How to avoid shortcomings and improve the quality

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How to avoid disadvantages and improve the quality of color reproduction in traditional offset lithography (Part 2)

4 points - there is a phenomenon of step jump

points have the following functions: ① they are responsible for reproducing hue, lightness and saturation in printing; ② It is the smallest unit of fat sensing and water repellent, and is the basic element of image transmission; ③ In color synthesis, it is the organizer of image color, hierarchy and outline. The main problem of points is to expand. The expansion is not only related to the printing pressure, the number of points and lines, but also to the point shape of points. Now the commonly used point shapes include square points, circular points and chain points. The square dot overlaps at the four corners of the 50% dot. Under the action of printing pressure, the 50% dot expands the most. From the perspective of hierarchical transition, a more obvious step jump is formed at the 50% dot; When the circular point reaches about 70%, it will have overlapping edges. At this time, it will also have embossing jump, but at the level, its jump position will be postponed backward; The chain point is divided into two angles, which are at 25% and 75%, respectively, and each produces a smaller step jump. The production of step jump will make the tone of printed matter discontinuous, so we should choose different shapes of dots according to the characteristics of the original level in prepress production. For example, circular dots can be used to reflect the pictures in which the dark tone level is not very important, and chain dots can be used to express the theme of middle tone. When making a figure, because most of the face is about 50% points, it is best to use chain points to avoid the expansion of 50% points. When making a landscape map, you can use either square points or chain points

5 "water" -- a double-edged sword

the main function of fountain solution is to form a water film that repels ink in the blank part of the printing plate and keep the blank part of the printing plate free from dirt. It can be said that without fountain solution, traditional offset printing is impossible. However, the use of fountain solution in offset printing is also a double-edged sword. Why do we say so? Here we don't talk about all kinds of printing faults caused by "large water and small water". That is, in the case of "water ink balance", when fountain solution competes with ink, "water" will scour and corrode the base of ink, making it difficult for small points (especially small points below 3%) to ink, This part of the idea is missing in the printing process, resulting in the merging of highlights. Therefore, as a printing operator, we should control the amount of water and try to reduce the amount of water in the layout on the premise that the printed matter is not dirty. As a prepress producer, we should not only pursue the delicacy of the image, but also consider the actual situation and select the appropriate number of lines, because the 3% 120 line/inch point is much larger than the 3% 175 line/inch point, and the point with large area is easier to stand on the printing plate

6 ink overprint total amount limit

the total amount of ink can be set in the color separation option in the CMYK palette of Photoshop. This parameter specifies the total percentage content of cyan, magenta, yellow and black in the black area of the printed image. Theoretically, the blackest area should be printed with 400% of the maximum total ink volume. However, in four-color wet overprint, this situation will lead to the following problems in the actual production operation of the printing machine

① it is easy to rub the back of the printing sheet

② the ink is not easy to dry

③ overprint difficulty and poor ink overprint

④ reverse ink transfer

in wet overprint, it is better to limit the total amount of ink to no more than 340%. In newspaper printing, the replacement of gray components in images can be larger, and the total amount of ink can also be reduced. Specifically, we should constantly summarize experience according to the actual situation, and find the appropriate setting value of various color separation parameters? However, under the condition of monochrome overprint, I think the replacement of gray components is only at the dark tone of the image. The overall amount of ink can be set larger, and other color parts should be expressed by CMY to increase the richness of color and image gloss

7 overprint problem

in CMYK four-color printing, overprint accuracy is the most basic requirement of color printing. The stability of the machine, the printing suitability of raw and auxiliary materials, and the temperature and humidity of the workshop may cause inaccurate overprint. Even if all conditions are good, there may still be slight dislocation between the two color blocks during the printing process, resulting in overprint deviation, It is specifically manifested in the leakage of white at the junction of different colors or the uneven edges and exposed colors of overprint colors. Now the way to make up for this defect is to trap when separating colors. The principle of trapping processing is to appropriately expand the light color at the color junction to the dark color. The specific expansion can be set according to the actual situation of printing (such as according to the registration accuracy of the printing machine). Now many software have trapping setting function, and some large companies also have professional trapping software, which provides better control of trapping. Attention should be paid to the use of color in trapping processing. The final trapping value setting should not damage the fine level of the image or blur the boundary of the image. Sometimes, trapping can not be done, such as printing with only one color. In addition, when using CMYK to design color patches, we should pay attention to the use of color patches or words with m+c=b as little as possible, because m and C are strong color inclined planes, and it is very ugly only when there is a slight misregister

I would like to give some suggestions to the printing colleagues who make newspaper advertising boards. Try to make as few printing boards with hollowed out thin lines or hollowed out small text as possible. I often read newspapers. Out of professional rare, refractory high-purity metal, high specific volume powder purification technology; The sintering and preparation habits of molybdenum, tantalum and niobium materials often need to turn over the advertisements in color newspapers, which can rarely be accurate to satisfaction. Therefore, prepress designers should not only pursue their own design effects, but also pay attention to the actual situation of printing, so that your design works will look better when they meet the audience

as a traditional offset lithography, due to its advantages in the cost performance of its products in the printing industry, it will continue to exist. Since there is a reason for its existence, we can't avoid it, including its advantages and disadvantages. I'm afraid the best way to master it is to fully recognize the above problems, so that we can develop strengths and avoid weaknesses in the design and production process, according to the different characteristics of products, Make prints satisfactory to the transferor. Source: PRINT world

Author: wuminggen Jiangsu publishing school

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