The use of lasers as a high-efficiency method for plate making is the goal of long-term unremitting efforts in the field of pre-press processing and platemaking. As a high-energy `high-performance recording tool, lasers have been in offset printing since the 1970s. The field of gravure platemaking plays an increasingly important role. On the basis of computer graphic information processing, the use of lasers to output the graphic records of films and offset printing plates is currently the most common and promising method for offset printing plate making, and the library "CTFilm" and "CTPlate" take for granted Contains laser plate records.
As we all know, the mechanical electromagnetic gravure electronic engraving machine was invented by Hell GmbH in 1962. This is very close to the time of laser invention. In fact, the company's technicians tried to use laser to engrave the copper-plated drum, but because of the high reflectivity of copper to light, they were unsuccessful. They turned to high-energy electron beam engraving research and achieved success.
The laser recording of the gravure printing plate began in 1977, when the British company Crosfield Electronics used a laser to engrave meshes on the gravure printing cylinder with a polymer resin layer to make a gravure printing cylinder. Although the system has not been put into practical use in large quantities due to quality stability and other reasons, as a beneficial technical research and exploration, it has pointed out a way for laser gravure plate to continue to develop.
At the printing event Drupa 2000 held in Düsseldorf, Germany in May this year, people saw that the laser plate recording technology has all entered the practical stage. In addition to the technology used for flat printing CTPlate has become a hot spot, many manufacturers have launched The laser production equipment for gravure printing plates and flexographic plates has become a highlight of CTP (Computer To Cylinder).
1. Types of laser engraving gravure technology
As far as the types of gravure meshes are concerned, there are generally four types, namely: area-variable meshes, recessed depth and right-variable meshes, both area and recessed depth are variable meshes, and FM meshes. At the current level of technological development, laser engraving of these four types of caves has been achieved.
Fig.1 Structure of a cell with variable area
1. Variable area
As the name implies, this type of network only reproduces the gradation of the image by changing its opening area. The area of ​​the net hole is large in the deep color, while the area of ​​the net hole is small in the light color, and the lower depth of the meat hole is unchanged. In terms of this feature, it is similar to the image reproduction principle of offset printing dots. Therefore, this type of gravure is also called "dot gravure". Its network structure is shown in Figure 1.
It should be noted that although the basic principle of image tone reproduction is roughly similar to that of offset printing, the structure of its mesh can not be separated from the basic technical requirements of gravure printing plate. This is: the mesh wall must be formed and retained. A large area of ​​netless walls appeared on the ground. Therefore, in terms of the microstructure of the net hole, it is not the same as the flat printing dot.
Figure 2 The structure of the recessed variable depth network
2. Recessed variable depth
This is the most typical gravure mesh, often referred to as "classic gravure" or "traditional gravure". This kind of network only changes the tone level of the reproduced image under the depth of its network. The recesses are deeper in the deeper colors, while the recesses are lighter in the lighter colors, and the area of ​​the holes is unchanged. Its network structure is shown in Figure 2.
Since the mesh area is the same, the thickness of the mesh wall is the same.
3. Variable area and recessed depth
This kind of net hole is often seen on the cylinder engraved by the mechanical electromagnetic electronic gravure engraving machine, and it is the most commonly used type of net hole for non-laser type electric engraving gravure. The characteristics of its mesh are: the opening area and the depth of the depression are deep in the deep color, while the opening area and the depth of the depression are small in the shallow color. Obviously, the thickness of the mesh wall is not equal. Its network structure is shown in Figure 3.
4. FM network
Applying the principle of FM screening to gravure used in gravure, FM gravure can be generated. The characteristics of its net hole are: the area is the same, and the spatial position appearing on the gravure plate changes randomly. Similarly, in order to avoid large-scale "zones" without mesh walls in the dark areas of the image, the spatial location of the FM cell should be reasonably controlled and not completely random. Its network structure is shown in Figure 4.
Second, the basic principle and implementation of laser gravure engraving
In principle, laser gravure engraving is to use one or more high-energy laser beams to ablate the mesh shape of the mesh or exposed copper on the material to be engraved (metal layer or base paint layer) on the surface of the drum to form directly. Mesh printing plate, or prepare for subsequent processing of mesh.
It can be known from the basic principles given above that this includes two slightly different carving techniques. The first is to directly engrave the metal surface of the drum with a high-energy laser to form a gravure mesh hole (see the left picture in Figure 5 on the next page). As far as the current technical level is concerned, the direct engraving of the copper layer has not been successful. The Swiss company Daetwyler has adopted a compromise method of engraving the zinc layer to achieve its laser engraving goals. The second method is to apply a black base paint layer on the copper roller first, and ablate the mesh area with a laser to expose the copper layer at the mesh hole. The non-net hole area is protected by the base paint to resist corrosion. A recessed net hole can be obtained. This is the technical solution adopted by HelioBeam C2000 launched by Dr. Hell of Germany on Drupa 2000 (see the right figure in the next page).
