During the offset printing process, reddish, yellowish, or blueish phenomena often do not match the color of the printed matter and the proofing sample, which may affect the quality of the product and seriously cause the entire batch of products to be scrapped. This article comprehensively analyzes the impact on the ink color of the printed matter from the production process of prepress platemaking, printing materials, printing color sequence, and the corresponding control measures to achieve the purpose of preventing the color deviation of the printed matter and accurately reproducing the colors and levels.
1. The problem of color shift during pre-press plate making
1. Color correction
Color correction is to reproduce the colors reflected in the original. In the operation process, it is mainly to correct the color deviation caused by the original or scanning, to ensure the color of the main body part is accurate, and to ensure the accuracy of the basic color. When adjusting the color, it is necessary to ensure that the screen effect matches the printing effect of this unit. When calibrating the color, you should pay attention to the specific parameter values ​​of the picture instead of displaying it on the screen. When judging the color shift of the scanned digital image, the concept of gray balance should be used. If you know the primary color components required to generate neutral gray with various brightness, you can use the neutral gray area in the original Perform color correction. Use the Screen Density Tool (Info) in Photoshop to measure the color value in the digital image. If the area should be neutral gray, but the value is not the gray balance value, it means that the image has a color shift. According to the gray balance ratio, It is easy to determine which color is more and which color is less. The highlight area in the image is the best area to check the neutral gray. The highlight area is not all neutral gray, but compared with other brightness colors, the gray component is more, so from here, you can judge the color Partial. In addition, there are many other experiences in judging the color deviation of colors, such as memory colors, such as blue sky and white clouds, grass and green land, etc. These colors have a deep memory in the human mind. For professional image processing personnel, more It is important to remember the CMYK ratio of these colors, the more you remember, the better you can judge the accuracy of the image color.
2. Network cable angle
In the process of plate making, if the arrangement of the screen angle is not correct, it will affect the printing color, and in severe cases, moire will also be produced. The current four-color plate-making printing is mainly based on the three primary colors of yellow, magenta, and cyan, expressing ever-changing colors. The black version only emphasizes the dark tone and plays the role of a skeleton. In terms of color rendering performance: among the four colors of yellow, magenta, cyan and black, yellow has the weakest visual sense, followed by magenta and cyan, and black has the strongest sense. Since the person's line of sight is particularly sensitive to the 45 ° angle, it feels poor for 0 ° or 90 ° dots, and generally feels for 15 ° or 75 ° dots. Therefore, when making a plate, the screen angle of each color plate should be determined according to the main color of the original. Therefore, the main color printing plate of the original should be made 45 °, and other colors should be made 15 °, 75 °, 90 °. For monochromatic mesh tone prints, we should choose a 45-degree dot. However, for multi-color mesh-adjusted printed products, the dot angle of each color version must be staggered. Arranging the screen angle according to the above method can effectively prevent the color deviation of the printed product.
3. Filming
In the process of film production, the process includes several processes such as RIP development, exposure film exposure, and processing and inspection. Controlling this process with data and standardization is an important means of ensuring quality. The digital control of film output has two aspects: one is to ensure the accuracy of the output dots, and the dot error should be controlled within 2%; the second is the bottom ash after the output of the film, the maximum density value meets the requirements, and the bottom ash Do <0.04, the maximum Density Dmax≥3.0. This requires multiple tests on the exposure and processing of the film to find a set of accurate system parameters, and to adjust different batches of raw materials at different times. For different films, through tests and multiple linear adjustments to make the films meet the above requirements. The impact of this process on quality mainly includes: ①The text becomes thick and fat due to the poor linearization of the output film, the printing effect is too black, the image color deviation and the poor reproduction of the gradation, and the color block color deviation. ②The bottom of the film is too large and the density is insufficient due to the exposure of the film and improper processing conditions. It is reflected in the layout that the layout is dirty, the field is false, the low-key dot level is lost, the net is dead, and the net is pasted. ③ The picture is pasted, extinct, and the picture layer performance is poor due to the high or low screen mesh. ④Due to the improper screen angle, the moire and color performance are poor. ⑤ Due to the low positioning accuracy of the film, the overprint error and the rule line are too thick, which leads to the imposition error. ⑥ The effect of different layouts that appear inconsistently before and after the phototypesetting parameters is different. ⑦ Scribing, horseshoe marks, dirty marks, etc. due to improper washing and inspection operations are directly reflected on the layout as scratches, virtual dots, dirty layouts, etc.
