Laser printing, which is a high-performance means, is a long-term goal in prepress processing and plate making. As a high-energy, high-performance recording tool, lasers have been used in offset printing since the 1970s. The gravure platemaking field is playing an increasingly important role. On the basis of computer graphics and text information processing, the use of lasers for image and document output of film and offset printing plates is currently the most common and most promising offset printing plate making method. The library “CTFilm†and “CTPlate†are taken for granted. Contains laser plate records.
As we all know, the mechanical electromagnetic gravure electronic engraving machine was invented in 1962 by Germany's Hell GmbH. This is very close to the laser invention time. In fact, the company's technical staff was trying to use the laser to carry out the engraving of the copper plated roller. However, due to the unsuccessful high reflectivity of the copper, they turned to high-energy electron beam engraving and succeeded.
Laser recording of gravure plates began in 1977, when Crosfield Electronics of the United Kingdom used lasers to sculpt cells in gravure cylinders with a layer of polymeric resin to make intaglio cylinders. Although due to quality stability and other reasons, the system has not been put into practical use in a large amount, but as a useful technology research and exploration, it has pointed out the way in which laser gravure printing plates 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 stage of practical application. In addition to the technology used to print CTPlate, it has been introduced by a number of manufacturers. The gravure printing plate and the flexographic laser production equipment became a highlight of the CTP (Computer To Cylinder).
First, the type of laser engraving gravure technology
As far as the types of gravure meshes are concerned, there are usually four types, ie, variable area mesh, recessed depth right variable mesh, area and recessed depth all with variable mesh, and frequency modulated mesh. In the current level of technological development, laser engraving of these four types of cyberspace has been realized.
Figure 1 Structure of an area-variable cell
1. Variable area network
As the name suggests, this type of network reproduces the tonality change of the image only by changing its open area. The depth of the network is larger in the area of ​​the network, and the area of ​​the network is smaller in the lighter color, and the depth of the meat cavity remains the same. In this regard, the principle of image reproduction is similar to offset printing. Therefore, this kind 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 gradation reproduction is similar to that of offset printing, the structure of the cell site cannot escape the basic technical requirements of the gravure printing plate. That is, the network wall must be formed and preserved, and the plate cannot be printed. There is a large area of ​​no net wall on the ground. Therefore, from the microscopic structure of the cell, it is not equal to the typographical dot.
Fig. 2 Structure of concave depth variable mesh
2. Concave depth variable mesh
This is the most typical gravure cell, often referred to as "classic gravure" or "traditional gravure." This type of network only changes the tone level of the image by changing its depth under the network. The depth of the network is deeper in the sunken pockets, while the shallower sunspots are shallower in the sunken pockets, and the area of ​​the net holes remains the same. Its network structure is shown in Figure 2.
Because the area of ​​the cell is the same, the thickness of the wall is the same.
3. Both the area and recess depth are variable
Such cells are often found on cylinders carved by mechanical electromagnetic electronic gravure engravers, and are the most commonly used types of cell types for non-laser type electric gravure. The characteristics of its nets are: the depth of the opening of the net hole and the depth of the concave are both large in the depth of the color, and the opening area and the depth of the concave of the net hole are small in the light color. Obviously, the thickness of the net wall is not equal. Its network structure is shown in Figure 3.
4. FM network
Applying the principle of FM screening to gravure to gravure can generate FM gravure gravure. The characteristics of its cells are: the same area, and the spatial position of the gravure plate randomly changes. Similarly, in order to avoid the appearance of a large-area network-free "zone" in the shadowed area of ​​the image, the spatial position of the FM-cell must be controlled reasonably and cannot be 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 the application of one or more high-energy laser beams to directly form a network of holes or exposed copper cells on the material to be engraved (metal layer or base paint layer) on the surface of the drum. Web site printing plate, or prepare for the subsequent processing of cell sites.
