The performance characteristics and causes of the printing bar phenomenon

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 phenomenon and characteristics, the bars can be divided into two situations.

1, the ink bar

Dots irregularly grow, stretch, ghost, and accentuate ink, forming a distinct dark streaks, called ink bars, that usually appear when printing lines, text, dot images, and on the ground.

2, the water bar

In the place where the bars appear, the image dots are irregularly reduced to form a distinct light-colored streak, which is called a water bar, also called a white bar, and the colorless white bar is also a result of a blot change caused by the thinning of the ink layer or the dot reduction. . Both of the above bars are caused by bad contact between the blanket and the water roller and the surface of the printing plate.

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. It mainly includes the following three aspects.

1. Design aspects

The arrangement position of the drum is not reasonable, and the commutation of the tandem roller is not performed when the roller is empty and the wall plate and the drum are deformed due to insufficient strength and rigidity.

2. Accuracy

The assembly accuracy of gears, eccentric sleeves, and rollers is not high, which affects the smoothness of the mechanical movement.

3. Dynamic load

The impact of the dynamic load on the ink bars is that the rotating parts are unbalanced, the reciprocating motion of the mechanism generates vibration, and the cam mechanism accelerates to cause an 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.