CTP plate evaluation index
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The introduction of CTP technology into China has made great progress in the past few years. More and more enterprises adopt CTP technology, and the technology is relatively mature. Looking at China's printing market, CTP still has a lot of room for development, so the major printing equipment and material suppliers also regard the Chinese market as a huge opportunity. The same is true for CTP plate suppliers. At present, CTP plates of all world-famous brands such as Kodak, Fujifilm, Agfa and so on can be seen in the domestic market. At the same time, domestic enterprises such as the Second Film Factory have also developed CTP plates with independent intellectual property rights in recent years. The CTP plate market can be described as a hundred flowers.
However, multi-brand and multi-technology has brought difficult troubles for printing companies to choose. How to choose the plate reasonably becomes an important issue, usually considering various factors such as economic factors and technical factors. Since the economic factors of each enterprise are different, it cannot be generalized. Therefore, the technical evaluation indicators of CTP plates will be analyzed for the printing companies, and it is hoped that they can help the enterprises to purchase.
Commonly used CTP plate evaluation methods such as sand size, shape and distribution, coating thickness, etc., require electron microscopy and related physical and chemical instruments for testing, which is expensive for printing plants. Moreover, these indicators are usually for the plate manufacturer, and the guidance for printing is not intuitive, so this paper proposes the following indicators of interest in printing.
First, the dot reproduction
The dot is the basis for printing and copying the graphic information, and has a very important position in printing. The quality of the printing plate access point will directly affect the quality of the printed Internet access point, which in turn affects the replication of the entire printing process. Therefore, the first step in controlling the outlet is to select a reproducible plate. There are several aspects to measuring the reproducibility of the dot of the plate.
1. Reproduction of AM outlets
The traditional CTF process is that the computer directly outputs the film and then uses the film to print the PS version through the printer. In this process, there is a certain degree of loss when the screen is dried due to the bottom diffusion phenomenon when the printing plate is exposed. At the same time, the development of the conventional PS plate material is small, the development time is slightly longer, the dot below 2% is lost, and the development time is short, the development is impervious, resulting in dirt. Therefore, in the process of printing the printing plate, the outlets have a certain proportion of reduction.
This phenomenon no longer exists after the CTP process, so CTP has the ability to replicate 1%-99% of the dots in terms of process. The remaining question is whether the CTP plate has the ability to support 1%-99% of the outlets. This is an important indicator of the quality of CTP plate production.
Like the traditional PS version, the mesh of the CTP plate is also composed of sand mesh. To achieve a mesh point that can support 3% or less, the sand of the plate base is small enough. Because the finer the sand, the more sands that support the high-gloss part of the dot, the stronger the dot, and the less likely it is to lose the plate during printing. However, the sands of the dots should not be too fine. If the fineness affects the wetting of the plate, the fineness of the plate with fine mesh in the printing is less latitude, which is not conducive to the control of printing quality.
The reproduction of the dots is also related to the coating thickness of the plate and the number of screen lines. The phenomenon of light penetration is a common phenomenon, both on the plate and on the printed matter. The phenomenon of light seepage is an important factor in the expansion of the dot in the printing. The thicker the thickness of the coating, the more obvious the phenomenon of light penetration on the dot on the printing plate. Therefore, the more serious the dot enlargement affects the quality of the plate making, the proper coating thickness is very reproducible for the dot. helpful. The more the number of screen lines, the larger the side length around the dot, the more serious the dot enlargement, so the reproduction of the plate dot is also related to the number of screens.
However, no matter what kind of production technology is used, it is the most important to meet the printing requirements for the traditional FM screening. The purpose of the technology is to meet the printing needs. Therefore, testing the CTP plate according to the printing needs is the most for the printing unit. Actually and most convenient.
According to the author's investigation of the market, the CTP plate on the market can reproduce 1%-99% of the dots under the 2001pi screen number. Compared with the quality, the quality is quite good, and can be reproduced at up to 2501pi. And most of what can be achieved is a thermal plate. Therefore, in terms of production capacity, it seems that 200-250lpi is already the upper limit.
After considering the capabilities of the plate manufacturer, it is also necessary to consider the need for printing. First, the dot reproduction of the CTP plate should be at least not lower than the standard of the PS version - 1501 pi, reproducing 2% - 98% of the dots, because if this is not possible It is required that even the printing quality under the traditional craft level may not be achieved. If there are special printing needs, such as high-line screening, etc., you should determine your own requirements for the plate according to the actual number of screens. In general, most of the high-line screenings are thermal plates.
