Complete collection of offset printing details

"Lithographic printing" and "offset printing" have the same meaning. This printing method is to transfer the ink on the rubber surface to the paper through a roller rubber stamp. Because the rubber surface is flat and there are no concave patterns, the patterns and patterns printed on paper are also flat, with no three-dimensional effect and poor anti-counterfeiting performance. Offset printing needs less ink, and the manufacturing cost of the mold is lower than that of gravure.

Basic introduction Chinese name: offset printing, also known as lithography technology type: printing technology is applied to: the third set of RMB printing introduction, principle and technology, basic principle, the choice of overprint or trapping, four-color printing, angle selection, spot color printing, waterless offset printing, gold and silver ink printing, printing color sequence arrangement, matters needing attention, and introduction. In the third set of RMB, 1960 version of "Remaining" Later, with the economic development, the purchasing power of 1, 2/and 5-cent notes became lower and lower, and no one forged them. Therefore, in order to reduce the printing cost, the banknote printing factory replaced their front with a lower-cost offset printing plate. Early intaglio corner coupons existed in a small amount, and the price was of course more expensive than offset corner coupons. Distinguish between "intaglio" and "offset printing" by looking at the color and three-dimensional sense of the pattern. If you compare these two kinds of banknotes, the difference is very obvious and it is easy to judge. In addition, for beginners, you can also use the following two methods: the first is "touch". The words "People's Bank of China" on the front of the intaglio RMB have obvious protruding feeling. Touch your finger gently from left to right, and you can clearly feel that every word is protruding. The second is to "look at the side." The characters and patterns printed by gravure printing will have tiny "burrs" on the edges of lines due to the diffusion of ink, which can be seen clearly with a magnifying glass. The edge of offset printing line is smooth. Principles and basic principles of technology: oil-water immiscible principle; Selective adsorption principle of printing plate; Principle of dot fineness; Principle of indirect printing. If the choice of overprinter or trapping is not accurate, trapping will easily affect the appearance, while overprinter is easy to rub the back because the ink layer is too thick, so monochrome machine wet pressing dry printing tends to use overprint, and multicolor machine wet pressing wet pressing tends to use trapping; Wet overprinting must consider the nature of ink, and the viscosity and adhesiveness of the ink printed first must be greater than that of the ink printed later, otherwise the ink printed later will appear reverse overprinting. The printed ink must also have high transparency; When printing gold ink and silver ink on coated paper, the color obtained by trapping gold ink and silver ink on white paper is thicker and purer than that obtained by printing on other background colors; When printing black, if there is only a large field of view image on the black plate, the ink amount of the black plate can be increased by trapping, and the ink color printed on white paper is thicker than that printed on other field of view background colors. However, thin lines are generally printed in an overlapping manner. In order to ensure that the printing plates arranged in front for trapping are covered by the ink layer at the back, the trapping amount of 0.03 ~ 0.05 mm is usually increased on the basis of the equipment registration accuracy. If the registration accuracy is 0. 15mm, the trapping part is generally 0. 18~0.20mm larger. When four-color printing technology is adopted, if a large area of black field appears, because the density of ink field does not exceed 1.8, printing a large area of black field with such density will lack stereoscopic visual effect. The common method is to overprint about 40% cyan on a large black field. Angle Selection Network cable * * * There are four angles to choose from, 15, 45, 75, 90. Four-color printing, the main color is generally 45, the yellow version is weak, choose 90; Three-color overprint, usually at angles of 30, 60 and 90; Two-color overprinter, usually 45 and 90; In the spot color printing, monochrome layered printing or monochrome layered superposition printing of the field edition, the layered edition with 45 angle is usually more beautiful; 45-degree screen line has the best visual effect, which is generally used to express the main color of characters, mostly magenta; The blue version and the black version constitute the skeleton of the whole picture, generally 15 and 75. Spot color printing A product uses both four-color printing and spot color printing: the four-color printing part correctly restores the picture by controlling the field density, and the background color part obtains the visual effect of uniform and thick ink color by appropriately increasing the ink amount. If the electronic document or film provided by the customer has 2~3 spot colors in addition to the four colors of CMYK, and the coverage area of 1 spot color ink is small and easy to match with CMYK, this spot color can be subtracted. If the manuscript provided by the customer only has CMYK four-color overprint, but its hue is difficult to control in actual printing, such as cyan and magenta overprint into purple, cyan ink is blue if it is large, and magenta ink is red if it is large. At this time, it may be better to change to the spot color purple version. Although there is one more version, the printing preparation time is shortened, the printing speed is improved and the hue control is stable. Principle of waterless offset printing Positive figure waterless offset printing is composed of aluminum base, bottom layer, photosensitive resin layer, silicone layer and covering film. After exposure, the siloxane layer in the exposed part (blank part) of the printing plate undergoes bridging reaction and photo-crosslinking; However, the silica gel layer of the unexposed part (graphic part) was removed by the