An Inquiry into the "Bar" Problem in Printing

In lithography practice, "bars" are print quality issues that often occur and are difficult to completely solve. The production of "bars" involves many aspects such as the design of the typographic machine, the material and processing technology of the mechanical parts, the operation of the staff, the selection of the printing materials, and the maintenance of the mechanical equipment. The main reason for the production of “bars” in printing is the vibration of the printing cylinders of the typograph and the process of transferring the ink and water conveying mechanism or paper, which causes the slippage of the relative positions of the prints. Therefore, it is impossible for our existing printing equipment and technology to eliminate the occurrence of “bars”. It is only possible to minimize and mitigate the “bars” by improving the technology and operating processes. This article explores the emergence and solution of "bars" and hopes to play a role in attracting the elixir.

1 The concept and classification of “bars”

1) The concept of "bar"

On the lithographic prints, there are banded imprints in the direction parallel to the axis of the drum, resulting in irregular shades of ink due to irregular deformation of the dots, also known as "bars" and "strip marks."

2) Classification of "bars"

According to the "bar" of the ink can be divided into two kinds of ink bars and water bars.

In the print area of ​​the printed matter, the dots are irregularly enlarged, and the ink layer is thicker than the ink printed at the same time. There is a dark banded print (also called a "color bar").

Plumbing: In the printed area of ​​the printed matter, the dots are irregularly reduced, and the ink layer is thinned, which is significantly lighter than the ink printed at the same time (also called "white bars").

According to "bars," the causes can be divided into gear bars, vibration bars, roller bars and other bars.

Gear bars: The accuracy of the transmission gears of the cylinders, in-transit components, and water delivery components of the offset press is insufficient, resulting in wear of the tooth surfaces or damage of the gear teeth. Vibrators are generated in the meshing drive, causing the printed images on the rollers to slip relative to each other. Move to "bar". It is characterized by the fact that the pitch of the bars in the printed sheets is equal to the pitch of the gears, and the layout of the sheets is similar to that of the "cooked board."

Vibrating bar: The offset machine suddenly vibrates at a moment when it runs at a constant speed. This vibration damages the drum at a constant speed and causes a slight amount of wobble between the printing rollers to produce a "bar." Such as the printing roller face and the neutral shift, roller clutch, the opening and closing of the roller bite teeth, the inertia of the cam link mechanism or bearing wear and other transmission parts, will cause shock vibration, resulting in relative slippage of printed graphics A "bar" appears. It is characterized by the presence of “bars” in the local position of the printed sheet and its position is fixed. The position of the “bar” is related to the periodic and regular vibration of the mechanical movement.

Roll bar: The roller bar is due to the uneven ink design of the offset press, the unsuitable design of the uniform water system, or the accuracy of the roller processing, and the improper matching accuracy, which causes the slippage between the water and ink rollers and the plate cylinder and roller during operation. Or, due to improper pressure adjustment, the contact position is recessed, disturbing the normal transfer of ink and ink, thereby forming a "bar." It is characterized by a slightly larger number of bars, a wider belt, a bar width equal to the surface perimeter of the roller, and an unfixed position. The inks are of different colors before and after printing.

Other bars: Bars generated by factors such as blankets, pads, printing plates, paper, inks, wetting fluids, and handling processes involved in the printing process. Because of its different characteristics, this article will not be discussed in detail.

2 Analysis of the causes of "bars" and their solutions

1) The backlash of the roller meshing gear is too large

1 The backlash caused by the center distance adjustment is too large. The roller gear of the offset printing press is basically meshed with a helical gear, and the gear exchange coefficient is large, and the space of the meshing side clearance is small, so as to reduce the impact in the transmission and chattering during printing, and maintain the smoothness and accuracy of printing. In actual work, the backlash of the roller gear mesh is changed with the printing of different thicknesses of paper. The thicker the paper is, the larger the center distance adjustment is, the smaller the exchange coefficient of the roller gear engagement surface is, and the larger the backlash is. The greater the difference in line speed on the surface of the printing cylinder, the greater the relative slippage produced by the printing surface and the more pronounced the bar. Solution: Under the premise of ensuring proper printing pressure, the center distance between the three rollers should be reduced as much as possible, and the meshing tooth gap should be properly pressed to reduce the degree of “bars”.

2 The backlash caused by gear wear is excessive. After a period of working operation, the roller gear is worn due to load or there is foreign matter in the tooth gap, which causes the gear meshing backlash to be too large. The involute of the gear cannot be uniformly rolled along the meshing line, or the “top gear” of the foreign object makes the gear in operation. Chattering occurs, causing the friction between the rollers to increase and slipping. At the instant of printing, the printing graphic part generates an ink ribbon equal to the pitch of the gears, that is, a gear bar. Solution: Strengthen the cleaning and maintenance work of machinery and equipment, clean and remove the debris between gears, and maintain a good lubrication condition; according to the wear of gears, recalculate the center distance, properly compress the backlash, and immediately replace the worn ones with new ones. gear.

2) Bearing wear

1 Roller bearing wear. When the offset press is used for a long period of time, especially if the rollers do not move the rollers, the impact vibration is large and the bearings are relatively easy to wear, making the gap between the journal and the bearing increase and the loosening of the sleeves. The vibrations occur due to the occurrence of separation, resulting in sliding friction forming "bars." In particular, when the load is not normal or the lubrication is not good, the shaft head and bushing of the drum wear more, and the “bar” is most likely to occur near the mouth. Solution: The inner part of the rubber liner can be cut into a trapezoidal shape to reduce the impact force when the pressure is applied. When the roller bearing wears badly, the bearing should be updated.

2 Ink roller bearing wear. Water, ink roller bearings, bearing seats, bearing locks containing debris or wear can also cause the water, ink roller in the movement of significant axial turbulence and radial runout resulting in "bars", especially 窜When the sleeve or transmission gear of the ink roller is worn out, the vibration generated due to the meshing of the roller gear affects the normal uniform water distribution, ink distribution and transmission of the roller, and 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 the cleaning dust particles, paper hair, ink knots and other impurities, or improper installation, uneven pressure adjustment and other factors, the surface of the ink roller is uneven or the axial line is not straight. In severe cases, the ink roller has two rough and thin middle or two coarse deformations. When the offset press is working, it will cause the ink roller to jump, causing the local friction to become heavier, resulting in an intermittent ribbon bar. The cloth sleeve of the water roll is incomplete due to incomplete stitching interface, or improper internal diameter size, or loose wrinkling of the cloth cover after a long time of use, or serious hair loss due to hair removal of the new water roll cloth cover. The occurrence of bars.

(to be continued)