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Digital addition technology in gravure printing

in ordinary printing, dots are the basic units that represent image levels and color levels of color images. Different dot coverage produces changes in color levels. Compared with other image processing technologies, digital image processing technology can obtain higher quality eye tone images, which has become an indispensable link in the current image prepress processing. The development from photographic addition, electronic addition to digital addition provides us with a method of forming points in a physical way. Computer digital addition is a technology widely used in the high-end connection system of Phototypesetter and electric extension. The technology and principle of electric extension plus and computer digital plus are the same, but the implementation methods are different. With the continuous improvement of the quality and speed of color desktop publishing system, especially the improvement of the performance of raster image processor (RIP) in key parts, the speed and quality of acceleration have been greatly improved. Compared with other addition technologies, digital addition technology shows its advantages in many aspects. It can achieve algorithms that are difficult or impossible to achieve in analog addition, and can obtain higher quality eye-catching images

first, the characteristics and new technology of gravure printing point

point is the basis for realizing the image gradation level. Different printing methods have different effects on the transmission and increase of points. Because the increase of the midpoint of gravure printing and flexo printing is more serious than offset printing, it has a greater impact on the image gradation. In the process of point transmission, point shape is an important factor affecting the image tone reproduction, especially in the printing stage. Traditional point shapes include square points and circular points. At present, there are chain points (or diamond points), letter points, compound points, frequency conversion points, frequency modulation points, etc. Under the same percentage of points, the sum of the perimeter of different point shapes is different, and the increase of points is also different. As the point increases, it expands outward along its edge, so the larger the circumference is, the more serious the point increases. In addition, in the process of the point from small to large, there will always be some overlapping parts, which will cause the density to rise suddenly. Thus, the continuity of the gradient curve is destroyed, resulting in the level loss of some gradient regions. For example, skin color, which happens to be in the middle tone of yellow and magenta versions, is very easy to cause harsh tones and lack of subtle changes. In skin color reproduction, chain points are better than circular points and square points, because the overlap of such points avoids the middle tone, and the overlap of points is divided into two times, and the jump caused by each time is weakened. In principle, the step jump caused by point overlap can also be overcome by adjusting the parameters of rip, but it has certain difficulties: first, the range of jump has the effect of many factors, and it has randomness within a certain range. Even if it is carefully preset, it is still difficult to offset all defects; The second is that the jumping position can only give a range, which is difficult to accurately locate. Therefore, the step jump caused by point overlapping can hardly be eliminated in actual production, and its influence can only be mitigated by the mutual cooperation between the point shape and the characteristics of the image. Gravure printing points are composed of walls and holes, which are different from other printing points such as letterpress and lithography. Relief and lithography express the level of printed matter by the size of dot area, while gravure expresses the level by the concave part (ink hole) of gravure. The gravure printing point needs a wall to support the doctor blade and strengthen the adsorption of ink on the hole. The concave part of gravure plate can be divided into concave parts with equal surface area and different depth according to different shapes; The depth of the concave part is equal, and the surface area is different; The depth and surface area of the concave part are all different. Compared with the printing plates of other printing methods, the production of these gravure printing plates, which have been completely practical today, is very complex. The color separation film (original) used in gravure production also has the characteristics of complex production process and great difficulty. In the gravure plate making process, although the range of dots is compressed a lot, because the ink layer is thick during printing, the color of gravure prints is usually more saturated and bright than that of offset prints

due to the particularity of gravure dot, its increasing law is also quite different from that of offset printing, and the hue of the ink used is also very different from that of ordinary offset printing ink. Therefore, the hierarchy curve and gray balance curve used in gravure plate making are different from offset printing. Electric extensions and most scanners are preset for offset printing, with a set of gradient curves to choose from. During plate making, you only need to convert the preset curve into glue concave. Now, plate manufacturers usually do offset conversion in image processing, and it is inconvenient to deal with the color level defined in the graphic part. The ideal way is to automatically do the glue concave conversion in the computer where rip processes the layout description file of graphic and text integration, so that most adjustments can be completed automatically in rip, and the special adjustments in image processing are only used as auxiliary means. In addition, in gravure plate making, it is sometimes necessary to use different points and different line numbers in different parts of the same layout. Most elements are point compressed, and some elements are not point compressed for compatibility. Some of these requirements can be solved by existing software, and some of them need to be realized by enhancing the functions of rip and design software

gravure printing has high requirements for point shape. In modern electric extension printing, gravure digital point systems are stored, such as white cross points, honeycomb hexagonal points and other special-shaped points. The electronic color separation copy of the image can be adjusted flexibly and the data is accurate, which can directly record the gravure point pictures. In a relatively complete solution ESK specially designed for gravure plate making prepress processing, it has the unique gravure dot technology of ESK, which is composed of compression

dot restore tool, special gravure dot, dot application tool, rapid proofreading printing tool and color management tool. After production application and further research, ESK has made new improvements in technology. At present, ESK point technology mainly includes the following points

