Offset "ink bar" causes analysis and solving skills
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In the practice of lithography, "ink bars" are print quality problems that often occur and are difficult to completely solve. The production of "ink bar" involves many factors such as the design of the printing machine, the materials and processing technology of the mechanical parts, the operation of the staff, the selection of printing materials, and the maintenance of mechanical equipment. The main reason for the occurrence of "bars" in printing is the vibration generated by the printing cylinder of the printing press and the inking, the water conveyance mechanism, or the paper web connection, causing the relative position of the print to slip. Therefore, our existing lithographic equipment and technology can not eliminate the "ink bar", only by improving the technology and operation process to minimize and reduce the "ink bar".
1. The backlash of the roller meshing gear is too large
1 The backlash caused by the center distance adjustment is too large.
The drum gear of the offset printing machine is basically meshed by helical gears. The gear exchange coefficient of the gear is large, and the meshing side clearance space is small, so as to reduce the impact in the transmission and the chattering during printing, and maintain the smoothness and accuracy of printing. In actual work, the backlash of the drum gear mesh is changed and adjusted according to the printing of paper of different thickness. The thicker the paper, the larger the center distance adjustment is, the smaller the exchange coefficient of the meshing surface of the drum gear is, and the side gap becomes larger. The greater the difference in line speed of the printed roll surface, the greater the relative slip f produced on the printed surface, and the more pronounced the bar.
Solution: Under the premise of ensuring proper printing pressure, the center distance between the three rolling rolls should be minimized, and the meshing backlash should be properly pressed to reduce the degree of "bar".
2 The side gap caused by gear wear is too large.
After a period of work, the drum gear is worn due to load or foreign matter in the tooth gap, resulting in excessive gear meshing backlash, the involute of the gear cannot be rolled at a constant speed according to the meshing line, or the foreign object "top tooth" makes the gear in operation. The chattering occurs, causing the friction between the rollers to increase to cause slippage, and the printed image portion produces an ink ribbon equal to the pitch of the gear at the moment of the sliding, that is, the gear bar. Solution: Strengthen the cleaning and maintenance work of mechanical equipment, clean and remove the debris between the gears, and maintain a good lubrication state; according to the gear wear condition, recalculate the adjustment center distance, properly press the backlash, and replace the new one immediately. gear.
2, bearing wear
1 Rolling bearing wear.
After the offset printing machine is used for a long time, especially the roller without the roller, the movement shock vibration is large, the bearing is easy to wear, and the gap between the journal and the bearing is increased and the sleeve is loosened. The mouth vibrates due to the disengagement, and the sliding friction creates a "bar". Especially when the load is abnormal or the lubrication is not good, the shaft head and the bushing of the drum are intensified, and the "bar" is most likely to be generated near the mouth.
Solution: The mouth part of the rubber inner liner can be cut into a trapezoidal shape to reduce the impact force when the pressure is combined. When the roller bearing wears seriously, the bearing should be updated in time.
2 Ink roller bearing wear.
Water, ink roller bearings, bearing housings, bearing locks, debris or wear can also cause water and ink rollers to undergo significant axial turbulence and radial runout during motion, resulting in "bars", especially 窜When the bushing or transmission gear of the ink roller wears, the vibration generated by the meshing of the roller gear affects the normal water leveling, ink leveling and transmission of the roller, which causes the occurrence of “bars”.
3, ink roller wear
After the ink roller is used for a period of time, the ink roller is worn due to cleaning of dust, paper, ink, and other debris, or improper installation, uneven pressure adjustment, etc., the surface of the ink roller is uneven or the axis is not straight. In severe cases, the ink rollers are thin at the middle of the middle or thin at the middle. When the offset printing machine is in operation, it will cause the ink roller to jump, causing the local friction to increase and causing intermittent strip-shaped "bars".
The cloth sleeve of the water roller is incomplete due to the suture interface, or the inner diameter is not suitable, or the cloth sleeve is loose and wrinkled after a long time of use, or the new water roller cloth sleeve is severely depilated to form pile piles, which may cause a normal strip. The occurrence of the bar.
4, improper pressure adjustment
1 The pressure between the rollers is not properly adjusted.
The roller of the offset printing machine completes the printing by relative extrusion. During the rolling process of the roller, the corresponding two rollers are repeatedly turned from the working surface (graphic contact surface) to the neutral position, and then the neutral position is transferred to the working surface twice. The conversion causes the load to mutate and produce an impact. This kind of impact causes instantaneous pressure changes and relative slippage of the printed image contact surface, resulting in "bars".