As can be seen from the basic principles given above, there are two slightly different engraving techniques. The first is to use a high-energy laser to directly engrave the metal surface of the roller to form a gravure net (see the left figure in Figure 5 on the next page). As far as the current level of technology is concerned, the direct engraving of copper layers has not been successful. Swiss company Daetwyler has adopted a compromise method for 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, and use a laser to ablate the network area so that the copper layer at the net hole is exposed. The non-cell hole is protected by the base paint and resist corrosion. A recessed cell can be obtained. This is the technical solution adopted by HelioBeam C2000 introduced by German Hell Company on Drupa 2000 (see the right figure in Figure 5 on the next page).
Fig. 3 Structure of variable cell and recessed depth
Although the difference between the two does not seem to be large in terms of basic principles, the analysis of the characteristics of the cell site and the details of the process are characteristic.
Third, the comparison of two laser gravure carving techniques
1. Laser engraving gravure base paint technology
Taking the HelioBeam C2000 system from Hell of Germany as the representative, the laser-engraved gravure base paint technology has become a new hot spot for laser engraving gravure on Drupa 2000.
The essence of the laser engraved gravure scheme of German Hell Company is: Make full use of the high resolution of the laser recording, and make the contour, text and figure outline of the laser ablation on the base paint reach high precision. The area of ​​the contours of the cells changes with the shades of color of the image. Therefore, the cells obtained by the subsequent etching process belong to the aforementioned "variable area, constant depth of the concave cell" (in fact, in the process of etching, the size of the cell outline area will still affect the network to some extent. Hole corrosion depth). Those who are familiar with offset printing will find that this is a "printed" gravure.
Figure 4 Structure of the FM cell
For a long time, the quality of text and graphics in gravure production was lower than that of offset printing. The reason is that the offset printing plate usually uses the recording resolution of 1200-3000 lines/inch to record the contours of the text and graphics, and the accuracy is higher; while the engraving resolution (the number of screen lines) of the electronic engraving machine is relatively low, generally the line is 60-140. /cm, that is, 150-356 lines/inch. This number of lines can reproduce the image hierarchy and detail, but it is insufficient for the outline quality of text and graphics. The edges of the text and graphics carved at this resolution are not smooth enough, and the quality and offset are not the same. In particular, the problem of the timing of writing small-size characters is even more prominent (see Figure 6).
Offset printing dots are composed of multiple laser exposure points. Using this feature, people can do two things: First, multi-level changes in dot area can be achieved. If the dot area of ​​100% area is composed of 16×16 record exposure points, the dot area rate can have 257 levels (including 0%); second, the dot shape can be more freely designed, such as different areas. The dots of the rate are designed into different shapes, which can compensate dot gain or dot reduction in the printing process accordingly, so that the image level and color reproduction are more perfect.
Figure 5 Principle of laser gravure engraving of metallic zinc layers (left) and base paint layers (right)
The engraving of a common mechanical electromagnetic engraving machine sculpting knife creates a net. The shape of the cell is determined only by the angle of the engraving knife, the rotation speed of the roller and the feed rate of the engraving head, etc. The degree of freedom in changing the shape of the cell is relatively small. It can be seen that the shape of non-combined cells has less variation, and because of the lower resolution, the engraving quality of the edges of text and graphics is not good. In order to solve this problem, Hell and Daetwyler-Ohio's technologists used methods such as Daetwyler-Ohio's TransCell technology to slightly change the spacing of the cells in the strokes of the sculpted text, making the pitch smaller, and partially increasing the engraving. Resolution; In addition to the micro-moving of the carving knife, Hell Company adopts the similar “anti-aliasing technique†method, and uses a small network point to supplement the gaps in the serrations of the strokes, and makes up for the visual effects.
The real solution to the problem of low precision of text and graphics is to increase the recording resolution. In the HelioBeam C2000 laser engraving machine, due to the "quasi-printing method" for laser-based paint ablation, images can be recorded with a resolution of 2540-5080 dots/inch (laser spot diameter of 5-10 microns), engraving resolution It is 10-20 times of the original resolution, so it fundamentally solves the above problems and improves the reproduction quality of text and graphics. At the same time, since each of the ablated cells is formed by combining a plurality of laser exposure points, the contour shape of the cells can be more conveniently designed to improve the transfer effect of the image layer. On the HelioBeam C2000 laser engraving machine, in addition to the original cell shape of Hell, other cell shapes can also be produced. For example, the recessed cells created on the system are useful for compensating dot gain in printing. In principle, it is also possible to engrave FM cells.
