Identification and elimination of printing bars
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Bar fault
The printed matter appears in parallel with the axial direction of the drum, and the ink marks of different shades of ink are called printed bars, commonly known as bars, slivers, roads, and strips. This is a kind of printing that is more common and difficult to solve 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 picture accuracy, effect or the customer does not accept, you must find a way to eliminate it. Complete elimination is not easy, even impossible, and can only minimize the impact.
The bars are often caused by a variety of 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 dot is irregularly enlarged, elongated, ghosted, and the ink is aggravated, forming an obvious dark streak, called the ink bar. Bars, usually appear in printed lines, text, dot images and on the ground; second is the water bar, where the image dots appear irregularly shrinking at the place where the bars appear, forming an obvious light-colored streak called the water bar. The white bar is called, and the colorless white bar is also a change in the imprint caused by the thinning of the ink layer or the reduction of the dot. Both of the above bars are caused by poor contact with the surface of the printing plate such as a blanket or a water roller.
Cause of the bar
Since the empty space of the plate cylinder and the impression cylinder collides and impacts during the process of switching with the working surface, the cam, the connecting rod and the like vibrate under the action of inertia, and the outer circular surface of the drum is no longer uniformly rolled at a constant speed. Therefore, between the ink roller and the printing plate, between the plate cylinder and the blanket cylinder, between the blanket cylinder and the impression cylinder, a slight slip occurs, which breaks the balance of the ink, and the ink layer is affected by the break of the ink layer. The thickness causes the dots to increase or decrease, and the bars appear in a direction parallel to the axis of the drum. The higher the speed of the machine, the more serious the bar phenomenon.
Design aspect
The arrangement position of the rollers is unreasonable, and the reversing of the squeezing rollers is not performed when the rollers are empty and the rolling, the wall panels and the rollers are deformed due to insufficient strength and rigidity.
2. Precision
The assembly accuracy of gears, eccentric sleeves and rollers is not high, which affects the smoothness of mechanical movement.
3. Dynamic load
The effect of the dynamic load on the ink stick is that the rotating parts are unbalanced in rotation, and the reciprocating motion of the mechanism generates vibration and the acceleration of the cam mechanism causes an impact. Also related to the factors of the bar are the embossing, leveling, wetting part of the drum, the bearing, the drum gear, the main drive gear, the roller roller gear, the ink roller, the rubber roller, the water roller, the transfer paper cam, Assembly and adjustment of the rack and machine chassis. The accuracy error of these components during printing is 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 bar analysis
Gear bar
(1) Fault phenomenon: The entire layout is distributed with bars, generally ink bars, which are shaped like a washboard. The distance between each bar is the pitch of the printing roller drive gear (t=πm).
(2) The cause of the failure: the gear manufacturing and installation accuracy is not enough, the tooth surface is worn, the tooth spacing is too large, or the damage of the individual teeth causes the gear teeth to mesh with the gear teeth during the transmission, and the surface of the roller is in sliding friction. , causing the dot of the printing plate to be deformed, oval, or enlarged by squeezing, and transferred to the printed matter to produce streaks. This type of failure not only causes the printed matter to appear, but also aggravates gear wear.
If the gear has a tooth damage or a foreign object between the teeth, there will be a bar on the print and it will be fixed at a certain position. If one of the pair of meshing gears is rotated by 90°, the position of the lever will change accordingly, then it can be concluded that the gear is a problem; if the position of the lever is unchanged, the other gear is rotated by 90°, if two After the gears are rotated and the position of the bars is still unchanged, it can be concluded that the gears are not the cause.
(3) Solution: Appropriately reduce the gear center distance or the flank clearance, and change the roller lining to a soft pad to increase the elasticity of the lining. In the case of serious conditions, a new pair of gears must be replaced.
2. Vibration bar
(1) Fault phenomenon: The local position of the printed sheet has a bar, and the position is fixed. The part where the bar is present is related to a certain regular vibration, and the bar is generally a ink stick.
(2) Cause of failure: There are many unbalanced factors in the operation of the offset printing machine. It is difficult to directly identify which vibration is caused by the vibration source. Impact and vibration caused by the roller when entering the rolling; impact and vibration generated when the impression cylinder is opened and closed; vibration generated when the roller is pressed and biased; the drum body itself is unbalanced, and the vibration generated by the inertia when rotating at high speed The impact and vibration of the reciprocating transfer mechanism due to the inertial force; the vibration of the take-up chain when the paper is opened and closed.
(3) Solution: According to the specific conditions of the bar, check the movement status of the above-mentioned mechanism one by one or replace the unqualified parts.
3. Roller bars
(1) Fault phenomenon: the bar is wide and the position is not fixed, and the color of the ink before and after the print is different.
