Identification and exclusion of printing bars

Broken down

Prints appear in parallel with the roller axis, ink shades of different traces, called the print bar, commonly known as bars, slivers, roads, heads, which is a more common and difficult to solve in offset offset printing malfunction. If the bar is light, it will not affect the overall picture, but if it is severe enough to affect the accuracy of the picture, the effect or the customer does not accept it, you must find ways to eliminate it. It is not easy to eliminate it completely or even impossible. It can only minimize the impact.

Bars are often caused by a variety of factors and it is difficult to identify and eliminate them at once. According to its phenomena and characteristics, the bars can be divided into two situations: First, the ink bars, dots irregularly grow, stretch, ghost, and ink become heavy, forming a distinct dark stripe, called ink bars. Bars usually appear when printing lines, text, dot images, and real-time images. Second, water bars, where the bars appear, the image dots are irregularly reduced to form a distinct light-colored streak, called a water bar. It is called white bars, and the colorless white bars are also the imprint changes caused by the thinning of the ink layer or the reduction of the dot. Both of the above bars are caused by bad contact between the blanket and the water roller and the surface of the printing plate.

Reasons for the emergence of bars

Due to the collision and impact of the plate cylinder and the impression cylinder during the transition from the working surface, the cams, connecting rods and other mechanisms will vibrate under inertia, and the outer circle of the cylinder will no longer be uniformly rolled at constant speed. Therefore, slight slippage occurs between the ink roller and the plate, between the plate cylinder and the blanket cylinder, between the blanket cylinder and the impression cylinder, and the ink and ink balance is damaged, and the ink layer is affected due to the breakdown of the ink layer. The thickness causes the dots to increase or decrease, and a bar appears parallel to the axis of the drum. The higher the speed of operation of the machine, the more serious the phenomenon of bars.
1. The design of the roller arrangement position is not reasonable, the commutation of the ink roller is not carried out when the roller is empty, and the wall plate and roller are deformed due to insufficient strength and rigidity.
2. Accuracy Accuracy of gears, eccentric sleeves, rollers, etc. is not high, affecting the smoothness of the mechanical movement.
3. Dynamic load aspects The impact of dynamic load on the ink bars is that the rotating parts are unbalanced, the mechanism reciprocates to generate vibration, and the cam mechanism accelerates to cause impact. Associated with the producing factors of the bars are the embossing, uniform ink, wetted parts of the drum, bearings, roller gears, main transmission gears, roller roller gears, ink rollers, rubber rollers, water rollers, paper feed cams, Rack and machine chassis assembly and adjustment. The accuracy error of these parts formed during printing is expressed directly 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 parsing

1. Gear wheels (1) Failure phenomenon: The entire layout is distributed with bars, usually ink bars, shaped like a washboard. The distance between each bar is the pitch of the printing roller transmission gear (t=Ï€m).
(2) Cause of failure: The manufacturing and installation accuracy of the gear is not enough, the tooth surface is worn, the distance between the teeth is too large, or the damage of the individual teeth causes the gears to mesh and become inaccurate when the gears are driven to generate vibration, which causes sliding friction on the surfaces of the rollers that are in contact with each other. The result is that the dots of the plate layout are deformed and become elliptical or enlarged due to squeezing, and transfer marks are produced on the printed material. This type of failure will not only make the printed material appear tapers, but will also exacerbate gear wear.
If the gear has a damaged tooth or a foreign object between the teeth, there will be a bar on the print and it will be fixed in a certain position. If one of the pair of meshing gears is rotated by 90°, the position of the lever will change correspondingly, then it can be concluded that the gear is a problem; if the position of the lever is not changed, rotate the other gear by 90°. After the gears are rotated respectively, the positions of the tapers remain unchanged, and it can be concluded that the gears are not the cause.
(3) Solution: Reduce the gear center-to-center distance or gear-to-gap clearance properly, and change the drum liner to a soft liner to increase the elasticity of the liner. In severe cases, a new gear must be replaced.
2. Vibrating bars (1) Symptoms: The partial position of the printed sheet has a bar, and the position is fixed. The position of the taping bar is related to the vibration of a certain rule. Such a bar is generally an ink stick.
(2) Cause of failure: There are many unbalanced factors in the running process of the offset printing press. It is difficult to directly identify what kind of vibration is caused by the vibration. It is necessary to analyze the vibration source. The impact and vibration caused by roller emptying when it enters the rolling; the shock and vibration generated when the stamping cylinder teeth open and close; the vibration generated when the roller is pressed off; the roller itself is unbalanced and the vibration caused by the inertia during high-speed rotation The impact and vibration of the reciprocating paper feed mechanism due to the inertial force; the vibration generated when the paper receiving chain releases the paper cutting board.
(3) Solution: According to the specific situation of the tap, check the movement of the above-mentioned organization one by one or replace the unqualified parts.
3. Roller (1) Symptoms: The bar is wider and its position is not fixed. The ink is different before and after the print.
(2) The cause of the malfunction: The roller is divided into two kinds, the ink bar and the water bar. Most of them are caused by the improper design of the uniform ink system or the inappropriate adjustment of the pressure of the rubber roller, which results in insufficient supply of the uniform ink system. The diameter, support point, cylindricity of the rubber roller, the clearance between the roller head and the supporting hole, and the surface hardness of the rubber roller all affect the roller.
4. Other causes of the lever (1) Blanket. After the blanket has slackened, it will swell and slip during stamping to form a bar. This slippage is intermittent, and the greater the pressure between the plate cylinder and the blanket cylinder, the more severe the slip phenomenon. Therefore, the problem of slackening of the blanket must be solved in time, otherwise it will not only form a bar, but also cause a series of related failures. To avoid the generation of ink bars, choose a rubber blanket with small elasticity, suitable elasticity, good ink absorption, uniform thickness, and smooth surface. At present, air-cushioned blankets are the best.
(2) Pads. If rigid liners are used, although the printed dots are clear and rich in depth, they are sensitive to the precision of the machine and easily form bars, so the medium and soft liners are better. However, the pressure between the blanket cylinder and the plate cylinder is too small, and ink bars are also prone to occur.
(3) Plates. The reticulation version has a large friction and is not prone to ink bars; the field version and the thin-line version have a small frictional force, are easy to slide, and the dots are easy to appear.
(4) Paper. Whiteboard paper is more sensitive to the reaction of ink bars; coated paper is smoother than offset paper, and it is also prone to ink bars.
(5) Ink. The viscosity of large ink cohesion is not easy to appear ink, thin, transparent ink, ink sticks will be very clear. In addition, the ink and the ink bar are also related to the production, black ink is easy to show the ink bar, yellow ink and paper color is relatively close, the ink bar is not obvious.

