Offset failures are broadly classified into mechanical failures, printing process technical failures, material-induced failures, operating environment and maintenance-related failures, and electrical failures.
a mechanical failure factor
Printing machinery produces the most failures. Even if a new offset press is produced, it will malfunction during work due to some defects in design assembly and installation and commissioning. An offset press consists of tens of thousands of parts, and the manufacturing precision of each part is extremely important. Due to the limitations of the working machine and operating technology of the machined parts, some parts do not reach the advanced level, which also causes mechanical failure. A factor. Some offset printing machines, such as the gears and rollers in the main machine, have good processing quality, but the technical performance of the matching parts is poor, which will also bring trouble to normal printing. For example, some vacuum water tanks equipped with water dispensers and water can not meet the vacuum requirements, and the automatic watering often fails, resulting in water shortage in the water bucket and the layout. The operator does not pay attention, and the dirty paste on the printed matter will occur.
The installation and commissioning of the offset press is also important. Many plants ignore the ground strength and hardness issues when they make the foundation of the foot before installing the machine. Nowadays, the 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. Adjust with the horizontal level of the bottom screw of the machine base. For example, there are 8 calibration screws under the base of J2108, and the contact point of the screw is 8cm square iron. After correcting the level, use the nut to fasten the base to prevent the foot screw from moving. The problem is: the base of the machine is not on the ground. Supported by 8 foot screws, because the oil pan is to be placed below, it is impossible to seal with cement. Therefore, the surface strength of the foundation is required to be high, and the specification uses 200 cement, and the actual use of 400 cement is better. And the steel is rolled on the upper layer of the foundation, otherwise the cement is easily damaged or cracked after the ground screw is touched, and the machine loses its level. In particular, the roller journal and the bushing wear quickly, which may cause printing failures such as ghosting and bumping.
After the machine has been used for a certain period of time, various faults will occur due to the wear and tear of the parts and the decrease in accuracy. The wear and tear of the parts is slow and slow. In addition to the factors of poor maintenance, it is also related to the materials used in the machine parts and the manufacturing process. For example, the paper feeding ball made of glue wood is easy to wear into an oval shape, causing the paper to be skewed. For example, the ball and the fork of the universal joint shaft with uneven force will cause paper feed failure after wear. Embossing the shaft swinging rod and the ball, due to the large uneven force, the push pin is loose, broken, and the ball bearing is broken, resulting in inaccurate overprint or equipment accident.
The surface of the drum is also worn. For example, the impression cylinder of the imported Roland four-color machine is made of ductile iron with good wear resistance. However, after printing the same specification, the surface of the impression cylinder will occur at the edge of the paper. The linear wear and depression, so that when the product of larger size is reprinted, a white bar appears on the mark, which affects the quality of the product.
The offset press should be cleaned, maintained, and refueled frequently to work properly. If the maintenance is not good, the fuel is not working, and there are more faults. If the roller of the offset printing machine of a factory is full of ink and glue to dry the dirt, it becomes a "contact roller" during the pressing process, and the roller is stretched open, causing serious wear of the roller journal and bushing. The ghosts, bars, greasy and other faults occur at the same time; some offset printers do not clean the teeth, do not refuel, the paper piles are piled up, the rust is streaked, the single bite paper rusts and bites, loses the bite force, and the printed matter is not printed. Quasi-, "peeling" faults are concurrent.
Mechanical wear or seizure failure caused by oil cuts is common in offset printing plants. Some factories have a refueling system, but they do not implement them. They have broken out the oil and killed the machine. Some of the offset printing machines have a pile of iron powder that is unmanned; the tubing inside the casing is broken or Equipment failure caused by insufficient oil delivery is also caused by lack of inspection. In general, equipment maintenance is not good, the system is not perfect, or the implementation is weak due to the system, which is the direct cause of many printing failures.
Two process technology factors
Due to the physical and chemical aspects of the offset printing process, there are high requirements for mastering the process and technology. Due to the problems of the process and technical level of the management personnel and operators, the offset failure is also more.
