Bar failure
The prints appear parallel to the axis of the cylinder, and the ink marks are of different shades of ink, which are called printing bars, commonly known as bars, bars, tracks, and heads. This is a common and difficult to print in lithographic offset printing. malfunction. If the bar is slight, it will not affect the overall picture, but if it is serious enough to affect the accuracy or effect of the picture or the customer does not accept it, you must find a way to exclude it. It is not easy, even impossible, to completely rule out, only to minimize the impact.
The bars are often caused by many factors, and it is difficult to identify and eliminate them at once. According to its phenomenon and characteristics, the bar can be divided into two situations: one is the ink bar, the dots irregularly increase, elongate, ghost, and the color of the ink becomes heavier, forming an obvious dark streak, called the color ink bar The bar usually appears when printing lines, text, dot images and on the ground; the second is the water bar, where the image dots shrink irregularly where the bar appears, forming an obvious light-colored streak, called the water bar, also Called the white bar, and the colorless white bar is also a change in the imprint caused by the thinning of the ink layer or the shrinkage of the dots. Both of the above bars are caused by poor contact between the blanket, water roller, etc. and the surface of the printing plate.
The cause of the bar
Due to the collision and impact of the plate cylinder and the impression cylinder during the process of conversion with the working surface, the cam, connecting rod and other mechanisms will vibrate under the action of inertia, and the working cylindrical surface of the cylinder no longer performs uniform speed and uniform rolling. Therefore, there will be slight slippage between the ink roller and the printing plate, between the plate cylinder and the rubber cylinder, and between the rubber cylinder and the impression cylinder, which will destroy the water-ink balance and affect the ink layer due to the cracking of the ink layer. The thickness causes the dot to increase or decrease, and a bar appears parallel to the axis of the drum. The higher the running speed of the machine, the more serious the bar phenomenon.
1. Design aspects
The arrangement position of the rollers is not reasonable. The reversing of the cross-ink rollers is not performed when the rollers are empty and when they are rolled. The wallboard and rollers are deformed due to insufficient strength and rigidity.
2. Accuracy
The assembly precision of gears, eccentric sleeves, rollers, etc. is not high, which affects the stability of mechanical movement.
3. Dynamic load
The effect of dynamic load on the ink stick is that the rotating parts rotate unbalanced, the mechanism reciprocates to generate vibration and the cam mechanism accelerates to cause impact. Related to the factors that produce the bar are imprinting, ink leveling, wetting part of the roller, bearing, roller gear, main transmission gear, ink roller gear, ink roller, rubber roller, water roller, paper delivery cam, Assembly and adjustment of racks and machine chassis. The accuracy errors formed by these components during printing are directly expressed in the form of a bar, so the severity of the bar is also an overall evaluation of the accuracy of the offset press.
Typical analysis
1. Gear bar
(1) Failure phenomenon: bars are distributed throughout the layout, generally ink bars, shaped like washboards, the distance between each bar is the pitch of the transmission gear of the printing cylinder (t = πm).
(2) Reasons for failure: insufficient gear manufacturing and installation accuracy, worn tooth surfaces, excessive tooth spacing, or damage to individual teeth causes gear teeth to mesh incorrectly during transmission and cause vibration, causing sliding friction on the surfaces of the rollers that are in contact with each other , Resulting in the deformation of the dots on the printing plate surface, which is elliptical or enlarged due to crushing, and transfer to the printed matter produces streaks. This type of failure not only causes bumps in printed products, but also aggravates gear wear.
If one tooth of the gear is damaged or there is a foreign object between the teeth, there will be a bar on the printed matter, which is fixed in a certain position. If you rotate one of a pair of meshing gears by 90 °, the position of the lever will change accordingly, you can conclude that it is the problem of the gear; if the position of the lever does not change, then rotate the other gear by 90 °, if the two After the gears rotate respectively, the position of the bar remains unchanged, it can be concluded that it is not the cause of the gears.
(3) Solution: appropriately reduce the center distance of the gears or the gap between the tooth sides, and change the roller lining to a soft lining to increase the elasticity of the lining. In severe cases, a new pair of gears must be replaced.
2. Vibration bar
(1) Failure phenomenon: There is a bar at the local position of the printed sheet, and the position is fixed. The part where the bar appears is related to a certain vibration. This bar is generally an ink bar.
(2) Cause of failure: There are many unbalanced factors during the operation of the offset printing press. It is difficult to directly identify what kind of vibration is caused, and the vibration source needs to be analyzed. Shock and vibration caused when the roller is empty when it enters rolling; impact and vibration generated when the impression roller gripper opens and closes; vibration generated when the roller is clutched and pressed; the drum body itself is unbalanced and vibrates due to inertia during high-speed rotation ; Shock and vibration generated by the inertial force of the reciprocating paper delivery mechanism; the vibration generated when the paper collection chain row grips the paper plate when opening and closing.
(3) Solution: According to the specific situation of the bar, check the movement status of the above mechanism one by one or replace the unqualified parts.
3. Roller bar
(1) Failure phenomenon: the bar is wide and the position is not fixed, and the ink color of the front and back of the printed matter is different.
(2) Causes of failure: There are two types of roller bars: ink bar and water bar. Most of them are caused by the unreasonable design of the ink leveling system or the improper adjustment of the pressure of the rubber roller, which makes the ink leveling system insufficiently supplemented. The diameter, support point, cylindricity of the rubber roller, the gap between the roller head and the support hole, and the surface hardness of the rubber roller all have an effect on the roller bar.
