Pre-coated photosensitive lithography (also known as PS plate) has been applied since the 1950s. With its high resolution, smooth dots, and high printing resistance, it has gradually replaced protein plates, multilayer metal plates and other plate materials, becoming a modern fast printing New plate materials. At present, under normal circumstances, the PS printing durability is generally around 100,000 prints. The baked PS printing durability can be increased by 3-5 times, reaching 300,000 to 500,000 prints, but many manufacturers now reflect PS The printing resistance of the edition is not high, generally only 210,000 to 30,000 printings, and even only a few thousands of copies are printed. Here, based on my personal experience, according to the production process of the PS version, I will make a rough discussion and analysis of the plate material production and plate making process of the PS version.
1. Plate selection
The base of the PS version is aluminum. The aluminum material used for the PS version belongs to general industrial grade pure aluminum with a thickness of 0.3mm and a content of 99.5%. The main impurities are iron and silicon, followed by copper, magnesium, zinc, manganese, titanium and so on. The presence of a small amount of impurities in the aluminum plate can improve the tensile strength and hardness of pure aluminum, reduce elongation, and reduce the electrical conductivity of aluminum.
2. Composition and coating of sensitizer
The sensitizer for PS plate is composed of photosensitive material, film-forming polymer resin, dye and organic solvent. For the photosensitive layer coated with a photosensitizer alone, it has strong alkali resistance, poor film-forming properties, and difficult development; add an appropriate amount of phenolic resin to the photosensitizer. Although the alkali resistance is poor, the film formation and adsorption are improved. The ratio of the two is 0.5: 1 or 0.62: 1. If there is less photosensitizer and more film-forming resin, the alkali resistance decreases and the dots are easy to lose , The residual film rate is low, directly reducing the printing plate durability.
The coating of PS plate sensitizer needs a special coating machine to ensure the uniform thickness of the photosensitive layer after coating. Coaters are generally divided into two types: single sheet version and reel version. In the coating method, there are spreading type, roller coating type, extrusion type and electrostatic coating type. We do not advocate the use of centrifugal coating. The roller coating type is commonly used in China. In order to ensure the coating quality, the solid content of the negative photosensitive layer is generally about 1g / m2. The positive pattern is 2-2.5g / m2, and the thickness is generally (20 ± 1.0) mg / dm2. When the coating has horizontal stripes and other failures, it should be eliminated in time.
Third, PS plate making
The key of the PS plate making is to maximize the reproduction of the dots and tone of the film, and form a firm lipophilic base and a stable hydrophilic base on the surface of the printing plate. Otherwise, it not only wastes time, but more seriously reduces the printing plate's durability and wastes plate materials. A series of factors that directly affect the quality of plate-making include the original plate-making. The reproducibility of the PS version of the outlets, plate-making equipment and conditions. Plate-making process and operation, especially exposure and developing conditions.
1. Copy the original
The original version of the positive or negative picture of the PS version can be made by traditional photo-engraving, electronic color separation and phototypesetting. In addition to the tone reproduction curve should be adjusted, we must also pay attention to the original density contrast and dot finish. Because the Y value of the original is too low, exposure, insufficient development or excessive thinning and other reasons. It is easy to form soft dots with low density or active dots with high intermediate density and small edge density. The printing stability of these two types of outlets is poor, and slight changes in exposure and development will make the printing plates have different shades, which will directly affect the printing durability of the printing plates. The original version of the hard dots, due to the high density and smooth dots, has greater latitude in exposure and development. So the quality is stable. Therefore, the original handwriting pattern for printing must be clear and dense, with a large contrast. Take Huaguang YZ-600 11 film as an example. The maximum density of the original version should be above 4.0. The minimum density is below 0.05, the contrast coefficient is above 8.0, and the layout is cleaned to reduce fog and collage.
2. Exposure light source For exposure light source. Because the light sensitivity of the sensitizer is a redox reaction. Only if the photosensitizer fully absorbs T enough light energy can occur, and the degree of its decomposition, cross-linking, combination is proportional to the required light intensity and light time, therefore, the emission spectrum of the printing light source and the photosensitizer photosensitive curve The sensitive area should be matched as closely as possible. The experiment proves that the spectroscopic sensitivity of the positive-type photosensitive thorns is within 300nm-460nm, and the drawing type is within 200nm-500nm. Consider the effects of the original printing plate and the printing glass. Spectral design is more suitable in the region of 350nm-460nm. Therefore, the ideal light source for the PS version is the iodine gallium lamp.