Figure 3 The structure of a variable-area cell with a reduced area and depth
Although the difference between the two in terms of basic principles does not seem to be great, but the analysis of the characteristics of the net hole, the process and other details, the two are still unique.
3. Comparison of two laser gravure engraving techniques
1. Laser engraving gravure base paint layer technology
Represented by the HelioBeam C2000 system from Hell in Germany, the laser-engraved intaglio base paint layer technology has become a new hot spot for laser-engraved intaglio on Drupa 2000.
The essence of the German laser engraving gravure solution of Hell Company is to make full use of the high resolution of the laser recording, so that the outline, text and graphic outline of the cavity ablated by the laser on the base paint can reach high precision. The outline area of ​​the net hole changes with the shade of the image color. Therefore, the mesh obtained after the subsequent corrosion treatment belongs to the aforementioned "mesh with variable area and constant depression depth" (in fact, during the etching process, the size of the mesh outline area will still affect the mesh to a certain extent. Corrosion depth). Those who are familiar with offset printing will find that this is a "lithographic" intaglio.
Figure 4 The structure of the FM cell
For a long time, the quality of text and graphics in gravure production is lower than that of offset printing. The reason is that offset printing plates usually use 1200-3000 lines / inch recording resolution to record the outline of text and graphics, and the accuracy is higher; while the engraving resolution (network line number) of electronic engraving machines is lower, generally 60-140 lines / Cm, that is 150-356 lines / inch. This number of lines is acceptable for the reproduction of image layers and details, but it is insufficient for the outline quality of text and graphics. The edges of text and graphics carved at this resolution are not smooth enough, and the quality is not the same as offset printing. , Especially the problem of the time of writing small font size is more prominent (see Figure 6).
Offset dots are composed of multiple laser exposure points. Using this feature, people can do two things: First, they can achieve multi-level changes in dot area. If a dot with a 100% area ratio is composed of 16 × 16 recorded exposure points, the dot area ratio can vary by 257 levels (including 0%); second, the shape of the dots can be designed more freely, such as different areas The dots of the rate are designed into different shapes, which can compensate for the dot expansion or the dot shrinkage in the printing process accordingly, so that the image gradation and color reproduction are more perfect.
Figure 5 The principle of laser gravure engraving metal zinc layer (left) and base paint layer (right)
A common mechanical electromagnetic electronic engraving machine adjusts the engraving action of a carving knife to generate a net hole. The shape of the cell is only determined by the angle of the engraving knife, the rotational speed of the drum, and the die feed speed of the engraving head. The degree of freedom to change the shape of the cell is relatively small. From this, it can be seen that the shape of the non-combined cell has little change, and because of the low resolution, the edge carving quality of text and graphics is not good. In order to solve this problem, the technical personnel of Hell and Daetwyler-Ohio have adopted some methods. For example, the TransCell technology of Daetwyler-Ohio company slightly changes the spacing of the net holes when engraving the strokes of the text, so that the spacing becomes smaller, and the engraving is increased locally. Resolution; In addition to the slight movement of the carving knife, Hell also uses a method similar to "anti-aliasing technology" to supplement the jagged gaps of the strokes with small net holes to make up for the visual effect.
The real solution to the problem of low accuracy of text and graphics edges is to improve the recording resolution. On the HelioBeam C2000 laser engraving machine, since the "quasi-flat printing method" is used for laser-based ablation, graphics and text can be recorded with a high resolution of 2540-5080 dots / inch (laser spot diameter 5-10 microns), engraving resolution It is 10-20 times the original resolution, so the above problem is fundamentally solved, and the reproduction quality of text and graphics is improved. At the same time, since each ablated cavity is formed by combining multiple laser exposure points, the contour shape of the cell can be designed more conveniently to improve the transfer effect of the image layer. In HelioBeam C2000 laser engraving machine, in addition to the original cell shape of Hell, other cell shapes can also be produced. For example, the concave recesses created on the system are useful for compensating for dot expansion in printing. In principle, it is also possible to engrave FM net holes.
Increasing the engraving resolution raises the question of how to ensure engraving efficiency. The technical staff of Hell Company used the method of parallel exposure of multiple laser beams on the HelioBeam C2000 laser engraving phase to divide the new fiber infrared laser (1110 nm) with 60 watts into 8 beams (7.5 watts each), The base paint layer on the copper roller is thermally ablated. The laser beam produced by the fiber laser has good quality and a large depth of focus (depth of field), and the properties of the base paint are stable and can only be vaporized under high-energy laser.
Observed from the engraving platemaking process route, adopting this system does not need to change the configuration of drum copper plating and drum surface processing (turning / grinding / polishing), and only needs to add two steps of base paint coating and corrosion in the process flow. The system can easily coexist with the original engraving process, that is, the processed copper drum can be used for mechanical electromagnetic engraving, or it can be applied to the HelioBeam C2000 for laser engraving after the base paint is applied, and then the base paint can be removed by corrosion The layer gets a gravure cylinder. Subsequent chrome plating and other process arrangements are also the same. Therefore, the system has high process flexibility.