Although each plate specification is marked with the ability to reproduce dots, it is necessary to perform relevant tests according to actual conditions. To complete the relevant tests, the printer needs to use a special density meter for the scale and measurement. Exposing the drying ruler on the printing plate according to the required number of screen lines, and then using a special density meter to measure the density of the corresponding step of the printing plate to see if the above requirements can be achieved is an effective one. method. It should be noted that the plate-making machine must be linearized during the drying process to achieve a 1:1 output, which is advantageous for the operation of the test plate.
2. Reproduction of FM sites
Different from the AM network, the FM network point controls the area ratio of the network by controlling the density of the network points in the recording unit. This article uses a 1200dpi direct plate-making machine to expose, the size of the dot on the plate is 20μm to explore the requirements of FM dot printing suitability.
The basis of the printing dot is the sand surface on the surface of the printed spot. The finer the sand, the more the number of sand meshes supporting the mesh, the stronger the adsorption force, and the smoother the edge of the dot, so for a frequency modulation dot of only 20 μm, In order to maintain the quality it should, it should be used as much as possible with a fine-grained, high-resolution plate. According to international standards, the ideal average size of the sand is 2.5 μm and the depth is 3 μm. Assuming each sand is considered a cone, there are 64 sand support for each 20 μm frequency modulation point, and the total contact area is about 817 μm 2 . However, for outlets above 1200 dpi, it is not covered by international standards, so it needs to be tested by actual printing tests.
In the printing, due to limited equipment conditions, the method of measuring sand is difficult to develop. The detection of FM points can be the same as the method of detecting amplitude modulation. However, in order to detect the ability of the plate to support FM points, it is necessary to adjust the resolution of the computer-to-plate equipment. See if the FM points obtained at different resolutions can reproduce 1%-100% of the dots, and find the smallest resolution that can reproduce the dots. At the same time, for the FM network, the shape of the dot and the sharpness of the edge are also very important factors, which can be measured by the method of magnifying glass observation. It is also possible to compare the frequency modulation dot and the amplitude modulation dot and the frequency modulation of various resolutions. The method of comparing the grid ladders is measured. If the effects obtained between FM and AM and different resolutions are the same, the support of the media for the FM network is very good.
Second, the line reproduction
Line reproduction is the same as dot reproduction, which is the embodiment of plate resolution test in printing. Because the algorithm of line screening and the screening algorithm of the dot are different, the dot highlights the optical characteristics of a specific area, while the line highlights the smoothness of the edge. Therefore, although the resolution is also tested, the test of line reproduction is also very necessary. The ability to reproduce lines usually includes the following two aspects:
1. Reproduction of extremely thin lines
The reproduction of extremely thin lines is related to the resolution of the printing plate, and the resolution is determined by the control of the process of sanding, coating and the like on the printing plate. The principle and the description of the dot reproduction have been described in detail. For the reproduction of the line, not only the size of the sand is concerned, but also the uniformity of the sand is more prominent, because the edge of the line supported by the uniformly distributed sand is likely to be smooth. Of course, the uniformity of coating is also a very important aspect.
To detect the reproduction of the extremely thin lines of the printing plate, straight lines and curves of different widths can be made on the CTP printing plate, the straight line and the curved line size which can be reproduced are observed by the naked eye, and the smoothness of the edge of the line is observed by means of a magnifying glass.
2. The size of the yin and yang lines
Due to the existence of light seepage, the visual width of the Yin and Yang lines on the printing plate will be changed to some extent. The thickness of the coating is also a key factor affecting this phenomenon. The comparison of the thickness of the Yinxian and Yangxian lines can reflect the fact that the lines are optically thick due to optical phenomena, and this is also a very important aspect for line reproduction. In the plate making, the adjacent negative and positive lines can be tanned on the printing plate to compare the changes of the lines.
Third, ink affinity and wetting properties
The imaging of the media discussed in the previous section is undoubtedly important for printing. But the plate is ultimately used for printing, and the printability of the plate is also a very important aspect. The most closely related to the printing plate is water and ink. The ink retention of the printing plate and the wetting property of the printing plate directly affect the balance of printing ink and thus affect the quality of printing.