Common solutions

Although the reasons for the emergence of bars are complex, as long as they are carefully analyzed, they can still find some rules.
(1) If the drive gear of the drum wears badly or the tooth gap is too large, it will cause a lever.
(2) Contamination of the tooth surface, intrusion of hard objects in the tooth gap, or blockage of the roots of the teeth, can cause problems. Solution: Reasonable maintenance, maintenance and elimination of foreign objects can reduce or avoid accidents.
(3) Roller bearing wear is severe and can cause a lever. Symptoms: There are several wide bars in the mouth of the paper, a few of which are equidistant. During the offset printing process, the shaft head and bushing are inevitably subject to wear. When the lubrication system is cut off oil or dirt enters the oil tank, it wears faster. When the shaft head and the sleeve wear, the pressure of the fistula will immediately yield, resulting in sliding friction, easy to cause ink. Solution: Place a liner in the blanket to make the jaw cushion trapezoidal to reduce the impact of the jaws when pressing together or replace them with wear-resistant bearings.
(4) If the pressure between the plate cylinder and the blanket cylinder is too large, the blanket will produce large sliding friction under the effect of the extrusion, causing the dots to deform and form the ink bars. Solution: Adjust the pressure between the two rollers, rather than blindly to the rubber bridge plus liner.
(5) The lining is not suitable. The linings of the printing plate and the blanket cylinder are not suitable, and the sliding friction force between them drives the plate cylinder, that is, the roller surface drive is in conflict with the driving of the roller gear. When such a failure occurs at a place where the pressure is low and the blanket is thick, the solution is to reduce the surface linear velocity error between the blanket cylinder and the plate cylinder, appropriately reduce the blanket lining and increase the plate cylinder. Lining.
(6) Improper bearing pressure. Contact roll-type offset printing presses will produce a bad lever when the pressure is not appropriate. The contact condition of the roll-bearing is related to the meshing condition of the gears. The improper roll-roller pressure, poor meshing of the gears, and vibration of the rollers may result in levers. Solution: Re-adjust the bearing pressure according to the instructions.
(7) The pressure between the ink roller and the plate surface is too high. If the line speed of the ink roller and the surface of the plate does not match, friction will occur between them. The last two of the ink rollers have a significant effect on the failure. When the contact width of the ink roller with the surface of the printing plate exceeds 6mm, the pressure on the surface of the printing plate is too large or the plate is too tight; when the ink roller passes through the empty space of the printing plate and hits the edge of the printing plate, impact bounce occurs. The ink layer on the ink roller is uneven, and an ink stick is produced at the mouth; when two or more ink rollers are pressed against the plate too tightly, the situation becomes worse. The first color of the bars is the most unfavorable to the quality of the printed matter. Such failures occur near the mouth.
(8) The pressure between the ink roller and the ink roller is excessive. The position of the ink stick is not fixed, and the pressure between the ink roller and the ink roller should be moderate, and the contact width within 5mm should be suitable.
(9) The pressure between the water roller and the plate is too high. At this time there will also be beating, forming a water bar. When the water roller is too close to the plate, after the roller is empty, it will hit the edge of the plate mouth and form a bar near the mouth. The solution is to reduce the pressure between the water roller and the plate.
(10) Intermeshing roller gear, intermeshing mesh is inaccurate or worn. Causes serious gear wear, large gaps, insufficient smoothness, high noise and numbness in hand. The ink bars are finely fenced and their pitch is equal to the pitch of the ink roller gears.
(11) The roller bearing is damaged. Ink roller bearing damage will cause the ink roller to run abnormally, often slipping on the plate, and form a lever, mainly in the water bar. Solution: Replace the ink roller bearing, and trouble can be eliminated.

Author: Qu Hao real

Reprinted from: Printing Technology