3) Ink roller wear After the ink roller is used for a period of time, the ink roller is worn due to cleaning dust, paper, ink and other impurities, or improper installation, uneven pressure adjustment and other factors, the surface of the ink roller is uneven or the axis line is not straight. In severe cases, the ink roller has two rough and thin middle or two coarse deformations. When the offset press is working, it will cause the ink roller to jump, causing the local friction to become heavier, resulting in an intermittent ribbon bar. The cloth sleeve of the water roll is incomplete due to incomplete stitching interface, or improper internal diameter size, or loose wrinkling of the cloth cover after a long time of use, or serious hair loss due to hair removal of the new water roll cloth cover. The occurrence of bars. 4) Improper pressure adjustment 1 Improper adjustment of the pressure between the rollers. The cylinder of the offset printing press completes the printing by relative extrusion. During the rolling process of the cylinder, the corresponding two rollers cycle from the working surface (graphic contact surface) to the neutral position, and then from the neutral position to the working surface, twice. The conversion causes the load to mutate and produce an impact. This shock causes a momentary pressure change and relative slippage at the contact surface of the printing image, resulting in a "bar." Roller Room 2 Ink roller and plate cylinder pressure is not proper. Although the layout of the four inking rollers of the offset printing press is thick and thin in the front and rear, different diameters are used to avoid overlapping of the “barsâ€, but because the four inking rollers are in contact with the printing plate in two groups, each two inking rollers are simultaneously Root roller contact. When the first inking roller comes into contact with the neutral position of the plate cylinder, the impact force causes the ink roller to be lifted up and then falls. The slip occurs between the ink roller and the inking roller, affecting the first ink. The amount of ink transferred from the roller to the plate changes, resulting in a "stick", which also affects the evenness of the second ink roller film; when the second ink roller contacts the plate cylinder in neutral, it also causes ink. The roller experiences a process of being topped up and falling down, at which point the first inking roller is in the plate, and the falling inked roller impacts the first inking roller to produce a "bar." In the same way, the third and fourth ink rollers will also produce “bars†on the printing plate, so that the whole page will appear light and heavy “barsâ€. Solution: Regularly check the tolerance of the diameter and axis of each ink roller to ensure the normal working status of the ink roller. Adjust the pressure of each roller and printing plate accurately according to the specification order. Generally, the width of the press track can be adjusted to 5mm, 4mm, 3mm in order, and ensure that the axial movement of the ink roller is less than 0.3mm. 3 The pressure in the roller and the ink roller is not proper. The ink roller of the ink-feeding part of the offset printing press basically adopts a soft-hard matching mode. When the pressure adjustment between the rollers is too large, the hard roller (the roller in the roller) and the soft roller (the ink roller and the ink roller) have serious problems. The extrusion, the soft roller under the pressure, the contact surface is sagging and the two sides of the micro drum affect the ink transfer to produce a "bar", or when the ink exchange roller reversing time and roll operation does not match, the ink roller Affect the amount of ink on the roller to produce a "bar." Solution: Install the working position of each roller in the order of specification, and accurately adjust the pressure of each roller and the ink roller. The width of the press trace is about 3-5mm. Each commutation of the ink roller should be completed within the idle time of the roller. 4 water roller pressure is not properly adjusted. When the pressure of the water roller and the plate cylinder is too large, the impact force generated when the plate cylinder is switched between the working surface and the neutral gear causes the water roller to vibrate, and the upper water is not uniform and generates a “barâ€. Or because of the increased friction between the water roller and the plate cylinder, the residual ink layer on the printing plate is destroyed, resulting in the destruction of the screen printing plate base and the lack of ink absorption to cause “barsâ€. Solution: When adjusting the pressure of the water roller and the printing plate roller, insert the test pressure with a feeler gauge of 0.15-0.2mm, and it is better to feel the friction resistance while still being able to withdraw smoothly. The upper roller pressure should be greater than the lower roller pressure, and the two head pressures should be the same. 5) Improper roller lining In practice, the blanket cylinder is an elastomer and there is a slight error in the pressure. The thick lining drum is not a pure rolling state, but has a certain range of sliding, prone to vibration, resulting in "bars"; when the lining is too large, the compressible rubber drum under the pressure, the blanket surface Forward or backward convex bulges, excessive rubbing plate layout and "bar"; the replacement blanket is not taut or loose with a long blanket, under the pressure of printing, the blanket in the drum Slippage of the surface can also cause "bars" to appear. Sometimes the paper is partially filled due to the shallow part of the cornice and the impact force of the drum's neutral period is increased, which can also cause "bars". Roller lining is generally soft, medium, hard, soft lining is generally not easy to appear bars, but due to the elasticity of the printing effect is not ideal and less use; hard lining printing outlets clear, but the lining elasticity Small, easy to appear "bars" on offset presses with low precision. Solution: The drum lining 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 that the print is clear and the dot deformation is small, the hard liner is used as far as possible. It is best to use an air-cushioned blanket. 6) transmission mechanism caused by the transfer of vibration The offset teeth of the offset roller are generally caused by the force of the brace spring to make the grippers close and bite the paper for printing. The rollers of the clenching shaft end rotate with the roller and press the holding spring when reaching the fixed cam opening plate on the impact wall plate. When the teeth are opened, the force acting on the brace spring disappears when the roller is released from the jaw and the teeth close. During the exercise, the teeth must be opened and closed twice to complete the transfer from the delivery paper to the finished delivery. Due to the large impact generated during opening and closing, the stability of the operation of the impression cylinder is affected, resulting in the occurrence of slippage between the impression cylinders. In severe cases, "bars" appear. Similarly, the drum clutch pressure and the cam drive mechanism can also cause vibrations and produce "bars."
Study on the Origin and Solution of Offset "Bar" (Part 2)