1 offset characteristics
Offset printing is performed using the principle of oil and water repulsion, on the same plate. The graphic part is lipophilic 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 case where both ink and water are present on the printing plate, the ink roller that transports the ink is in contact with water. Due to the high-speed rotation of the ink roller, water is mixed into the ink under the action of mechanical force, causing ink emulsification and normal printing. A reasonable emulsification value is reached. Therefore, offset printing actually utilizes the principle of oil and water repelling, and utilizes the characteristics of oil and water to dissolve and emulsify. According to foreign experimental data, when offset printing is generally normal printing, the reasonable emulsification value of ink, water accounts for about 20 to 25%. There is no instrument measurement in China, which is determined by the operator's technology and experience. This includes unstable factors. , which increases the possibility of failure.
The water used for offset printing is not pure water. The PVA version contains phosphorus and chromic acid in water, which is very corrosive to the layout. It not only corrodes the blank part of the blank part of the layout, but also continuously accumulates the salt layer of the machine while corroding the graphic part. The base paint reduces the printing durability of the printing plate. The wetting powder added to the PS version 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 foundation for process technology. Because the offset printing is indirect printing, the image is transferred from the plate cylinder to the blanket for the first imprinting. After the blanket cylinder receives the ink, the imprint is transferred to the substrate. This is the second imprinting and two pressing. The pressure required for printing is different. How to achieve the "ideal pressure" of the realm, we must make some efforts. Generally, the pressure between the offset printing press and the plate cylinder and the blanket cylinder is about 0.10 mm; the pressure between the blanket cylinder and the impression cylinder is about 0.20 to 0.25 mm. With the use of a better flatness PS plate and imported air cushion blanket, the pressure between the plate cylinder and the blanket cylinder can be smaller, such as the imported Heidelberg four-color machine can be printed at 0.05mm.
The pressure used in offset printing is achieved by calculating the radius of the drum, measuring the center distance, measuring and increasing or decreasing the number of linings. The offset pressure must be standardized and controlled by data. The liner should not be added arbitrarily, the pressure will be increased blindly, and the pressure will suddenly change. The line speed of the roller surface will be different, the friction of the contact surface will increase, and even greasy, ghosting and ink will appear. Other faults such as bars. Therefore, the correct use and adjustment of printing pressure is an important part of the process technology required to print products.
3 sets are not printed
Imprinting is not allowed to be classified into vertical overprinting and horizontal overprinting. There is a saying that "the paper is always firmly controlled by the organic parts during the transmission process, and the overprinting can be guaranteed." In fact, this sentence is not comprehensive. For example, the embossed biting shaft or bushing wear is strong, but it can be firmly controlled when transferring paper, but it cannot be properly controlled. This is like a person holding a ball in both hands, the two arms can be stretched straight, and the two arms can be bent, but the balls held in two different positions are different from the distance and stability of the human body. . Therefore, it should be said that "the correct control is in place to ensure accurate overprinting."
There are many reasons for inaccurate overprinting. Common ones are: paper stretching, paper feeding, inaccurate positioning, insufficient biting force, wear of the biting shaft, unsatisfactory handover, and wear of the transmission teeth. In addition, some special factors may cause inaccuracies. For example, if the rubber layout is deformed, the image may be partially overprinted; if the transfer shaft and the impression cylinder axis are axially moved, the lateral overprint may be inaccurate.
4 color order arrangement
In the past, the color sequence of the monochrome machine was always yellow, red, blue, and ink. This color sequence was arranged. The larger difficulty was to sign the yellow and red prints, especially the first color. Edition, related to the color reproduction and reproduction of printed matter. If the yellow print is too deep, the color of the printed person has no sense of vitality, and the house is as old as the old temple, which makes it difficult to accept; if the yellow print is too light, the copied picture is dry and the color is biased toward blue-violet. Also not good-looking. There are two main reasons for the yellow color of the yellow version: one is the retinal color-resolving cells of the human eye, which has the lowest resolution for yellow; the other is that people do not understand the oil absorption of paper, lack of instrument detection, and the same size. It often causes too deep and too weak.
