Lithographic print color cast solution
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In the offset printing process, the phenomenon of reddish, yellowish or partial green which does not match the color of the proofing sample often occurs, and the lighter affects the quality of the product, which seriously causes the whole batch of products to be scrapped. This paper analyzes the influence of the printing ink color and the corresponding control measures from the pre-press plate making, printing materials, printing color sequence and other production processes to prevent the color deviation of the printed matter and accurately reproduce the color and level.
First, the color shift problem during prepress printing
1. School color
The color correction is to reproduce the color reflected by the original. In the course of operation, the main purpose is to correct the color shift caused by the original or scanning, to ensure the color of the main part is accurate, and to ensure the basic color is accurate. When adjusting the color of the school, first ensure that the screen effect is compatible with the printing effect of the unit. When you check the color, you should pay attention to the specific parameter values of the picture and not just the screen. When discriminating the color shift of the scanned digital image, the concept of gray balance is used. If the primary color component required to generate the neutral gray of various brightness is known, the neutral gray area in the original can be utilized. Perform color correction. In Photoshop, the screen density tool (Info) is used to measure the color value in the digital image. If the value is supposed to be neutral gray, the value is not the gray balance value, indicating that the image has a color shift, according to the proportion of gray balance. It's easy to tell which color is more and which color is less. The highlight area in the image is the best area for checking neutral gray. The highlight area is not all neutral gray, but the gray component is more than the other brightness colors, so check it from here to judge the color. Partial. In addition, there are many other experiences in judging the color color deviation, such as memory color, like blue sky and white clouds, grass green space, etc. These colors have deep memories in people's minds. For professional image processing personnel, It's important to remember the proportion of CMYK for these colors. The more you remember, the better you can judge the color of the image.
2. Cable angle
In the process of plate making, if the arrangement angle of the cable is not correct, it will affect the printing color, and in severe cases, it will produce moiré. At present, the four-color plate printing is mainly composed of yellow, magenta and cyan, and the performance is ever-changing. The black version only emphasizes the dark part and plays the role of skeleton. From the color rendering performance: yellow, magenta, cyan, black four colors, the yellow visual perception is the weakest, magenta, cyan followed by black, the strongest black. Since the line of sight of a person is particularly sensitive to a 45° angle, it feels poor to a dot of 0° or 90°, and feels generally at a dot angle of 15° or 75°. Therefore, the angle of the screen of each color plate should be determined according to the main color of the original when making the plate. Therefore, the main color printing plate of the original should be made 45°, and the other colors should be 15°, 75°, 90°. For monochrome mesh prints, we should use 45 degree dots. However, the multi-color mesh prints the prints, and the angles of the dots of each color plate must be staggered. According to the above method, the angle of the network cable can be arranged to effectively prevent the color shift of the printed matter.
3. Release
The filming process includes several processes such as RIP hairline, photo-exposure film exposure, and rinsing and inspection. Controlling this process with data and standardization is an important means of ensuring quality. The data control of the film output has two aspects. One is to ensure the accuracy of the output network, the dot error should be controlled within 2%; the second is the bottom ash after the film is output, the maximum density value meets the requirements, the bottom gray Do<0.04, the maximum The density Dmax ≥ 3.0. This requires a series of tests on the exposure and rinsing process of the film to find a set of accurate system parameters, and adjust the different batches of raw materials at different times. For different films, through trial and multiple linear adjustments, the film meets the above requirements. The main influences of this process on quality are as follows: (1) The text is thick and fat due to the poor linearization of the output film, the printing effect is too dark, the picture color deviation and the layer reproduction are not good, and the color deviation of the color block. 2 Due to the exposure of the film and improper washing conditions, the bottom of the film is too large and the density is insufficient. Reflected in the layout is the dirty face, the field is imaginary, the low-key network level is lost, the net, the paste network. 3 Due to the high or low number of screens, the picture is ruined, and the picture is poor. 4 The moiré and color appear poorly due to improper screening angle. 5 The overprinting error and the rule line appear too thick due to the low positioning accuracy of the film, resulting in imposition error. 6 Different layouts of the inconsistent appearance of the parameters are different. 7 Due to improper operation such as washing and inspection, scribes, horseshoe prints, and dirty marks are directly reflected on the surface of scratches, imaginary marks, and dirty surfaces.
4. Printing
During the printing process, the intensity and spectrum of the printing plate do not match the photosensitive characteristics of the printing plate, or the exposure time is improper; the pH of the developing solution and the length of the developing time are not properly controlled, etc., which may cause the printing plate to be too deep or Too light. When such a plate is printed on the machine, the color deviation of the screen is inevitable. Therefore, the printing plate must first identify its quality before it is put on the machine. The identification of the printing plate dots can be compared with the original film of yellow, magenta, cyan and black by means of a magnifying glass, and the corresponding parts of the corresponding printing plate pattern can be carefully observed. Adjusted, adjusted, and dark-adjusted regional outlets, that is, 2% of the small dots in the bright-adjusted part, the printing plate should be able to be equally exposed, and 3% of the printed dots should be printed. If 2% of the small dots on the plate are not exposed, the plate is light. In the same way, the 97% dot on the printing plate with the same darkness as the original film was identified. If the dots on the plate are pasted, the plate is sun-dried; if the area of the blank between the dots is enlarged, it means that the sun is light, and it should be re-tanned. Of course, Brunel signal strips can also be used to control the quality of the print, which is accurate and easy to operate. The identification method is to observe 12 yin and yang dots in the 5th 50% fine mesh test block of the Brunel signal strip on the printing plate by means of a high magnification magnifying glass, and the area of the yin and yang mesh points should be in the order of large to small. 6 in 6; 6% of the 6 small cells in the 6 small grid to be exposed, if the printing plate 3% of the dot exists, and 2% of the dot disappeared, indicating that the plate is light. Conversely, not only does 2% of the outlets exist, but 1% of the small outlets also exist, indicating that the plates are too deep.
