Color deviation and color mixing in the most popul

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Color deviation and color mixing in offset printing process during offset printing process, there are often red, yellow or blue color deviation between printed products and proofed samples, or color mixing failure after pattern and picture overprinting, which will affect product quality in light cases, or cause the whole batch of products to be scrapped in serious cases, causing serious economic losses to the enterprise. This paper summarizes and sorts out my own opinions on this issue in combination with my own experience in offset printing process for many years, with a view to discussing with my peers

the causes of color deviation and color mixing can be summarized into four aspects: (1) the printing plate spots of yellow, magenta and cyan are too deep or too shallow due to nonstandard plate making operation; (2) Improper ink color allocation or mismatching of three primary color inks; (3) Unreasonable arrangement of printing color sequence; (4) The printing paper itself has serious color deviation

1. color deviation caused by the depth fault of the printing plate point

during the printing plate drying process, the intensity and spectrum of the printing plate light source do not match the photosensitive characteristics of the printing plate photosensitive adhesive, or the exposure time is improper; Improper control of the pH value of the developing solution and the length of the developing time may cause too deep or too shallow printing plate spots. Such a printing plate on the machine printing, the picture color deviation is inevitable

therefore, the quality of the printing plate must be identified before it is put into the machine. The identification of the printing plate points can be made by comparing the yellow, magenta, cyan and black original films with the help of a magnifying glass, and carefully observing the appearance of the bright, middle note and dark spots in the same part of the corresponding printing plate pattern, that is, the small spots at 2% of the bright spots can be equally exposed on the printing plate, and 3% of the spots on the printing product can be printed. If 2% of the dots on the printing plate cannot be exposed to the sun, it means that the printing plate is shallow and not applicable. The method of identifying 2% dots on the same bright spot of the same film on the printing plate is used to identify 97% dots on the same dark spot of the same film on the printing plate. If this part of the printing plate is not pasted, it means that the printing plate is deeply sunned; If the area of the blank area between the points in this part is expanded, it means that the printing plate is shallower, so it should be re dried

of course, if conditions permit, using Brunel signal bar to control the printing quality can be accurate and easy to operate. The identification method is to use a high-power magnifying glass to observe 12 Yin and Yang points in the 5th 50% fine test block of the Bruner signal strip on the printing plate, and the area of the Yin and Yang points should be 6 in order from large to small; The 6 single-layer steel sheet stamping parts, die-casting aluminum alloy parts, (steel plate/plastic/steel plate) sandwich composite structural parts, thermosetting plastic (SMC) molded parts and thermoplastic (nylon) injection molded parts of block 6 need to be exposed to the sun. If 3% of the exposed plates exist and 2% of the points disappear, it indicates that the plates are shallow. On the contrary, if not only 2% of the dots but also 1% of the dots exist, it indicates that the printing plate is too deep

2. color deviation caused by ink problems

in the printing process, the urbanization and sustainability of oil also affect the color deviation caused by ink. Most of them are because the existing three primary color inks produced in various places have different degrees of color deviation, such as peony medium yellow (red 0.16) and 05-32 sky blue (red 0.35), which will have different degrees of color deviation on the whole picture, especially the middle tone. However, a small amount of color deviation can be corrected from the aspect of ink compatibility. For example, a group of three primary color inks selected in East China for a long time, such as transparent yellow, magenta and sky blue produced by Shanghai ink factory and gs-135 magenta, gs-425 sky blue and gs-236 medium yellow inks produced by Tianjin ink factory, have the characteristics of dry maintenance, fast fixation, bright color, less color deviation, strong hydrophobicity, good printing adaptability, etc. for high-speed The multicolor machine has played a good role in printing fine products with coated paper

3. color deviation caused by color sequence arrangement

compared with the proofing sheet, sometimes although the ink, paper and field density used are the same, the printed matter is still yellow, red or blue, or the color mixing phenomenon of overprint ink occurs. What is the reason? First, the proofing machine is generally monochrome wet press dry printing, and the ink adsorption and transfer rate are appropriate; Second, the machine structure, pressure and speed are different from the actual printing; Third, the arrangement of color sequence in high-speed multi-color printing is unreasonable. The author believes that the unreasonable color sequence will not only cause the color deviation fault, but also cause the printing pattern color to be mixed due to wet press and wet overprint

principles to be followed for color sequence arrangement of multi-color printing machine:

(1) arrange according to the order of brightness difference of 4-color printing ink, that is, dark color is printed first, bright light color is printed later

(2) arrange according to the graphic area of the printing plate. That is, the printing plate with small graphic area shall be printed first, and the printing plate with large graphic area shall be printed later

(3) arrange according to the order of ink viscosity, that is, the color ink with large connection degree is printed first, and the color ink with small teaching degree is printed later. For example, Shanghai Peony brand 05 black ink is 1188.2pa, blue ink is 488.5pa> magenta ink is 477.1pa> yellow ink is 147pa. For the color ink with high viscosity that must be placed in the back color deck, it is very important that the viscosity of the ink should be properly reduced first, otherwise it will cause the color mixing problem of reverse overprint

(4) according to the arrangement of the transparency and coverage of the color ink, that is, the color ink with low transparency and high coverage is printed after the color ink with high transparency and low coverage, which can improve the color rendering effect after ink overprint

(5) the color sequence is arranged according to the characteristics of the original. The monochrome machine is a wet and dry stamping method. It usually adopts the color sequence of yellow, magenta, cyan and black, or the color sequence of yellow, cyan, magenta and black. This is based on the overprint between the colors of the multi-color sheet, and the next color can be overprinted at least several hours, which may cause the paper to stretch. From the perspective of visual requirements for overprint, the first color is printed with weak yellow ink first. Even if the printing sheet is expanded, the limited error effect on the overprint of the later color sequence blue, magenta and black is not obvious. The two-color machine printing usually adopts the alternate color sequence of light and dark ink, that is, yellow, magenta, cyan and black sequence, or yellow, cyan, magenta and black sequence. 4-color machine printing mostly adopts the reverse color sequence of black, cyan, magenta, yellow or black, magenta, cyan and yellow

in a word, as long as offset printing operators can seriously and dialectically understand the main regular causes of color deviation and color mixing faults, and take corresponding preventive measures with a clear aim, they can prevent the trouble before it happens and get twice the result with half the effort

however, these disadvantages can be overcome by copolymerization with other monomers

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