Analysis of key technologies in the hottest CTP im

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Analysis of key technologies in CTP imaging system

ctp technology is the most noteworthy digital printing technology in the printing field at present. It mainly reflects its advantages from two aspects, that is, it can improve printing efficiency and print quality. This is because CTP technology adopts full digital management, which makes it many times faster than traditional plate making in ink balance, plate making, plate installation and so on, and makes full use of the production capacity of the printing machine; On the other hand, using CTP plate making, the obtained points are accurate and stable, the colors are uniform, and FM processing can be applied on a large scale, which helps the quality of printed matter get a qualitative leap and improvement. Based on the above advantages, it is not surprising that CTP technology has developed so rapidly in the printing industry

a complete CTP system consists of three subsystems, namely, the computer control system, the impact resistance of the film pendulum impact testing machine: the film pendulum impact testing machine uses a high-precision photoelectric encoder and a computer with special measurement and control and calculation software to detect the impact absorption energy, the impact toughness system of materials, the printing plate imaging system and the plate storage and supply system by detecting the difference between the potential energy before and after the impact of the pendulum. Among them, the plate imaging system plays a very important role in the whole CTP system, and its performance determines the comprehensive performance of the whole CTP system. At present, when CTP system is popular in the market, many high-tech control technologies are integrated into the imaging system in order to improve the competitiveness of the whole system. Take the CTP system produced by Cleo company as an example. In the imaging system, it adopts many high-tech technologies, such as external drum imaging, square light spot, self persuasion focusing, plate fine registration and temperature compensation, which ensure that the whole system operates in a high-performance state

I. external drum technology

according to the way the printing plate is placed, the imaging system of CTP can be divided into three types: external drum, internal drum and flat table. The external drum imaging device is shown in Figure 1. Its remarkable feature is that the printing plate is installed on the external drum as it is installed on the printing plate cylinder of the printing unit. During imaging, the printing plate rotates around the axis of the drum with the drum, and the laser moves horizontally along the axis of the drum. The advantage of this printing plate structure is that multiple laser heads can be used to expose the printing plate at the same time (for example, 240 laser beams are exposed at the same time, and the width of the laser head can reach 20mm), The imaging speed of the external drum type is greatly improved; Another advantage is that the external drum imaging device generally uses a short light path laser system. The light beam emitted by the laser only passes through a single lens to reach the printing plate surface, without a long light transmission path, which eliminates the possibility of light vibration during the transmission of the laser, so that the optical performance and imaging performance of the laser beam are very stable, the formed points are uniform, and the printing adaptability is good. At present, the external drum imaging device is the most widely used in the flexible CTP system

of course, the external drum type also has some shortcomings, such as slow drum speed, high requirements for drum uniformity and high requirements for beam uniformity. These deficiencies are being solved with the improvement and compensation of technology

second, square light spot technology

square light spot technology is a patented technology of Cleo company. Its advantage is to make up for the shortcomings of circular light spots, such as the overlap between light spots, the expansion of dots on the printing plate caused by overlap, the edge of dots is not sharp, and it is difficult to reproduce delicate levels. The square light spot can match the edge of the grid exactly, without the dot expansion generated in the plate making stage, and the performance of the fine level is accurate

the energy curve of the traditional circular light spot belongs to the Gaussian energy curve, as shown in Figure 2 (a). The circular light spot with the characteristics of the Gaussian energy curve has a wide edge halo, as shown in Figure 2 (b). Due to the existence of edge halo, the edges of circular points formed on the printing plate are blurred, and the fine layers of the image cannot be faithfully reproduced

times, as shown in Figure 2 (c). Using square light spot technology can eliminate the above defects of circular light spots. The principle of forming square light spots is shown in Figure 3

first, the laser bar composed of 20 laser tubes in parallel emits laser beams, and the power of each laser tube is 40 watts. The 20 laser beams overlap each other to form a laser waterfall, which is directed to the lens system: the function of the lens system is to evenly distribute the laser energy, and then the laser passes through a special light valve, which is the key device to form square light points. After passing through the light valve, the beam with uniform energy distribution is dispersed into square light points, The power of each light spot is 8 watts. When it is shot on the thermal plate, the surface temperature of the thermal plate can reach 400 degrees, making the printing plate exposed to form square spots

such a square light spot generator has high safety performance, and the service life of the laser tube is very long, usually more than 5 years. Even if a laser tube is broken, it will not affect the energy of the square light spot, and it can also produce completely qualified points

the characteristics of square light spots generated by the system in Figure 3 are completely different from those of circular light spots. Its energy curve is no longer a Gaussian curve, but an approximate trapezoid, as shown in Figure 4 (a). In this way, there is little halo around the square light spot, and the energy distribution is very uniform. It has a perfect marketing system and high-quality marketing personnel, as shown in Figure 4 (b). The edges of the square dots generated on the printing plate are very neat. As shown in Figure 4 (c), the dots formed are very "cut", which can beautifully copy the images with delicate layers

