During the offset printing process, the reddish, yellowish or blueish phenomenon that the color of the printed product and the proof sample does not match often occurs, or the color mixing failure after overprinting of the pattern screen, which affects the quality of the product, and will seriously make the entire batch of products scrap , Causing severe economic losses to the enterprise. This article combines this experience with my experience in the offset printing process for many years, summarizes and sorts out my own opinions, in order to discuss with colleagues.

The color deviation and color mixing failure of the printed product can be summarized into four reasons:

(1) The printing dots of a certain color of yellow, magenta and cyan are too dark or too light caused by the irregular plate-making operation;

(2) Improper color adjustment of ink or mismatch of three primary colors ink;

(3) The arrangement of printing color sequence is unreasonable;

(4) The printing paper itself has a serious color shift phenomenon.

1. The color deviation caused by the failure of the printing plate dot depth

During the printing process of the printing plate, the intensity and spectrum of the printing light source do not match the photosensitive characteristics of the printing plate photoresist, or the exposure time is inappropriate; the pH value of the developer and the length of the development time are not properly controlled, etc. Too shallow. Such printing plate printing on the machine, the color cast of the picture is inevitable.

Therefore, the quality of the printing plate must be identified before the machine is used. The identification of the printing plate dots can be compared with the original films of yellow, magenta, cyan, and black by means of a magnifying glass, and carefully check the brightness of the same part of the corresponding printing plate pattern. The appearance of dots in the tone, middle tone and dark tone areas, that is, the small dots of 2% in the bright tone area should be equally exposed on the printing plate, and 3% of the dots on the printed product should be able to be printed. If 2% of the small dots on the printing plate cannot be exposed, it means that the printing plate is light and not applicable. Using the method of identifying 2% small dots on the bright tone of the printing plate and the film, identify 97% of the dark dots on the same portion of the printing plate as the original film. If the dots on this part of the printing plate are mixed, it means that the printing plate is sunburned; if the blank area between the dots of this part is enlarged, it means that the printing plate is light and should be reprinted.

Of course, under conditional conditions, the use of Brunel signal strips to control the quality of printing can be accurate and easy to operate. The identification method is to observe the 12 yin and yang dots in the fifth 50% fine mesh test block of the Brunal signal strip on the printing plate with the help of a high magnification magnifier, and the yin and yang dot areas should be in order from large to small. There are 6 of them; 2% of the small dots in the 6 small blocks of the sixth block should be exposed. If 3% of the dots of the printed plate exist, and 2% of the dots disappear, it means that the printing plate is light. On the contrary, not only 2% of the dots exist, but also 1% of the small dots exist, which means that the printing plate is too deep.

2. Color shift caused by ink problem

During the printing process, the color shift failure caused by the ink is mostly due to the existence of different color shifts in the existing three primary color inks, such as Peony 05-24 medium yellow (reddish 0.16), 05 ~ 32 sky blue (Reddish 0.35), this will have a color cast effect to the entire picture to varying degrees, especially the effect of the middle tone is more serious. But a small amount of color shift can be corrected from the ink compatibility. For example, a set of three primary color inks used in East China for a long time, such as 05-28 transparent yellow, 05-14 magenta, 05-32 sky blue produced by Shanghai Ink Factory and GS-135 magenta, GS-425 sky blue, GS produced by Tianjin Ink Factory -236 medium yellow inks have the characteristics of fast drying, fast fixing, bright colors, less color, strong hydrophobicity, good printability, etc., which play a good role in the high-speed, multi-color machine used for printing fine products on coating paper .

3. Color shift caused by color sequence arrangement

Comparing offset printings with proofing proofs, although the ink, paper and solid density used are the same, there are still yellowish, reddish or bluish or mixed colors of overprinted inks.

What is the reason? First, the proofing machine is generally monochromatic wet press dry printing, and the ink absorption and transfer rate are more suitable; second, the machine structure, pressure, and speed are not the same as the actual printing; third, the color sequence arrangement is not reasonable when high-speed multi-color printing. The author believes that the unreasonable color sequence arrangement will not only cause color deviation failure, but also cause the color mixing of the printed product pattern due to wet pressing and wet overprinting.

Principles to be followed when arranging the printing sequence of multi-color machines:

(1) Arranged according to the order of difference in the brightness of the 4-color ink, that is, dark and dark colors are printed first, and light and light colors are printed later.

(2) Arrange according to the graphic area of ​​the printing plate. That is, the printed image with a small area is printed first, and the printed area with a large area is printed later.

(3) Arranged according to the order of ink viscosity, that is, the color ink with large degree is printed first, and the color ink with small degree is printed after. For example, Shanghai Peony brand 05 type black ink 1188.2Pa, blue ink 488.5Pa> magenta ink 477.1Pa> yellow ink 147Pa are arranged in this order. For those inks with large viscosity that must be placed in the back color group, the viscosity should be lowered properly. This is very important, otherwise it will cause the color mixing disadvantage of reverse overprinting.

(4) According to the arrangement of the transparency and coverage of the color ink, that is, the color ink with low transparency and large coverage will be printed before the color ink with high transparency and low coverage, which can improve the color rendering effect of the ink after overprinting.

(5) Arrange the color sequence according to the characteristics of the original. The monochromatic machine belongs to the wet pressing dry imprinting method. The color sequence of yellow, magenta, cyan and black is mostly used, or the color sequence of yellow, cyan, magenta and black is used. This is based on overprinting between the colors of multiple sheets of paper, and at least a few hours apart to overprint the next color, which may cause the paper to stretch. From the perspective of favoring visual requirements for overprinting, the first color is printed with a weak yellow ink first. Even if the printed sheet is stretched, it will not have a significant impact on the limited errors produced by the subsequent color sequence cyan, magenta, and black overprinting. For two-color machine printing, the color sequence of light and dark inks is used alternately, that is, yellow, magenta and cyan, and black, or yellow, cyan and magenta and black. For 4-color machine printing, the reverse order of black, cyan, magenta, yellow or black, magenta, cyan, and yellow is used.

In short, as long as the offset operator can carefully and dialectically understand the main regular causes of color misregistration and color mixing faults, and take appropriate preventive measures, he can prevent them from happening and get twice the result with half the effort.

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