Teach you how to buy CTP? (two)

2. Several evaluation indicators that should be noted when purchasing CTP plates

The introduction of CTP technology in China has made considerable progress in the past few years. There are more and more companies using CTP technology, and the technology is more mature. Looking at China's printing market, CTP still has a lot of room for development. Therefore, major printing equipment and material suppliers also regard China's market as a huge opportunity. The same is true for CTP plate suppliers. At present, almost all world famous brands of CTP plates can be seen in the domestic market, such as Kodak, Fuji Film, Agfa and so on. At the same time, domestic companies, 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 blooming.

However, multi-brand and multi-technology has brought troubles to the printing companies that are difficult to choose. How to reasonably select the plate material has become an important issue. It is usually necessary to consider various factors such as economic factors and technical factors. Since the economic factors of various companies are not the same, they cannot be generalized. Therefore, the technical evaluation indicators of CTP plates will be analyzed for the majority of printing companies. It is hoped that they can help enterprises to purchase.

The commonly used CTP plate evaluation methods such as the size, shape and distribution of the grain size, the coating thickness, etc., are expensive for the printing factory due to the electron microscopy and related physical and chemical instruments required for the test. Moreover, these indicators are usually for the manufacturers of printing plates, and the guidance for printing is not intuitive. Therefore, the following indicators that are concerned about printing are proposed in this paper.

First, network reproduction

The outlets are the basis for printing and copying graphic and text information, and have a very important position in printing. The quality of the printing plate's Internet site will directly affect the quality of printed Internet sites, and then affect the reproduction of the entire printing process. Therefore, the first step in controlling the outlets is to select a site with good reproducibility. Measure the dot reproducibility of the plate mainly in the following aspects.

1. Reproduction of AM sites

Traditional CTF technology is the computer directly output film, and then according to the film through the printing machine drying PS version. In this process, there is a certain degree of loss in the drying of the dots due to bottom diffusion during exposure of the printing plate. At the same time, the latitude of the traditional PS plate material development is small, the development time is slightly longer, the dots below 2% will be lost, and the development time will be short and the development will be impenetrable, resulting in dirtiness. Therefore, a certain proportion of the outlets are reduced in the process of drying the printing plate.

This phenomenon no longer exists after using the CTP process, so the CTP has the ability to duplicate 1%-99% of the dots from the process. The remaining question is whether CTP media has the ability to support 1%-99% of its outlets. This is an important indicator of the quality of CTP plate production.

As with the traditional PS version, the dots of the CTP plate are also composed of sand grains. To make it possible to support 3% or less of the dots, the grains of the plate base must be small enough. Because the finer the sand, the more the number of sands that support the high-light part of the mesh, the more firm the dot is, and the less likely it is that there will be blanks during printing. However, the grain size of the dot cannot be too fine. If it is too fine, it will affect the wetting of the plate, and the fine balance of the ink content of the plate with fine grain in the printing is small, which is not conducive to the control of printing quality.

The reproduction of dots is also related to the coating thickness of the plate and the number of screens. The phenomenon of light penetration is a common phenomenon and it exists both on the printing plate and in the printed matter. The phenomenon of light penetration is an important factor in the dot enlargement during printing. The thicker the coating thickness, the more obvious the light penetration phenomenon of the dots on the printing plate, so the more serious the dot expansion, which affects the quality of the plate making, and the appropriate coating thickness reproduces the dots. helpful. The more screen lines are added, the larger the edge length around the network points is, and the more serious the network points are expanded. Therefore, the reproduction of the Internet point of the printing plate is also related to the number of screen lines.

However, no matter what kind of production technology is adopted, it is the most important to meet the printing requirements for traditional FM screening, and the purpose of technology is also to meet the needs of printing. Therefore, testing CTP media is the most important for printing units according to the needs of printing. Actually it is the most convenient.

The CTP plate on the market can reproduce 1%-99% of the dots under the 2001pi screen number. The quality is quite good. It can be reproduced at 2501pi. And most of them are thermal plates. So from the perspective of productivity, it seems that 200-250 lpi is the upper limit.

Taking into account the capabilities of the media manufacturers, the same need to consider the need for printing, first of all CTP media outlets can not be less than the standard version of the PS version - 1501pi, 2% -98% reproduction of the network, because if you can not reach this Requirements, even the level of traditional printing process quality may not be achieved. If there are special printing needs, such as the number of high-line screening, etc., according to the actual number of screen lines to determine their needs for the plate. In general, most thermal screens that can achieve a high number of screens are screened.

