Production and solution of the most popular offset

2022-08-06
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The production and solution of offset bar and bar in the practice of lithography, bar and bar is a printing quality problem that often appears and is difficult to be completely solved. The production of bar involves many factors, such as the design of lithography machine, the material and processing technology of mechanical parts, the operation of staff, the selection of printing materials, the maintenance of mechanical equipment and so on. The main reason for the bar in printing is the vibration caused by the movement of the printing cylinder and the ink and water conveying mechanism of the lithography machine or the paper connection process, which causes the slip of the relative position of the imprint. Therefore, our existing lithography equipment and technology can not eliminate the production of bars. We can only reduce and reduce the production of bars by improving technology and operation process. This paper explores the production and solutions of bar, hoping to play a role in attracting jade

1. The concept and classification of bars

1) the concept of bars

there are banded marks parallel to the direction of the roll axis on the lithographic printing, which are also called bars and stripes, due to the irregular deformation of the points

2) classification of bars

according to the ink color of bars, they can be divided into ink bars and water bars

ink bar: in the printing service promised brush area of the printed matter, the dots are irregularly increased, the ink layer is thickened, and there are obvious dark banded marks (also known as color bars) than the ink color printed at the same time

water bar: in the printing area of the printed matter, the dots are irregularly reduced, the ink layer is thinner, and there are obvious light color band marks (also known as white bars) than the ink printed at the same time

the causes of bar pressing can be divided into gear bar, vibration bar, roller bar and other bars

gear bar: the accuracy of the transmission gears of each drum, ink conveying part and water conveying part of the offset press is insufficient, resulting in the wear of the tooth surface or the damage of the tooth, resulting in vibration in the meshing transmission, resulting in the relative sliding of the printed text on the drum and the bar. The utility model is characterized in that the spacing between the bars in the printing sheet is equal to the pitch of the gears, and the printing sheet is densely covered in the form of a washboard

vibrating bar: the offset press suddenly vibrates at a certain moment when it is running at a uniform speed. This vibration destroys the uniform rotation of the drum and causes slight shaking between the printing rollers to produce bars. For example, the shift between the working face of the printing cylinder and the neutral gear, the clutch of the cylinder, the opening and closing of the paper teeth of the cylinder, the motion inertia of the cam linkage mechanism or the bearing wear of each transmission component will cause impact vibration, resulting in the relative sliding of the printed text

bar. It is characterized by the presence and fixed position of bars at the local position of the printing sheet. The position of the bars is related to the periodic and regular vibration of the mechanical movement

roller bar: the roller bar is caused by the unreasonable design of the ink and water equalizing system of the offset press or the improper processing accuracy and matching accuracy of the new concept and new concept roller, which causes the water and ink roller to slip with the plate cylinder and the shifting roller during operation, or the contact position is sunken due to improper pressure adjustment, which interferes with the normal transfer of ink and water, thus forming a bar. The utility model is characterized in that the number of bars f is slightly more, the bar band is wider, the bar width is equal to the surface perimeter of the ink roller, the position is not fixed, and the ink color before and after the printing is different

other bars: bars produced by blanket, pad, printing plate, paper, ink, wetting solution, operation process and other factors involved in the printing process. Because of its different characteristics, this paper will not discuss it in detail for the time being

1) excessive side clearance of drum meshing gear

① excessive side clearance caused by center distance adjustment. The cylinder gear of offset press is basically meshed with helical gear. The transmission exchange coefficient of the gear is large, and the meshing backlash space is small, so as to reduce the impact in transmission and the vibration during printing, and maintain the stability and accuracy of printing. In practical work, the meshing backlash of the drum gear changes and adjusts with the printing of paper of different thickness. The thicker the paper is, the larger the center distance adjustment, the exchange coefficient of the meshing surface of the drum gear becomes relatively small, and the backlash becomes larger. The greater the difference of linear velocity on the surface of the printing roll, the greater the relative slip f on the printing surface and the more obvious the bar. Solution: on the premise of ensuring appropriate printing pressure, the center distance between the three rollers shall be reduced as far as possible, and the meshing backlash shall be properly pressed to reduce the degree of bar

② excessive side clearance caused by gear wear. After a period of operation, the drum gear is worn due to load or there is foreign matter in the tooth groove, resulting in excessive gear meshing side clearance, the involute of the gear can not roll at a uniform speed according to the meshing line, or the top tooth of the foreign matter makes the gear vibrate during operation, which increases the friction between the drums and causes sliding. At this sliding moment, the printed text part produces an ink belt equal to the gear pitch, that is, the gear bar. Solution: at ordinary times, strengthen the cleaning and maintenance of mechanical equipment, clean and remove the sundries between gears, and maintain a good lubrication state; According to the gear wear, recalculate and adjust the center distance, properly compress the backlash, and replace the new gear immediately in case of serious wear

