Gravure printing plate technology innovation and application (in)

Photographic direct screen plate making technology

In addition to traditional photogravure, direct photogravure processes have also been around for a long time, especially in package gravure applications. Photographic Direct Screening The basic principle of the gravure platemaking process is that the platen roller, after copper plating and polishing, is first covered with a protective layer, exposed and then imaged, and then the copper layer is etched.

The photosensitive layer was first coated on the copper layer of the platen roller and exposed with a high-pressure mercury lamp. Later laser etching technology emerged. Instead of film, the image data directly controls the fine laser beam imaging on the photosensitive layer. There have been two kinds of digital imaging platemaking methods: one is using a photosensitive film as a mask layer, and the exposed photosensitive layer needs to be developed before the etching to expose the copper layer of the graphic part; the other is to use A special thin coating is used as a mask. The laser beam can completely ablate the protective coating of the graphic part, leaving the copper layer exposed. Therefore, it can be directly etched without development, but the laser beam must be increased. energy.

Regardless of whether the imaging process uses digital or traditional screening films, the mesh structure is only created on the mask layer to change the area, and depth information cannot be reflected. Corroding the exposed part of the copper layer is a rather backward process. If the operator momentarily neglects to perform the same level of corrosion on the exposed part of the copper layer, the continuous tone characteristic of the image cannot be expressed. With this type of printing plate, lines and text can be reproduced well, but the copying effect of the image is less, because the image part contains rich tone levels, and the requirements for the depth change of the cell site are higher.

Recently, there has been developed a screen corrosion technique which requires the following operations on a plate cylinder: coating a photosensitive layer on the surface of the copper layer, imaging with a laser on the photosensitive layer, developing, etching, and stripping the film. Finally, the surface of the copper layer is cleaned and chrome plated.

One of the features of the process is the use of digital imaging heads that can simultaneously generate a variety of graphics with different line counts. Another feature is the addition of a process control navigation system that controls the electrolytic corrosion process. This plate-making process can use the hybrid screening method, that is, using the FM screening method in the bright-tuned part of the image, while the other parts still use the traditional AM screening method. But this kind of craft can't solve the problem fundamentally, even though the mixed screening method can produce very fine printing elements, but there is still no depth information. In addition, the individual pixels or groups of pixels etched are small in area but deep in depth, which is very unfavorable for transfer and transfer of ink during high-speed printing.

Another focus of the controversy is that the platemaking process uses pulsed current for electrolytic corrosion, and it is said that many of the disadvantages of conventional corrosion can be avoided. And we think this is not entirely correct. The electrolytic corrosion process was studied decades ago in Germany and found a special problem. As shown in Fig. 1, during the electrolytic etching, only the copper portion of the platen is exposed on the platen roller, and the surface of the plate is not covered with a protective layer. The electrolyte ions do not contact the copper layer at this portion. Therefore, No corrosion will occur on this part. These ions are all adsorbed to the graphic part that can be eroded, especially the edge part of the graphic part, which increases the erosive intensity of the part, causing the depth of the cell edge of the graphic part to be too large, and the image after printing In the midtone region, dark edges occur at the edge of the dot.

In addition, the photographic direct-screening process has a disadvantage that the process is complicated and has many steps, which greatly reduces the reliability of the platemaking process. In general, the more links passed, the worse the reliability of the process. Why? Because the overall reliability of the entire plate making process is bound to be affected by various links and steps, if the entire plate making process is divided into 5 steps to complete, the reliability of each step is 98%, then the total reliability of the platemaking process is only 90 %, that is to say 10% of the drums produced may be discarded, and the scrap rate of the electronic engraving process is much lower, generally less than 2%.

In short, compared with the electronic engraving platemaking process, all the photogravure direct-gravure gravure platemaking processes (even with digital imaging technology are no exception) have three fundamental problems: the imaging method is not suitable for gravure printing, the disadvantages of the corrosion process itself, and Complex process steps.

Engraving the zinc layer directly with a laser beam

In addition to electron beam engraving developed by HELL more than 20 years ago, another method is to directly bombard the surface of the platen cylinder with a very short wavelength (approximately 1 μm) of laser pulses and generate a net pocket. A cell is formed by one or two pulses and the pulse is exactly aligned with the center of the cell. However, there is also a problem with this type of engraving method, which is the mirror reflection effect of the copper layer on the laser beam. One of the solutions is to carry out engraving on the zinc layer, but we believe that it is more difficult to carry out engraving on the surface of the zinc layer than on the copper layer. This drawback also limits the further promotion of direct laser beam engraving technology. development of.

Another fundamental problem is the use of short-wavelength laser pulse engraving, the formation of the tone is very steep, not soft, not delicate. At the same time, using a short-wavelength laser pulse to directly sculpt on the metal layer also involves the sensitivity of the metal layer to the laser. The small metal sensitivity to the laser beam will affect the energy threshold, because only the pulse energy above the threshold can As a result, the energy threshold changes, of course, affecting the cell size. Moreover, if the laser beam focuses on the fluctuation of the energy density, it will also cause the change of the cell size, that is to say, the change of the two static processes will cause the deviation of the cell size. If this change happens at the same time, it will surely Cause greater deviation. As shown in FIG. 2 , the engraving material on the left side has a lower engraving threshold because the process variation is smaller and the focusing range of the laser beam is slightly wider than the normal value; the case on the right side is exactly the opposite. From this we can better understand why the cell size changes.

One or two laser beam pulses are used to form the cells, and the entire gradation range of the image is represented by a single cell. This process can be considered as a simulation process. why? Because the pulse energy required by the cells of different sizes and sizes is also different, with the gradual increase of the area of ​​the cell, the required pulse energy will be higher, and the resulting ratio of the cell and the wall will be very unsatisfactory. For example, the area of ​​the net wall in the field is still relatively large. Compared with electronically-sculpted cells, in the printing process, these cells have a much smaller area of ​​contact with the paper, and the ink transfer performance is far less than that of electronically-engraved cells, especially during high-speed printing.

Since the laser beam is focused through the glass lens, only a fixed imaging area can be set. This leads to a fixed diameter of the laser beam. If you do not consider the depth of the cell, the diameter of the sculpted cell in this way should be fixed, and the change of the pulse energy of the focused single laser will affect and change the depth of the cell. This is impossible to achieve in the direct screening process. When engraving directly with a laser beam, special methods must be used to change the area of ​​the cell. If two coaxial laser beams are used, one of the laser beams has a larger focusing diameter, and the other laser beam has a smaller focusing diameter. By adjusting and controlling the relative pulse energies of the two laser beams, it is possible to obtain cells of different diameters. However, we believe that the reproducibility of the cell size generated by this method is not reliable.

Another way to increase the engraving stability and increase the resolution of the zinc layer is to use a few small cells to form a grid, so that the resulting hexagonal grid can accommodate 7 relatively small independent cells. However, this method will greatly reduce the engraving speed. (to be continued)