Although the ability to reproduce the dots has been marked on the specifications of each edition, it is still necessary to carry out relevant tests according to the actual situation. The printing factory to complete the relevant test requires a dedicated density meter with a step scale and a measurement version. Exposing the solar bench on the plate according to the required number of screens, and then using a dedicated densitometer to measure the density of the corresponding step on the plate, to see if it can meet the requirements described above is a valid one. method. It should be noted that the platemaker must be linearized to achieve a 1:1 output during the drying process, which is beneficial to the operation of the test plate.

2. The reproduction of FM network

Different from the AM network point, the FM network point controls the dot area rate by controlling the density of the dot in the recording unit. This paper uses a 1200dpi direct plate-making machine to expose, the dot size on the printing plate is 20μm size to discuss the printing dot adaptability requirements of the FM network.

The printing dot is based on the sand spot on the surface of the blotch. The finer the sand, the more the number of sand spots supporting the dot, the stronger the adsorption force, and the smoother the edge of the dot, so for a FM dot with a size of only 20 μm, In order to maintain its proper quality, it is best to use fine-grained, high-resolution printing plates. According to international standards, the ideal sand size is 2.5 μm and the depth is 3 μm. Assuming each sand spot is considered as a cone, there are 64 sand supports for each 20 μm frequency modulation point, and the total contact area is about 817 μm 2 . However, for sites above 1200dpi, they are not covered by international standards and therefore need to be tested by actual printing tests.

Due to limited equipment conditions in printing, the method of measuring the sand is difficult to develop. The detection of FM mesh can pass the same method as the detection of amplitude modulation. However, in order to detect the ability of the printing plate to support the FM mesh, the resolution of the computer-to-plate system needs to be adjusted. To see if the resulting FM network at different resolutions can reproduce 1%-100% of the network, to find the smallest resolution that can reproduce the network. At the same time, for the FM network point, the shape of the dot and the sharpness of the edge are also very important factors, which can be measured by using a magnifying glass observation method, or by comparing the FM network point with the amplitude modulation dot and various kinds of resolution of the FM. Measured by the comparison method of outlet ladders. If the results obtained between FM and AM and FM with different resolutions are consistent, it indicates that the media support for FM is very good.

Second, line reproduction

Line reproduction is the same as dot reproduction, which is a concrete embodiment of plate resolution test in printing. Because of the difference between the algorithm of line screening and the screening algorithm of screen points, the network points highlight the optical characteristics of a specific area, and the line highlights the smoothness of the edges. Therefore, although the same resolution is tested, the test of line reproduction is also very necessary. The reproducibility of lines usually includes the following two aspects:

1. The reproduction of very thin lines

The reproduction of the very thin lines is related to the resolution of the printing plate, and the resolution is determined by the control of the process of the plate, such as the sand, coating and the like in the production. The principle and the reproduction of the dots have been described in more detail. Regarding the reproduction of the lines, not only the size of the sand, but also the evenness of the sand is more prominent, because evenly distributed sand supports the edges of the lines may be smooth. Of course, the uniformity of the coating is also a very important aspect.

To detect the reproduction of the fine lines of the printing plate, it is possible to print lines and curves of different widths on the CTP plate, visually observe the rectilinear and curvilinear line sizes that can be reproduced, and observe the smoothness of the line edges by means of a magnifying glass.

2. The size of Yin Yang line

Due to the phenomenon of light penetration, the visual width of the Yin and Yang lines on the printing plate will change to some extent. The thickness of the coating is also a key factor affecting this phenomenon. The comparison of the thickness of the Yin and Yang lines can reflect the optical thickening of the lines due to optical phenomena. This is also a very important aspect for line copying. Plates can be printed on the plate adjacent to the Yin and Yang Xian to compare the line changes.

Third, the ink performance and wetting properties

The previous discussion on the imaging of the plates was undoubtedly very important for printing. However, the plate is ultimately used for printing, and the printability of the plate is also a very important aspect. Water and ink are the most closely related to the printing plate. The ink affinity of the printing plate and the wettability of the printing plate directly affect the printing ink balance and thus the printing quality.

1. Ink-producing performance

As we all know, offset printing is a process of forming a mark by controlling ink. In the graphic part, due to ink affinity, it can accept the ink on the transfer roller, then transfer to the blanket, and finally transfer to the paper through the blanket. It is clear that the thickness of the ink on the plate directly affects the thickness of the ink on the blanket and the paper. To control the density and color of prints, the thickness of the ink layer is a very important factor. The larger the thickness of the ink layer can be controlled, the greater the room for printing to adjust the print quality, and the more favorable the control of product quality. Studies have shown that when a certain thickness of ink film has adhered to the surface of the transfer roller, the amount of ink transferred on the plate depends on the level of ink affinity of the plate. If the ink-printing portion of the printing plate is low in ink receptivity, the range of controlling the amount of ink in the entire printing is smaller, and the more it is unfavorable to the product quality. In addition, the use of poor ink affinity printing plate, the remaining ink will accumulate in the transfer roller, so that the transfer roller ink film is thickened, both ends of the ink accumulation, resulting in ink splash phenomenon, but also reduce the ink cover force. Therefore, CTP plates with good ink receptivity are very important.