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 just plays the role of blue glass color filter, which is convenient for identifying whether the thickness of the yellow plate ink layer is suitable. However, due to the wet-stack printing, the two-color machine often produces irregular A and B ghost images on the two corners of the trailer. This requires that the color sequence be appropriately changed during the process arrangement.
Multi-color offset printing machine with more than four colors, because the three colors are wet and wet printing, the color sequence is arranged according to the viscosity of each color ink from high to low, otherwise the overprinting failure will occur, and the dark color of the ink will appear. Sometimes the first few colors will also produce A and B ghosts, and the color sequence should be adjusted as necessary.
Three printing material factors
Offset printing materials, mainly paper, ink, printing plates, blankets and padding. All materials must meet the printing suitability requirements of offset characteristics, otherwise various printing failures will occur.
1 paper factor
Physical and chemical properties and technical indicators of paper, including physical properties, optical properties, printing performance indicators, chemical performance indicators. Among them, closely related to the suitability of offset printing are: tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, pH and the like.
The tightness of the paper and the value of the tensile strength. When the pressure is separated during the printing process, the viscosity of the ink and the adhesive force generated by the surface moisture of the blanket may cause hair loss and powder drop, resulting in burr-like spots or solid spots on the printed matter. Whitening failure.
The stretchability of the paper is related to the direction in which the fibers are arranged, and the expansion ratio of the filaments in the direction of the filaments is smaller than the stretch ratio in the direction of the two sides of the fiber array, so that the offset printing of the filaments in the direction parallel to the axis of the drum is performed to minimize the printing. The image is overprinted due to wet elongation.
The whiteness and smoothness of the paper are related to the color vividness and saturation of the printed image. After the color white ink is printed on the paper with high whiteness, the light projected onto the paper is less absorbed by the paper and has more reflection. Better to present the original color of the ink color. The paper with poor whiteness has a partner, absorbs more projection light, less reflected light, and the printed image is gray. The paper with poor smoothness has rough surface, strong oil absorption, dullness after blotting, and the projection light is absorbed more, and the reflected light is diffusely reflected. The visual effect of the printed matter is poor, and the color is gray and not bright.
The oil-absorbing paper absorbs the binder in the imprinting ink in a large amount before the imprinting has not dried the conjunctiva, resulting in a rough conjunctiva on the surface of the imprint and a flawless product. In severe cases, there is also a blotting failure.
Generally, the pH of the paper is slightly acidic at 5 or more. When the pH drops below 5, the acidity of the paper increases, and the paper can destroy the drying property of the ink, resulting in a failure of the print.
2 ink factors
The ink used to express the image and color of the image is a plastic fluid, which is transported by an ink roller, coated on the graphic on the printing plate, coated with a blanket, and finally transferred to the paper surface, and dried and conjunctiva. It becomes solid afterwards. Offset printing requires the main indicators of ink printing suitability: fixability, dryness, coloration, fineness, viscosity, fluidity, and resistance to acid, alkali, water, light and light.
With the increase in the speed of offset presses and the emergence of multicolor machines, the fixation of fast-fixing inks is becoming more and more important. The printing speed of the multi-color machine is mostly above 8000 sheets/hour, and each printing sheet takes about 0.5 seconds (s) to be transferred to the second color printing. If the fixing speed of the ink is slow, the front color imprint will be more retained on the back color blanket, and the reverse color will be gradually reversed, so that the back color will cause color mixing defects. The drying of the ink blot is generally more than four hours, and the early drying ink is crusted on the rubber roller, causing trouble for normal printing and cleaning work. If the print printed on the paper is too dry, crystallization (vitrification) occurs, so that the back color is not printed. On the contrary, if the print is not dry, the back color cannot be printed, which reduces the production efficiency.
The higher the coloring rate of the ink, the better, it is required to reach 100%, but some ink coloring rate can only reach 80-90%. In order to achieve the color saturation degree, the offset printing operator has to take the method of improving the thickness of the ink layer to remedy. However, the ink layer is printed too thick, which tends to cause the back of the product to be dirty and the dots in the dark area to be blurred.
