The first section of the formation of offset printing faults Offset printing faults are roughly divided into mechanical faults, printing process technical faults, faults caused by materials, faults caused by operating environment and maintenance and electrical faults. 1. Mechanical failure factors Printing machinery produces the most failures. Even if a new offset press is produced, due to certain defects in the design, assembly, installation and commissioning, failures will occur in the work. An offset press is composed of tens of thousands of parts, and the accuracy of the manufacturing and processing of each part is extremely important. Due to the limitations of the working machine and the operation technology of domestic machine parts, some parts cannot reach the advanced level, which also causes mechanical failures. A factor. Some offset presses, such as the gears and cylinders in the host, have better processing quality, but the technical performance of the supporting parts is poor, which will also cause trouble for normal printing. For example, some offset presses are equipped with a vacuum water tank that does not meet the vacuum requirements. The automatic water addition often fails, resulting in a lack of water in the hopper and the layout. If the operator is not careful, a dirty paste on the printed matter will occur. The installation and commissioning of offset presses are also important. Many factories ignore the strength and hardness of the ground when making foundations before installing machines. At present, medium and high speed offset presses have extremely high requirements on the level of the machine base: the horizontal allowable horizontal error cannot exceed 0.08 / 1000, and the horizontal horizontal error cannot exceed 0.04 / 1000. Use the foot screws to correct the level under the machine base. For example, there are 8 correction screws under the base of the J2108 machine. The contact point of the screw is 8cm square iron. After correcting the level, fasten the base with a nut to prevent the foot screws from walking. The problem is: the base of the machine is not on the ground. It is supported by 8 foot screws, because the oil pan is below, it is impossible to fill with cement, so the surface strength of the foundation is required to be higher, and the specification indicates that 200 cement is used, and it is better to use 400 cement. And roll the steel bar on top of the foundation, otherwise the cement will easily be damaged or cracked after the ground screw touches the site and the machine will lose its level. In particular, roller journals and bushings wear fast, which can cause printing problems such as ghosting and bars. After the machine is used for a certain period of time, various failures will occur due to the gradual wear of the machine parts and the decline in accuracy. The wear and tear of the machine parts is not only due to poor maintenance, but also related to the materials used and the manufacturing process of the machine parts. For example, the paper feed ball made of bakelite is easy to wear into an oval shape, causing the paper to skew and malfunction. As another example, the unevenly loaded universal joint ball and fork, will cause a paper feed failure after wear. The swing rod and the ball of the imprinting grit shaft will be loose and broken due to the large force, and the ball bearing will be broken, resulting in inaccurate overprinting or equipment accidents. The surface of the cylinder will also wear. For example, the impression cylinder of the imported Roland four-color machine is made of nodular cast iron with better wear resistance. However, after printing products of the same specification frequently, the surface of the impression cylinder will occur at the edge of the paper. Linear wear dented, so that when re-printing products of larger size, a white bar will appear on the mark of the place, which affects the quality of the product. Offset presses should be cleaned, maintained, and lubricated frequently to work properly. If maintenance is not good and refueling is not frequent, more failures will occur. If the roller pillow of the offset press of a factory is filled with ink and glue to dry and dirt, it becomes a "contact roller pillow" during the pressing process, and the roller is stretched hard, causing the roller journal and bushing to wear seriously, the printed product Double ghosting, bumps, greasy and other faults occur at the same time; some offset presses do not clean and grind their teeth, and the paper hairs accumulate and become rusty. A single bite is rusted and bites. Accurate and "peeling" failures are concurrent. The failure of the machine parts due to oil cut-off or biting failure is common in offset printing plants. Some factories have a refueling system, but it is not implemented to people, and there is a failure to break the oil and bite the machine; some factories have no one to grind a pile of iron powder next to the cam of the offset press; the oil pipe in the hood is broken or Equipment failures caused by inadequate oil delivery are also caused by lack of inspection. In general, poor equipment maintenance, unsound systems, or poor implementation despite the systems are the direct causes of many printing failures. 2. Technological factors Due to the physical and chemical aspects involved in the offset printing process, there are higher requirements for mastering the process and technology. Often due to the process and technical level of managers and operators, offset printing problems are also caused. 