Development of laser engraving gravure and flexographic plate making technology
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The use of lasers as a high-performance means for plate making is a long-term goal of pre-press processing and platemaking. As a high-energy, high-performance recording tool, lasers have been offset since the 1970s. The field of gravure printing plays an increasingly important role. On the basis of computer graphic information processing, the use of laser to image and output the film and offset printing plate is the most common and most promising offset printing method. The library "CTFilm" and "CTPlate" are taken for granted. Contains laser plate records.
It is well known that the mechanical electromagnetic gravure electronic engraving machine was invented in 1962 by the German company Hell GmbH. This is very close to the laser invention time. In fact, the company's technicians tried to use lasers to engrave copper-plated cylinders at the time. However, due to the high reflectivity of copper to light, they turned to high-energy electron beam engraving and succeeded.
The gravure printing of the gravure plate began in 1977, when Crosfield Electronics of the United Kingdom used a laser to engrave a cell on a gravure cylinder with a polymer resin layer to make a gravure cylinder. Although the system has not really been put into practical use due to reasons such as quality stability, etc., as a useful technology research and exploration, it has pointed out that the laser gravure printing plate can continue to develop.
At Drupa 2000, a printing event held in Düsseldorf, Germany, in May this year, people saw that laser plate recording technology has entered the practical stage. In addition to the technology used to print CTPlate has become a hot spot, many manufacturers have launched The gravure printing plate and the flexographic laser manufacturing equipment have become a highlight of CTP (Computer To Cylinder).
First, the type of laser engraving gravure technology
As far as the types of gravure cells are concerned, there are generally four types, namely: variable area cells, concave depths, right-changing cells, area and concave depth, variable cell and frequency-modulated cells. At the current state of the art, laser engraving of these four cells has been achieved.
1. Variable area cell
As the name implies, this type of cell reproduces the gradation level change of the image only by changing its open area. The area of the cell in the depth of the color is large, while the area of the cell in the shallow color is small, and the depth of the hole is unchanged. In this regard, it is similar to the principle of image reproduction of offset dots. Therefore, this type of intaglio is also called "dot gravure".
It should be noted that although the basic principle of image tone reproduction is similar to that of offset printing, the structure of the cavity cannot be separated from the basic technical requirements of the gravure printing plate. This is: the mesh wall must be formed and retained, not in the printing plate. There is a large area of no-net wall on the ground. Therefore, from the microscopic structure of the cell, it is not equivalent to the plain dot.
2. Concave depth variable mesh
This is the most typical gravure cell, often referred to as "classic gravure" or "traditional gravure." This type of cell only changes the tone level of the image by changing the depth of the cell. The color of the cells is deeper in the depth of the color, while the cells in the shallow color are shallower and the area of the cells is unchanged.
Since the cell area is the same, the thickness of the mesh wall is equivalent.
3. Variable mesh area both in area and under recessed depth
Such a cell is often found on a cylinder engraved by a mechanical electromagnetic gravure engraving machine, which is the most commonly used cell type for non-laser electroglossogravure. The characteristics of the cells are: the opening area of the cell in the depth of the color and the depth of the recess are large, and the opening area and the depth of the recess in the shallow color are small, and it is obvious that the thickness of the mesh wall is not equal.
4. FM network
Applying the principle of FM screening to the intaglio for gravure, you can generate FM gravure. The characteristics of the cells are: the area is the same, and the spatial position appearing on the gravure plate changes randomly. Similarly, in order to avoid large-area wall-free "zones" in the dark-adjusted areas of the image, the spatial position of the FM cells should be reasonably controlled and not completely random.
Second, the basic principle and implementation of laser gravure engraving
In principle, laser gravure engraving is the application of one or more high-energy laser beams. On the surface of the drum to be carved (metal layer or base paint layer), the shape of the cells of the cells or exposed copper is ablated and directly formed. The cell print plate is ready for subsequent processing of the cell.
As can be seen from the basic principles given above, this includes two slightly different engraving techniques. The first is to directly engrave the metal surface of the drum with a high-energy laser to form a gravure cell (see Figure 5 on the left). As far as the current technical level is concerned, 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 goal of laser engraving. The second method is to apply a black base paint layer on the copper roller, ablate the mesh area with a laser, and expose the copper layer at the cell cavity. The non-cell site is protected by the base paint and is corrosion-resistant. A recessed cell can be obtained. This is the technical solution adopted by the HellBeam C2000 launched by German Hell Company at Drupa 2000.