4. Other reasons
(1) Blanket. The blanket will swell after slack, and will slip during imprinting, forming a bar. This slip is intermittent. The greater the pressure between the plate cylinder and the rubber cylinder, the more severe the slip phenomenon. Therefore, the problem of blanket slack must be resolved in time, otherwise it will not only form a bar, but also cause a series of related failures. To avoid the occurrence of ink sticks, you should choose a rubber cloth with low elasticity, appropriate elasticity, good ink absorption, uniform thickness and smooth surface. At present, the air cushion rubber is the best.
(2) Liner. If you use a hard liner, although the printed dots are clear and rich in layers, but it is sensitive to the accuracy of the machine and it is easy to form a bar, so medium and soft liners are better. The pressure between the rubber cylinder and the plate cylinder is too small, and ink sticks are also prone to appear.
(3) Printing plate. The mesh-adjusted version has a large friction force and is not prone to ink sticks; the field version and the thin-line version have a small friction force, are easy to slide, and the dots increase easily appear.
(4) Paper. Whiteboard paper is more sensitive to ink sticks; coated paper is smoother than offset paper, and ink sticks are also prone to appear.
(5) Ink. Ink with a large viscosity has a large cohesion, and it is not easy for ink sticks to appear. For thin, transparent ink, the ink stick will be obvious. In addition, the ink color is also related to the generation of ink bars. Black ink is easy to show ink bars, yellow ink is relatively close to the color of paper, and ink bars are not obvious.
Solutions for common bars
Although the causes of the bars are complex, as long as they are carefully analyzed, some laws can still be found.
(1) The driving gear of the drum is worn seriously or the tooth gap is too large, which will cause the bar.
(2) Sticky dirt on the tooth surface, hard objects embedded in the tooth gap or clogged roots of the tooth will cause the bar. Solution: Reasonable maintenance, maintenance and elimination of foreign objects can reduce or avoid accidents.
(3) The roller bearings are seriously worn, which can cause the bars. Fault phenomenon: There are several wide ink bars at the paper mouth, and a few of them are equidistant. In the offset printing process, the shaft head and the sleeve will inevitably wear. When the lubrication system cuts off oil or dirt enters the oil groove, its wear will be faster. When the shaft head and the sleeve are worn, the gripper will immediately give way to compression, resulting in sliding friction, which is easy to cause ink sticks. Solution: pad the paper in the blanket to make the gripper pad trapezoidal, to reduce the impact when the gripper is pressed, or replace it with a wear-resistant bearing.
(4) If the pressure between the plate cylinder and the rubber cylinder is too large, the blanket will produce large sliding friction under the squeezing action, causing the dots to deform and form ink sticks. Solution: Adjust the pressure between the two rollers, instead of blindly adding backing paper to the rubber briquettes.
(5) The lining is not suitable. The lining of the printing plate and rubber cylinder is inappropriate, and the sliding friction between them will drive the printing plate cylinder, that is, the surface drive of the cylinder conflicts with the drive of the cylinder gear. When this kind of trouble occurs in places where the pressure is low and the blanket cylinder liner is thick, the solution is to reduce the surface linear velocity error between the blanket cylinder and the plate cylinder, appropriately reduce the blanket cylinder liner and increase the plate cylinder Lining.
(6) Improper pressure of rolling pillow. When the pressure of the roller pillow in contact with the roller pillow type offset press is inappropriate, a bar will be generated. The contact condition of the roller pillow is related to the meshing of the gear. If the pressure of the roller pillow is improper and the gear is not well meshed, the roller will easily cause vibration and produce a bar. Solution: readjust the pressure of the rolling pillow according to the instructions.
(7) The pressure between the inking roller and the surface of the printing plate is too large. If the linear speed of the inking roller and the surface of the printing plate are not consistent, friction will occur between them. The last two of the inking rollers have a more significant impact on the fault. When the width of the contact between the inking roller and the surface of the printing plate exceeds 6mm, it means that the pressure on the surface of the printing plate is too large or the plate is too tight; when the inking roller passes through the cylinder and hits the edge of the plate opening, there will be shock and jump. If the ink layer on the ink roller is uneven, an ink bar will be produced at the mouth; when two or more ink rollers are too close to the printing plate, the situation will be more serious. The first color of the bar is the most detrimental to the quality of the print, this failure occurs near the mouth.
(8) The pressure between the inking roller and the inking roller is too high. The position of the ink bar is not fixed, so the pressure between the inking roller and the inking roller should be moderate, and the contact width within 5mm should be suitable.
(9) The pressure between the water roller and the printing plate is too large. At this time, there will also be a jump, forming a water bar. When the water roller is too close to the printing plate, after passing through the cylinder, it will hit the edge of the gripper of the printing plate, forming a bar near the gripper. The solution is to reduce the pressure between the water roller and the printing plate.
(10) The gears and rollers of the ink roller are not meshed correctly or are worn. Causes serious gear wear, large gaps, insufficient finish, noisy noise, and tingling feel. The ink bar is in the form of a thin fence with a pitch equal to the pitch of the ink roller gears.
(11) The ink roller bearing is damaged. Damage to the ink roller bearing will result in abnormal operation of the ink roller, often slipping on the printing plate, and forming a bar, mainly water bar. Solution: replace the ink roller bearing, the fault can be eliminated.
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