Troubleshooting three of the most popular offset p

2022-09-21
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Troubleshooting of offset printing (III)

the elasticity of paper is related to the direction of fiber arrangement, and the expansion rate in the filament (along the filament) direction is less than that in the direction of both sides of the fiber arrangement. Therefore, offset printing is used to print with paper whose filament direction is parallel to the axis of the drum, so as to minimize the inaccurate image overprint caused by the wet elongation of the paper

the whiteness and smoothness of the paper are related to the brightness and saturation of the printed image. After the paper with high whiteness is printed with color ink, the light projected on the paper surface is less absorbed by the paper and reflects more, so it can better present the original appearance of the ink color. The paper with poor whiteness is divided into groups, which absorbs more projected light, reflects less light, and the printed image is gray. The paper with poor smoothness has rough surface and strong oil absorption. The imprint is dull after drying, and the projected light is absorbed more, and the reflected light is diffuse. The visual effect of the print is poor, and the color is gray and not bright

the oil absorbing paper absorbs a large amount of binder in the imprinting ink before the imprinting has dried the conjunctiva, resulting in rough conjunctiva on the imprinted surface and glossy products. In serious cases, there will also be imprinting pulverization failure

generally, the pH value of paper is slightly acidic when it is above 5.5. When the pH value drops below 5.5, the acidity of paper increases. This kind of paper can destroy the dryness of ink and produce imprinting failure

2. Ink factor

the ink used to express the image image and color is a kind of plastic fluidity, which is transmitted by the ink roller, coated on the graphics and text on the printing plate, and finally transferred to the paper through the rubber blanket. After drying the conjunctiva, it becomes solid. The main indicators of ink printability required by offset printing are: fixation, dryness, coloring rate, fineness, viscosity, fluidity, and acid, alkali, water, light and other durability

with the improvement of the speed of offset press and the emergence of multicolor machine, the fixation of fast fixing ink is becoming more and more important. The printing speed of multi-color machine is mostly more than 8000 pieces/hour, and it takes about 0.5 seconds (s) to transfer to the second color printing. If the fixing speed of the ink is slow, the front color imprint will stay more on the back color blanket, and gradually reverse transfer, causing the back color to have the disadvantage of color mixing. The drying of ink imprinting usually takes more than four hours. The early drying ink crusts on the cot, which brings trouble to the normal printing and cleaning work. If the imprint printed on the paper is too dry, crystallization (vitrification) will occur, so that the back color cannot be printed. On the contrary, if the imprint is not dry, the back color cannot be printed, reducing the production efficiency

the higher the coloring rate of the ink, the better. It is required to reach 100%. But some such special structures cause changes in performance. The coloring rate of the ink can only reach 80 ~ 90%. In order to achieve color saturation, offset printing operators have to take measures to improve the thickness of the imprinted ink layer to remedy it. However, if the ink layer is printed too thick, it is easy to cause sticky dirt on the back of the product and blurred dark tone areas

generally, the diameter of ink particles is about 7.5 μ m. If the area of one point is 1613 according to line 200 μ Calculated by m2, it can accommodate 68 pigment particles. According to the requirements of Printability 11, we must pay attention to the change of refrigeration material volume caused by temperature. More than 5 particles are enough. However, in actual printing, the anti aggregation property of domestic ink is poor. Due to the mutual friction of ink rollers during machine operation, the viscosity of ink increases, and the mixing of powder and paper wool on the paper can make the particles coarser and coarser, affecting the clarity and hierarchical expressiveness of printed matter

3. Printing plate factor

offset printing plates have entered the PS version stage at present, but a considerable number of factories are still using protein version, PVA version, multi-layer metal version, and in recent years, they have developed a zinc oxide paper base for special printing of reprinted books. The plate making processes of these printing plates are different. As for the plate printing process, PS plate drying is the simplest, while PVA plate and multi-layer metal plate technology are more complex

pva plate development, corrosion and base paint coating operations are all manual, so the painting of the printing plate is difficult to control, resulting in the color restoration of the printed matter often fails to meet the requirements of the original. Moreover, the thickness of the frosting purpose of the zinc skin plate is controlled by experience. The quality is not guaranteed. The experimental data are accurate and reliable. Sometimes the grinding times of zinc skin are more, and the plate material is reduced from the original 0.55mm to 0.45mm. The machine operator also pads the printing plate based on experience. Often, the pad lining is too much or too little, which is equivalent to changing the radius of the printing plate cylinder, and also changing the size of the internal image, making the internal image overprint inaccurate

the printing resistance of multi-layer metal plates can reach millions of sheets, which is suitable for printing invoices, tax bills, thin books, account books, etc. with unity and large printing volume. However, in the process of printing, in case of sand friction falling from the paper, the chromium layer of the layout is often crushed and exposed copper, resulting in grease sensitive dirt. It is very troublesome to eliminate the dirt on the crushed chromium plating layer. It is necessary to remove the dirt with potassium xanthate first, and then electrify the chromium plating with special equipment, which is more time-consuming

the cost of zinc oxide paper base plate is low. Although the production process is simple, it has high requirements for environmental temperature and humidity, current and voltage of electrostatic plate making machine, water content of the basic body of the paper plate, ink sensitivity, electrostatic conductivity of toner and other indicators. There is often a slight deviation in some links, and the prepared version text is gray or full of dirt, and there are many bad versions

IV. operating environment factors

at present, there are many new offset printing machines and new offset printing plants in China. However, due to the lack of understanding of offset printing process by managers, the temperature and humidity of the environment such as simple circuit, easy design, small interference, high transmission rate and good planting cannot be strictly controlled. The problems such as inaccurate overprint caused by paper bowing and stretching, and the electrostatic influence of paper on printing have been difficult to be completely solved for a long time, and they cannot be produced normally and give full play to their due benefits. The operating environment here refers to the area, height, level, paper drying land occupation, doors and windows and ground, workshop orientation and lighting, process arrangement, logistics direction, as well as air pump and pool, safety, dust prevention, shock prevention, fire prevention, heating, ventilation Lighting, etc. The operating environment has an important impact on the quality of printed matter. A good operating environment can reduce the occurrence of faults

v. maintenance factors

practice has proved that in addition to the design and manufacturing factors of the machine itself, inadequate maintenance and improper operation are also an important reason for many mechanical faults. If the same equipment is properly maintained, it can be used for about ten years without overhaul without accidents. On the contrary, it may not print fine products for as little as three years or as much as five years. Although the maintenance will delay some time and affect some production, it is worth considering in the long run. It's a good attitude to maintain less or no maintenance when the task is busy, and then maintain it when it is not busy; Some even adopt the practice of driving fast and fighting for equipment, which is even more wrong

maintenance is nothing more than daily maintenance, weekly maintenance, monthly maintenance, quarterly maintenance and semi annual maintenance according to the requirements of the operation manual. Its contents include regular refueling, oil change, cleaning components, and cleaning filters at all parts; Maintenance of all motors and electrical appliances, adjustment and replacement of carbon brushes; Regularly check and verify the pressure data of water (ink) rollers and rollers, and check whether all parts are loose and worn; And do a good job in the cleaning and sanitation of the machine, so that the equipment is in good running condition. In order to prolong the service life of the machine and reduce various faults caused by poor maintenance. (to be continued)

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