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.
A mechanical failure factor
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 failures occur at the same time; some offset presses do not clean the teeth and do not add oil, paper hair accumulation, rust stains, single bite teeth rust and bite dead, losing bite, resulting in printing overprint 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 of the machine to break the oil and bite; 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 cover 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.
Second, 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 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 overprints are 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 paper 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 blue and purple. It's not pretty. There are two main reasons for the color deviation of the yellow label: one is the retinal colorimetric cells of the human eye, which has the lowest resolving power for yellow; the second is that people do not know much about the oil absorption of paper, and lack of instrument detection. 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.
Three printed material factors
Offset materials mainly include paper, ink, printing plates, blankets and cushions. Various materials must meet the printability requirements of offset printing characteristics, otherwise various printing failures will occur.
1 Paper factor
The physical and chemical properties and technical indexes of paper, including physical performance indexes, optical performance indexes, printing performance indexes, chemical performance indexes, etc. Among them are closely related to the suitability of offset printing: tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, pH, etc.
The paper's tightness and anti-brush strength value, when the pressure is separated during the printing process, the viscosity of the ink and the adhesive force generated by the moisture on the surface of the blanket will cause hair loss and powder loss, resulting in burr-like spots or solid spots on the printed product. Failure to spend money.
The stretchability of the paper is related to the direction of the fiber arrangement. The stretch rate in the filament (sin) direction is less than the stretch rate in both directions of the fiber arrangement. Therefore, offset printing is usually used for printing on paper with the filament direction parallel to the cylinder axis to minimize Image overprint caused by wet elongation of paper is not accurate.
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 color ink, the light projected on the paper is absorbed less by the paper and reflected more, so it can Better present the original appearance of ink color. The paper with poor whiteness has a member, which absorbs more projection light and reflects less light, and the printed image is gray. The surface of the paper with poor smoothness is rough, strong in oil absorption, dull after the print is dried, the projected light is absorbed more, and the reflected light is diffusely reflected, the visual effect of the printed matter is poor, and the color is dark and not bright.
Before the blotting has dried the conjunctiva, the oil-absorbent paper absorbs a large amount of the binding material in the blotting ink, resulting in a rough conjunctiva on the surface of the blot, and the product is not tarnished. When it is serious, it will also produce a pulverization failure.
Generally, the pH value of paper is above 5 and it is slightly acidic. When the pH value drops below 5, the acidity of the paper increases. This kind of paper can destroy the dryness of the ink and cause the printing to dry.
2 Ink factor
The ink used to express the image and color of the image is a kind of plastic fluidity, which is transmitted through the ink roller, coated on the graphics on the printing plate, used by the blanket, and finally transferred to the paper surface, after drying the conjunctiva Later it becomes solid. The main indicators of ink printing suitability for offset printing are: fixability, dryness, coloring rate, fineness, viscosity, fluidity, and acid resistance, alkali resistance, water resistance, light resistance and other resistance.
With the increase in the speed of offset presses and the advent of multi-color 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 it takes about 0.5 seconds (s) to print the second color for each printing. If the fixation speed of the ink is slow, the front color imprint will stay more on the back color blanket, and gradually reverse the transfer, so that the back color will produce a color mixing disadvantage. The drying of ink blots usually takes more than four hours. The early drying of the ink will cause skinning on the rubber roller, which will cause trouble for normal printing and cleaning work. If the imprint printed on the paper is over-dried, crystallization (vitrification) occurs, so that the post-color cannot be printed. On the contrary, if the imprint is not dry, the post-color cannot be printed, which reduces the production efficiency.
The higher the coloring rate of the ink, the better, it is required to achieve 100%, but some ink coloring rate can only reach 80 ~ 90%, in order to achieve color saturation, offset operators have to take the method of increasing the thickness of the printed ink layer to remedy , But the ink layer is printed too thick, it is easy to cause the back of the product to be dirty and the dots of the dark tone area are blurred.