Figure 6 Edge quality of text on offset and gravure plates
On Drupa 2000, the Digilas laser engraving system introduced by Daetwyler-Ohio also belongs to the same type of laser ablation base paint layer system. The system uses 1 or 2YAG laser (1064 nm), and the resolution of engraving and recording is 1250-2540 dots / inch (laser spot diameter 10-20 microns).
Japan Think Lab also launched its TB-21 series laser engraving system FP-20 / 40/80 on Drupa 2000. The system uses multiple laser beams to expose the photosensitive resist layer, and then develops to expose the copper layer treated by the mesh, and then performs copper layer mesh etching to obtain a gravure cylinder. Although its basic characteristics are similar to the systems of the above two companies, its laser recording is the photosensitive layer, which needs to be developed, without the ablation method of base paint.
2. Laser engraving metal zinc layer technology
Swiss Daetwyler company is a company that has the courage to explore. Before Drupa 2000 was held, they merged with the Ohio Electric Carving Machine Company to form Daetwyler-Ohio Company to face competition in the field of engraving technology. At Drupa in 1995, the company first introduced LaserStar, a system for engraving metallic zinc. The system uses single-beam argon ion laser engraving, and the engraving depth is 35000-70000 net holes / second. At that time, under the slogan "Renaissance of Traditional Intaglio", it exhibited its caves with variable indentation depth and the characteristics of "shadow intaglio". The company is also developing FM image laser engraving systems.
Figure 7 7 laser points combined into 1 net hole
From the point of view of laser recording technology, the size of the exposure spot remains unchanged, and the image recording signal is used to modulate the recording intensity of the laser to engrave the above-mentioned features. A laser exposure produces a net hole. The key point of this technology is: in order to ensure the gradation of the image, accurate control of the laser exposure intensity. If the image digital signal is 8 bits, it can carry 256-level image hierarchy information, and the laser energy is also required to be accurately controlled to 256 levels, and a multi-level depth network is carved in the range of tens to hundreds of microns. The reproduction of the image hierarchy depends on the precision control of laser engraving. Since the number of network lines is 70-200 lines / cm, at this engraving resolution, the outline accuracy of text and graphics is acceptable, but not very high.
Table 1 Comparison table of two gravure laser engraving techniques
Main parameters System type Hell C2000 Daetwyler LaserStar
Engraving medium Base paint layer Metal zinc layer Laser type Fiber laser Argon ion laser beam 8 1
Number of wire holes (line / inch, LP1) 152-356 178-508
Engraving record resolution (/ inch, dpi) 2540-5080 178-508 (non-combined network hole)
530-1500 (combined net hole)
The process flow is compatible with the original copper layer processing technology, adding base paint coating and copper layer corrosion to establish a zinc layer processing line
Table 2 Main parameters of the two laser flexographic engraving systems
Main parameter system type Hell F2000 Daetwyler SaserStar
Engraving medium Photopolymer material Photopolymer material Laser type Fiber laser (1110 nm) YAG laser (1064 nm)
Laser beam 8 1ï¼2
Plate material size (mm) 1600 × 1200 1100 × 1600 or 1524 × 2032
Carving record resolution (dots / inch, dpi) 1270-2540 1270-2540
In order to improve the engraving quality, the company has launched a multi-beam combination mesh on the basis of improving the engraving resolution. The specific implementation method is to use 7 laser exposure points to form a network (engraving resolution is 3 times the original), such carved network (see Figure 7) is of "variable area and depth of depression" type. The area ratio of the net hole can be changed in 7 levels and the concave depth can be changed in multiple levels. The increase in resolution can improve the engraving quality of text and graphics, and at the same time, the multi-level change of the cell area can reduce the precision requirements for laser intensity modulation.
From the perspective of the engraving process, because the engraving object is a metal zinc layer, it is necessary to establish a production line for galvanizing and zinc layer surface processing.
From the technical comparisons made above, we can get the current laser gravure engraving technology type, technical level, network features and process roll composition. Table 1 summarizes the above items briefly.
4. Overview of flexible laser engraving technology
Laser engraving of flexographic materials can also use high-energy laser exposure to form relief dots. At Drupa 2000, both Hell and Daetwyler-Ohio demonstrated a flexible laser engraving system. Both companies' systems use a laser to directly expose a single flexographic plate or flexographic sleeve (see Table 2) to form a flexographic printing plate. The basic configuration of the system is similar to the gravure laser engraving system. Specific properties are shown in Table 2.
In summary, laser engraving plate-making technology is in the process of steady development. With the development of laser technology, printing plate materials and other technologies, it will also make greater progress in the field of gravure and flexographic plate-making.
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