Although the difference between the two seems to be small, the two are still unique in terms of details such as the characteristics of the cells and the process.
Third, the comparison of two laser gravure engraving techniques
1. Laser engraving gravure-based paint layer technology
Represented by HellBeam C2000 system of Hell Company of Germany, laser engraving gravure base paint technology has become a new hot spot of laser engraving gravure on Drupa 2000.
The essence of the laser engraving gravure scheme of Hell Company of Germany is to make full use of the high resolution of laser recording to achieve high precision of the outline, text and graphics of the cavity ablated by the laser on the base paint. The area of the cell contour varies with the shade of the image. Therefore, the cells obtained through the subsequent etching treatment belong to the aforementioned "area variable, concave depth constant mesh" (in fact, during the corrosion process, the size of the mesh contour area will still affect the network to some extent. Hole corrosion depth). Those familiar with offset printing will find that this is a gravure of "printing".
For a long time, there has been a problem that the quality of text and graphics is lower than offset printing in gravure production. The reason is that offset printing usually records the outline of text and graphics with a recording resolution of 1200-3000 lines/inch with high precision; while the engraving resolution (number of network lines) of the electronic engraving machine is low, generally 60-140 lines. /cm, which is 150-356 lines/inch. Such line numbers are possible for the reproduction of image levels and details, but they are insufficient for the outline quality of text and graphics. The edges of text and graphics carved with this resolution are not smooth enough, and the quality is not the same as offset printing. In particular, the problem of writing time for small characters is more prominent (see Figure 6).
Offset dots are composed of multiple laser exposure points. With this feature, one can do two things: First, multiple levels of dot area can be achieved. If the 100% area rate dot is composed of 16×16 recording exposure points, the dot area ratio can be changed by 257 (including 0%). Second, the shape of the dot can be relatively freely designed, such as different areas. The rate of dots is designed into different shapes, which can compensate for the dot enlargement or dot reduction in the printing process, and the image layer and color reproduction are more perfect.
A common mechanical electromagnetic type electronic engraving machine embosses a knife to create a cell. The shape of the cell is determined only by the angle of the engraving knives, the rotational speed of the drum, and the die feed speed of the engraving head. The degree of freedom in changing the shape of the cells is relatively small. It can be seen that the shape of the non-combined cells is less changed, and the edge carving quality of the characters and graphics is not good due to the low resolution. In order to solve this problem, the technicians of Hell and Daetwyler-Ohio have adopted some methods. For example, Daetwyler-Ohio's TransCell technology slightly changes the spacing of the cells when engraving the strokes of the characters, making the pitch smaller and locally increasing the engraving. Resolution; in addition to the micro-movement of the engraving knives, Hell also uses a method similar to “anti-aliasing technology” to supplement the serrated space of the stroke with small meshes to compensate for the visual effect.
The real solution to the problem of low edge precision of text and graphics is to improve the recording resolution. On the HelioBeam C2000 laser engraving machine, due to the "quasi-printing" laser ablation, the image 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 of the original resolution, so the above problems are fundamentally solved, and the reproduction quality of text and graphics is improved. At the same time, since each ablation cavity is composed of a plurality of laser exposure points, it is convenient to design the shape of the cavity contour to improve the image layer transmission effect. On the HelioBeam C2000 laser engraving machine, in addition to the original cell shape of Hell, other cell shapes can be created. For example, the concave mesh created on the system is beneficial for compensating for dot gain in printing. In principle, engraving FM grids is also achievable.
Increasing the engraving resolution leads to the question of how to ensure engraving efficiency. Hell's technicians used a multi-beam laser parallel exposure method on the HelioBeam C2000 laser engraving phase to divide a new fiber infrared laser (1110 nm) with a power of 60 watts into 8 bundles (7.5 watts per bundle). The base paint layer on the copper cylinder is thermally ablated. The fiber laser produces a good laser beam with a large depth of focus (depth of field), while the base paint is stable and vaporized only under high energy lasers.
Observing from the engraving and making process route, the system does not need to change the copper plating and the surface treatment of the drum (car/grinding/polishing). It only needs to add two steps of coating and etching in the process. The system can coexist conveniently with the original engraving process, that is, the processed copper layer roller can be mechanically and mechanically engraved, or can be laser-engraved through the base paint into HelioBeam C2000, and then corroded and removed. The layer is given a gravure cylinder. The subsequent process arrangement of chrome plating and the like is also the same. Therefore, the system has high process flexibility.
