Reasons for the generation of offset printing bars and analysis of their exclusion

"Beings" is a common quality problem in offset printing. Water and ink bars are one of the most troublesome problems for printing companies (including printer manufacturers). It is more difficult to solve it in actual production, because the appearance of the bar is often caused by a variety of factors, and it is difficult to immediately identify and rule out. In order to systematically and quickly determine the reasons for the emergence of the bars, and to eliminate them quickly, I combined my knowledge and summarized my own and other comrades' practical experience to make a cursory discussion of the causes and elimination methods of the bars. Some of these experiences were written for peer reference and correction.

1, the definition of bars

The so-called "cone" refers to a stripe-like print with a density different from that of the surroundings that is parallel to the axis of the drum when the print is printed. It is due to the reason that the ink roller and the plate cylinder, between the plate cylinder and the blanket cylinder, or between the blanket cylinder and the paper, cause slight slippage. This slight slip during ink transfer changes the thickness of the ink film or the shape and size of the dot, resulting in a stripe-like change in ink, which is the bar.

2, the characteristics of leverage and the reasons

According to the appearance, the offset bar is divided into two types: black bar (ink bar) and white bar (water bar). The reason for this is due to mechanical wear, sudden pressure change, uneven line speed, smooth contact surface, and low precision of gear machining. As a result, the blanket, ink roller, water roller, etc. come into contact with the surface of the printing plate to make a momentary slip. This changes the shape of the image point and the ink deposition thickness of a certain straight line parallel to the axis line of the printing plate to form a "stripe." Bar." The causes and characteristics of the bars are as follows.

1) Causes and characteristics of ink bars: 1 The roller bars are seriously worn out due to wear of the roller gear, the distance between the ink bars is equal to the gear pitch; 2 the roller bars are heavily worn by the abrasion of the roller bearings, and these types of ink bars often appear in the mouth of the mouth. A relatively wide, a small number of equally spaced ink bars; 3 pressure between the plate cylinder and the blanket cylinder is too large, the resulting ink bars are mostly in a fixed area, the position is often the same; 4 ink roller and plate or The pressure of the ink fountain roller is too large. The characteristic of the ink roller caused by this is that the position of the ink roller is not fixed. The ink bar may appear on one side only. 5 The drum will vibrate at a certain moment of uniform rotation, making the plate cylinder and the ink roller. The blanket roller and the impression cylinder produce a slight amount of shaking, resulting in the production of ink bars. The regularity of the ink bar is not strong and appears randomly on the prints.

2) Causes of white bars: 1 The white bars produced by the excessive pressure caused by the contact between the water roller and the surface of the printing plate. Most of the white bars are present in the mouth and the position is relatively fixed. The resulting white bars usually appear in a certain area when full-size images or fields are printed; 3 The white bars that are caused by improper roll-swing time are usually present in the fixed parts; 4 The low speed offset press is inking. The white bars produced by the roller and the squeegee during the rolling process. The white bars are light and heavy and have a wide gap. Their positions are not fixed, and they sometimes appear on one side.

3, exclusion of examples

1) Rubber roller gear bar. A J4104 four-color monochrome offset press, after normal installation and commissioning test machine, the printed material appears more like a "squeeze board" bar, from the mouth to the uniform distribution, width of about 5mm, spacing 5mm, when the machine stops, the bars can be clearly seen on the plate and the blanket. After we recheck the drum pressure, ink roller pressure, water roller pressure, etc., are normal. After many aspects of debugging can not be resolved. We contacted the manufacturer. They sent engineers to the site to participate in the 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 test machine was reinstalled and debugged, and the lever disappeared.

2) Water roller bridge gear bar. Newly installed and commissioned a J4104 offset press. The test machine found that the printed product had vaguely resembled a "squeeze board". When the machine was shut down, it was found that the bar was not visible on the blanket and the printing plate was clearly visible. After further careful observation and testing, it was found that when the machine was running down the water roller and the ink roller, there was no lever on the printing plate. After lifting the ink roller to idle the machine for 1 to 2 minutes, the printing plate appeared obvious bars, and the ink roller disappeared. Based on this analysis, this is considered to be a white bar, that is, a gear bar caused by a water roller failure. Check the water roller transmission gear, found that the water roller bridge gear shaft is eccentric, and the gap between it and the plate cylinder is adjustable, due to the top gear phenomenon caused by the assembly problem of the manufacturer, the white bars disappeared after debugging. In the adjustment of this gear should pay attention to the first mark between the eccentric shaft and the clamp with a hacksaw, in order to grasp the range of mobility.

3) The ink roller bridge gear bar. In April 2001, a new J4104B offset press was installed and commissioned. When the test machine was printing, there were inconspicuous bars on the printed product. This model is very different from the J4104 model. The transmission gears are all moved outward. The water and ink roller structures are fixed on the wall plate. The ink roller bridge gear shaft is not eccentric, and the gap between the roller and the plate cylinder is not adjustable. of. At this time, the center distance between the plate cylinder and the blanket cylinder can only be properly reduced by 0.05 to 0.1 mm, and the plate cylinder and the blanket cylinder liner can be properly reduced, so that the gap between the ink roller bridge gear and the plate cylinder can be opened. Eliminate top teeth phenomenon. After the above adjustments, the bars disappear.

4) When using a J210lS off-color monochrome offset press that used 8 years to print the text version, a bar approximately 12mm wide is generated about 28cm from the nip. The bar runs across the entire layout and the bar width is uniform. Because it is a printed version of the text, the blotters appear to be somewhat blurred. After careful observation, the author found that the noise of this machine is much larger than usual, and the machine can feel the vibration when touching the machine wallboard. After a closer inspection, it was found that the shock was caused by the impact of the “pick-up” on the pinhead shaft on the impression cylinder and the open-toe collision head. Disassemble the machine and check the collision head core. It is found that the wearer has a serious wear. After the replacement of the part, the machine noise is reduced and the lever disappears.

5) A J4104B four-color monochrome offset press that used for two years produced several irregular bars with a fixed position width of about 8mm in the middle of the print. After careful observation, I found that this machine is running abnormal noise and abnormal sound from the machine main motor, open the machine protective cover, see the main motor output shaft on the small belt pulley jumping injury. After the inspection was dismantled, 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 1 mm. Replacement of these two parts after the test machine, the machine noise is reduced, the bars disappear.