Second, the color cast problem caused by printed materials
1. Ink
The three primary color inks have different degrees of color shift, which will have a color shift effect on the entire screen to varying degrees. However, a small amount of color shift can be corrected from the ink formulation. When printing, we must grasp the gray balance of the ink used. The same amount of yellow, yellow, and green do not produce neutral gray. They produce a lighter, turbid brownish gray rather than a real gray. The reason is due to the undesired absorption of the color light by the ink described above (the spectral curve of the ink is not ideal), that is, caused by the impureness of the ink used. In the CMYK space of the actual ink, in order to obtain the solid gray, it is necessary to increase the green color. The amount of ink. The excess cyan makes the other two colors cleaner, for example, 30% cyan, 2l% magenta, and 21% yellow mixed to produce 30% neutral ash, if it is 30% cyan, 30% magenta, and 30% yellow mixed The resulting neutral ash will have a dark brown color. For a certain type of ink product, the CMY value of the mixed gray color is constant, that is, the gray balance ratio value of a certain ink is constant, so that we can measure the gray balance of the ink, and then according to the balance The image is corrected so that the image to be printed can accurately reproduce the appearance of the color after correction, thereby compensating for the defect of the ink.
2. Paper itself
The influence on the tone reproducibility and color reproducibility of the print is relatively large, such as smoothness, absorbency, whiteness and glossiness of the paper.
Smoothness is divided into apparent smoothness and print smoothness. Apparent smoothness refers to the flatness of the surface of the paper, which is dependent on the surface topography of the paper and describes the structural characteristics of the surface of the paper. The paper with high apparent smoothness is advantageous for ink coverage and leveling, the formed ink film is glossy, and the tone reproduction of the printed matter is good. The apparent smoothness of paper has a certain relationship with its glossiness, and the paper with high apparent smoothness has a relatively high gloss. The print smoothness refers to the flatness exhibited by the paper under the printing pressure in the printing mode in which the ink is transferred by pressure. Printing smoothness is the combined effect of apparent smoothness and surface compressibility of paper, which has a direct impact on the quality of 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. An important factor in whether the ink transfer is comprehensive and the image is clear.
Paper absorbency refers to the acceptability of the paper to the ink and the ability to absorb the binder in the ink, which directly affects the penetration of the ink into the paper and the conjunctiva. Many printing failures are caused by the inability of the paper to absorb ink and the printing conditions used. Paper absorbs ink too much, which can cause the print to be dull and even produce offset and chalking. If the paper absorbs ink too little, the ink fixing speed is lowered, which may cause the back surface to be dirty. It can be seen that the absorption of ink by paper is also an important factor affecting the quality of printed matter. The absorbency of the paper can be measured by the IGT printability meter.
The whiteness of the paper refers to the amount of light diffused by the surface of the paper to the visible light. The whiteness of the paper will directly affect the contrast of the printed matter and the reproducibility of the color of the image. Paper with a high whiteness can reduce the color shift and enlarge the color gamut of the three primary colors. Paper whiteness measurement measures the ability of a paper to reflect against directional blue light. Therefore, strictly speaking, paper whiteness should be called "brightness" rather than whiteness.
Gloss is the closeness of the specular reflection ability of the paper to the total specular reflection ability. The gloss of the paper is directly related to the gloss of the printed matter. At the same time, there is a correlation between the gloss of the paper and the apparent smoothness of the paper. The gloss of the paper is directly related to the inking rate of the paper. Paper with a high glossiness can achieve a higher printing density when printing the same ink film thickness than paper with a low gloss.
Third, the color shift problem caused by the printing color sequence
In color printing, how to properly arrange the order of color printing is an important topic for process technicians. Yellow, magenta, and cyan primary printing inks, regardless of brand, have a certain gray component, and more or less have some color cast. Choosing a reasonable printing color sequence can compensate for these deficiencies to a certain extent and play a role in correcting the color shift. The arrangement of the color sequence is scientific, which will make the color of the printed matter closer to the original, and even enhance the atmosphere of a certain color, so that the level is clear, the dots are clear, the overprint is accurate, and the color is true, natural and harmonious. 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 of expressing layout design, and the emotional components of each color are different. Therefore, the arrangement of color sequence must first consider the original content and characteristics. When designing graphic layouts, editors often choose the tone of the layout color. This tone is the overall feeling of the whole color, and becomes the dominant color in printing. In terms of color, red, orange, and yellow are called warm colors; green and blue are called cool colors. Because of the hiding power of the ink, the first is mainly black and green, and the later printed magenta and yellow are mainly warm-toned; the first printed magenta is mainly cold-pressed, and the cyan is printed later. Anyone who wants ripe watermelons is red, and they don't want to see purple and greenish prints.
In short, in the production process of offset printing, there are many factors that cause the color difference of printed products. It is necessary to analyze the various influencing factors and corresponding control measures, and use the density control method and the color control method to control the whole production process to ensure The color of the print.