Research on the "Bar" Problem in the Simplicity (2)

4) Improper pressure adjustment

1 Improper roller pressure adjustment. 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." If the pressure between the rollers is too large, the bars produced are noticeable. Solution: Adjust the printing pressure to a suitable or air-cushion blanket.

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 “bars”, 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, creating a "bar," which also affects the evenness of the second ink roller film; when the second ink roller comes in contact with the plate roller, it also experiences the ink roller. The process of being thrown high and then falling down, the first inking roller is now in the plate, and the falling ink 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 “bars”. Solution: Regularly check the tolerance of the diameter and axis of each ink roller to ensure the normal working status of the ink roller. According to the specification order, each ink roller and plate pressure can be accurately adjusted. Generally, the width of the press track can be adjusted to 5mm, 5mm, 4mm, and 3mm, and the axial movement of the ink roller is less than 0.3mm.

3 The pressure of the ink roller and each ink roller is not proper. The ink roller of the ink-jet part of the offset printing machine basically adopts a soft-hard matching mode. When the pressure between the rollers is adjusted excessively, the hard roller (ink roller) and the soft roller (ink roller, ink roller) will 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 "bar", or when the reversing time of the ink roller does not match with the drum operation, the ink roller is agitated Affects the amount of ink on the ink roller and creates 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 reversing of the ink roller should be completed in 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 work surface and the neutral gear causes the water roller to vibrate, and the water is not even and generates a “bar”. Or due to the intensification of friction between the water roller and the plate cylinder, the residual ink layer on the printing plate will be destroyed, resulting in damage to the printing plate base, and insufficient ink absorption resulting in “bars”. Solution: When adjusting the pressure of the water roller and the printing plate roller, insert the test pressure with a feeler of 0.15-0.2mm, it is better to feel the friction resistance but still 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 lining thick roller is not a pure rolling state, but has a certain range of sliding, prone to chatter, resulting in "bars"; when the lining is too large, the compressible rubber roller under pressure, the surface of the blanket Produces forward or backward convex hulls, excessive rubbing of the plate layout and "bars" appear; when the newly replaced blanket is not taut or when the slack is produced with a long blanket, the rubber is pressed under the effect of the printing press force. Slippage of the cloth on the surface of the roller can also lead to "bars." Sometimes, due to the shallow part of the cornices and the partial paper filling, the impact force of the drum's neutral period will also cause "bars". Roller lining is generally soft, medium (soft, medium hard), hard several, although the soft lining is not easy to appear "bar", but due to the elasticity of the printing effect is not ideal and less; use hard lining printing The outlets are clear, but the elasticity of the lining is small, and “bars” are easy to appear 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 the clearness of the print and the small deformation of the dots, it is preferable to use a medium-hard lining (a blanket + a sheet of adhesive + paper, or a blanket + a thick paper + a thin paper, preferably a copper paper, and a rubber sheet paper). It is best to use an air-cushion blanket (hard liner is applied at this time).

6) transmission mechanism caused by the transfer of vibration

The offset teeth of the offset roller are generally printed on the gripper by the action of a strong expansion spring. The roller on the end of the gripper rotates with the roller and squeezes the retainer when it reaches and hits the fixed cam plate on the wall plate. When the teeth are opened, the force acting on the brace will disappear when the roller is released from the jaws and the teeth will close. During the exercise process, the cavities need to be opened and closed twice to complete the transfer from the delivery paper to the finished delivery. Due to the short switching time of opening and closing, a large impact occurs during opening and closing, which affects the stability of the operation of the impression cylinder, resulting in slippage between the impression cylinder and the blanket cylinder. In severe cases, "bars" appear. Similarly, the drum clutch pressure, the cam drive mechanism, etc., can also cause vibration and produce "bars."

Source: "Printing World"

Author: Huang degree

Resin&Lighting

Bishan Ceramic Import And Export Company Limited , http://www.ll-ceramic.com