Reasons and Influencing Factors of Dirty Printed Backs

In offset printing, the ink on the previous sheet is not dry, and the next sheet is superimposed on it. The ink on the previous sheet may be partially transferred to the back of the back sheet. This is called back-scratching and back-smearing. Both prints show signs of dirt on the back of the printed sheets, but the mechanism of their formation is completely different. The back stains are likely to occur on the printed coating paper, the thickness of the ink layer, and the number of overprinted colors. There are no dirty marks in the middle of the paper; In the back of printing non-coated paper, the first color printing and the ink directly printed on the white paper, there are dirty marks in the middle of the paper and cannot be wiped off.

Different types of inks of different colors have different drying methods and different drying speeds. At the same time, inks with large thixotropic properties have decreased viscosity in the printing process due to the shearing action between the ink rollers. After the transfer process, the viscosity has increased rapidly. Since the viscosity is the external manifestation of cohesion, the increase of viscosity indicates the increase of cohesive force, which is conducive to the prevention of back caries. Therefore, most modern high-speed printing machines use color inks that are fast drying and thixotropic. In order to adjust the printability of the ink, it is sometimes necessary to add an ink to reduce the viscosity and viscosity of the ink. Therefore, when printing is easy to produce coated paper on the back, it is necessary to strictly control the amount of the detackifier and control the viscosity of the ink. Stickiness.

The back blemishes often occur at thicker parts of the printing ink layer. For resin inks, which are commonly used for sheetfed offsets, once the ink layer exceeds a certain thickness, back-scratching occurs quickly. Therefore, the main measures to prevent back rubbing 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 front and back sheets not too close. For color images with mainly dark shadows, the method of removing the background color should be adopted. The thickness of the ink layer can be reduced. Increasing the distance between the papers can not only reduce the intermolecular binding force between the ink on the surface of the ink film and the next print sheet, but also Conducive to the penetration of air and accelerate the drying of the ink.

In actual production, controlling the drying speed of the ink is the most important means to prevent back-scratching. The main factors affecting the drying of the print are paper, ink and ink layer thickness, wetting liquid 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 of the imprinting. After the smooth paper is piled into paper basket, the air is not easy to be introduced into the lower part of the paper stack, which affects the drying speed of oxidation polymerization, and the pH of the paper is low. High water content will delay ink drying.

b Effect of adjuvants. The ink itself contains a driers. In order to speed up the drying process, the driers are sometimes added to the ink during printing. However, excessive driers can cause the ink to emulsify, which is not conducive to drying. When the paper is napped, a certain amount of de-bonding agent is usually added to reduce the viscosity of the ink. However, the de-bonding agent is a non-drying auxiliary material and will reduce the drying speed of the ink layer.

c Wetting fluid and degree of emulsification. The stronger the acidity of the wetting fluid, the slower the drying of the ink. The more serious the ink emulsification, the slower the drying. When the large-format printing machine prints the solid background color of the small-format paper, the back-side pick-up is most likely to occur on both sides of the paper in the axial direction. The solution is to use the water blowing device or the water roller as much as possible. The method of sticker strips reduces the amount of water supply at both ends of the printing plate, and reduces the ink emulsification value so that the imprinting ink can be dried in time.

d Printing pressure, printing speed. Increased printing pressure can promote the pressurization of the ink; reduce the printing speed, can increase the stamping time, can play a role in preventing the back of the dirt.

e Ambient temperature and humidity. High ambient temperature, fast chemical reaction, rapid drying of imprints, summer, general fast-drying inks, fast drying without drying oil, and some drying agents in winter, so the control of workshop temperature and humidity It is very important for mastering the drying speed of the blot.

f Sheet stacking and dusting. After printing the finished product into a stack, the air hardly penetrates into the middle of the lower part of the stack. Due to the lack of oxygen, this part of the printed sheet is difficult to dry. Therefore, in order to prevent slow drying and smudging on the back, it is important to strengthen the management of semi-finished products after printing. Printing products 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 interval, the printed sheet should be gently shaken to make it breathable, increase ink and air contact, in order to accelerate the drying of the ink . In the sheet-fed offset printing, dusting is commonly used to prevent back-scratching. The powder-spraying is the use of powder support to increase the distance between printed sheets, which facilitates air circulation and speeds up drying.