In order to improve the ink-receptive performance of the CTP plate, a photosensitive material having a chemical property close to that of the pigment, the resin, and the solvent in the ink should be used in the development of the plate material. The ink-receptive performance of the test plate can be determined by measuring the density of each color block by adjusting the ink transfer roller ink amount to conduct a trial printing.

2. Wetting properties

The wettability of a plate refers to the property of selectively retaining water on the non-graphic area of ​​the plate. The control of the ink in printing is not only based on the amount of ink supply but also on the amount of water, and it can be printed only when the state of ink and ink is balanced. Therefore, in printing, water and ink are a pair of contradictory bodies that fight against each other and are unified with each other. The amount of water in the printing is too small, and parts that are not imprinted are prone to blemishes. If the amount of water in the printing is too large, imprints may appear dull, the paper may not be collected, and the paper may be deformed and overprinting may occur.

When the FOGRA of Germany tested the fountain solution in 1999, the wettability of the plate was also analyzed. The analysis shows that because the surface of the CTP plate is extremely smooth, it has a higher requirement for wetting, and its surface requires a dampening liquid film that is thicker than a normal plate. The thickness of the ideal fountain film layer is 0.6-1.5 μm. Because after this thickness is exceeded, the ink is over-emulsified, the resulting imprint will have no gloss, and at the same time the amount of water is too large. In order to achieve ink balance, the amount of ink required will also increase. At this time, the range of controllable ink volume will be reduced, and it is also unfavorable. QC. Therefore, it is very important to adjust the hydrophilicity of the printing plate to ensure that the thickness of the dampening liquid film is between 0.6-1.5 μm when the ink-water balance is reached.

Obviously, the wetting performance of the printing plate is directly related to the grain of the printing plate. The traditional printing plate thinks that the grains are denser and deeper, and the wettability is better, but for the CTP plate, due to the fine sand, the printing The surface of the blank part of the plate, on the other hand, becomes smooth and difficult to wet. The resolution of the printing plate and the stability of the dot and the wetting characteristics of the printing plate become a pair of contradictory contradictions under the special conditions of CTP plate manufacturing. Resolution and dot stability require finer sand, but wetting conditions require that the grain size is not too fine. Of course, the situation is not as bad as it seems. In most cases, the two can be balanced. Some manufacturers (FUJIFILM) have also developed a composite sand technology: coarse sand head is water-retaining, not easy to get dirty; middle sand head improves the developability and enhances the resistance to imprinting force; the fine sand head maintains the balance of water and ink and improves Wear resistance.

Hydrophilicity of the printing plate can be achieved through actual printing tests, adjusting the ink-water balance, and the wetting characteristics of the printing plate can be measured by testing the print ink balance tolerance.

Fourth, other indicators

In addition to the above indicators, there are some requirements for the CTP plate. Such as bottom ash, sensitivity, size specifications, coating uniformity, resistance to force and so on.

Bottom ash is caused by improper processing of aluminum bases in production, and bottom ash is most likely to appear on the top and bottom of aluminum base production. Generally, the bottom ash can be removed by reasonably controlling the time of development and exposure, but the removal of bottom ash should be performed without affecting the quality of the graphic part. If the bottom ash is removed, it is necessary to explain the non-compliance with the loss of image quality. Claim.

Sensitivity refers to the requirements of the CTP plate to expose the light source. This index is particularly important for thermal CTP plates because the laser source of computer-to-plate machines 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 higher-sensitivity plates requires the use of higher power lasers to accelerate the laser's aging. At the same time, due to the high energy requirements of the exposure of the plate, 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 CTP plates. The appropriate adjustments can be made with reference to the relevant requirements of the PS version of the national standard. The resistance force of India is usually different according to the type of the plate material. In general, the more dense the grain is, the higher the printing plate resistance is, and the influence of the printing operation on the resistance force is also very large. It should be determined according to the actual situation.

Some of the above indicators are items that are very concerned about printing and can be easily measured. They are more instructive for the printing process. Only by controlling the above aspects in the choice of media can the printing quality be guaranteed.