In general, the diameter of ink particles is about 7.5μm. If calculated according to the area of ​​200 lines and a dot area of ​​1613μm2, 68 pigment particles can be accommodated. According to the printability requirements, more than 15 particles are more than enough. However, in actual printing, the resistance of domestic inks is poor. Due to the friction of the ink rollers when the machine is running, the viscosity of the ink increases, and the powder on the paper is mixed with the paper wool, which can make the particles thicker 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 a considerable number of factories are still using protein plates, PVA plates, and multi-layer metal plates. In recent years, zinc oxide paper base plates have been developed for printing printed copies. The plate-making processes of these printing plates are different. As the plate-making process, the PS plate-making process is the simplest, and the PVA version and the multi-layer metal plate process are more complicated.
The operation of developing, corroding, and applying base paint on the PVA plate are all manually controlled. Therefore, the coating of the printing plate is difficult to control, so that the color reproduction of the printed matter often fails to meet the original requirements. Moreover, the thickness of the zinc plate frosting is controlled by experience. Quality is not guaranteed. Sometimes the number of grinding times of zinc skin is increased, the plate material is reduced from the original 0.55mm to 0.45mm, and the machine operator also pads the printing plate based on experience. Often, too much or too little padding is equivalent to changing the plate cylinder The radius also changes the size of the inner picture, making the overprint of the inner picture inaccurate.
The multi-layer metal plate can have a printing durability of up to several million sheets, which is suitable for the printing of invoices, tax bills, thin books, account books, etc. with a uniform and large printing volume. However, in the printing process, if the paper is rubbed by sand particles, the chromium layer of the plate is often crushed and exposed to copper, resulting in fat-sensing and ill-effects. To eliminate the chromium-plating layer is very troublesome, and potassium flavinate must be used first Decontamination, and then use special equipment to pass chrome plating, which takes more time.
The cost of zinc oxide paper base plate is low. Although the manufacturing process is simple, it has high requirements on the environment temperature and humidity, the current and voltage of the electrostatic plate making machine, the water content of the basic body of the paper plate, the ink sensitivity, and the electrostatic conductivity of the toner. Often there are some deviations in some links, and the produced text is white or dirty, and there are many bad versions.
4 Operating environmental factors
At present, there are many new offset printing machines and new offset printing workshops in China. However, due to the insufficient understanding of the offset printing process by managers, the temperature and humidity of the operating environment cannot be strictly controlled. Problems such as inaccurate overprinting caused by paper bowing and wrinkling, and the impact of static electricity on paper have long been difficult to solve completely. They cannot produce properly and exert their due benefits. The operating environment here refers to area, height, level, Paper drying area, doors and windows and ground, workshop orientation and lighting, process arrangement, logistics direction, and air pump and pool and safety, dustproof, shockproof, fire prevention, heating, ventilation, lighting and other contents. The operating environment has an important influence on the printing quality. A good operating environment can reduce the occurrence of failures.
5 Maintenance factors
Practice has proved that in addition to the design and manufacturing factors of the machine itself, many mechanical failures are not well maintained, and improper operation is also an important cause of failures. Similarly, if a piece of equipment is properly maintained, it can be used for about ten years without major repairs without accidents. On the contrary, it may take as little as three years or more than five years to print fine products. Although it may delay some time and affect some output during maintenance, it is worth it in the long run. That kind of task requires little or no maintenance when busy, and maintenance when not busy is actually an irresponsible attitude; some even take the express train and fight for equipment, which is even more wrong.
Maintenance is nothing more than according to the requirements in the instruction manual, according to the parts are divided into daily maintenance, weekly maintenance, monthly maintenance, seasonal maintenance and half-year maintenance. Its content includes regular oiling, oil changing, cleaning parts, cleaning various parts of the filter; maintenance of various motors and electrical appliances, adjustment and replacement of carbon Brushes; regular inspection verification and inspection of various data of water (ink) roller and roller pressure Whether each part is loose and worn; and the cleaning and sanitation of the machine are well done, so that the equipment is always in good operation. In order to prolong the service life of the machine and reduce various failures caused by poor maintenance.
