Analysis of the causes and exclusion examples of offset printing bars
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"Bar" is a common quality problem in offset printing. The appearance of water and ink sticks in offset presses is one of the most difficult problems for printing companies (including printer manufacturers). It is difficult to solve in actual production because The appearance of the bar is often caused by a variety of factors, so it is difficult to identify and eliminate it immediately. In order to be able to reasonably and quickly determine the cause of the bar, and quickly eliminate the fault, I combine the knowledge I have learned and sum up myself and others. The comrades' practical experience has made a rough discussion on the causes and elimination methods of the bars, and some of them are written for peer reference and correction.
1 definition of the bar
The so-called "bar" refers to a strip-like imprint that appears on the print at the time of printing and is parallel to the axis of the drum and has a different density from the surrounding. It is caused by the slight difference between the plate roller and the plate cylinder, between the plate cylinder and the blanket cylinder, or between the blanket cylinder and the paper. This slight slip in the ink transfer process changes the thickness or dot shape and size of the ink film, forming a strip of ink color change, which is the bar.
2 characteristics of the bar and the cause
According to the appearance, the offset pole is divided into two types: black bar (ink bar) and white bar (water bar). The reason is due to wear of the machine, sudden pressure change, uneven line speed, smooth contact surface, and low gear machining accuracy. , causing the blanket, the ink roller, the water roller and the like to instantaneously slide after contacting the surface of the printing plate, thereby changing the shape of the graphic dot and the thickness of the ink deposited on a straight line parallel to the axis line of the printing plate cylinder, forming a “bar” Bar". The causes and characteristics of the bars are as follows.
1) Causes and characteristics of the ink stick: 1 The ink stick is seriously generated by the roller gear wear, the ink stick pitch is equal to the gear pitch; 2 The ink stick is seriously generated by the roller bearing wear, and such ink stick often appears in the mouth of the mouth, and Relatively wide, a small number of equally spaced ink sticks; 3 between the plate cylinder and the blanket cylinder is too much pressure, the resulting ink sticks are mostly found in a fixed area, the position is often unchanged; 4 Ink roller and plate or The pressure of the squeezing roller is too large, and the characteristic of the ink ray caused by the ink roller is that the position is not fixed, and it is possible that the ink bar is only unilaterally present; 5 the roller generates vibration at a certain moment of constant rotation, so that the plate cylinder, the ink roller, The blanket cylinder and the impression cylinder produce a slight amount of sloshing, thereby producing a ink stick which is not regularly regular and appears randomly on the printed matter.
2) Reasons for the white bar: 1 The white bar produced by the pressure when the water roller is in contact with the surface of the printing plate. Most of the white bars appear in the mouth of the mouth, and the position is relatively fixed. 2 The water roller drive gear is seriously worn. The resulting white bar usually appears in a certain area when printed on the graphic or in the field; 3 the white bar generated by the incorrect watering roller pendulum time, this white bar usually appears in the fixed part; 4 the low speed offset printing machine is inked The white bar produced by the roller and the inking roller during the rolling process. The white bar is light and heavy and has a wide spacing. The position is not fixed and sometimes appears on one side.
3 bar exclusion example
1) Rubber roller gear bar. A J4104 four-color monochrome offset printing machine, after the normal installation and commissioning test machine, the printed matter appears more obvious like the "washing board" of the bar, evenly distributed from the bite to the drag, the bar width is about 5mm, the spacing is about 5mm, when the machine is stopped, the bar can be clearly seen on the printing plate and the blanket. After we re-check the drum pressure, ink roller pressure, water roller pressure, etc., it is normal. After many aspects of debugging, it still cannot be solved. We contacted the manufacturer, and they sent engineers to the site to participate in debugging and inspection. They thought that there was a problem with the rubber roller gear. After disassembling the machine and replacing the new rubber roller gear, the machine was reinstalled and debugged, and the bar disappeared.
2) Water roller bridge gear bar. A new installation and commissioning of a J4104 offset printing machine, the test machine found that the printed product has a looming shape like a "dressing plate", the shutdown inspection found that the rubber bar is not obvious and the plate is obvious. After further careful observation and testing, it was found that when the machine was running, the water-falling roller and the ink roller had no bars on the printing plate. After lifting the ink roller and letting the machine idle for 1 to 2 minutes, the plate appeared obvious, and the roller roller disappeared. According to this analysis, this is a white bar, that is, a gear bar caused by a water roller failure. Check the water roller drive gear and find that the water roller bridge gear shaft is eccentric. The gap between the water roller and the plate cylinder is adjustable. Due to the assembly problem caused by the manufacturer, there is a top tooth phenomenon, and the white bar disappears after debugging. When adjusting this gear, it should be noted that the hacksaw strip is marked between the eccentric shaft and the clamp to grasp the movement range.
3) Ink roller bridge gear bar. Installation and commissioning of a new J4104B offset press, when the test machine is printed, there are not obvious bars on the printed product. This type of machine is very different from the J4104 type. The transmission gears are moved outwards. The water and ink roller structures are fixed on the wall plate. The ink roller bridge gear shaft is not eccentric, and its gap with the plate cylinder is not adjustable. of. At this time, only the center distance between the plate cylinder and the blanket cylinder can be appropriately reduced by 0.05 to 0.1 mm, and the plate cylinder and the blanket cylinder liner are appropriately reduced, thereby opening the gap between the ink roller bridge gear and the plate cylinder gear. To eliminate the top teeth phenomenon. After the above adjustment, the bar disappears.
4) A J210lS folio offset printing machine that has been used for 8 years produces a bar with a width of about 12 mm from the bite at a distance of 28 cm. The bar traverses the entire layout and the bar width is even. Because of the printed version, the bar prints are a bit fuzzy. After careful observation, the author found that the noise of this machine is much larger than usual, and the hand feels vibration when it touches the machine wall panel. After careful inspection, it was found that the vibration was caused by the impact of the "dump" on the axis of the paper feed head on the impression cylinder in contact with the core of the opener. Disassemble the machine and check the core of the striker. It is found that the wear is serious. After replacing the part, the machine is tested and the machine noise is reduced and the bar disappears.
5) A J4104B four-color monochrome offset printing machine that has been used for two years produces several irregular bars with a fixed position of about 8 mm in the middle of the printed matter. After careful observation, the author found that this machine has abnormal noise during operation and the abnormal noise comes from the main motor of the machine. The machine protective cover is opened, and the small pulley on the output shaft of the main motor is seen to be pulsating. After dismantling and checking, the two bearings on the output shaft of the main motor are broken, and the main motor output shaft is also worn by about 1 mm. After replacing the two parts and testing the machine, the machine noise is reduced and the bars disappear.