1. Ink performance
As is well known, offset printing is a process of forming an imprint by controlling ink, and the image portion is capable of accepting the ink on the transfer roller, which is then transferred to the blanket and finally transferred to the paper by the blanket. It is obvious that the thickness of the ink on the plate directly affects the thickness of the ink on the blanket and paper. To control the density and color of the printed matter, the thickness of the ink layer is a very important factor. The larger the range of the thickness of the ink layer that can be controlled, the greater the room for adjusting the printing quality in printing, and the more favorable the control of product quality. Studies have shown that when a certain thickness of the ink film has been attached to the surface of the transfer roller, the amount of ink transfer on the printing plate depends on the ink retention of the printing plate. If the ink retention of the printed image portion is low, the range of the amount of ink controlled in the entire printing is smaller, which is more detrimental to the product quality. Moreover, by using a printing plate with poor ink affinity, the remaining ink will accumulate on the transfer roller, so that the ink film of the transfer roller is thickened, and the ink is accumulated too much at both ends, thereby causing splashing and reducing the hiding of the ink. force. Therefore, CTP plates with good ink retention properties are very important.
In order to improve the ink-receptive properties of CTP plates, photosensitive materials with similar pigments, resins and solvents should be used in the development of plates. The ink-receiving performance of the test plate can be determined by measuring the density of each color block by adjusting the ink amount of the transfer roller.
2. Wetting properties
The wettability of the plate refers to the property of selectively retaining water on the non-image portion of the plate. The control of the ink in printing is not only based on the amount of ink but also on the amount of water, and can be printed when the state of ink balance is reached. Therefore, in printing, water and ink are a pair of contradictions that fight each other and unite with each other. The amount of water in the printing is too small, and the part without the imprint is easy to crepe. If the amount of water in the printing is too large, the imprinting is dull, the paper is not uniform, the paper is deformed, and the overprinting is not allowed.
In 1999, when FOGRA in Germany tested the dampening solution, the wetting of the plate was also analyzed. Analysis shows that due to the exceptionally smooth surface of the CTP plate, it has a higher requirement for wetting, and its surface requires a dampening film that is thicker than a normal plate. The thickness of the ideal fountain solution layer is 0.6-1.5 μm. Because the ink is excessively emulsified after the thickness is exceeded, the resulting print has no gloss, and the amount of water is too large. In order to achieve the balance of ink and ink, the amount of ink required is also increased. At this time, the range of controllable ink amount is reduced, which is not conducive to QC. Therefore, it is very important to adjust the hydrophilicity of the printing plate to ensure that the thickness of the dampening solution layer is between 0.6 and 1.5 μm when the ink is balanced.
It is obvious that the wetting property of the printing plate is directly related to the sand of the printing plate. The traditional printing plate thinks that the sand is denser and deeper, and the wetting performance is better. However, for the CTP plate, the sand is too thin and printed. The surface of the blank portion of the plate thus becomes smooth and difficult to wet. The resolution of the plate and the stability of the dot and the wetting characteristics of the plate become a contradiction between the special conditions of CTP plate manufacture. The resolution and dot stability require a finer sand, but the conditions of wetting require that the sand should not be too thin. Of course, the situation is not as bad as imagined, and in most cases the two can be balanced. Some manufacturers (FUJI FILM) have also developed a composite sand screen technology: the coarse sand has water retention and is not easy to get dirty; the middle sand has improved developability and enhanced printing resistance; the fine sand maintains the balance between water and ink. Improve wear resistance.
The hydrophilicity of the plate can be achieved by actual printing tests, adjusting the ink balance, and measuring the wetting characteristics of the plate by testing the ink balance latitude.
Fourth, other indicators
In addition to the above indicators, there are some requirements for CTP plates. Such as bottom ash, sensitivity, size specifications, coating uniformity, printing resistance and so on.
Bottom ash is caused by improper processing of aluminum in production, especially in the upper and lower winding parts of aluminum-based production. Generally, the bottom ash can be removed by reasonable control of the development and exposure time, but the removal of the bottom ash should be carried out without affecting the quality of the graphic part. If the bottom ash is removed, the image quality must be lost. Claim.
Sensitivity refers to the requirement of the CTP plate material for the energy of the exposure source. This indicator is especially important for thermal CTP plates because the laser source of a computer-to-plate machine is expensive and difficult to replace, so it is necessary to extend the life of the light source as much as possible. The use of a higher sensitivity plate requires a larger power for the laser to accelerate the aging of the laser. At the same time, because the energy requirement of the plate exposure is too high, the laser is more easily attenuated, further shortening the life. Therefore, the lower the sensitivity, the better.
There are no special requirements for the size and coating uniformity of the CTP plate. It can be tested by referring to the relevant requirements of the PS version of the national standard. The printing durability usually varies according to the type of the plate. Generally, the denser the plate, the higher the printing durability, and the printing operation has a great influence on the printing durability. It should be determined according to the actual situation.
Some of the above indicators are items that are of great interest in printing and can be easily measured. They are more instructive for the printing process. Only by controlling the above aspects in the selection of the plates can the printing quality be guaranteed.