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.
2 Cause analysis and solution of "bar"
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".
3) Ink roller wear
After using the ink roller for a period of time, the ink roller is worn due to factors such as cleaning dust, paper hair, ink skin and other debris, improper installation, uneven pressure adjustment, etc. The surface of the ink roller is uneven or the axis line is not straight , In severe cases, the deformation of the ink roller is thick at both ends and thin at the middle or thin at both ends. When the offset press is working, it will cause the ink roller to jump, which will cause the local friction to increase and produce intermittent band-shaped "bars". The water cover of the water roller is incomplete, or the inner diameter is not suitable, or the cloth cover is loose and wrinkled after a long time of use, or the new water roller cloth cover is depilated, and the fluff accumulation is serious, which can affect the normal water delivery. Bar ".
4) Improper pressure adjustment
â‘ Improper pressure adjustment between rollers. The cylinder of the offset press is printed by relative squeezing. During the roller rolling process, the corresponding two cylinders are moved from the working surface (graphic contact surface) to the neutral position, and then from the neutral position to the working surface, twice The transition makes the load jump and produce shock. This impact causes instantaneous pressure changes and relative slippage on the printed graphic contact surface, resulting in "bars". If the pressure between the rollers is too large, the resulting bars are obvious. Solution: Adjust the printing pressure to an appropriate one or use an air cushion blanket.
â‘¡ Improper pressure between the inking roller and the plate cylinder. Although the layout of the four inking rollers of the offset press is thicker and thinner at the front and back, and different diameters are used to avoid the overlap of the "bars", because the four inking rollers are in two groups and are in contact with the plate, each two inking rollers are simultaneously Root channel roller contact. When the first inking roller is in contact with the plate cylinder neutral position (grip position), the impact force causes the ink channeling roller to be lifted up, and then falls, the ink channeling roller and the ink roller will slip between them, affecting the first ink inking The amount of ink transferred from the roller to the plate surface changes, resulting in a "bar", which also affects the uniformity of the ink film of the second toner roller; when the second toner roller comes into contact with the plate cylinder in neutral, it also causes ink to channel. The roller undergoes the process of being lifted and dropped. At this time, the first inking roller is in the printing plate, and the falling ink roller impacts the first inking roller to produce a “barâ€. In the same way, the third and fourth inking rollers will also produce "bars" for the printing plate, so the entire layout will appear as "bars" groups that are light and heavy. Solution: Always check the tolerance accuracy of the diameter and axis of each ink roller to ensure the normal working state of the ink roller. Accurately adjust the pressure of each roller and plate according to the standard order. Generally, it can be adjusted to the width of the indentation in order of 5mm, 5mm, 4mm, 3mm, and ensure that the axial movement of the inking roller is less than 0.3mm.
â‘¢ The pressure between the roller and each ink roller is not appropriate. The ink rollers of the inking part of the offset printing machine basically adopt a soft-hard matching mode. When the pressure between the rollers is adjusted too much, a serious problem occurs between the hard roller (floating roller) and the soft roller (inking roller, uniform ink roller) When the soft roller is under pressure, the contact surface sags, and the micro drums on both sides affect the ink transfer to produce "bars", or when the reversing time of the ink blow roller does not match the roller operation, the ink roller rolls. Motion affects the amount of ink on the ink roller and produces a "bar". Solution: Install the working position of each roller according to the standard order, accurately adjust the pressure of each roller and the ink channel, the width of the indentation is about 3-5mm, and each change of the direction of the ink channel should be completed within the idle time of the roller.
â‘£ Improper adjustment of water roller pressure. When the pressure between the water roller and the plate cylinder is too large, the impact force generated by the plate cylinder at the conversion of the working surface and the neutral position causes the water roller to vibrate, and the uneven water supply produces "bars". Or due to the increased friction between the water stick and the plate cylinder, the residual ink layer of the dots on the printing plate is destroyed, causing the dot base to be destroyed there, and the lack of ink absorption causes "bars". Solution: When adjusting the pressure of the water roller and the plate cylinder, use a feeler gauge of 0.15-0.2mm to insert the test pressure therebetween. It is advisable to feel the friction resistance but still pull it out smoothly. The pressure of the upper water roller should be greater than the pressure of the lower water roller, and the pressure at both ends should be the same.
5) Improper roller lining
In actual work, the rubber roller is an elastic body, and there is a slight error when it is under pressure. The thick-lined roller is not in pure rolling state, but has a certain range of sliding, which is prone to vibration and produce "bars"; when the liner is too large, the compressible rubber roller is produced under the pressure on the surface of the blanket The convex hull of the front or back, excessive friction of the printing plate surface appears "bars"; when the replacement blanket is not tight or the long-lasting blanket is slack, under the pressure of printing, the blanket is in the cylinder Surface slippage can also cause "bars" to appear. Sometimes, due to the shallow imprinting of the mouth part and the partial padding of paper, the impact force of the idle cycle of the drum becomes larger, which can also cause "bars". Roller linings are generally soft, medium and hard. Soft linings are generally not prone to bars, but due to their elasticity, the printing effect is not ideal and rarely used; the dots printed with hard linings are clear, but the linings are elastic Small, easy to appear "bars" on offset presses with low accuracy.
Solution: The lining of the roller must be accurately calculated to determine the thickness of the lining under the premise of ensuring the minimum printing pressure. Under the premise of ensuring clear imprinting and small dot deformation, try to use medium-hard lining. It is best to use air cushion blanket.
6) Caused by the transfer vibration of the transmission mechanism
The rollers of the offset press generally grind their teeth by the action of a strong support spring to close the gripper and bite the paper for printing. The roller at the end of the bite shaft rotates with the roller. The teeth are opened, and when the roller comes off the tooth plate, the force acting on the support spring disappears and the teeth are closed. In the process of movement, the teeth must be opened and closed twice to complete the transfer from the paper delivery to the paper printing to the completion of the delivery. Due to the large impact during opening and closing, which affects the stability of the operation of the impression cylinder, it causes slippage between the impression cylinders. In severe cases, "bars" appear. Similarly, the clutch pressure of the drum and the operation of the cam drive mechanism will also cause vibration and produce "bars".
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