In the practice of lithography, "bars" are printing quality problems that often occur and are difficult to solve completely. The production of "bars" involves many factors such as the design of flat printing machines, the materials and processing techniques of mechanical parts, the operation of workers, the selection of printing materials, and the maintenance of mechanical equipment. The main reason for the "bars" in printing is the vibration generated when the printing cylinder of the offset printing press is in contact with the ink and water conveyance mechanisms or during the paper connection process, causing the relative position of the print to slip. Therefore, our existing lithographic equipment and technology can not eliminate the occurrence of "bars", only by improving the technology and operation process to minimize and reduce the occurrence of "bars". This article explores the generation and solution of "bars", hoping to play a role in inspiring others.
1. The concept and classification of "bars"
1) The concept of "bar"
Lithographic prints appear parallel to the axis of the roll-to-roll axis, and irregularly deformed dots cause ink bands with different shades of ink, also known as "bars" and "stripes."
2) Classification of "bars"
According to the ink color of "bar", it can be divided into two types: ink bar and water bar.
Ink bar: In the printing area of ​​the printed matter, the dots increase irregularly, the ink layer thickens, and there is a clear dark band mark (also called "color bar") than the ink color printed at the same time.
Water bar: In the printing area of ​​the printed matter, the dots shrink irregularly, the ink layer is thinned, and there is a noticeable light color band imprint (also called "white bar") than the ink color printed at the same time.
According to the cause of "bar", it can be divided into gear bar, vibration bar, roller bar and other bars.
Gear bar: Insufficient precision of the transmission gears of the cylinders, ink conveying parts and water conveying parts of the offset press, resulting in wear of the tooth surface or damage of the gear teeth, resulting in chattering in the meshing drive, causing the printed graphics on the cylinder to slip "Bar" appears. The characteristic is that the spacing of bars in the printed sheet is equal to the pitch of the gears, and the dense layout is like a "washing board".
Vibration bar: Vibration suddenly occurs at an instant when the offset printing machine is operating at a constant speed. This vibration destroys the uniform rotation of the cylinder and causes a small amount of shaking between the printing rollers to produce a "bar". Such as the shifting of the working surface of the printing cylinder and the neutral gear, the clutch of the cylinder, the opening and closing of the cylinder bite, the motion inertia of the cam link mechanism or the bearing wear of each transmission component, etc., will cause shock and vibration, which will cause the relative slip of the printed graphics And appeared.
"Bars". It is characterized by the presence of "bars" in the local position of the printed sheet and their positions are fixed. The position of the "bars" is related to the periodic and regular vibration of mechanical motion.
Roller bar: The roller bar is due to the unreasonable design of the ink leveling and water leveling system of the offset printing machine or the inappropriate processing accuracy and matching accuracy of the rubber roller, which causes slippage between the water, the ink roller and the plate cylinder, and the roller. Or, due to improper pressure adjustment, the contact position is depressed, which interferes with the normal transfer of ink and ink, thereby forming a "bar". The characteristic is that the number of bars f is slightly larger, the bar band is wider, the bar width is equal to the surface circumference of the ink roller, the position is not fixed, and the ink color of the front and back of the printed product is different.
Other bars: Bars produced by factors such as blankets, pads, printing plates, paper, inks, wetting fluids, operating techniques and other factors involved in the printing process. Because of its different characteristics, this article will not elaborate.
1) The backlash of the meshing gear of the drum is too large
â‘ The backlash caused by the center distance adjustment is too large. The cylinder gears of offset presses are basically meshed with helical gears. The gears have a large transmission exchange coefficient and a small meshing backlash space to reduce impact in the transmission and chatter during printing, and to maintain the stability and accuracy of printing. In actual work, the backlash of the roller gear mesh is changed and adjusted as paper of different thickness is printed. The thicker the paper, the larger the center distance adjustment, the exchange coefficient of the roller gear meshing surface becomes relatively smaller, and the backlash becomes larger. The greater the difference in the linear speed of the printing roller surface, the greater the relative slip f produced by the printing surface, and the more obvious the bar. Solution: Under the premise of ensuring proper printing pressure, the center distance between the three rollers should be reduced as much as possible, the meshing tooth gap should be properly compressed, and the degree of "bars" should be reduced.
â‘¡ The backlash caused by gear wear is too large. After a period of working operation, due to the wear of the drum gear or the presence of foreign objects in the tooth groove, the backlash of the gear mesh is too large, the involute of the gear cannot be rolled at a constant speed according to the mesh line, or the "top tooth" of the foreign object makes the gear in operation Vibration occurs, causing friction between the cylinders to increase and produce slippage. At this instant, the printed image and text produce an ink ribbon equal to the gear pitch, that is, the gear bar. Solution: Strengthen the cleaning and maintenance of mechanical equipment at ordinary times, clean and remove the debris between the gears, maintain a good lubrication state; recalculate and adjust the center distance according to the wear of the gears, properly tighten the backlash, and replace the new ones immediately if the wear is serious gear.
2) Bearing wear
â‘ Roller bearing wear. After the offset press is used for a long time, especially the roller that does not walk the roller pillow, its motion shock vibration is large, and the bearing is prone to wear, which makes the gap between the journal and the bearing increase and the sleeve is loose. The mouth trembles due to yielding, resulting in sliding friction to form a "bar". Especially when the load is abnormal or the lubrication is not good, the abrasion of the shaft head and sleeve of the drum is increased, and it is most likely to produce "bars" near the gripper. Solution: The gripping part of the rubber inner liner can be cut into a trapezoid shape to reduce the impact force when closing the pressure. When the roller bearing is seriously worn, the bearing should be updated in time.
â‘¡ The ink roller bearing is worn. Debris or wear in water, ink roller bearings, bearing seats, and bearing locks can also cause water and ink rollers to undergo significant axial and radial jumps in motion, resulting in "bars", especially channeling When the sleeve or transmission gear of the ink roller wears out, the vibration generated by the meshing of the roller gear affects the normal water distribution, ink distribution and transmission of the channeling roller, causing the appearance of "bars".
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