(1) honeycomb point. The point has an angle range of 0 ~ 60 degrees, and the collision can be avoided if the angle difference is 15 degrees. Therefore, it is easy to get four available angles without collision. The traditional method can have three non colliding titanium alloys as the shell. Of course, the angle is not used for brightness and blindness for the main color plate. Usually, the fourth color plate is a weak color plate, and there will be a slight collision when it is superimposed with the three main color plates. Gravure printing usually needs to print a white plate first. In some cases, the white plate will also collide with other color plates. Making a white plate with honeycomb dots will reduce the chance of such collision. To sum up, the loss of honeycomb structure points with regular hexagon in the field during printing and corrosion is smaller than that of square points, and the printing resistance is stronger. Under the same printing difficulty, the honeycomb structure points can achieve a larger point density, that is, when the number of printing lines is 200 lines/inch, the printing difficulty of honeycomb structure points is equivalent to that of 175 lines/inch of permanent orthogonal grid arrangement point printing. Because in 50% of the plane, the size of points and the spacing between points are equal. When the honeycomb structure points are superimposed on the multicolor version, the chance of collision is small. The hierarchical points, shape and arrangement of points of honeycomb structure are relatively good, which is a promising point

(2) T-shaped point. Also known as the gravure square dot of communication. This point can increase the leveling of the field ink, reduce the water ripple phenomenon, and make the field printing even and full. Japanese shinko company has also used a similar point in its laser direct plate making system, which has achieved success in production. ESK software also uses this function, so users can apply a technology similar to that developed by Japanese shinko company for its film free laser plate making system in the coating corrosion process. In practice, we found that the requirements of T-point on the process should be just right in the depth of corrosion and the width of ink communication in order to have a better effect

(3) chain point. Its shape and arrangement are closest to the point of the electric carving machine. The width of the chain point is adjustable, which is similar to adjusting the axial step of the electric carving machine. This adjustment can be independent of the radial step. This kind of dot provides a new means for etching gravure to learn from some successful experience of dot carving process. In the early stage of the etching process, the laser Imagesetter is used to output the film. Technically, there are many methods to adjust the shape and arrangement of points, and the hierarchical curve is also relatively easy to adjust

(4 in many utilization fields with safety and fire protection requirements) frequency conversion points. Mainly used for high-speed offset printing. For example, when an image of 330 lines/inch is offset printed, the dots in the highlight part are too small (and there are few white spaces in the dark part), which is difficult to realize in the process. HMS solves this problem by using low line numbers in highlights and shades. According to this idea, we designed a frequency conversion point for gravure printing. The application of frequency conversion point in gravure printing is not mainly to solve the problem of high-line printing. It is suitable for improving the quality of the front flat and highlight areas. Under the commonly used condition of 175 Wires/inch, the points below 10% are very unstable in technology. Although the image of 150 lines/inch or 133 lines/inch is better in the light tone level, the middle tone part of the main body of the layout is too thick. The application of frequency conversion point in gravure printing is much more complicated than that in offset printing. The point range of frequency conversion (for example, 1% - 12% is the frequency conversion range) and the size of the starting point should be adjusted according to the production process. But it also has negative effects. For example, small dirty spots will be clearly displayed, and a step will appear in the highlight part of the gradient, but it has little impact on the image. There are still visually visible steps at the large area of the gradient. Therefore, it is also necessary to decide whether to use it according to the way of touch key operation and the characteristics of the layout

(5) star point. The spot on the field part can reduce the wall area between ink holes, and increase the spot area without reducing the wall strength, so the ink content is large, which is good for field printing

(6) hierarchical points. The shape of traditional gravure dots is the same from highlight to field. This is a simplified method. In fact, dots in the hierarchy are the best solution. However, in the field, the ink volume of dots is not as large as that of other dots, so it is not easy to make it plain. Therefore, we designed a hierarchical dot that combines dots with other gravure dot shapes. It is easier to make small dots in the highlight part of this kind of hierarchical points. The transition from the middle to the high-key and low-key levels is also smoother

source: printed in China

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