Increasing the engraving resolution leads to the question of how to ensure engraving efficiency. Hell's technicians used a multi-laser parallel exposure method on the HelioBeam C2000 laser engraved phase. The new fiber infrared laser (1110 nm) with a power of 60 watts was divided into 8 beams (7.5 watts per bundle). The base paint layer on the copper drum is thermally ablated. The quality of the laser beam produced by the fiber laser is good, and the depth of focus (depth of field) is large, while the base paint is stable in nature and vaporized only under high-energy laser light.
Observing from the engraving and engraving process route, this system does not need to change the configuration of roller copper plating and roller surface processing (car/grinding/throwing), and only needs to add two steps of base paint coating and corrosion in the process flow. The system can coexist with the original engraving process more conveniently, that is, the machined copper drum can be mechanically electromagnetically engraved, or it can be applied to a HelioBeam C2000 for laser engraving, and then etched and removed. 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 Intaglio
On Drupa 2000, the Digilas laser engraving system introduced by Daetwyler-Ohio also belongs to the same laser ablation base paint system. The system uses a 1 or 2 YAG laser (1064 nm) with engraving resolution of 1250-2540 dots/inch (laser spot diameter 10-20 microns).
Japan's Think Lab also launched its TB-21 series laser engraving system FP-20/40/80 on Drupa 2000. The system uses a plurality of lasers to expose the photosensitive resist layer, and then exposes the exposed copper layer through a development process, and then performs copper layer network etching to obtain a gravure cylinder. Although its basic characteristics are similar to those of the above two companies, the laser records a photosensitive layer and requires a development process without using a base paint ablation method.
2. Laser engraving metal zinc layer technology
Swiss company Daetwyler is a company that has the courage to explore. Before Drupa 2000 was held, they merged with the American electric engraving machine company to establish Daetwyler-Ohio company to face the 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 a single beam of argon ion laser engraving, engraving depth of 35000-70000 net points / second. At that time, with "traditional intaglio
As we all know, the mechanical electromagnetic gravure electronic engraving machine was invented in 1962 by Germany's Hell GmbH. This is very close to the laser invention time. In fact, the company's technical staff was trying to use the laser to carry out the engraving of the copper plated roller. However, due to the unsuccessful high reflectivity of the copper, they turned to high-energy electron beam engraving and succeeded.
Laser recording of gravure plates began in 1977, when Crosfield Electronics of the United Kingdom used lasers to sculpt cells in gravure cylinders with a layer of polymeric resin to make intaglio cylinders. Although due to quality stability and other reasons, the system has not been put into practical use in a large amount, but as a useful technology research and exploration, it has pointed out the way in which laser gravure printing plates 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 stage of practical application. In addition to the technology used to print CTPlate, it has been introduced by a number of manufacturers. The gravure printing plate and the flexographic laser production equipment became a highlight of the CTP (Computer To Cylinder).
First, the type of laser engraving gravure technology
As far as the types of gravure meshes are concerned, there are usually four types, ie, variable area mesh, recessed depth right variable mesh, area and recessed depth all with variable mesh, and frequency modulated mesh. In the current level of technological development, laser engraving of these four types of cyberspace has been realized.
Figure 1 Structure of an area-variable cell
1. Variable area network
As the name suggests, this type of network reproduces the tonality change of the image only by changing its open area. The depth of the network is larger in the area of ​​the network, and the area of ​​the network is smaller in the lighter color, and the depth of the meat cavity remains the same. In this regard, the principle of image reproduction is similar to offset printing. Therefore, this kind 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 gradation reproduction is similar to that of offset printing, the structure of the cell site cannot escape the basic technical requirements of the gravure printing plate. That is, the network wall must be formed and preserved, and the plate cannot be printed. There is a large area of ​​no net wall on the ground. Therefore, from the microscopic structure of the cell, it is not equal to the typographical dot.
