Reasons for the dirty back of printed products and the influencing factors
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In offset printing, the ink of the previous sheet is not dried, and the next sheet is placed on top of it. The ink on the previous sheet may be partially transferred to the back of the next sheet, which is called the back side and the back side is dirty. Both the print and the print mark have dirty marks on the back side of the printed sheet, but the mechanism of their formation is completely different. The back surface is easy to occur when the printing paper, the ink layer is thick, and the number of overprinted colors is large, there is no dirt in the middle of the paper; In the printing of non-coated paper, the first color printing and the ink directly printed on the back side of the white paper, the paper has dirty marks in the middle and cannot be wiped off.
Different types of inks of different colors have different drying methods, and the drying speed is very different. At the same time, the ink with large thixotropy decreases in viscosity during the printing process due to the shearing action between the ink rollers. After the transfer is completed, the viscosity increases rapidly. Since viscosity is an external manifestation of cohesion, an increase in viscosity indicates an increase in cohesive force, which is conducive to the prevention of back smear. Therefore, most modern high-speed machine printing color products use fast drying and thixotropy inks. In order to adjust the printing suitability of the ink, it is sometimes necessary to add varnish to reduce the viscosity and viscosity of the ink. Therefore, when printing is easy to produce a smear coated paper, the amount of the viscous agent should be strictly controlled to control the viscosity of the ink and Sticky.
The back smear occurs mostly in areas where the printed ink layer is thick. For resin-based inks commonly used in sheetfed offset printing, once the ink layer exceeds a certain thickness, the backside smear will appear quickly. Therefore, the main measures to prevent back smear are to control the thickness of the ink layer, increase the cohesion of the ink itself, speed up the drying speed of the ink on the substrate, and control the distance between the two sheets before and after the control. For the color image mainly composed of dark tone, it is better to use the method of background color removal to reduce the thickness of the ink layer and increase the distance between the papers, not only to reduce the intermolecular bond between the ink on the surface of the ink film and the latter. Conducive to the infiltration of air, speed up the drying of the ink.
In actual production, controlling the drying speed of the ink is the most important means to prevent back smudging. The main factors affecting the drying of the blotting are paper, ink and ink layer thickness, dampening solution and emulsification degree, ambient temperature and humidity, and production process.
a. Paper. The paper with rough surface, loose structure and small sizing degree has good absorption and fast drying. After the smooth paper is piled into paper, the air is not easily introduced into the lower part of the paper pile, which affects the drying speed of oxidative polymerization, and the pH value of the paper is low. High water content will delay the drying of the ink.
b. Effect of adjuvants. The ink itself contains a drier. In order to speed up the drying process, it is necessary to add a drier to the ink. However, an excessive amount of the drier may cause the ink to be emulsified seriously, which is not conducive to drying. When the paper is pulled, a certain amount of detacking agent is usually added to reduce the viscosity of the ink. However, the detacking agent is a non-drying auxiliary material, which reduces the drying speed of the ink layer.
c. Wetting fluid and degree of emulsification. The stronger the acidity of the dampening solution, the slower the drying speed of the ink. The more serious the ink emulsification, the slower the drying. When the large-format printing machine prints the solid ground color of the small-format paper, the back smear is most likely to appear on both sides of the paper. The solution is to use the water pipe blowing device or the water roller. The method of the sticker strip reduces the amount of water supplied at both ends of the printing plate and reduces the emulsification value of the ink so that the printing ink is dried in time.
d. Printing pressure, printing speed. Increasing the printing pressure can promote the pressure penetration of the ink; reducing the printing speed, increasing the imprinting time, and preventing the back surface from being smeared.
e. Ambient temperature and humidity. The ambient temperature is high, the chemical reaction speed is fast, and the blotting is fast. In summer, the general bright and fast drying ink can be dried quickly without adding dry oil. In winter, some drier is needed. Therefore, the temperature and humidity of the workshop are controlled. It is very important for mastering the drying speed of the blot.
f. Sheet stacking method and dusting. After the printed product is stacked, the air is difficult to penetrate into the lower middle of the paper roll. Due to insufficient oxygen supply, this part of the printed sheet is difficult to dry. Therefore, in order to prevent the drying from being too slow and the back side to be dirty, it is important to strengthen the management of the semi-finished products after printing. Printing products that are not easy to dry, on the one hand, stacking should not be too high; on the other hand, after the ink layer is fixed, at a certain time, the sheets should be gently shaken to make them breathable, and the contact between ink and air is increased to accelerate the drying of the ink. . In the sheetfed offset printing, dusting is commonly used to prevent back smudging. Powder spraying is the use of powder support to increase the distance between the sheets, which is conducive to air circulation and quick drying.