3. The exposure is in the normal plate-making process. The amount of exposure is the most important factor that affects the change of the dot and the depth of the layout. We know that the exposure of the printing plate = illuminance × printing time. Because the commonly used PS version of the positive image type is a photodecomposition type. Therefore, when the exposure exceeds the critical light value, that is, overexposure, the dots will shrink, and fine strokes and small dots are difficult to reproduce; when the exposure is less than the critical light value, that is, underexposure. The outlets will be enlarged and thickened accordingly. Dirty the layout, affecting the depth of the layout. Therefore, in the process of printing, the control of the exposure amount. Continuous adjustment of ladders or Brunel test strips should be used for monitoring. Its function is to check whether the exposure is appropriate after development. Generally, the proofing plate is controlled at level 3-4 (density 0.4-0.55), and the printing plate is controlled at level 4.5-5.5 (density 0.625-0.775). When printing a positive plate, if 0.5% of small ideas and 99.5% of hollow ideas can be paired out, such a printing plate is considered qualified.
4. Development
The development of the PS plate is to remove the photosensitive layer of the blank part, so that the surface of the PS plate base remains hydrophilic. It is an important link in the plate making process of the PS plate. It is not only restricted by the nature of the photosensitizer and the state of the original plate, but also affected by the development conditions such as the concentration of the developer, temperature, fatigue and development time.
(1) The concentration affects the concentration of the developer. Generally refers to the content of developing main agent. Insufficient developer concentration will not only increase the development time, but also cause incomplete development. Causes the plate surface to rise; if the concentration is too large, the development time is accelerated, and the entire process is difficult to control. It is easy to cause excessive development, shrink or damage the dots, dissolve the unexposed photosensitive layer, and reduce the grease sensitivity and durability of the printing plate. Therefore, in order to accurately control the density, after the plate is developed for 30-60s, observe whether the 98% -99% hollow dots of the Bruner test strip are clear and the 1% -2% small black dots are firm. If the idea is clear and strong, the concentration is appropriate.
(2) The effect of temperature is relatively stable under other development conditions. The higher the development temperature, the faster the development speed, and vice versa. Generally, the developing temperature should be controlled at about 20 ℃. The automatic developing machine should be controlled at 25 ° C. If the temperature exceeds 30 ° C, the dots will dissolve. The binding force between the molecules of the photosensitive material and the film-forming material in the photosensitive layer is weakened, the alkali resistance is reduced, the percentage of the dot area of ​​the plate is reduced, and the reproduction of the tone of the printing plate is destroyed.
(3) The degree of fatigue of the developer affects the fatigue of the developer means that the developing ability of the developer decreases. If the developer is developed by manual development, the developer is exposed to the air, and the absorbed moisture and CO2 accelerate the decline of the developer and affect the development performance. Another cause of developer fatigue is that the photo-decomposed product dissolves in it, significantly reducing the developing ability. Therefore, the currently used alkaline developer should minimize contact with air during storage and use, reduce the impact of moisture and CO2 on it, and periodically replace and quantify the dosage.
(4) Influence of development time Because the development time is affected by factors such as exposure to exposure, performance of the sensitizer, development method, and state of the developer, so to control the degree of development, you should first grasp and stabilize the above volatile factors, and then The gray ladder ruler is used for trial display to determine the development conditions and the correct development time. Generally, the development time is about 1 minute.
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4. After the baking plate is baked, the photosensitive group in the photosensitive layer disappears, and the phenolic resin undergoes a thermal cross-linking reaction when heated, changing from linear molecules to large network molecules. At this time, the solvent resistance, corrosion resistance, abrasion resistance of the printing plate and the adsorption of the photosensitive layer to the plate surface are greatly improved. However, during baking, in addition to the change in the photosensitive layer in the picture and text, the micropores closed by the sealing treatment on the surface of the anodized aluminum plate may increase the water flow due to water loss, causing the plate surface to become dirty. Therefore, before baking the plate, it is necessary to apply a layer of protective agent to the plate surface uniformly with absorbent cotton to protect the blank part from being dirty. The general coating amount is 20ml-30ml for one pair of openings. The baking plate temperature is controlled at 230 ℃ -250 ℃. The time is 5-8 minutes. It must be noted that the special baking machine used for baking plates must have a constant temperature control device, the temperature difference range is between ± 2 ℃, too high baking temperature or too long time will make the photosensitive layer carbonized and deformed, aluminum plate deformation and mechanical strength The reduction will affect the printing plate durability and the life of the plate base; the temperature is too low or the time is too short, because the photosensitive layer is not cured enough by heat, it will also affect the printing resistance. After baking, it must be cooled naturally, and it is not advisable to use water for forced cooling to avoid deformation of the base. After cooling, the protective agent is washed away with waste developer, and then neutralized with 3% phosphoric acid, and washed with water to dry. If it is not used for a while, it should be coated with gum arabic for protection.
The production and drying of the PS version play a decisive role in improving the printing durability of the printing plate. When the printing process is operated in accordance with data, standardization and standardization, the production and printing of printing plates are particularly important. therefore. We only have to deal with the above problems carefully, eliminate external and human factors in the production and printing process, and vigorously promote data, standardization, and standardized printing to ensure the quality of the plate making to the greatest extent and improve the printing plate durability. Put it into practice.
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