Fig. 2 Structure of concave depth variable mesh
2. Concave depth variable mesh
This is the most typical gravure cell, often referred to as "classic gravure" or "traditional gravure." This type of network only changes the tone level of the image by changing its depth under the network. The depth of the network is deeper in the sunken pockets, while the shallower sunspots are shallower in the sunken pockets, and the area of ​​the net holes remains the same. Its network structure is shown in Figure 2.
Because the area of ​​the cell is the same, the thickness of the wall is the same.
3. Both the area and recess depth are variable
Such cells are often found on cylinders carved by mechanical electromagnetic electronic gravure engravers, and are the most commonly used types of cell types for non-laser type electric gravure. The characteristics of its nets are: the depth of the opening of the net hole and the depth of the concave are both large in the depth of the color, and the opening area and the depth of the concave of the net hole are small in the light color. Obviously, the thickness of the net wall is not equal. Its network structure is shown in Figure 3.
4. FM network
Applying the principle of FM screening to gravure to gravure can generate FM gravure gravure. The characteristics of its cells are: the same area, and the spatial position of the gravure plate randomly changes. Similarly, in order to avoid the appearance of a large-area network-free "zone" in the shadowed area of ​​the image, the spatial position of the FM-cell must be controlled reasonably and cannot be 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 the application of one or more high-energy laser beams to directly form a network of holes or exposed copper cells on the material to be engraved (metal layer or base paint layer) on the surface of the drum. Web site printing plate, or prepare for the subsequent processing of cell sites.
As can be seen from the basic principles given above, there are two slightly different engraving techniques. The first is to use a high-energy laser to directly engrave the metal surface of the roller to form a gravure net (see the left figure in Figure 5 on the next page). As far as the current level of technology is concerned, the direct engraving of copper layers has not been successful. Swiss company Daetwyler has adopted a compromise method for 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, and use a laser to ablate the network area so that the copper layer at the net hole is exposed. The non-cell hole is protected by the base paint and resist corrosion. A recessed cell can be obtained. This is the technical solution adopted by HelioBeam C2000 introduced by German Hell Company on Drupa 2000 (see the right figure in Figure 5 on the next page).
Fig. 3 Structure of variable cell and recessed depth
Although the difference between the two does not seem to be large in terms of basic principles, the analysis of the characteristics of the cell site and the details of the process are characteristic.
Third, the comparison of two laser gravure carving techniques
1. Laser engraving gravure base paint technology
Taking the HelioBeam C2000 system from Hell of Germany as the representative, the laser-engraved gravure base paint technology has become a new hot spot for laser engraving gravure on Drupa 2000.
The essence of the laser engraved gravure scheme of German Hell Company is: Make full use of the high resolution of the laser recording, and make the contour, text and figure outline of the laser ablation on the base paint reach high precision. The area of ​​the contours of the cells changes with the shades of color of the image. Therefore, the cells obtained by the subsequent etching process belong to the aforementioned "variable area, constant depth of the concave cell" (in fact, in the process of etching, the size of the cell outline area will still affect the network to some extent. Hole corrosion depth). Those who are familiar with offset printing will find that this is a "printed" gravure.
Figure 4 Structure of the FM cell
For a long time, the quality of text and graphics in gravure production was lower than that of offset printing. The reason is that the offset printing plate usually uses the recording resolution of 1200-3000 lines/inch to record the contours of the text and graphics, and the accuracy is higher; while the engraving resolution (the number of screen lines) of the electronic engraving machine is relatively low, generally the line is 60-140. /cm, that is, 150-356 lines/inch. This number of lines can reproduce the image hierarchy and detail, but it is insufficient for the outline quality of text and graphics. The edges of the text and graphics carved at this resolution are not smooth enough, and the quality and offset are not the same. In particular, the problem of the timing of writing small-size characters is even more prominent (see Figure 6).