(2) Causes of failure: The roller is divided into two types: the ink stick and the 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 causes the ink system to be insufficiently replenished. The diameter of the rubber roller, the support point, the cylindricity, the fit clearance between the roller head and the support hole, and the surface hardness of the rubber roller all affect the roller.
4. Other causes of the bar
(1) Blanket. When the blanket is relaxed, it will swell and slip during embossing 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, it is necessary to solve the problem of blanket slack in time, otherwise it will not only form a bar, but also cause a series of related faults. To avoid the generation of the ink bar, a blanket with small stretchability, suitable elasticity, good ink absorption, uniform thickness and smooth surface should be selected. At present, the air cushion blanket is the best.
(2) Liner. If a hard pad is used, although the printed dots are clear and rich in layers, they are sensitive to the accuracy of the machine and are easy to form a bar, so the medium and soft pads are better. The pressure between the blanket cylinder and the plate cylinder is too small, and the ink stick is also prone to occur.
(3) Plate. The friction of the mesh version is large, and the ink stick is not easy to appear; the friction of the solid version and the thin line version is small, and it is easy to slide, and the dot gain is easy to appear.
(4) Paper. The whiteboard paper is more sensitive to the reaction of the ink stick; the coated paper is smoother than the offset paper, and the ink stick is also prone to occur.
(5) Ink. The ink with high viscosity has a large cohesive force, and it is not easy to have a liquid bar, and the thin, transparent ink can make the ink stick very obvious. In addition, the ink color is also related to the generation of the ink stick, the black ink is easy to appear the ink stick, the yellow ink is relatively close to the color of the paper, and the ink stick is not obvious.
Common bar solution
Although the reasons for the generation of the bars are very complicated, as long as they are carefully analyzed, some rules can be found.
(1) The drive gear of the drum is seriously worn or the tooth gap is too large, which may cause the lever.
(2) The tooth surface is dirty, the hard object is embedded in the tooth gap or the root of the tooth is blocked, which will cause the bar. Solution: Reduce or avoid accidents by properly maintaining, maintaining and eliminating foreign objects.
(3) The roller bearing is seriously worn and will cause the bar. Symptom: 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 bushing are inevitably worn. When the lubricating system is cut off or dirt enters the oil groove, the wear will be faster. When the shaft head and the bushing are worn, the pressure of the mouth will immediately cause the transfer, resulting in sliding friction, which is easy to cause the ink stick. Solution: pad the paper in the blanket so that the mouthpiece is trapezoidal to reduce the impact of the jaws when pressed, 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 generate a large sliding friction under the action of the extrusion, causing the dot to be deformed to form a ink stick. Solution: Adjust the pressure between the two rollers instead of blindly adding the rubber to the rubber.
(5) The lining is not suitable. The lining of the printing plate and the blanket cylinder is not suitable, and the sliding friction between them drives the plate cylinder, that is, the surface driving of the drum is in conflict with the driving of the drum gear. When such a failure occurs in a place where the pressure is small and the blanket cylinder is thick, the solution is to reduce the surface line speed error of the blanket cylinder and the plate cylinder, appropriately reduce the blanket cylinder liner and increase the plate cylinder. Lining.
(6) Improper pressure on the roller. When the pressure of the roller of the contact roller type offset printing machine is not normal, the lever is generated. The contact condition of the roller is related to the meshing condition of the gear. The pressure of the roller is not good, the gear meshing is not good, and the roller is easy to cause vibration and generate a lever. Solution: Re-adjust the pressure of the roller according to the instructions.
(7) The pressure between the inking roller and the surface of the printing plate is too large. If the line speed of the inking roller and the surface of the printing plate does not match, friction will occur between them. The latter two of the inking rollers have a significant effect on the failure. When the contact width of the inking roller with the surface of the printing plate exceeds 6 mm, 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 empty space of the roller and hits the edge of the printing plate, impact beating occurs. The ink layer on the ink roller is uneven, and a rubber bar is generated at the mouth; when two or more ink rollers are too tightly pressed against the printing plate, the situation is more serious. The first color of the bar is the most unfavorable for the quality of the print, and this kind of failure occurs near the mouth.
(8) The pressure between the inking roller and the inking roller is too large. The position of the ink stick is not fixed, so the pressure between the inking roller and the tandem roller should be moderate, and the contact width within 5 mm is 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 plate, after passing through the roller, 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) The inching roller gear and the intermediate wheel are not meshed or worn. The gear wear is serious, the gap is large, the finish is not enough, the noise is large and the hand feels numb, and the ink bar is in the shape of a thin fence, and the pitch is equal to the pitch of the roller of the tandem roller.
(11) The ink roller bearing is damaged. Damage to the ink roller bearing will cause the ink roller to operate abnormally, often slipping on the printing plate and forming a bar, mainly based on the water bar. Solution: Replace the ink roller bearing and the fault can be eliminated.