printing, the smoothness and flatness of the blanket must be emphasized in particular to maintain the integrity and clarity of the outlets. It is best to use a smaller air-cushioned blanket with a printed dot.
4.4 Appropriateness of Printing Jobs The correct transfer of printing outlets is one of the key elements for evaluating the quality of printing. However, the concept of controlling percentages and tonality in traditional screening printing is no longer applicable. FM printing and dot printing can only inspect ideas. Whether complete or not lost, the increase in outlets is unavoidable.
We know that if the pressure is too high in the printing, or if the amount of grinding is too large, the idea will definitely increase. If you press the theoretical rule of the perimeter of the network, the dot gain is calculated by the perimeter of all printing dots, then because the dot is small and almost isolated, the sum of the dots around the dot is much larger than the traditional dot, but all dots are normal. The increase in the situation is even. At the same time, there is almost no limitation on the high-accuracy recording of the FM network, the fineness of the dots, and the reproducible tone. Therefore, the image is still soft and clear, but it does not mean that the pressure or ink can be increased. Because the FM network is isolated and unevenly distributed, the force on each part of the unit area is also uneven. In particular, the highlights are few and dilute, the stress on the unit area is the largest, the wear is severe, and the risk of network loss is greatest. . Therefore, the use of rigid pads is more suitable for the printing of FM networks.
In addition, because the entire circumference of the inked dot on the print is surrounded by the wetting liquid, it is prone to be wetted by the liquid, and excessive water consumption or inappropriate pH may cause dot loss. Therefore, the key to printing is to master the printing pressure and ink balance.
At the same time, the print quality control strips used in the original printing are not suitable for FM network printing. It is necessary to develop new tools for measuring and controlling the printing quality.
5 FM network is conducive to printing In summary, FM network printing in the entire printing process, all aspects of operations must be standardized, environmental requirements are extremely high, especially at the plate making stage more prominent. However, in the printing operation, it is obviously easier than traditional dot printing.
a. Insensitive to overprinting accuracy requirements. Because the frequency modulation points on each color version are irregularly distributed, even if the distribution of dots of different colors in the same site is different, slight errors do not reflect major changes in color, and the gray balance is easily obtained, so it is possible to save costs due to adjustment of overlay errors. A lot of time.
b. The key to printing is to pay attention to the fact that FM outlets cannot be lost. Even if the outlets are increased, they are uniform and will not affect the change of the adjustment. At the same time do not have to worry about will produce white spots, moire and other phenomena.
c. Because the FM network is fine, dark tone is not easy to produce paste, so the ink balance is not very sensitive.
d, because the FM network is randomly distributed, and was isolated, the printing is not easy to cause the paper to pull hair, the phenomenon of increase in the back side is also greatly reduced, so the viscosity of the printing ink can be slightly larger.
According to the test, a 1200 dpi laser spot was used as an FM printing dot network printing