Analysis on the causes and elimination of the hott

Analysis of the causes and elimination examples of offset printing bar

bar is a common quality problem in offset printing. The emergence of water and ink bars in offset printing machines is one of the most troublesome problems for printing enterprises (including printing machine manufacturers). It is difficult to solve it in actual production, because the appearance of the bar is often caused by many factors, so it is difficult to identify and eliminate it immediately. In order to be able to judge the causes of the bar in an organized way and quickly, so that the faults can be eliminated quickly, combined with the knowledge learned and summarizing the practical experience of myself and other comrades, I have a rough discussion on the causes of the bar and the elimination methods, and write some of my experiences for peer reference and correction

1. Definition of bar

the so-called bar refers to the strip-shaped imprint parallel to the axis of the drum and different in density from the surrounding when printing. It is caused by slight slippage between the printing plate roller and the printing plate roller, between the printing plate roller and the rubber roller, or between the rubber roller and the paper. In the process of ink transfer, this small slip changes the thickness of the ink film or the shape and size of the dots, forming a strip of ink color change, which is called a bar

2. Bar characteristics and causes

according to the appearance, the offset bar is divided into black bar (ink bar) and white bar (water bar). The causes are due to wear of machine parts, sudden pressure change, uneven linear speed, smooth contact surface, low gear processing accuracy, resulting in instantaneous sliding of rubber cloth, ink roller, water roller, etc. after contact with the printing plate surface, Thus, the graphic point shape and ink stacking thickness of a straight line parallel to the axis line of the printing plate cylinder are changed, forming a bar. The causes and characteristics of the bar are as follows

1) causes and characteristics of ink bars: ① for ink bars produced by serious wear of drum gears, the spacing of ink bars is equal to the gear pitch; ② Ink bars produced by serious wear of roller bearings, which often appear in its mouth and are relatively wide, and a few are equidistant ink bars; ③ The pressure between the printing plate cylinder and the rubber cylinder is too large, and the ink bars produced are mostly in the fixed area, and the position is often unchanged; ④ The pressure between the inking roller and the printing plate or the inking roller is too large, and the ink bar caused by this is characterized by unstable position, which may only appear on one side; ⑤ The drum vibrates at a moment when it rotates at a uniform speed, causing slight shaking of the plate cylinder, ink roller, blanket cylinder and impression cylinder, resulting in ink bars, which are not regular and appear randomly on prints

2) causes of white bars: ① white bars caused by excessive pressure when the landing roller contacts the surface of the printing plate. Most of these white bars appear in the mouth, and the position is relatively fixed; ② The white bars caused by the serious wear of the driving gear of the water roller usually appear in a certain area when the pictures and texts are printed full or on the spot; ③ The white bar caused by the incorrect swing time of the water transfer roller usually appears in the fixed part; ④ The white bar produced by the inking roller and the ink string roller in the mutual rolling process of the low-speed offset press. This kind of white bar is light in front and heavy in the back, with a wide spacing. Its position is not fixed, and sometimes appears on one side

3. Examples of bar exclusion

1) rubber drum gear bar. A J4104 four way monochrome Offset Press is tested after normal installation and commissioning. There are obvious bars in the shape of washboard in the printed matter, which are evenly distributed from bite to drag. The bar width is about 5mm. Room 21. The effective experimental width of the experimental machine is 395mm, and the distance is about 5mm. When the machine is stopped, the bars can be clearly seen on the printing plate and the blanket. After our re inspection, the roller pressure, ink roller pressure, water roller pressure, etc. are normal. It still cannot be solved after various debugging. We contacted the manufacturer, and they sent engineers to the site to participate in the commissioning and inspection. We thought that there was a problem with the rubber drum gear. Then we disassembled the machine and replaced it with a new rubber drum gear. After reinstallation and commissioning, the machine was tested, and the bar disappeared

2) water roll bridge gear bar. A new J4104 offset press was installed and debugged. It was found that there were bars in the shape of a washboard on the printed matter during the trial printing. After stopping the machine for inspection, it was found that the bars on the blanket were not obvious, but the bars on the printing plate were obvious. After further careful observation and test, it was found that there was no bar on the printing plate when the machine was running. After lifting the ink roller and idling the machine for 1 ~ 2min, there was an obvious bar on the printing plate, and then the bar of the ink roller disappeared. According to the analysis, it is considered that this is the white bar, that is, the gear bar caused by the failure of the water roller. Check the water roll transmission gear, and find that the water roll bridge gear shaft is eccentric, and the gap between it and the printing plate cylinder is adjustable. Due to the assembly problems of the manufacturer, there are top teeth, and the white bar disappears after debugging. When adjusting this gear, you should first make a mark between the eccentric shaft and the clamp with a hacksaw blade in order to master the adjustment range

3) ink roller bridge gear bar. In April, 2001, a new j4104b offset press was installed and commissioned. When the press was tested, there were invisible bars on the prints. This model is very different from J4104 model. The transmission gears move outward, and the structures of water and ink rollers are fixed on the wallboard. The ink roller bridge gear shaft is not eccentric, which will cause many problems. It is not adjustable with the roller tooth clearance of the actual version of the tension, contraction, low cycle and high cycle fatigue of printing materials and parts. At this time, the center distance between the printing plate cylinder and the rubber cylinder can only be appropriately reduced by 0.05 ~ 0.1mm, and the lining between the printing plate cylinder and the rubber cylinder can be appropriately reduced, so as to open the gap between the ink roller bridge gear and the printing plate cylinder gear, and eliminate the jacking phenomenon. After the above adjustment, the bar disappears

4) when a j210ls folio monochrome Offset Press, which has been used for 8 years, prints a text version, a bar with a width of about 12mm is generated at a distance of about 28cm from the bite. The bar runs across the whole page, and the bar width is uniform. Because it is a printed version, the bar imprint appears a little fuzzy. After careful observation, the author found that the stability of the machine is good. Now the noise of this machine is much higher than usual, and you can feel the vibration by touching the wall of the machine. After careful inspection, it was found that the vibration was caused by the impact caused by the contact between the tipping bucket on the paper head shaft on the imprinting drum and the open tooth ram core. Disassemble the machine and check the ram core. It is found that it is seriously worn. After replacing the part, the machine noise is reduced and the bar disappears

5) a j4104b quarto monochrome Offset Press, which has been used for two years, produces several irregular bars with a fixed position and a width of about 8mm in the middle of the printed matter. After careful observation, the author found that there was abnormal noise when the machine was running, and the abnormal noise came from the main motor of the machine. Open the protective cover of the machine, and see the small pulley on the output shaft of the main motor jumping. After disassembly and inspection, the two bearings on the output shaft of the main motor were damaged, and the output shaft of the main motor was also worn by about 1mm. After replacing these two parts, the machine noise is reduced and the bar disappears