4. Exposure
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 are not exposed, it means that the printing plate is light. Use the same method to identify 97% dark spots on the same part 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 area of ​​the blank area between the dots of this part is enlarged, it means that the exposure is light and should be re-dried. Of course, Brunel signal bar can also be used to control the quality of printing, which 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 deviation caused by printed materials
1. Ink
The three primary color inks have different degrees of color shift, which will have different degrees of color shift impact on the entire picture. However, a small amount of color shift can be corrected from the aspect of ink formulation. When printing, it is necessary to grasp the gray balance of the ink used. Equal amounts of yellow, magenta, and cyan do not produce neutral gray. They produce a lighter, more turbid gray with brown, rather than real gray. The reason is due to the undesirable absorption of colored light by the ink described above (the spectral curve of the ink is not ideal), which is caused by the impureness of the ink used. In the CMYK space of the actual ink, to obtain a solid gray, it is necessary to increase the green The amount of ink. Excess cyan makes the remaining two colors cleaner, for example, mixing 30% cyan, 21% magenta, and 21% yellow produces 30% neutral gray, if it is 30% cyan, 30% magenta, and 30% yellow The neutral gray produced will have a dark brown color. For a certain type of ink product, the CMY value of the gray produced by mixing is constant, that is, the gray balance ratio of a certain ink is constant, so that after we measure the gray balance of this ink, we can use this balance to Correct the image so that the image to be printed can accurately reproduce the color appearance after correction, thereby making up for the defects of this ink.
2. The paper itself
The smoothness, absorbency, whiteness, and glossiness of the paper have a greater impact on the tone reproducibility and color reproducibility of the printed matter.
Smoothness is divided into apparent smoothness and printing smoothness. Apparent smoothness refers to the smoothness of the paper surface, which depends on the surface morphology of the paper and describes the structural characteristics of the paper surface. Paper with high apparent smoothness is conducive to ink coverage and leveling, the resulting ink film is shiny, and the tone reproduction of the printed matter is good. The apparent smoothness of paper has a certain relationship with its glossiness. The glossiness of paper with high apparent smoothness is also relatively high. Printing smoothness refers to the flatness of paper under printing pressure in a printing mode that uses pressure to achieve ink transfer. Printing smoothness is the combined effect of the apparent smoothness of the paper and the surface compressibility, which has a direct impact on the quality of the printed matter. It determines the degree of contact between the surface of the paper and the surface of the printing plate or blanket at the moment of imprinting The important factor is whether the ink transfer is comprehensive and the graphics are clear.
Paper absorbency refers to the paper's acceptance of ink and the absorption capacity of the binder in the ink, which directly affects the penetration of ink into the paper and the film formation. Many printing failures are caused by the paper's ability to absorb ink and the printing conditions used are not compatible. If the paper absorbs ink too much, it will cause matt imprinting, and even produce through-printing and powdering. If the paper's ability to absorb ink is too small, the speed of ink fixation will be reduced, which will easily cause the back side to smear. It can be seen that the absorption of ink by paper is also an important factor affecting the quality of printing. The absorbency of paper can be measured by the IGT printing suitability tester.
The whiteness of the paper refers to the amount of light that the surface of the paper diffuses evenly to the visible light. The whiteness of the paper will directly affect the contrast of the printed matter and the reproducibility of the image color. Paper with high whiteness can reduce color shift and expand the color gamut of the three primary colors. The measurement of paper whiteness refers to the measurement of the paper's ability to reflect blue light. Therefore, strictly speaking, the paper whiteness should be called "brightness", not whiteness.
Glossiness is the degree to which the specular reflection ability of the paper is close to the complete specular reflection ability. The glossiness of the paper is directly related to the glossiness of the printed matter. The gloss of the paper is directly related to the inking rate of the paper. Compared with the paper with low gloss, the paper with high gloss can obtain higher printing density when printing the same ink film thickness.
Third, the color shift problem caused by the printing color sequence
In color printing, how to reasonably arrange the color overprinting sequence is an important subject studied by process technicians. The printing inks of the three primary colors of yellow, magenta, and cyan have certain gray components, and some have more or less color cast. Choosing a reasonable printing color sequence can make up for these deficiencies to a certain extent and play a role in correcting color deviation. The scientific arrangement of color sequence will make the color of the printed matter closer to the original, and even strengthen a certain color atmosphere to make the level clear, the dots clear, the overprint accurate, and the colors true, natural and coordinated. The principle of arranging the color sequence is to arrange the color sequence according to the content and characteristics of the original. Printing is a means to express the graphic design of the layout, and the emotional components of each color are also different. Therefore, the arrangement of the color sequence must first consider the content and characteristics of the original. When designing graphic layouts, editors often choose the tone of the layout color. This tone represents the overall feeling of the entire color tone and becomes the dominant color in printing. In terms of chromaticity, those based on red, orange, and yellow are called warm colors; those based on green and blue are called cool colors. Due to the hiding power of ink, black and cyan are printed first mainly in warm tone, and magenta and yellow are printed later in cold tone; Everyone hopes that the ripened watermelon crumbs are red, and they don't want to see the prints appear purple or green.
In short, in the production process of offset printing, there are many factors that cause the color difference of the printed product. It is necessary to analyze the various influencing factors and corresponding control measures, and use the density control method and the chromaticity control method to control the entire production process in order to ensure The color of the print.
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