Four methods for checking and eliminating offset printing faults
The biggest distress for the operators of offset presses is that a fault occurs during the printing process and the cause of the fault cannot be found. Often, the school bus has to be shut down for failures such as inaccurate overprinting, bumps, ghosting, paper crepe, and inverted smooth hair. It is common to spend two or three hours each time. Sometimes it takes a few days to solve a failure. Serious loss of working hours.
There are many factors for offset printing faults. Sometimes there are dozens of possible causes for a fault. If you want to find the root cause, take fewer or no detours, and avoid wasting unnecessary and effective working hours, you can use the following methods.
1 analysis method
After the offset printing fault occurs, there are many ways to find 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 based on the analysis of the time, location, condition and shape of the fault. List the subjective factors of these four aspects one by one, and then conduct a comprehensive analysis, and many failures can be solved. In actual production, some operators lack comprehensive analysis of these four aspects after encountering a fault, resulting in extended downtime and unnecessary loss of working hours.
An example is as follows: During the work of an offset press, the operator found that the paper was bowed and deformed. The product was a magazine cover. Instead, it had two colors and four colors on the front. When printing on the reverse side, the paper is normal, and the reverse side is printed on the front side. When printing to the 25th order paper, it is found that the trailing edge of the paper has a 50mm long crepe phenomenon. Analysis, preliminary conclusions:
â‘ The fault was just generated, indicating that there are new factors leading to bowing.
â‘¡ The part is on the trailing edge of the paper, which may be a machine factor or a paper factor.
â‘¢ Judging from the length of the paper crepe is only 50mm, it is impossible to be caused by mechanical failure.
â‘£ From the analysis of the paper conditions that produce more than 50mm long bow crepe, it is generally possible under the condition of "lotus leaf edge", but intuitively the paper is very flat and it is impossible to produce bow crepe.
After this analysis, the machine factor and the paper factor were negated, and the focus was on a problem that had just arisen, which immediately reminded people of the possibility of the back imprint stack sticking to the impression cylinder. Because the two-color prints printed on the back are often accumulated on the surface of the impression cylinder because the imprint has not yet dried out, this ink accumulation is both irregular and thicker, which is equivalent to gradually increasing the rubber cylinder and the irregular The pressure between the impression cylinders causes the paper to be pressed on the uneven surface, which causes the trailing bow crepe failure. After checking the impression cylinder, it is indeed the accumulation of ink. This problem disappeared after printing after cleaning, and it took more than ten minutes before and after.
Of course, some faults are more complicated, and the analysis method alone cannot solve the problem, and the detection method and the test method must be used together.
2 detection method
The so-called detection method is to use tools and testing instruments such as magnifying glass, micro-card, iron absorption dial gauge, diameter gauge, vibration oscilloscope, density meter, etc. to observe the fault, inspect and measure the machine parts, and find out The cause of the failure comes. The detection method is often used to find faults such as bars, ghosting, and empty dots.
The detection method requires much longer time to resolve the fault than the analysis method. Generally, before using the detection method, try to use the analysis method to analyze, and then focus on one or two points for the necessary detection to find the cause of the fault.
3 test method
The test method is commonly referred to as "try it out". Some inexperienced new workers and those who do not have comprehensive analytical skills often use this method. The test method takes a long time, is easy to take detours, and brings a certain loss to production. But once successful, the operator not only accumulated experience, but also improved the technology.
The test method is usually more effective for the use of new paper, ink, and plate materials. But sometimes the common faults that occur in the printing process are caused by the lack of experience of new workers, and they are also regarded as new problems for experimentation. In this way, simple problems are often complicated and a lot of man-hours are wasted.
For example, the overprint that is not in place at one end is not allowed, and it is not allowed to be around 2mm. The cause of this failure is either the paper is skewed and the end is not in place, or the hard paper with more than 80g / m2 is ejected from the front regulation. . However, some operators spent a lot of time to correct the front gauge, teeth pressure, and handover relationship, and people were also very tired. As a result, the failure was still not ruled out. Therefore, it is recommended to use a negotiated method to find the cause of the failure after a failure occurs during the printing process. New workers must consult with middle-aged and elderly workers with higher skills in order to eliminate the failure quickly, which is conducive to improving product quality. I also learned the technique of troubleshooting and improved the operation level.
