Solution to the "bar" problem in offset printing
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The most prone problem in printing is the "bar". It is very difficult for the master to do anything. This requires calm thinking and less detours. Here is a brief explanation of how to judge and solve the "bar" problem for reference.
Generally "bar" is divided into ink stick, water bar, roller bar, gear bar and so on.
1. Roller meshing gear side gap 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 produced by 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 elements should be minimized, and the meshing backlash should be properly pressed to reduce the degree of "bar".
Excessive backlash caused by gear wear
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 gear involute 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
After the offset printing machine is used for a long time, especially the roller without the roller, the impact 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 loose, and when the roller is pressed and pressed, The mouth vibrates due to the deviation, and the sliding friction creates a "bar". Especially when the load is not normal 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.
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 two ends of the ink roller are thin or thin at the middle.
When the offset printing machine is working, 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 of the waterwheel 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 seriously depilated to form a pile pile, which may affect the normal water delivery. Raises the occurrence of "bars".
4. Improper pressure adjustment
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 produces an impact. This impact causes instantaneous pressure changes and relative slippage of the printed image contact surface, resulting in "bars".
Ink roller and plate cylinder pressure is not proper
Although the layout of the four inking rollers of the offset printing machine is thick and thin in the front and the back, different diameters are used to avoid the overlap of the “bars”, but the four inking rollers are respectively in contact with the printing plates, and each of the two inking rollers simultaneously and one 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, and a slip occurs 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", which also affects 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 this time, the first inking roller is in the printing plate, and the falling inking roller impacts the first inking roller to generate 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 the specification, generally adjustable to the width of the indentation is 5mm, 4mm, 3mm, and ensure that the axial movement of the inking roller is less than 0.3mm.
Improper pressure on the roller and each roller
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 between the hard roller (the roller) and the soft roller (the ink roller and the roller) occurs seriously. Extrusion, the soft roller under the action of the pressure, the contact surface is sunken and the micro-drum on both sides affects the ink transfer to produce a "bar", or the ink roller reversing time does not match the rolling operation, the roller is 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.
Improper adjustment of water roller pressure
When the pressure of the water roller and the plate cylinder is too large, the impact force generated by the plate cylinder during the switching between the working surface and the neutral gear 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 between them, so that the frictional resistance of the hand is felt, but it can still be smoothly extracted. The water roll pressure above should be greater than the water roll pressure below, 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 used for a long time to produce 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 as a bar, but it is less used because of its elasticity, and the printing effect is clear, but the lining is flexible. 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. Transmission mechanism caused by vibration
The offset roller of the offset printing machine generally relies on the strong spring to make the gripper close the bite to print the paper. The roller at the end of the biting shaft rotates with the drum. 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".
Basically, these problems can produce "bars". If the bars are improperly installed, they are not solved by the master.