2) bearing wear

① wear of roller bearing. After the offset press has been used for a long time, especially the roller without roller pillow, its movement impact and vibration are large, and the bearing is easy to be worn, which increases the gap between the journal and the bearing and looses the sleeve. When the roller is simply closed and pressed, its mouth vibrates due to separation, resulting in sliding friction to form a bar. Especially when the load is abnormal or the lubrication is not good, the wear of the shaft head and the shaft sleeve of the drum is intensified, and it is most likely to produce a bar near its mouth. Solution: the mouth part of the rubber inner liner can be cut into a trapezoid to reduce the impact force during pressure closing. When the roller bearing is seriously worn, the bearing should be updated in time

② the ink roller bearing is worn. Sundries or wear in the bearings, bearing seats and bearing locks of the ink and water rollers can also cause obvious axial movement and radial runout of the ink and water rollers during movement, resulting in bars. In particular, when the shaft sleeve or transmission gear of the ink channeling roller is worn, the vibration generated by the meshing of the drum gear affects the normal water and ink leveling and

3) wear of the ink and water roller

after the ink roller is used for a period of time, due to the cleaning of dust particles, paper wool Ink cover and other sundries, or improper installation, uneven pressure adjustment and other factors cause the ink roller to be worn, the ink roller surface is uneven or the axis line is not straight. In serious cases, the ink roller is thick at both ends and thin in the middle or thick at both ends. When the offset press works, it will cause the ink roller to jump, causing local friction to increase and produce intermittent strip bars. The cloth sleeve of the water roller is not complete due to the incomplete sewing interface, or the internal diameter size is inappropriate, or the cloth sleeve is loose and wrinkled after long-term use, or the new water roller cloth sleeve is depilated seriously to form fluff accumulation, which affects the normal water delivery and can also cause the occurrence of stripes

4) improper pressure adjustment

① improper pressure adjustment between rollers. The cylinder of the offset press completes the printing by relative extrusion. In the process of cylinder rolling, the corresponding two cylinders turn from the working face (picture and text contact surface) to the neutral gear, and then from the neutral gear to the working face. The two conversions cause sudden load change and impact. This kind of impact causes the instantaneous pressure change and relative slip on the contact surface of printed text, resulting in bars

② improper pressure between inking roller and plate cylinder. Although the layout of the four inking rollers of the offset press is thick in the front and back and thin in the middle, different diameters are used to avoid the overlap of the bars, because the four inking rollers are in two groups to contact the printing plate respectively, each two inking rollers are in contact with an ink channeling roller at the same time. When the first inking roller is in contact with the plate cylinder in the neutral position (its mouth position), the impact force generated causes the ink channeling roller to be jacked up and then fall down, resulting in slippage between the ink channeling roller and the inking roller, which affects the change of the ink volume transferred from the first inking roller to the plate, resulting in bars, and also affects the uniformity of the ink film of the second toner roller; When the second toner roller is in neutral contact with the printing plate cylinder, the ink channeling roller also experiences the process of being raised and falling. At this time, the first ink channeling roller is in the printing plate, and the falling ink channeling roller impacts the first ink channeling roller to produce bars. In the same way, the third and fourth inking rollers will also produce bars on the printing plate, so the whole plate appears light and heavy bars. Solution: regularly check the diameter and axis tolerance accuracy of each ink roller to ensure the normal working state of the ink roller. Accurately adjust the pressure of each inner roll and printing plate according to the specification sequence. Generally, the width of the impression can be adjusted to 5mm in turn. Generally, the hydraulic universal testing machine can be divided into 100kN, 300KN, 600kN, 1000KN, 2000kN, 3000kN, 4mm and 3mm from the range, and ensure that the axial movement of the ink roller is less than 0.3mm

③ the pressure between the channeling roller and each ink roller is improper. Each ink roller of the ink conveying part of the offset press basically adopts a soft hard matching mode. When the pressure between the rollers is adjusted too much, serious extrusion occurs between the hard roller (channeling roller) and the soft roller (inking roller and ink leveling roller). Under the action of pressure, the contact surface of the soft roller sinks and the micro drums on both sides affect the ink transfer, resulting in bars, or when the reversing time of the channeling roller does not match the operation of the roll, The movement of the ink channeling roller affects the ink volume on the ink channeling roller and produces a bar. Solution: install the working position of each roll according to the standard sequence, accurately adjust the pressure of each roll and the ink channeling roll, the width of the impression is about mm, and each reversing of the ink channeling roll should be completed within the neutral time of the drum

electric poles made of new composite materials came out ④ improper pressure adjustment of water rollers. When the pressure between the water roller and the plate cylinder is too high, the impact force generated by the plate cylinder when the working face and neutral shift

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