In short, for the various faults encountered in the production of offset printing, it is recommended to use analytical methods and detection methods to find out the cause of the fault. When encountering complex faults, it is necessary to comprehensively use the analysis method and the detection method, so as to eliminate various faults quickly and well, and achieve the purpose of improving product quality and increasing effective working hours.
Judgment of abnormal sound of five parts
As a skilled technician in the operation of offset presses, it is necessary to develop ear hearing skills in daily production. Because there are many parts of the offset press, and the sounds of various parts during operation have a fixed tone and beat, the operator has to identify which part of the sound has changed, so as to find the problem. Hearing skills are particularly important to ensure the safe operation of the equipment, because most of the components of modern offset printing machines are enclosed in the casing, invisible to the eyes, and cannot be touched by the hands. The only thing that can be felt by people is that they are emitted during operation. Sound, this requires that in normal work, pay attention to the sound that the machine parts make when they are normal, and listen to and remember clearly. The average operator's minimum requirement is to "check the disease by smelling sound." If it stays at this stage, it is obviously not enough. Operators with real skills should strive to "know the disease by smelling sound." This "knowing the sound and knowing the disease" is a great skill, and even the provisions that should be known should not be included. The following are some rough experiences of the author in practice.
1 The abnormal sound made by the components outside the hood
â‘ J2108, J2203 and J2201 offset presses sometimes make a "squeak-slap" sound. The "squeak" sound is sharper, and the "slap" sound is louder but it seems not neat. This is the sound that the tip of the swinging tooth touches the empty edge of the impression cylinder. Or the sound of the paper collecting teeth and the tip of the teeth touching the empty edge of the impression cylinder. Because the tip of the tooth is sharp, when the anti-point is scraped, it slides against the neutral edge of the roller, and there must be a sharp "squeak" sound. The "slap" sound when closing the tooth is not neat. This is because the length of the tooth is short Close your teeth, and slowly close your teeth while scratching the gap, so the "slap" sound will not be neat.
The swinging tooth tip touches the blank edge of the impression cylinder, the position of the large cam can be adjusted or the diameter of the roller can be increased; the paper collection tip touches the blank edge of the impression cylinder, and the running time of the paper row can be adjusted as a whole .
â‘¡ The sound of "Boom, Boom" is made when the drum is pressed and operated, and the pressure used by the machine is too high. Sometimes the sound of "clap, crap, crap" is that the printing plate is not strained or broken. The former needs to reduce the pressure between the rollers, the latter needs to be tightened or replaced with a new printing plate.
â‘¢ When the paper stack is full of paper and printing is started, the paper feeder makes a sharp "squeak-squeak" sound like every piece of paper. This is because a small paperboard (crushing) hits the paper transfer roller. Loosen the screw in the middle, and let the fixed foot sink a little.
â‘£ The paper stack rises up and the suction iron keeps ringing continuously, but the paper stack does not rise. This is because the pin of the continuous iron core is broken or is about to slide out. It should be stopped immediately and connected Get it right.
⑤ During the printing process, every time a piece of paper is printed, a "burst-burst-burst" sound is emitted, which shows that the pressure of a certain water and ink roller on the printing plate is too high. Because the ink roller and water roller are made of rubber, the sound is relatively dull. To check which one? Is the adjustment screw moving? Then adjust it.
â‘¥ The sound of "snap, slap, slap" from the bottom of the machine's paper collection part, this is because the paper collection chain is too long, and the swaying force of the tooth row when turning is generated on the guide rail. .
⑦ The paper-receiving stacking platen rises too high and hits the paper-collecting suction rod. The suction wheel will make a "squeak-squeak" sound. It can move the suction eye down and the sound should be Will disappear.
The above are frequently heard abnormal sounds and troubleshooting methods.
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