The most prone problem when printing is the "bar". Very masters are helpless. This requires calm thinking and trying to avoid detours. Here is a brief explanation of how to judge and solve the "bar" problem for reference.
The general "bar" is divided into ink bar, water bar, roller bar, gear bar, etc.
1. The backlash of the roller meshing gear 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 slippage on the printing surface and the more obvious the bars.
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.
Excessive backlash caused by gear wear
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
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 more prone to wear, so that the gap between the journal and the bearing increases and the sleeve is loose, and the roller is pressed when pressed. 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.
Ink roller bearing wear
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 cloth cover of the water roller of the waterwheel is incomplete, or the inner diameter size is not suitable, or the cloth cover is loose and wrinkled after a long period of use, or the new water roller cloth cover depilation severely forms fluff accumulation, affecting the normal water delivery Triggered the occurrence of "bars".
4. Improper pressure adjustment
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".
Improper pressure between inking roller and 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 overlapping of the "bars", because the four inking rollers are in two groups and are in contact with the printing plate, each two inking rollers are in contact with one 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, 4mm, 3mm, and ensure that the axial movement of the inking roller is less than 0.3mm.
Improper pressure between the roller and each ink roller
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.
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 "ink bar"; 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. The transmission mechanism causes vibration
The rollers of the offset press generally grind their teeth by the action of a strong supporting spring to close the gripper to the paper for printing. The roller at the shaft end of the teeth 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".
Basically, these problems can produce "bars". If the bar is formed by improper installation, it cannot be solved by a master.
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