1. Offset printing characteristics Offset printing is based on the principle that oil and water repel, and it is printed on the same plate. The graphic part is oleophilic and hydrophobic, and the blank part is hydrophilic and oleophobic. This is the most obvious difference between offset printing and other printing, and it is also the key to offset printing technology. In the presence of both ink and water on the plate surface, the ink roller that transports the ink is in contact with water. Due to the high-speed rotation of the ink roller, the water is mixed into the ink under the squeeze of mechanical force, causing the ink to emulsify and print normally. A reasonable emulsification value is reached. Therefore, offset printing actually uses both the principle of oil and water repulsion, and the characteristics of oil and water compatible emulsification. According to foreign experimental data, when the offset printing can normally be printed normally, the reasonable emulsification value of the ink, water accounts for about 20 to 25%, there is no domestic instrument to determine, it is determined by the operator's technology and experience, which contains unstable factors , Which increases the possibility of failure. The water used for offset printing is not pure water. The water used in PVA plates contains phosphorus and chromic acid, which greatly corrode the plate surface. It not only corrods the blanks of the blank part of the plate, but also continuously accumulates the salt layer of the husband, and also corrodes the graphic parts Base paint, which lowers the printing durability of the printing plate. The wetting powder added to the PS plate water contains more than ten chemical components. It is a slightly acidic wetting agent with a cleaning effect and is extremely corrosive to the printing plate. 2. Printing pressure It is also a big foundation of process technology. Because offset printing is indirect printing, the image is transferred from the plate cylinder to the rubber blanket for the first imprint. After the rubber cylinder receives ink, the imprint is transferred to the substrate. This is the second imprint and two presses. The pressure required by India is different, and how to achieve the "ideal pressure" level requires some effort. In general offset presses, the pressure between the plate cylinder and the rubber cylinder is about 0.10mm; the pressure between the rubber cylinder and the impression cylinder is about 0.20 ~ 0.25mm. With the adoption of PS plates with better flatness and imported air cushion blankets, the pressure between the plate cylinder and the blanket cylinder can be lower. For example, the imported Heidelberg four-color machine can print at 0.05mm. The pressure used for offset printing is achieved by calculating the radius of the cylinder, measuring the center distance, measuring and increasing or decreasing the number of pads. The pressure of offset printing must be standardized and digitally controlled. Do not add pads at will, increase the pressure blindly, so that the pressure changes suddenly, the surface speed of the roller surface varies, the friction of the contact surface increases, and even greasy, ghosting, ink Bar and other failures. Therefore, the correct use and adjustment of printing pressure is an important part of the process technology required for printing products. 3. Overprint is not allowed Overprint inaccuracy can be divided into two types: vertical overprint inaccuracy and horizontal overprint inaccuracy. There is a saying that "the paper is always tightly controlled with organic parts during the transmission process, which can ensure accurate overprinting." In fact, this sentence is not comprehensive. For example, the embossed grit shaft or sleeve wear, although it can also be firmly controlled when transferring paper, it can not be controlled correctly. This is like a person holding a ball in both hands. The arms can be held straight when the arms are straight, but they can also be held firmly when the arms are bent, but the distance and stability of the two balls held in different postures are different from the human body. . Therefore, it should be said that "correctly and firmly controlled to ensure accurate overprinting". There are many reasons for inaccurate overprinting. Common ones include: paper expansion and contraction, paper misalignment, inaccurate positioning of the teeth, insufficient bite force, abrasion of the bite shaft, unsatisfactory handover, and wear of the transmission components of the delivery tooth. Other special factors will also cause inaccurate registration, such as large deformation of the rubber layout part, which can also make the partial registration of the image inaccurate; the axial movement of the delivery shaft and the impression cylinder axis will cause inaccurate lateral registration. 4. Color sequence arrangement In the past, the printing color sequence of the monochrome machine was always in the order of yellow, red, blue, and ink. In this color sequence arrangement, the greater difficulty is to sign the yellow and red printing samples, especially the first color yellow. Version, related to the color reproduction and reproduction of printed matter. If the yellow version is printed too dark, the person's skin color is not vivid, and the house is as old as the ancient temple, which makes it difficult to accept; if the yellow version is printed too light, the copied picture is shriveled and dull, and the color tone is biased towards blue and purple It's not pretty. There are two main reasons for the color deviation of the yellow version: one is the retinal colorimetric cells of the human eye, which has the lowest resolution of yellow; the second is that people do n’t know much about the oil absorption of paper. It often causes too deep and too weak ills. The color sequence arrangement of the two-color machine can put the yellow version and the blue version in the same printing process. The blue version just plays the role of filtering the blue glass, which is convenient for identifying whether the ink layer thickness of the yellow version is appropriate. However, due to the wet stacking and wet printing, the two-color machine often produces irregular A and B ghost images on the two corners of the trailing edge. This requires that the color sequence be appropriately changed during the process arrangement to solve. For multi-color offset presses with more than four colors, because the three colors are wet stacked wet printing, the color sequence is arranged from high to low according to the viscosity of each color ink, otherwise reverse overprinting faults will occur, resulting in the dark color mixing of color. Sometimes the first few colors will also produce A, B ghosting, the color sequence must be adjusted. Hard Case Luggage,Luggage Sets,Traveling Luggage,Hard Shell Luggage Yongxin Juanhua Leather & Bags Co., Ltd , https://www.luggagegld.com
Trouble shooting for offset printing (1)