Research on the Origin and Solution of "Stripe" in Lithography

In lithography practice, "bars" are print quality issues that often occur and are difficult to completely solve. The production of "bars" involves many aspects such as the design of the typographic machine, the material and processing technology of the mechanical parts, the operation of the staff, the selection of the printing materials, and the maintenance of the mechanical equipment. The main reason for the production of “bars” in printing is the vibration of the printing cylinders of the typograph and the process of transferring the ink and water conveying mechanism or paper, which causes the slippage of the relative positions of the prints. Therefore, it is impossible for our existing printing equipment and technology to eliminate the occurrence of “bars”. It is only possible to minimize and mitigate the “bars” by improving the technology and operating processes.

1, the roller gear backlash is too large

1 Center clearance is too large due to backlash

The roller gear of the offset printing press is basically meshed with a helical gear, and the gear exchange coefficient is large, and the space of the meshing side clearance is small, so as to reduce the impact in the transmission and chattering during printing, and maintain the smoothness and accuracy of printing. In actual work, the backlash of the roller gear mesh is changed with the printing of different thicknesses of paper. The thicker the paper is, the larger the center distance adjustment is, the smaller the exchange coefficient of the roller gear engagement surface is, and the larger the backlash is. The greater the difference in line speed on the surface of the printing cylinder, the greater the relative slippage produced by the printing surface and the more pronounced 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, and the meshing tooth gap should be properly pressed to reduce the degree of “bars”.

2 gear wear caused by excessive backlash

After a period of working operation, the roller gear is worn due to load or there is foreign matter in the tooth gap, which causes the gear meshing backlash to be too large. The involute of the gear cannot be uniformly rolled along the meshing line, or the “top gear” of the foreign object makes the gear in operation. Chattering occurs, causing the friction between the rollers to increase and slipping. At the instant of printing, the printing graphic part generates an ink ribbon equal to the pitch of the gears, that is, a gear bar.

Solution: Strengthen the cleaning and maintenance work of machinery and equipment, clean and remove the debris between gears, and maintain a good lubrication condition; according to the wear of gears, recalculate the center distance, properly compress the backlash, and immediately replace the worn ones with new ones. gear.

2, bearing wear

1 roller bearing wear

When the offset press is used for a long period of time, especially if the rollers do not move the rollers, the impact vibration is large and the bearings are relatively easy to wear, making the gap between the journal and the bearing increase and the loosening of the sleeves. The vibrations occur due to the occurrence of separation, resulting in sliding friction forming "bars." In particular, when the load is not normal or the lubrication is not good, the shaft head and bushing of the drum wear more, and the “bar” is most likely to occur near the mouth.

Solution: The inner part of the rubber liner can be cut into a trapezoidal shape to reduce the impact force when the pressure is applied. When the roller bearing wears badly, the bearing should be updated.

2 Ink roller bearing wear

Water, ink roller bearings, bearing seats, bearing locks containing debris or wear can also cause the water, ink roller in the movement of significant axial turbulence and radial runout resulting in "bars", especially 窜When the sleeve or transmission gear of the ink roller is worn out, the vibration generated due to the meshing of the roller gear affects the normal uniform water distribution, ink distribution and transmission of the roller, causing the appearance of “bars”.

3, ink roller wear

After the ink roller is used for a period of time, the ink roller is worn due to the cleaning dust particles, paper hair, ink knots and other impurities, or improper installation, uneven pressure adjustment and other factors, the surface of the ink roller is uneven or the axial 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 pressure between 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." 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 Improper pressure on the ink roller and plate cylinder

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 the 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 improper water roller pressure adjustment

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 because of 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."