Generally, the diameter of the ink particles is about 7.5 μm. If it is calculated according to the line area of ​​1613 μm 2 of 200 lines, it can accommodate 68 pigment particles, and the printing suitability requirement is more than 15 pieces, which is more than enough. However, in actual printing, the domestic ink has poor resistance to polymerization. Because the ink rollers rub against each other during the operation of the machine, the viscosity of the ink increases, and the powder on the paper is mixed with the paper wool, which makes the particles coarser and thicker, affecting the printed matter. The clarity and level of expressiveness.
3 plate factors
Offset printing plates have now entered the PS version stage, but there are still a considerable number of factories using protein plates, PVA plates, multi-layer metal plates, and in recent years, the Zinc Oxide paper base plates have been developed. These plates have different plate making processes. As a printing process, the PS plate is the simplest, and the PVA plate and multi-layer metal plate process are more complicated.
The development, corrosion and painting of the PVA version are all controlled by hand. Therefore, the printing of the printing plate is difficult to control, and the color reproduction of the printed matter often fails to meet the original requirements. Moreover, the thickness of the zinc-coated version of the matte is controlled by experience. Quality is not guaranteed. Sometimes the number of grinding of the zinc skin is much higher, the plate material is reduced from the original 0.55mm to 0.45mm, and the machine operator also relies on the experience to lining the printing plate, often the padding is too much or too little, which is equivalent to changing the plate cylinder. The radius also changes the size of the inner image, making the inner image overprinted.
The multi-layer metal version has a printing durability of several million sheets, and is suitable for printing invoices, tax bills, thin books, books, etc. with uniformity and large printing volume. However, in the printing process, if the sand particles falling from the paper are rubbed, the chrome layer of the plate is often crushed and exposed to copper, which causes the viscous viscous ills. It is very troublesome to eliminate the chrome plating and crushing the smear. It is necessary to use potassium citrate first. It is more time-consuming to remove the dirt and then use special equipment to chrome.
The zinc oxide paper base plate has low cost, and the manufacturing process is simple, but the environmental temperature and humidity, the current voltage of the electrostatic plate making machine, the water content of the basic body of the paper plate, the ink sensibility, and the electrostatic conductivity of the toner are required to be high. Often, some links are slightly biased, and the printed version of the text is white or full of dirty, and there are many bad versions.
4 operating environment factors
At present, there are many new offset printing machines and new offset printing factories in China. However, due to the lack of understanding of the offset printing process by managers, the temperature and humidity of the operating environment cannot be strictly controlled. Problems such as paper wrinkles, inaccurate overprinting caused by stretching, and problems such as paper with static electricity affecting printing have long been difficult to completely solve, and it is impossible to produce and exert its proper benefits. The operating environment here refers to the area, height, level, Air paper occupies, doors and windows and ground, workshop seating and lighting, process arrangement, logistics direction, as well as air pump and pool and safety, dust, shock, fire, heating, ventilation, lighting and other content. The operating environment has an important impact on the quality of the print, and a good operating environment can reduce the occurrence of faults.
5 maintenance factors
Practice has proved that many mechanical failures, in addition to the design and manufacturing factors of the machine itself, are not adequately maintained, and improper operation is also an important cause of failure. If the same equipment is properly maintained, it can be used for about ten years without major accidents, and vice versa. If it is less than three years, it will not be able to print fine products for five years. Although it takes some time to maintain and affect some production, it is worth considering in the long run. When the task is busy, it is less maintenance or maintenance, and when it is not busy, it is actually irresponsible attitude; some even take the practice of driving fast and assembling equipment, it is even worse.
Maintenance is based on the requirements of the instruction manual, divided into daily maintenance, weekly maintenance, monthly maintenance, quarter maintenance and half-year maintenance. The contents include regular refueling, oil change, cleaning parts, cleaning of filters at various parts; maintenance of various motors and electrical appliances, adjustment and replacement of carbon brushes; periodic inspection verification and inspection of various data of water (ink) rollers and rollers Whether the parts are loose or worn; and do a good job in cleaning and sanitizing the machine, so that the equipment is in good working condition. To extend the life of the machine and reduce various failures caused by poor maintenance.
