Digital Screening Technology and Its Application in Gravure Printing

Dots are the basic units of the color printing color, and the dot coverage can represent changes in color levels. The process from continuous image adjustment to screen printing is called screening. The development of screening technology has experienced three stages of development: contact screen photographic screening, electronic color separation electronic screening, and computational technology digital screening. Digital screening technology is mainly composed of Guangshanhua calculation and outlet generation technology. This article mainly discusses dot generation software technology.
Computer digital screening is a technology widely used in the high-end networking systems of the imagesetters and point machines. Electric extension screening and computer digital screening are the same in terms of technology and principle, but the implementation method is different. Now, the performance of the computer has been greatly improved. The speed and quality of RIP screening can be comparable to those of conventional extensions. The adjustment of parameters and the ease of use and flexibility are even better.

First, the network point and tone In the process of passing through the outlets, the shape of the outlets is an important factor in order to influence the order. Especially in the printing stage. The increase of dots in gravure printing and flexo printing is more serious, and the effect on image tone is even greater. The traditional dot shapes are square dots and circular dots. There are also chain points (or diamonds), letter points, composite points, frequency conversion points, and frequency modulation points. One of the reasons that outlets affect the reproduction of printing credits is due to the difference in dot lines. The sum of the perimeters of the outlets at the same percentage of outlets is different because the increase in outlets is also different. Because the dot increases outward along its edge, the larger the perimeter, the more serious the dot increases. The second reason is that in the process of small and large outlets, there are always sites where the outlets overlap, and the density suddenly increases due to the overlapping connection. Thus the wave broke the continuity of the tone curve and Zhao Cheng lost some of the tone levels of the tone region. For example, skin tones, midtones that are just in the yellow and magenta versions, can easily cause stiffening of the tone and lack of subtle changes. In the reproduction of the skin tone, the diamond points are better than the round points and the square points, and it should be avoided that the laps of the dots avoid the intermediate tone, and the laps of the dots are divided into two times, and the jump caused by each time is weakened.
The tone jump caused by the lap of the network can also be overcome in principle by adjusting the adjustment parameters of the RIP, but there are also certain difficulties; the first is that the amplitude of the jump has many factors, and there is randomness within a certain range, even if With careful presupposition, it is still difficult to offset all the deficiencies; the second is that the position of jump can only be given a range, and it is difficult to precisely locate. Therefore, the gradation jump caused by the overlapping of outlets can hardly be eliminated in actual production and can only be mitigated by the cooperation of the shape of the outlets and the characteristics of the manufactured images.
Electrically embossed gravure printing dots are not overlapped in principle, and only the solid part is also prone to unevenness during printing. This is one of the reasons for gravure printing. The traditional gravure dots will also produce gradation jumps between the field and midtones. The reproduction of highlights is also printed with visually perceptible jumps in the conditions. If the use of hierarchical network and variable frequency network point machine can avoid sudden changes in the tone.

Second, the special nature of the gravure network points The gravure network is composed of network walls and network cells. His particularity is that he learns to have a mesh wall to support the squeegee and to enhance the adsorption of ink on the network. In the gravure platemaking process, although the range of dots is compressed a lot, because of the thick ink layer during printing, the color of a gravure printing product is usually more saturated and more vivid than the color of an offset printing product.
The law of gravure dot enlargement is different from that of offset printing. The hue of all inks is also quite different from common offset inks. Therefore, the level curve and gray balance curve used in gravure platemaking are different from those of offset printing. Electric extensions and most scanners are preset for offset printing and have a set of tone curves to choose from. The platemaking factory only needs to make a concave and convex conversion of the preset curve. At present, plate-making manufacturers usually perform offset conversion during image processing, and inconveniently handle the color levels defined in the graphic portion. The ideal way is to automatically do glue-concave conversion in the computer where RIP processes the text and the layout description file, so that most of the adjustments can be done automatically in RIP. The special adjustment in image processing is only used as an auxiliary means.
In gravure plates, it is sometimes necessary to use different dots and different screens in different parts of the same layout. Most elements do dot compression, and some elements do not do dot compression. Some of these requirements can be solved by existing software, and some can only be achieved by enhancing the functionality of RIP and design software.
Third, ESK's unique gravure dot technology ESK is a relatively complete solution designed for gravure plate prepress processing. He consists of a compression point recovery tool, a special gravure dot, dot application tool, a quick renderer, and a color management tool (This issue was introduced in the first issue of this issue in 1999). After more than a year of production applications and further research, there has been a new improvement in ESK technology. Here, we mainly introduce new advances in ESK network technology.
1、Hexagon and exagonPro Screening
Honeycomb shaped points (see Figure 1) have the following advantages:
(1) In the field, the loss of square hexagonal cells is less than that of the square dots in printing and etching.
(2) strong resistance to India.
(3) Under the same printing difficulty, the honeycomb structure dot can achieve a larger dot density, that is, when the number of printing lines is 200 lines/inch, it is difficult to print with a honeycomb dot 7 in an orthogonal grid arrangement dot printing 175 lines/inch. The image is quite. Because in 50% of flat screens, the size of outlets is equivalent to the spacing between outlets.
(4) When the honeycomb structure network is superimposed on a multi-color version, the chance of hitting the net is smaller.
Cellular network points can be hit by a network angle range of 0 ̇ to 60 ,, and the network angle can be 15 ̇ to avoid collisions. Therefore, it is easy to obtain 4 available angles without collision. The traditional method can have three angles not hitting the net for the main color version, usually the fourth color version is the weak color version, and when the three main color versions are superimposed together, there will be slight collisions. Gravure printing is usually preceded by a white base plate. In some cases, the white base plate will crash into other color plates, and white spots will be made from honeycomb spots, which will reduce the chance of collision.

