Offset trouble diagnosis and elimination

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 tanks equipped with hydraulic presses for offset presses do not meet the vacuum requirements. Automatic water addition often fails, causing water buckets and layouts to be deficient in water. Operators do not pay much attention, and there is a problem with 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. The 8 foot screws are used to support the oil pan because it is necessary to put the oil pan below and it is impossible to use cement to fill the seal. Therefore, it is required that the surface strength of the foundation is high, and the specification states that the 200th cement should be used instead of the 400th cement. , and rolling steel bars on the foundation, otherwise the feet of the place where the screw contact is shaken, 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, a cardan shaft and a fork joint with uneven force will cause a paper feed stall 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 roller may also be worn. For example, the roller of an imported Roland four-color machine is made of ductile iron with good abrasion resistance. However, after the product of the same specification is often printed, 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 is 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, causing 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 to 25%, the domestic no-instrument measurement, depends on the operator's technology and experience to decide, which contains 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. Third, the printing material factors Offset materials, mainly paper, ink, printing plates, blankets and pad lining four categories. Various materials must meet the printability requirements of offset printing properties, otherwise there will be various printing failures. 1. Paper-based paper physicochemical properties and technical indicators, including physical performance indicators, optical performance indicators, printing performance indicators, chemical performance indicators. Among them, tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, glossiness, pH, etc. are closely related to offset printing suitability. The tightness and tensile strength of the paper, and the pressure and separation during the printing process, the viscosity of the ink and the sticky force generated by the surface of the blanket can cause hair loss and powder fall, resulting in burr-like spots on the printed material or in the field. Loss of money. The retractility of the paper is related to the direction in which the fibers are arranged. The stretch rate in the direction of the filaments is smaller than the retractility in the direction of the fibers at both sides, so the offset printing is usually done with the paper in the direction of the filaments parallel to the axis of the cylinder to minimize the size. The overprint of the paper due to wet stretch is not allowed. The whiteness and smoothness of the paper are related to the vividness and saturation of the printed image. After the paper with high whiteness is printed with colored ink, the light projected on the paper is absorbed less by the paper and reflects more. The original appearance of ink color is better presented. The paper with poor whiteness has confetti, absorbs much of the projected light, has less reflected light, and the printed image is dark. Papers with poor smoothness have a rough surface, strong oil absorption, dullness after imprinting, more projection light, and a diffuse reflection of reflected light, poor visual quality of printed matter, and dullness of colors. The oil-absorbing paper absorbs a large amount of the binder in the imprinted ink before the blot has dried and the conjunctiva, so that the surface of the imprinted film is rough, and the product does not stain. In severe cases, imprinting failures may also occur. General paper pH value, above 5.5, was slightly acidic, when the pH dropped below 5.5, the acidity of the paper increased, this paper can damage the drying of the ink, resulting in imprinting failure. 2. Ink is used to represent the image image and color of the ink, is a plastic fluidity, through the ink roller transmission, coated on the plate on the graphic, with a blanket, and finally transferred to the paper After drying the conjunctiva, it becomes solid. Offset printing requirements The main indicators of printing ink suitability are: fixing, drying, coloring rate, fineness, viscosity, fluidity, and resistance to acids, alkalis, water, and light.