Analysis of the Causes of Color Pitch and Color Mismatching in Offset Printing

During the offset printing process, reddish, yellowish or partial blue color that does not correspond to the color of the printed product and the proof sample often occurs. After the pattern screen is overprinted, color mixing fails. Light affects the quality of the product, and the entire product is severely scrapped. The company suffered serious economic losses. This article combines this experience with the experience of many years in the offset printing process and summarizes its own opinions in order to discuss it with peers.
The color shift and color mixing failures of printed products can be roughly classified into four reasons:

(1) The printing plate of yellow, magenta and cyan caused by non-standard plate making operation is too dark or too shallow;

(2) Improper ink color matching or three primary inks do not match;

(3) Arrangement of printing color sequence is irrational;

(4) There is a serious color shift in the printed paper itself.

1, color deviation caused by the depth of printing plate network

In the printing process of the printing plate, the strength of the printing plate light source, spectrum does not match the photosensitive properties of the printing plate, or the exposure time is not proper; the pH of the developer and the length of the development time are not properly grasped, etc., which may cause the printing plate spot to be too deep or Too shallow. Printing on this type of printing plate, the color shift of the picture is inevitable.

Therefore, the quality of the printing plate must be identified before it is on the machine. The identification of the printing plate outlets can be controlled by yellow, magenta, cyan, and black original plates, respectively, with the help of a magnifying glass. Carefully look at the bright spots of the corresponding parts of the plate. The appearance of regional outlets in the tone, middle tone, and shadow tone areas is 2% of bright spots. The printing plate should be able to produce the same amount of sunshine. 3% of the printed materials should be printed. If 2% of the small dots on the printing plate do not get out of the sun, it means that the printing plate is light and not applicable. With the method of discriminating the printing plate and the 2% small spot of the bright spot of the film, identify the dark spots of 97% of the dots in the same part of the plate on the printing plate. If this part of the printing plate is not fused, it indicates that the printing plate is basking in depth; if the area of ​​the blank area between the sites is enlarged, it means that the printing plate is basking in light and it should be re-baked.

Of course, using the Bruner signal bar to control the quality of prints can be accurate and easy to operate under conditions. The identification method is to use the high magnification magnifying glass to observe the 12 yin and yang network points in the 50% fine mesh test block of the 5th block of the Brunel signal strip on the printing plate, and the yin and yang dot area should be sorted out from the largest to the smallest. Qi 6; 6th small grid 2% of the small grid to be out of the sun, if the drying out of the plate 3% of the existence of the network, and 2% of the dot disappeared, indicating that the printing plate sun. Conversely, not only 2% of outlets exist, but also 1% of small outlets exist, indicating that the printing plate is too deep.

2. Color shift caused by ink problem

During the printing process, the color shift caused by the ink is mostly due to the fact that existing three-color inks produced in various places have different degrees of color shift. For example, Peony Card 05-24 Yellow (reddish 0.16), 05-32 Sky Blue (Reddish 0.35), this will have a color cast effect on the whole picture to varying degrees, especially the influence of midtones is more serious. However, a small amount of color shift can be corrected in terms of ink compatibility. For example, a group of three primary inks that have been used in East China for a long time, such as 05-28 Clear 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 yellow ink has fast drying, fast fixing, colorful, less color, strong hydrophobicity, good printability and other characteristics, high-speed, multi-color machine for coated paper printing fine products play a good role .

3, color shift caused by the arrangement of color order

Comparing offset printing with proofing, sometimes the prints are still yellow, reddish or bluish, or the color of the overprinting ink is mixed even though the density of ink, paper, and field are the same.

What is the reason for this? First, the proofing machine is generally monochrome wet-dry printing, ink adsorption, transfer rate is more appropriate; Second, the machine structure, pressure, speed is not the same as the actual printing; Third, high-speed multi-color printing when the color sequence is not reasonable. The author believes that the arrangement of the color sequence is not only unreasonable, it will not only cause color misalignment, but also make the color of the printed product pattern caused by the wet pressure overprinting caused the result of color mixing.

The principle that should be followed in the arrangement of multi-color printing:

(1) According to the four-color version of the ink brightness difference order arrangement, namely dark dark color first printing, bright light color after printing.

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

(3) According to the order of the viscosity of the ink to arrange, it is the first large-scale color ink printing, taught a small color ink after printing. Such as the Shanghai Mudan Brand 05 black ink 1188.2Pa, blue ink 488.5Pa> magenta ink 477.1Pa> yellow ink 147Pa the sequence. For inks with large viscosity, they must be placed on the printer of the post-color group. The viscosity of the ink must be reduced appropriately. This is of vital importance. Otherwise, the color misregistration of the reverse printing will be caused.

(4) According to the color ink's transparency and coverage arrangement, that is, low transparency and high coverage of the color ink first team high transparency, coverage of the small color ink after printing, this can improve the ink after the overprint of the color effect.

(5) Arrange the color sequence with the characteristics of the manuscript. The monochromator is a wet and dry embossing method. The color sequence of yellow, magenta, cyan, and black is often used, or the sequence of yellow, cyan, magenta, and black is used. This is based on overprinting between multi-color sheetfed paper and color, which can overprint the next color at least several hours apart, which may cause the paper to expand or contract. From the perspective of facilitating visual overprinting requirements, the first color is the first to print the weak yellow ink. Even if the printed sheet is stretched, the effect of the limited error generated by the subsequent color sequential, magenta, and black printing is not obvious. Two-color printing machine is to adopt alternate color sequence of light and dark ink, that is, yellow, magenta and cyan, black sequence, or yellow, cyan and magenta, black color sequence. 4-color machine printing uses black, cyan, magenta, yellow or black, magenta, cyan, and yellow.

In short, as long as offset press operators can seriously and dialectically understand the main regular causes of color misalignment and color mixing failures, and take appropriate preventive measures in a targeted manner, they will be able to take preventive measures and receive more effective results.