Offset fault diagnosis and elimination (1)

Section 1 The formation of offset faults The offset faults can be broadly classified into mechanical faults, faults in the printing process technology, faults caused by materials, faults caused by the operating environment and maintenance, and electrical faults.
First, mechanical failure factors The printing machinery produces the most failures. Even if a new offset press is produced, due to certain defects in the design, assembly and installation and debugging, failures may occur during work. An offset press consists of tens of thousands of parts, and the precision of manufacturing and processing of each part is extremely important. Due to the limitations of the working machine and the operating technology of the machine parts, some parts cannot reach the advanced level, which also causes mechanical failure. A factor. Some offset presses such as gears and drums in the main machine are of good quality, but the technical performance of the parts is poor, which will also cause troubles for normal printing. For example, some vacuum drums equipped with water for offset presses do not meet the vacuum requirements. Automatic water addition often fails, resulting in lack of water in the buckets and layout. Operators will notice little if they do not pay attention, and there will be dirty prints on the prints.
Offset press installation and commissioning work is also very important. Many factories ignore the ground strength and hardness issues when setting up the foundation before installing the machine. The medium and high-speed offset presses now require extremely high levels of the machine base: the horizontal allowable horizontal error cannot exceed 0.08/1000, and the horizontal horizontal error cannot exceed 0.04/1000, and is adjusted with the foot base screw of the machine base correction level. For example, there are 8 correction screws under the base of J2108 machine, 8cm square iron for the place of contact with the screws. After adjusting the level, tighten the base with a nut to prevent the bottom legs screws from walking. The problem is: Now the base of the machine is not grounded. And relying on 8 foot screws to support, because the following oil pan to put, can not be filled with cement, it is required that the surface strength of the foundation is higher, and the instructions indicate that using 200 cement, the actual use of 400 cement is better , And on the foundation layer rolled steel bar, or after the ground screw contact site is subject to vibration, the cement is easy to damage or crack, the machine loses its level. In particular, roller journals and bushes wear quickly, which can cause ghosting, sticks, and other printing problems.
After the machine is used for a certain number of years, due to the gradual wear and tear of the parts, the accuracy will be reduced and various failures will occur. The wear speed of the parts is not only due to poor maintenance but also related to the materials used in the parts and the manufacturing process. For example, paper-feed pressure balls made of plastic wood are easily worn out into an oval shape, causing paper feed skew failure. For another example, the cardan shaft and the fork joint with uneven force will produce a paper feed pause after wear. Embossing the crusher shaft and the ball, due to the large force and inequality, make the push pin loose, break, ball bearing broken, resulting in overprint or equipment accident.
The surface of the cylinder is also worn. For example, the cylinder of the imported Roland four-color machine is made of ductile iron with good abrasion resistance. However, after printing the same size of product often, the surface of the cylinder will occur at the edge of the paper. Linear wear depressions, so that when printing a larger size product, there will be a white bar on the print at that location, affecting product quality.
Offset presses should always be cleaned, cleaned and refueled before they can work properly. If the maintenance is not good, refueling is not working, and more failures will occur. For example, if the roll of an offset press of a home factory is filled with ink and the dirt on the glue is dry, it becomes a “contact roll” during the pressurization process, and it hardly pushes the roll apart, causing severe abrasion of the drum journal and bushing. On the other hand, the ghosting, stickiness, greasy and other faults occurred at the same time; some offset presses did not clean the paper teeth, did not add oil, piled up the paper, and rusted. The single bite of the paper teeth was rusted and bitten and the bite force was lost, resulting in overprinting of the prints. Quasi-, "skinned" faults occur concurrently.
The abrasion or seizure of mechanical parts caused by broken oil is widespread in offset printing plants. Although some factories have refueling systems, they have not been implemented. They have lost their oil and have died. Some mills grind a pile of iron powder next to the cam; the tubing in the housing is broken or Failure of equipment caused by failure of oil delivery is also due to lack of inspections. In general, the maintenance of equipment is not good, the system is not perfect, or the system is not implemented effectively, which is the direct cause of many current printing failures.
Second, process technology factors Offset process because it involves both physical and chemical content, to master the process and technology have higher requirements, often due to management personnel, operators of the process, technical level and other issues, resulting in offset failure many.
1. Offset Printing Offset printing is based on the principle of repulsion of oil and water on the same plate. Graphic part of the lipophilic hydrophobic, blank part of the hydrophilic oleophobic, which is the most obvious difference between offset printing and other printing, but also the key to offset printing technology.