2. Star and Starpro Screening
The star network is shown in Figure 2. In the field part, the star point can reduce the area of ​​the wall between the ink caverns. Without reducing the strength of the net wall, the dot area is increased, so the ink content is larger, which has certain benefits for the field printing.

3.T-Shape and T-Shapepro Screening
T-shaped outlets (as shown in Fig. 3), also known as communicative gravure square outlets, can increase the leveling of ink in the field, reduce water ripples, and make printing on the field even and full. Nippon Shinco also uses similar outlets in its laser direct plate making system, which has achieved remarkable success in production. ESK software also uses this function, so users can apply a similar Japanese Shinco company to the coating corrosion process. The technology developed by its filmless laser platemaking system. In practice, we have found that the requirements of T-junctions on the process must be justified by the depth of corrosion and the width of ink communication.

4. Diamond and diamondPro Screening
The shape and arrangement of the chain points (see Figure 4 and Figure 5) are closest to those of the electric engraving machine. The width of the chain point is adjustable, which is similar to the adjustment of the axial step of the electric engraving machine. This adjustment can be independent of the radial step. This network provides a new tool for the success of the etching method to draw on the gravure process. In the early process of the etching method, the laser photographing machine outputs the film. There are many methods for adjusting the shape and arrangement of the dots in the technique, and the level curve is also relatively easy to adjust.

5. QFCSA Screening
Harleqlin Micro Screening), mainly used ten-high line offset printing. For example, when 330 lines/inch images are printed with offset printing, the dots in the high-light part are too small (the dark tone is also very little in the white space), which is difficult to achieve in the process. HMS solves this problem by using the baseline in highlights and shadows. Based on this idea, we designed a frequency conversion network for gravure printing. The application of frequency conversion dots in gravure printing is not mainly to solve the problem of high-line number printing. It is suitable for improving the quality of front plane and highlight areas. From the usual 175 lines/inch condition. Below 10% of the outlets are very unstable in terms of process. It should not be easy to evenly spread the front level screen, and it is difficult to reproduce the high-level level steadily. Although the 150 lines/inch or 133 lines/inch image is better at the shallow tone level, the middle portion of the layout's main body appears to be too thick. The application of frequency conversion dots in gravure printing is much more complicated than offset printing. The frequency range of the variable frequency {eg, 1% - 12% for the frequency conversion range} and the starting point size should be adjusted for the production process. However, it also has a negative effect, such as small dirty spots will be clearly shown; a gradient of high-light part will appear a ladder, but has little effect on the image, there are still visible steps in the large area of ​​the gradient. Therefore, it is still necessary to decide whether to use it according to the characteristics of the layout.

6. The shape of the traditional gravure network in the hierarchical network is a column from the highlight to the field. This is a simplified approach. In fact, the dots in the hierarchy are the best solutions. However, the amount of ink in the dots in the field is not as large as other dots. It is not easy to make it flat. Therefore, we have designed hierarchical meshes that combine dots with other gravure dots. This kind of hierarchical network is easier to make in the highlights of small points. Middle tone to the high-profile and low-key levels are also more smooth. The hierarchical structure of the honeycomb structure and the shape and arrangement of the outlets are all in a good state. It is a relatively promising outlet. In the ESK outlets, all the outlets have special configurations for lines and take into account both the line version and the hierarchical version, ie the "-PRO" series of outlets.