Four methods for checking and eliminating offset faults
The biggest distress of the offset press operator is that a fault has occurred during the printing process and the cause of the malfunction has not been found. Often, in order to overprint, bar, ghost, paper, and hair, you have to stop the school bus. It is common to use two or three hours each time. Sometimes it takes a few days to solve a fault. The working hours are severely damaged.
There are many factors in offset printing. Sometimes there are dozens of possible causes for a fault. If you want to find the root cause, take less or no detours, and avoid wasting unnecessary effective working hours, the following methods can be used.
1 analysis method
After the offset fault occurs, there are a variety of ways to find out the cause of the fault, and among the various methods, the analysis method is the fastest to find the cause of the fault.
The so-called analysis method is a method of analyzing the four aspects of the time, location, condition, and shape of the failure. The subjective factors of these four aspects are listed item by item, and then comprehensive analysis, many failures can be solved. In actual production, some operators lack comprehensive analysis of these four aspects after encountering the fault, which leads to prolonged downtime and unnecessary work time loss.
An example is as follows: During the working process of an offset printing machine, the operator finds that the paper is twisted and deformed. The product is a magazine cover, but two colors and four colors on the front. On the reverse side, the paper was normal when printing first, and the front side was printed on the reverse side. When printing on the 25th order paper, the paper was found to have a 50mm long bowing phenomenon. After the machine was stopped, the experienced people used the analysis method to analyze various possible factors. Analysis, preliminary conclusions:
1 The fault was just created, indicating that there are new factors that are causing the bow.
2 The location of the paper at the tip of the paper may be a machine factor or a paper factor.
3 From the perspective of the shape of the paper cassette only 50mm, it is impossible to cause mechanical failure.
4 From the analysis of paper conditions that produce 50mm long bows, it is generally possible in the case of “ruffled edgesâ€, but the intuitive paper is very flat and it is impossible to produce bows.
After this analysis, the machine factor and paper factor were negated, and it was concentrated on a problem that had just arisen, which immediately reminded people of the possibility of the backside imprinted on the impression cylinder. Because the two-color printed on the back is often deposited on the surface of the impression cylinder because the imprint has not dried out, the ink deposit is irregular and thicker, which is equivalent to gradually increasing the blanket cylinder and irregularly. The pressure between the impression cylinders causes the paper to be pressed on a rugged plane, thus creating a bowing fault. After viewing the impression cylinder, it is indeed ink accumulation. This problem was lost when printing and then printing, and it took more than ten minutes before and after.
Of course, some faults are more complicated, and the problem can not be solved only by the analysis method. It is necessary to use the detection method and the test method.
2 detection method
The so-called detection method is to use a magnifying glass, a thousandth card, an iron percentage dial gauge, a homometer, a vibrating oscilloscope, a density meter and other tools and testing instruments to observe the fault, check and measure the machine parts, and find out The cause of the malfunction comes. Detection methods are often used to find faults such as bars, ghosts, and dot emptiness.
The time required for the detection method to solve the fault is much longer than the analysis method. Generally, before using the detection method, analyze the analysis method as much as possible, and then concentrate one or two points to perform the necessary detection to find out the cause of the failure.
3 test method
The test method is commonly referred to as "try it out." This method is often used by some inexperienced new workers and those who do not have comprehensive analytical skills. The test method takes a long time and is easy to take detours, which brings certain losses to production. But once successful, the operator not only accumulated experience, but also improved the technology.
The test method is generally effective for using new paper, ink, plate, and the like. But sometimes the common faults that occur during the printing process are caused by the lack of experience of new workers, and they are also regarded as new problems for testing. This often complicates simple problems and wastes a lot of man-hours.