2 Ink roller and plate cylinder pressure is not good.
Although the layout of the four inking rollers of the offset printing machine is thick in the front and back, different diameters are used to avoid the overlap of the "bars", but the two inking rollers are respectively in contact with the printing plates, and each of the two inking rollers simultaneously The root roller is in contact with the roller. When the first inking roller is in contact with the plate cylinder neutral position (the mouth position), the impact force is generated to cause the inking roller to be lifted up, and then falls, causing a slip between the inking roller and the inking roller, affecting the first inking The amount of ink transferred from the roller to the layout changes, thereby producing a "bar" and also affecting the uniformity of the ink film of the second toner roller; when the second toner roller is in contact with the neutral of the plate cylinder, the ink is also inked. The roller undergoes the process of being lowered and lowered, at which time the first inking roller is in the printing plate, and the falling inking roller impacts the first inking roller to produce a "bar". In the same way, the third and fourth inking rollers will also produce "bars" on the plates, so that the entire layout appears as a light and heavy "bar" group. Solution: Always check the diameter and axis tolerance accuracy of each ink roller to ensure the normal working state of the ink roller. Accurately adjust the pressure of each roller and plate in the order of specification, generally can be adjusted to the width of the indentation of 5mm, 4mm, 3mm, and ensure that the axial movement of the inking roller is less than 0.3mm.
3 The pressure of the roller and the ink roller is improper.
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, the hard roller (the inner roller) and the soft roller (the ink roller and the ink roller) are severely affected. Extrusion, under the action of pressure, the contact surface is sag and the micro-drum on both sides affects the ink transfer to produce a "bar", or the squeezing roller is not matched with the squeezing roller and the squeezing roller The amount of ink on the ink roller is affected to produce a "bar".
Solution: Install the working positions of each roller in the order of specification, and accurately adjust the pressure of each roller and the squeezing roller. The width of the embossing roller is about 3-5mm. Each reversing of the squeezing roller should be completed within the neutral 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 by the plate cylinder when the working surface and the neutral gear are switched causes the water roller to vibrate, and the water supply is uneven and a "bar" is generated. Or because the friction between the water stick and the plate cylinder is intensified, the residual ink layer of the dot on the printing plate is destroyed, so that the base of the dot is destroyed, and the lack of ink causes a "bar".
Solution: When adjusting the water roller and plate cylinder pressure, use a 0.15-0.2mm feeler gauge to insert the test pressure therebetween, so that the hand feels frictional resistance but can still be smoothly extracted. The upper water roll pressure should be greater than the lower water roll pressure and the pressure at both ends should be the same.
5, the drum is not properly lining
In actual work, the blanket cylinder is an elastomer with a slight error when subjected to pressure. The drum with thick liner is not in pure rolling state, but has a certain range of sliding, which is easy to vibrate and produce “bars”; when the lining is too large, the compressible rubber cylinder is under pressure and the surface of the blanket is generated. Forward or backward convex hull, excessive rubbing plate layout and "ink bar"; when the replaced blanket is not stretched or a long blanket is used for slack, under the printing pressure, the blanket is on the roller Slippage on the surface can also cause "bars" to appear. Sometimes, because the part of the mouth is too shallow and partially padded, the impact force of the drum neutral period becomes large, and "bars" are also caused. The lining of the drum is generally soft, medium and hard. The soft lining is generally not easy to appear on the bar, but it is less used because of the elasticity and the printing effect is not ideal; the dot with the hard lining printing is clear, but the lining is elastic. Small, "bars" are prone to appear on offset presses with low precision.
Solution: The lining of the drum must be determined by precise calculations to ensure the thickness of the lining under the premise of ensuring the minimum print pressure. In the premise of ensuring clear imprinting and small deformation of the dot, the medium-hard lining should be used as much as possible. It is best to use an air cushion blanket.
6, the transmission mechanism caused by vibration
The offset roller of the offset printing machine generally relies on the strong spring to make the gripper close and bite the paper for printing. The roller at the end of the biting shaft rotates with the drum, and when the cam opening plate fixed on the impact wall panel is reached, the pressing spring will be pressed. The teeth are opened, and when the roller is separated from the open plate, the force acting on the spring is lost, and the teeth are closed. During the exercise, the teeth need to be opened and closed twice to complete the transfer from the transfer paper to the completion of the delivery of the delivery. Due to the large impact during opening and closing, the stability of the operation of the impression cylinder is affected, resulting in slippage between the impression cylinders, and "bars" appear in serious cases. Similarly, the drum clutch pressure and the cam drive mechanism work also cause vibration to produce a "bar".