At Drupa 2000, the Digilas laser engraving system from Daetwyler-Ohio is also part of the laser ablation based paint system. The system uses a 1 or 2YAG laser (1064 nm) with an engraving record resolution of 1250-2540 dots per inch (laser spot diameter 10-20 microns).
Japan's Think Lab also introduced its TB-21 series laser engraving system FP-20/40/80 at Drupa 2000. The system exposes the photosensitive resist layer by using a plurality of laser beams, and then exposes the copper layer treated by the cavity by developing processing, and then performs copper layer cell etching to obtain a gravure cylinder. Although the basic characteristics are similar to those of the above two companies, the laser recording is a photosensitive layer, which requires development processing without using a base paint ablation method.
2. Laser engraving metal zinc layer technology
Daetwyler, Switzerland, is a company that has been brave to explore. Before the Drupa 2000, they merged with Ohio Electric Carving Machine Company to form Daetwyler-Ohio to face competition in the field of engraving technology. At Drupa in 1995, the company first introduced LaserStar, a system for engraving metal zinc. The system uses a single beam of argon ion laser engraving with a depth of 35,000-70000 cells per second. At that time, with the slogan "Revival of the traditional gravure", the cell with a variable depth and a "gravity intaglio" feature was exhibited. The company also develops a laser image engraving system for FM images.
From the point of view of laser recording technology, the size of the exposure spot is unchanged, and the recording intensity of the laser is modulated by the image recording signal, and the cells of the above characteristics can be engraved. A laser exposure produces a cell. The key to this technology is the precise control of the laser exposure intensity in order to ensure image level reproduction. If the image digital signal is 8 bits, it can carry 256 levels of image hierarchy information, and the laser energy is required to be accurately controlled to 256 levels, and a plurality of depths of the cells are engraved in the range of tens to hundreds of micrometers. The reproduction of image levels relies on laser engraving precision control. Since the number of screen lines is 70-200 lines/cm, the outline accuracy of text and graphics is acceptable at this engraving resolution, but it is not very high.
Table 1 Comparison table of two kinds of gravure laser engraving techniques
Main parameters System type Hell C2000 Daetwyler LaserStar
Engraving medium base paint layer zinc metal layer
Laser type fiber laser argon ion laser
Laser beam 8 1
Number of lines
(line/inch, LP1) 152-356 178-508
Engraving record resolution
(/inch, dpi) 2540-5080 178-508 (non-combined cell)
530-1500 (combined cell)
Process flow Compatible with the original copper layer processing technology, increase the base paint coating, copper layer corrosion, establish a zinc layer processing line
Table 2 Main parameters of two laser flexographic engraving systems
Main parameters 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 1-2
Plate material size (mm) 1600 × 1200 1100 × 1600 or 1524 × 2032
Engraving record resolution
(dots/inch, dpi) 1270-2540 1270-2540
In order to improve the quality of engraving, the company has introduced a multi-beam combination cell in addition to improving the engraving resolution. The specific implementation method is to form a cell with 7 laser exposure points (the engraving resolution is 3 times of the original), so that the engraved cells (see Fig. 7) are of the type "variable area and concave depth". The area ratio of the cells can be changed in 7 steps and the depth of the recesses can be changed in multiple steps. The improved resolution can improve the engraving quality of text and graphics. At the same time, the multi-level variation of the cell area can reduce the precision requirements for laser intensity modulation.
From the perspective of the engraving process, since the object to be engraved is a metal zinc layer, it is necessary to establish a production line for galvanizing and zinc layer surface processing.
From the comparison of the techniques carried out above, the types, technical levels, cell features and process rolling configurations of the current laser gravure engraving technology can be obtained. Table 1 summarizes the above items briefly.
Fourth, the flexographic laser engraving technology overview
Laser engraving of flexographic materials can also be performed by exposure to high-energy lasers to form relief dots. At Drupa 2000, both Hell and Daetwyler-Ohio showcased a flexographic laser engraving system. Both companies' systems use a laser to directly expose a single flexographic or flexographic sleeve (see Table 2) to form a flexographic plate. The basic configuration of the system is similar to the gravure laser engraving system. The specific performance is shown in Table 2.
In summary, laser engraving and plate-making technology is in the process of steady development. With the development of technologies such as laser technology and printing plate materials, it will make greater progress in the field of gravure and flexographic plate making.