Technological factors of offset printing failure

Because the offset printing process involves physics, chemistry and other aspects, it requires high technical mastery, and often due to the technical and technical level of managers and operators, many offset printing failures will occur. Offset printing is based on the principle that oil and water repel each other, printing on the same printing plate. The most obvious difference between offset printing and other printing is that the graphic part is hydrophilic and hydrophobic, and the blank part is hydrophilic and hydrophobic, which is also the key of offset printing technology.

In the case of ink and water in the printing plate, the ink roller that conveys the ink is in contact with the water. Due to the high-speed rotation of the ink roller, water is mixed into the ink under the extrusion of mechanical force, which leads to the emulsification of the ink and reaches a reasonable emulsification value in normal printing. So in fact, offset printing not only uses the principle of oil-water repulsion, but also uses the characteristics of oil-water phase emulsification. According to foreign experimental data, when offset printing can be printed normally, the reasonable emulsification value of ink is about 20 ~ 25%. There is no instrument measurement in China, which depends entirely on the skill and experience of operators, including unstable factors, which increases the possibility of failure.

The water used for offset printing is not pure water. The water used for PVA board contains phosphorus and chromic acid, which is very corrosive to the layout. It not only corrodes the sand screen in the blank part of the layout, increases the organic salt layer, but also corrodes the base paint in the graphic part, resulting in the reduction of printing resistance of the printing plate. The wetting powder added to PS plate water contains more than ten kinds of chemical components, belonging to weak acid wetting agent, which has cleaning effect and little corrosion to printing plate. This is also a great technical foundation. Because offset printing is indirect printing, the image is transferred from the plate cylinder to the blanket during the first impression. After the blanket cylinder receives the ink, the imprint is transferred to the substrate. This is the second impression, and the two impressions need different pressures. How to reach the level of "ideal pressure", we must make some efforts. Generally, the pressure between offset press, printing plate cylinder and rubber cylinder is about 0.10 mm; The pressure between the rubber drum and impression cylinder is about 0.20 ~ 0.25mm. By using PS plate with good flatness and imported air cushion rubber blanket, the pressure between the printing plate drum and the rubber drum can be even smaller. For example, the imported Heidelberg four-color machine can print 0.05mm..

The pressure used in offset printing is realized by calculating the radius of the drum, measuring the center distance, measuring and increasing or decreasing the number of pads. The pressure of offset printing must be controlled in a standardized and digital way. Never arbitrarily increase the padding, blindly increase the pressure, so that the pressure changes suddenly, the linear speed of the roller surface varies, the friction of the contact surface increases, and even other faults such as greasy, ghosting and ink sticks appear. Therefore, the correct use and adjustment of printing pressure is an important part of the technology needed for printing products. Overprinting prohibition can be divided into vertical overprinting prohibition and horizontal overprinting prohibition. There is a saying that "paper is always firmly controlled by parts during transmission, which can ensure the accuracy of overprinter." Actually, this sentence is not comprehensive. For example, when conveying paper, the wear of the embossing grinding shaft or shaft sleeve can be firmly controlled, but it cannot be correctly controlled. It's like a person holding the ball with both hands, holding it firmly with both arms straight and holding it firmly with both arms bent, but the distance and stability of holding the ball in two different postures are different. Therefore, it should be said that "correct and firm control can ensure the accuracy of overprinter".

There are many reasons why overprinter is forbidden. The common ones are: paper stretching, paper feeding deviation, inaccurate positioning of embedding, insufficient biting force, wear of grinding shaft, unqualified handover, wear of transmission parts of paper transfer teeth, etc. Other special factors will also cause overprinter ban, such as large local deformation of blanket, which will also cause local overprint ban of image; The axial movement of the paper transfer gear shaft and the impression cylinder shaft will lead to inaccurate lateral overprinting. In the past, the printing color sequence of monochromator always followed the color sequence of yellow, red, blue and ink. The difficulty of this color sequence arrangement is the signing of the yellow and red printing proofs, especially the yellow version of the first color, which is related to the color reproduction and reproduction effect of printed matter. If the yellow edition is printed too deeply, the printed characters will be lifeless, and the house will be as old as an ancient temple, which is unacceptable; If the yellow version is printed too lightly, the copied picture will be dry and dull, and the color tone will be blue and purple, which will not look good. There are two main reasons for the color deviation of yellow version: first, the retinal color discrimination cells of human eyes have the lowest ability to distinguish yellow; Second, people don't know much about the oil absorption of paper, lack of instrument detection, and the size is the same, which often causes the disadvantages of being too deep and too light.

The color sequence arrangement of the two-color machine can put the yellow plate and the blue plate in the same printing process, and the blue plate only plays the role of color filtering of the blue glass, which is convenient for identifying whether the ink layer thickness of the yellow plate is appropriate. However, because the two-color printing machine is wet-stacked and wet-printed, irregular A and B double images often appear on the two corners of the trailer, which requires appropriate changes in the color sequence in the process arrangement.

For a multi-color offset press with more than four colors, because the three colors are wet-stacked and wet-printed, the color sequence is arranged from high to low according to the viscosity of each color ink, otherwise, the fault of reverse overprint will occur, resulting in the color mixing defect of dark ink. Sometimes the first few colors will also produce a and b ghosts, and the color order should be adjusted if necessary.