In the case where there are both ink and water in the printing plate layout, the ink roller that conveys the ink comes into contact with water. Due to the high speed rotation of the ink roller, the water is mixed into the ink under the action of the mechanical force, resulting in ink emulsification and normal printing. Achieve a reasonable emulsification value. Therefore, offset printing actually uses both the principle of oil and water repulsion and the characteristics of oil-water emulsification. According to foreign experimental data, when the offset printing can normally print normally, the reasonable emulsification value of the ink, water accounts for about 20-25%, and there is no instrument measurement in the country, and it depends on the operator's technology and experience to decide. This includes unstable factors. It also increases the possibility of failure.
The water used for offset printing is not pure water. PVA water contains phosphorous and chromic acid. The corrosion of the layout is very large. It erodes the blank part of the layout, continuously accumulates the salt layer of the jet engine, and corrodes the graphic part. Base paint, resulting in reduced printing plate printing rate. The wetting powder added to the PS plate water contains more than ten chemical components and is a slightly acidic wetting agent with a cleaning action and minimal corrosion to the printing plate.
2. Printing pressure is also a major foundation of process technology. Since offset printing is indirect printing, the image is transferred from the plate cylinder to the blanket for the first impression. After the blanket cylinder receives the ink, the imprint is transferred to the substrate. This is the second press. Two presses The pressure required for India is different. It is necessary to make some efforts to achieve the "ideal pressure" realm. The pressure between a typical offset press, a plate cylinder and a blanket cylinder is about 0.10 mm; the pressure between a blanket cylinder and an impression cylinder is about 0.20-0.25 mm. With the adoption of the PS plate and imported air-cushion blankets with better flatness, the pressure between the plate cylinder and the blanket cylinder can be smaller. For example, the Heidelberg four-color machine imported can be printed at 0.05 mm.
The pressure used for offset printing is achieved by calculating the drum radius, measuring the center distance, measuring and adding or subtracting the number of pad linings. Offset pressure must be standardized and data controlled. Must not increase the pad lining, increase pressure blindly, resulting in sudden changes in pressure, drum surface line speed, contact surface friction increases, and even there will be greasy, ghosting, ink Bars and other faults. Therefore, the correct use and adjustment of printing pressure is an important part of the process technology needed to print products.
3. Misregistration is not allowed. It can be divided into two types: vertical misregistration and horizontal misregistration. There is a saying that "Paper has always been firmly controlled by the organic parts during the transmission process, which can guarantee the accuracy of overprinting." In fact, this sentence is not comprehensive. For example, the embossing clincher shaft or sleeve wear, although it can also be firmly controlled in the transmission of paper, but it can not be properly controlled. This is like a person holding a ball in both hands. The arms are straight and they can hold firmly. The bending of the arms can hold them firmly. However, the distance and stability of the ball from two different positions are different from each other. . Therefore, it should be said that "it is firmly controlled to ensure accurate overprinting."
There are many reasons for overprinting. The common ones are: paper stretching, paper feeding, improper positioning, insufficient bite force, wear of the gnawing shaft, non-conformity of the transfer, and wear and tear of the conveying parts. In addition, some special factors may cause misalignment, such as large deformation of the layout part of the rubber, and the partial overprinting of the image may be inaccurate; the axial transfer of the paper feed shaft and the impression roller shaft may cause misalignment in the horizontal direction.
4. Chromatic sequence arrangement In the past, the color sequence of monochrome machines was always sequential yellow, red, blue, and ink. This kind of color sequence arrangement, the greater difficulty is the signing of the yellow version, the red version of the printing, especially It is the first yellow version, which is related to the reproduction and reproduction of prints. If the yellow version is printed too deeply, the skin color of the characters printed is not lifelike, and the houses are as old as the ancient temples, making them unacceptable; if the yellow edition is printed too light, the reproduced images will be dark and the colors will be blue-violet. It doesn't look good either. There are two main reasons why the yellow version has a color cast: First, the human eye's retinal color cells have the lowest resolving power to yellow; second, people lack understanding of the oil-absorbing properties of the paper, and lack of instrumental testing. Often cause too deep, too weak.
The color sequence arrangement of the two-color machine allows the yellow printing plate and the blue printing plate to be placed in the same printing process, and the blue plate just functions as a color filter for the blue glass, which is convenient for identifying whether the ink layer thickness of the yellow printing plate is appropriate. However, due to wet and wet printing, two-color machines often produce irregular A and B ghosts on the two corners of the trailing edge. This requires that the color sequence be properly changed during the process.
Four-color multi-color offset press, because the three colors are wet and wet printing, so the color sequence is based on the viscosity of each color ink, arranged from high to low, otherwise there will be reverse overprint failure, resulting in dark ink color miscellaneous ills. Sometimes the first few colors will produce A, B ghosts, and the color sequence must be adjusted as necessary.