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". Drum room
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, 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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