In the production of offset printing, the ink of the previous printing sheet is not dry, and the latter printing sheet is stacked on it, the ink on the previous printing sheet may be partially transferred to the back of the latter sheet, which is called back rubbing, and the back rubbing Both the through-print and the through-printing show that the back of the printed sheet has dirty marks, but the mechanism of their formation is completely different. The back-face smearing is easy to occur when printing coated paper, thick ink layer, and many overprint colors. There is no stain in the middle of the paper; On the back side of printing non-coated paper, first-color printing and ink directly on white paper, there is a dirty mark in the middle of the paper, which cannot be wiped off.
Different types of inks with different colors have different drying methods, and the drying speed varies greatly. At the same time, the thixotropic inks have a lower viscosity due to the shearing effect between the ink rollers during the printing process. Because viscosity is the external manifestation of cohesion, an increase in viscosity indicates an increase in cohesion, which is helpful to prevent back rubbing. Therefore, most modern high-speed machine printing color products use ink that dries quickly and has a large thixotropy. In order to adjust the printing suitability of the ink, it is sometimes necessary to add ink oil to reduce the viscosity and viscosity of the ink. Therefore, when printing coated paper that is easily stained on the back side, the amount of debonding agent should be strictly controlled to control the viscosity and viscosity of the ink. Sticky.
Smearing on the back side occurs mostly in the thick parts of the printing ink. For the resin-based inks commonly used in sheetfed offset printing, once the ink layer exceeds a certain thickness, smudging on the back will quickly occur. Therefore, the main measures to prevent backside smearing 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 printed sheets before and after not to be too close. For printing color images mainly with dark tone, it is advisable 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 can reduce the binding force between the ink on the surface of the ink film and the next sheet, but also It is conducive to the penetration of air and accelerates the drying of ink.
In actual production, controlling the drying speed of the ink is the most important means to prevent backside smearing. The main factors affecting the drying of the blot are the thickness of the paper, ink and ink layer, the degree of wetting fluid and emulsification, the environmental temperature and humidity, and the 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 up in a stack, the air is not easy to be introduced into the lower part of the pile, which affects the drying rate of oxidative polymerization. The pH 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 sometimes necessary to add a drier to the ink, but excessive drier will make the ink emulsify seriously, but it is not conducive to drying. When the paper is rough, a certain amount of debonding agent is usually added to reduce the viscosity of the ink. However, the debonding agent is a non-drying auxiliary material, which will reduce the drying speed of the printed ink layer.
c Wetting fluid and emulsification degree. The stronger the acidity of the wetting fluid, the slower the drying speed of the ink. The more serious the emulsification of the ink, the slower the drying. When printing large-format printers on the solid background of small-format paper, the back staining is most likely to occur on both sides of the paper's axis. The solution is to use a waterway blower or a bucket roller The method of sticker strips reduces the water supply at both ends of the printing plate and reduces the emulsification value of the ink, so that the blotting ink can be dried in time.
d Printing pressure and printing speed. Increasing the printing pressure can promote the pressurized penetration of the ink; reducing the printing speed can increase the printing time and can prevent the back side from being stained.
e Environmental temperature and humidity. The environment temperature is high, the chemical reaction speed is fast, and the blot drying is fast. In summer, the general bright quick-drying ink can be dried quickly without adding drying oil. In winter, some drier is needed. Therefore, the temperature and humidity of the workshop are controlled. , Is very important for mastering the drying speed of blots.
f Stacking method and powder spraying. After stacking the printed products into a stack, it is difficult for air to penetrate into the middle of the lower part of the paper stack. Due to insufficient oxygen supply, this part of the printed sheet is difficult to dry. Therefore, it is very important to strengthen the management of semi-finished products after printing in order to prevent slow drying and dirty back. For printing products that are not easy to dry, on the one hand, the stacking should not be too high; on the other hand, after the ink layer is fixed, at a certain time, the printed sheet should be shaken gently to make it breathable, increasing the contact between the ink and the air to accelerate the drying of the ink . In single sheet offset printing, powder spraying is generally used to prevent backside smearing. Powder spraying uses the support of powder to increase the distance between the printed sheets, which is beneficial to air circulation and speeds up drying.
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