Offset printing dots are composed of multiple laser exposure points. Using this feature, people can do two things: First, multi-level changes in dot area can be achieved. If the dot area of ​​100% area is composed of 16×16 record exposure points, the dot area rate can have 257 levels (including 0%); second, the dot shape can be more freely designed, such as different areas. The dots of the rate are designed into different shapes, which can compensate dot gain or dot reduction in the printing process accordingly, so that the image level and color reproduction are more perfect.
Figure 5 Principle of laser gravure engraving of metallic zinc layers (left) and base paint layers (right)
The engraving of a common mechanical electromagnetic engraving machine sculpting knife creates a net. The shape of the cell is determined only by the angle of the engraving knife, the rotation speed of the roller and the feed rate of the engraving head, etc. The degree of freedom in changing the shape of the cell is relatively small. It can be seen that the shape of non-combined cells has less variation, and because of the lower resolution, the engraving quality of the edges of text and graphics is not good. In order to solve this problem, Hell and Daetwyler-Ohio's technologists used methods such as Daetwyler-Ohio's TransCell technology to slightly change the spacing of the cells in the strokes of the sculpted text, making the pitch smaller, and partially increasing the engraving. Resolution; In addition to the micro-moving of the carving knife, Hell Company adopts the similar “anti-aliasing technique†method, and uses a small network point to supplement the gaps in the serrations of the strokes, and makes up for the visual effects.
The real solution to the problem of low precision of text and graphics is to increase the recording resolution. In the HelioBeam C2000 laser engraving machine, due to the "quasi-printing method" for laser-based paint ablation, images can be recorded with a resolution of 2540-5080 dots/inch (laser spot diameter of 5-10 microns), engraving resolution It is 10-20 times of the original resolution, so it fundamentally solves the above problems and improves the reproduction quality of text and graphics. At the same time, since each of the ablated cells is formed by combining a plurality of laser exposure points, the contour shape of the cells can be more conveniently designed to improve the transfer effect of the image layer. On the HelioBeam C2000 laser engraving machine, in addition to the original cell shape of Hell, other cell shapes can also be produced. For example, the recessed cells created on the system are useful for compensating dot gain in printing. In principle, it is also possible to engrave FM cells.
Increasing the engraving resolution leads to the question of how to ensure engraving efficiency. Hell's technicians used a multi-laser parallel exposure method on the HelioBeam C2000 laser engraved phase. The new fiber infrared laser (1110 nm) with a power of 60 watts was divided into 8 beams (7.5 watts per bundle). The base paint layer on the copper drum is thermally ablated. The quality of the laser beam produced by the fiber laser is good, and the depth of focus (depth of field) is large, while the base paint is stable in nature and vaporized only under high-energy laser light.
Observing from the engraving and engraving process route, this system does not need to change the configuration of roller copper plating and roller surface processing (car/grinding/throwing), and only needs to add two steps of base paint coating and corrosion in the process flow. The system can coexist with the original engraving process more conveniently, that is, the machined copper drum can be mechanically electromagnetically engraved, or it can be applied to a HelioBeam C2000 for laser engraving, and then etched and removed. 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 Intaglio
On Drupa 2000, the Digilas laser engraving system introduced by Daetwyler-Ohio also belongs to the same laser ablation base paint system. The system uses a 1 or 2 YAG laser (1064 nm) with engraving resolution of 1250-2540 dots/inch (laser spot diameter 10-20 microns).
Japan's Think Lab also launched its TB-21 series laser engraving system FP-20/40/80 on Drupa 2000. The system uses a plurality of lasers to expose the photosensitive resist layer, and then exposes the exposed copper layer through a development process, and then performs copper layer network etching to obtain a gravure cylinder. Although its basic characteristics are similar to those of the above two companies, the laser records a photosensitive layer and requires a development process without using a base paint ablation method.
2. Laser engraving metal zinc layer technology
Swiss company Daetwyler is a company that has the courage to explore. Before Drupa 2000 was held, they merged with the American electric engraving machine company to establish Daetwyler-Ohio company to face the 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 a single beam of argon ion laser engraving, engraving depth of 35000-70000 net points / second. At that time, with "traditional intaglio
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