For example, if the one end is not in place, the overprinting is not allowed, and it is not allowed to be around 2mm. The reason for this failure is that the paper is skewed at the end, or the hard paper of 80g/m2 or more is ejected from the front gauge. . Some operators have to correct the pre-regulation, the pressure on the teeth, and the handover relationship. It takes a lot of time and people are very tired. As a result, the fault has not been ruled out. Therefore, after the failure in the printing process, it is recommended to use the means of consultation to find the cause of the failure. The new workers should especially discuss with the middle-aged and older workers with higher technology, so that the fault can be quickly eliminated, which is beneficial to improve product quality. I also learned the techniques to troubleshoot and improve the level of operation.
In short, for the various faults encountered in offset printing, it is recommended to use analytical methods and detection methods to find out the cause of the fault. In the case of complex faults, it is necessary to use the analysis method and the detection method in a comprehensive manner, so as to quickly and well eliminate various faults, thereby achieving the purpose of improving product quality and increasing effective working hours.
Judgment of abnormal sound of five parts
As a skilled craftsman who operates as an offset press, he or she should practice the auditory skills of the ear in daily production. Because there are many parts in the offset printing machine, the sounds of various parts in operation have fixed sounds and beats, and the operator has to identify which part of the sound is changed and find the problem. The auditory skills are especially important for ensuring the safe operation of the equipment, because most of the components of the modern offset press are enclosed in the casing, the eyes are invisible and the hands are not touched. The only thing that can be felt is that it is issued during operation. Sound, this requires that in the usual work, pay attention to the sound of the normal parts of the machine, familiar with the record. The general operator, at least the requirement is to "sound the disease", if it stays at this stage, it is obviously not enough. The operators with real skills must strive to "sound the disease." This "sounding and ill" is an amazing skill, and even the provisions that should be known should not be booked. The following is some of the rough experience of the author in practice.
1 The sound of the parts outside the hood is abnormal when it is not working properly.
1 J2108, J2203, J2201 offset presses sometimes emit a "å±--啪哒" sound when the reverse point is made, the "å±" sound is sharper, and the "啪哒" sound is louder but does not seem to be neat. This is the sound that the swinging tooth tip touches the neutral side of the impression cylinder. Or it is the sound of the picking nipple touching the neutral side of the impression cylinder. Because the tip of the tooth is sharp, when the point is reversed, the roller slides along the neutral side of the drum, which will inevitably produce a sharp "beep" sound. The "beep" sound when closing the teeth is not neat, because the length of the tooth tip is short and short. Close the teeth, long scratching the gap and slowly closing the teeth, so the "squeaky" sound will not be neat.
Swing the cusp against the neutral side of the impression cylinder, adjust the position of the large cam or increase the diameter of the roller; the tip of the delivery nipper touches the neutral side of the impression cylinder, which can solve the running time of the paper platoon as a whole. .
2 When the drum is pressed and pressed, it will make a “beep, bang†sound. Once you hear it, the pressure used by the machine is too great. Sometimes the sound of "squeaky, slightly, slightly" is made, the plate is not tightened or broken. The former needs to reduce the pressure between the rollers, and the latter needs to be tightened or replaced with a new printing plate.
3 When the paper stack is full of paper, the paper machine makes a sharp “interrupted – å±--†sound like each sheet of paper, which is the small paper-blocking paper that meets the paper-feeding roller. Loosen the middle solid screw and let the foot sink down to solve it.
4 The stack of paper rises and rises, and the iron and steel “嗒, 嗒, 嗒†continue to ring and the pile does not rise. This is the pin of the continuous iron core that is broken or will be slipped out. Get it done.
5 During the printing process, a “sudden-burst-burst†sound is produced for each printed sheet of paper, which indicates that the pressure of a certain water and ink roller on the printing plate is too great. Because the inking roller and the water roller are made of rubber, the sound is relatively dull. To check which one? Adjust the screw to move? Then adjust it.
6 The sound of “嚓, 嚓, 嚓†from the delivery position of the machine. This is because the delivery chain is too long. The force of the tooth row on the guide is generated on the guide rail. The delivery chain on both sides should be properly tightened. .
7 The stacking platen rises too high, and it hits the delivery suction rod. The suction wheel will make a “sound-sound†sound, which can move the suction eye down and the sound should be Will disappear.
The above are commonly heard abnormal sounds and troubleshooting methods.
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