developer, exposing the photosensitive resin layer below. Because the photosensitive resin layer is ink-loving, and the unexposed silicone layer is ink-repellent, the selective adsorption of ink is completed. Water-free offset printing has high requirements on the environment: the temperature of printing workshop is 23~25℃, and the ink will solidify at low temperature; When the temperature is high, the paste will fail. In addition, waterless offset printing plates are difficult to decontaminate after being soiled. When printing four-color offset printing with gold and silver ink, use medium hard liner, and put the one with short tone and thin ink layer in front of the gold and silver ink to increase the amount of ink; Using monochrome offset printing "wet drying" has more advantages: fast drying speed, strong gloss of printed matter, good visual effect and so on. You can use neutral or soft packaging, use neutral packaging, cooperate with domestic blanket, add French pad cloth or cardboard (good hardness), and the effect will be good if you increase the pressure slightly. However, the printed semi-finished products can not be stored for a long time, otherwise crystallization will easily occur, leading to subsequent color ink printing; Moisturizing can't use mercuric chloride to avoid oxidation reaction, so it can be replaced by "Lide" powder. It is best to wet the plate with alcohol; Influence of printing speed on printing pressure In order to transfer the pictures and texts coated with ink on the printing plate to the substrate surface clearly, there must be enough printing pressure. However, the applied printing pressure should not be too great. Excessive printing pressure will make the substrate continue to deform, the imprint will become thicker, the dots will be at the same level, and the printing plate will paste, which will affect the clarity of graphics and text, and will also aggravate the wear of the printing plate and reduce the printing resistance of the printing plate. When the printing pressure is insufficient, there is not sufficient and reliable contact between the printing plate and the blanket, and the imprint will become shallow, which will affect the clarity of pictures and texts. In general, when the printing color sequence is wet color, the ink color printed first is dominant. For example, first print cyan and then magenta, and the overprint color will be blue; Printing magenta first and then Yin Qing, the overprint color will be red. In the case of wet pressing and drying, the color of post-printing ink is dominant. From the optical point of view, lithography should adopt CMYK color sequence. Generally speaking, the color sequence of warm-tone prints mainly based on figure paintings should be black, cyan, magenta and yellow; Cold color prints such as landscape paintings and snow scenes should be printed in black, magenta, cyan and yellow; The color sequence of two-color machine printing is cyan+magenta and black+yellow, and the color sequence of monochrome machine printing is cyan, magenta, yellow, black or black, cyan, magenta and yellow (generally, when the black version is a skeleton version, black is printed first); In order to facilitate overprinting, the color sequence should be arranged in order of ink layer thickness from small to large, such as black ink 0.8um—- cyan ink 0.9um—- magenta ink1.0 um-yellow ink1.1um; Arrange the printing color sequence according to the order of ink viscosity, such as black ink 1 188 Pa. S- cyan ink 488 pa. S- magenta ink 477 pa. S- yellow ink 147Pa.s, if the ink viscosity cannot meet the arrangement requirements, the ink viscosity can be adjusted in advance; Print the ink with small coverage area first, and print the dark ink first (because of high viscosity and large hiding power, avoid sticking to the back); When printing UV ink, the color sequence of printing should be black-cyan-yellow-magenta because of its different degree of absorbing UV photons and different curing speed. Precautions A. Try to avoid using dots with a small percentage, such as flat screen dots below 20%. Because the color block composed of small dots is easy to get local sunburn due to insufficient inhalation or fine particles attached to the negative and printer glass; When printing, it is easy to lose the printing plate because of too much water, dirty blanket or worn printing plate. The above two reasons will cause uneven ink color of color blocks. As for the dot below 5%, the ordinary offset printing process is difficult to restore and should be avoided. B at the same time, try to avoid using a large proportion of outlets, such as more than 80% of flat screen outlets. Because the color block composed of large dots is slightly short of water or the blanket is dirty when printing, it is easy to paste the plate. As for more than 95% of the outlets, they should be avoided. C. Avoid overprinting color blocks on spot colors with too many colors or high percentage of dots, which is easy to cause dirt on the back due to thick ink layer. D. When using spot color printing technology, try not to use color blocks that need to be mixed with too many basic color inks. Mixing too many kinds of inks will greatly increase the difficulty of ink blending, which not only increases the time of ink blending, but also makes it difficult to blend excellent and similar colors. E. When printing a certain color in a large area with spot color, try to avoid printing small white characters and lines with the same color. Because large-scale printing often increases the thickness of the ink layer, the ink layer is easy to spread when it is thick, but it will lead to small white characters and line paste. F for text, customers are advised to choose bold when printing small words with white background in the middle of the field. G. When printing full plates on site, you can use the plates left after the last printing, stop water supply, and do not need special plate making. However, because there is a slight difference in ink absorption between the photosensitive adhesive in the graphic part and the alumina in the blank part of PS plate, it is best to choose a completely folded printing plate when printing a color with strong color rendering (such as dark blue) in the whole plate.