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 version's print durability is generally around 100,000 prints. The baked PS print resistance can be increased by 3-5 times, reaching 300,000-500,000 prints, but now there are many manufacturers It reflects that the PS version has low printing resistance, generally only 20,000-30,000 prints, and even only a few thousand 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. The thickness is generally 0.3mm and the content is 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 the 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, not only a waste of time, but more serious is to reduce the printing plate resistance, wasting plate material.
A series of factors that directly affect the quality of plate-making include the plate-making original version. PS version of the dot reproducibility, plate-making equipment and conditions. Plate-making process and operation, especially exposure and developing conditions.
1. The original version
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 hard dots has stable quality because of its high density, smooth dots, and greater latitude in exposure and development.
Therefore, the original handwriting pattern for printing must be clear and dense, with a large contrast. Take Huaguang YZ-600 11 type 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 printing 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 pattern photothorn 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 350nm-460nm region. Therefore, the ideal light source for the PS version is the iodine gallium lamp.
3. Exposure
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 printing process, for the control of the exposure amount, continuous adjustment of the ladder or Brunel test strip should be used for monitoring. Its function is to check whether the exposure amount 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 are paired out, such a printing plate is considered qualified.
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