III. auto focusing technology

the plates used in CTP system are divided into photosensitive type, thermal type and silver salt type. Take the commonly used thermal plate as an example. This kind of plate is sensitive to temperature rather than visible light, that is, when the temperature exceeds a specific threshold, the printing plate will be exposed, otherwise, the printing plate will not be exposed, so it can be operated in the bright room. Because

is the point on the thermal plate produced by heat, the temperature must reach a printing plate with stable quality, long storage time, no accumulation point expansion during plate making, the produced point is more exquisite, and the printing plate has a high print resistance rate, reaching 1million prints after baking

thermal plates are generally composed of plate base and coating. The plate base is mostly aluminum plate. A layer of hydrophilic resin is first coated on the plate base, and the thermal layer is on the resin layer. Due to the coating technology and the overstock deformation during transportation and storage, the surface of the thermal layer is often deformed, resulting in the thermal layer is not absolutely flat. During plate making, if no remedial measures are taken, the photosensitive area of the uneven surface is different, resulting in the size of the formed dot area, which affects the quality of the copied image. In order to solve this problem, automatic focusing technology is used in imaging

the principle of automatic focusing technology is shown in Figure 5. The detection laser tube installed in front of the exposure laser tube sends out a detection beam. According to the return time of the detection beam, the detection receiver determines the convexity and concavity of the printing plate surface, and then sends an action command to direct the forward or backward of the focusing lens. When using auto focusing technology, the rotation speed of the external drum plate making device should be set to 180 rpm. The condensing lens is driven by magnetic guide rail, and the minimum stroke is 1 micron. Using auto focusing technology, the exposure of the printing plate is uniform, which ensures the consistent size of the points generated on the printing plate, which is conducive to the quality control of the printing process and improve the overall quality of the printing product

IV. accurate registration and temperature compensation technology

. Registration is a key factor affecting the quality of color printing image. For amplitude modulation, inaccurate overprinting intensifies the effect of moire and rose spots, and produces waste products in serious cases. Creo company has ensured the accurate positioning of imaging on the printing plate with three accurate registration methods,

1, three-point positioning method. As shown in Figure 6, in the three-point positioning method, there are two points for positioning the long side of the printing plate, which belong to mechanical positioning. The other point is located on the side of the printing plate, that is, the short side, which belongs to optical positioning. The printing plate is first located by the reference line of two mechanical printing plates, and then imaged according to the reference line. Because the optical positioning edge can be moved flexibly, even if the placement position of the printing plate is very different, the imaging position is also very accurate. This method belongs to imaging first and then punching, so the imaged printing plate needs to be punched before it can be printed on the machine

2. Virtual hole location method. The virtual hole positioning method is divided into two steps. First, three holes are punched on the long side of the printing plate, as shown in Figure 7 (a). The triangular hole on the right is still the most widely used virtual hole in the construction market. Then the printing plate is installed on the imaging drum. As shown in Figure 7 (b), the long side is positioned by the mechanical positioning pin, and the side is positioned by the laser. With the virtual hole as the reference line, the printing plate is exposed according to the reference line. This method belongs to punching first and then imaging. After printing plate imaging, it can be directly printed on the machine without additional punching

3. Combination of three-point positioning and punching. The first two methods are combined to form the third method, which is the combination of three-point positioning and punching. In this method, only two mounting holes are punched, and no virtual holes are punched. After punching, the printing plate is positioned and imaged according to the first method

during the printing process, due to the mutual friction between the printing plate and the rubber drum, inking roller, and the unsatisfactory temperature control device in the printing workshop, the temperature of the printing plate surface will change from time to time, resulting in the elongation or shortening of the printing plate, causing great trouble to the printing registration. The built-in temperature compensation device can automatically compensate the thermal expansion and cold contraction of the

printing plate according to the changes of the environment, and the printing plate exposed at any time can be overprinted accurately. Using the temperature compensation device, the repetition accuracy of the printing plate made by the same CTP system can be controlled within ± sum, and the repetition error of the printing plate made by different CTP systems is less than 20um, which greatly improves the registration accuracy of the printing plate

v. conclusion

because CTP can improve printing efficiency and print quality, and can use FM plus plate making, CTP system will inevitably replace electronic color separation machine and become the first choice of plate making equipment in the printing field. As a new thing, it also needs to be constantly improved and improved in practice. In particular, some key technologies in the imaging system that directly affect the overall performance of CTP need researchers to innovate and improve constantly. I believe that with the progress of related technology, CTP system will be more and more welcomed by the majority of users

(author/Huang Lingge, Duan Huawei)

author unit: Department of packaging engineering, Zhengzhou University

information source: China Printing Market

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