How to Solve the Problem of Bars in the Offset Printing Process

The most common problem we encounter during printing is bars. Many masters are at a loss, which requires us to think calmly and try to take fewer detours. How do we judge these bars? Let me explain in detail.

Generally, bars are classified into ink bars, water bars, cylinder bars, gear bars, etc.

1 Excessive backlash of the cylinder meshing gears

The cylinder gears of offset presses basically adopt helical gear meshing, with a large gear transmission exchange coefficient and a small meshing backlash space, so as to reduce impact during transmission and vibration during printing, and maintain the stability and accuracy of printing. In actual work, the meshing backlash of the cylinder gears is adjusted and changed according to the different thicknesses of paper being printed. The thicker the paper, the larger the center distance adjustment, the relatively smaller the exchange coefficient of the meshing surface of the cylinder gears, and the larger the backlash. The greater the difference in linear speed between the surfaces of the printing cylinders, the greater the relative slip produced on the printing surface, and the more obvious the bars.

Solution: Under the premise of ensuring appropriate printing pressure, the center distance between the three cylinders should be reduced as much as possible, and the meshing backlash should be appropriately tightened to reduce the degree of barring.

Excessive backlash caused by gear wear.

After a period of operation, the cylinder gears, due to load wear or foreign matter stored in the tooth grooves, cause excessive meshing backlash; the involute of the gear cannot roll uniformly along the meshing line, or foreign matter jacking the teeth causes the gear to vibrate during operation, increasing friction between the cylinders and producing sliding. The printed image and text area, at the instant of this sliding, produces an ink band equal to the gear pitch, namely gear bars. Solution: Strengthen the cleaning and maintenance of mechanical equipment in daily work, clean and remove debris between gears, and maintain good lubrication; according to the gear wear condition, recalculate and adjust the center distance, appropriately tighten the backlash, and immediately replace severely worn gears with new ones.

2 Bearing wear

After an offset press has been used for a long time, especially cylinders that do not run on bearers, its motion impact and vibration are large, and the bearings are more prone to wear, causing the clearance between the journal and the bearing to increase and the fit to become loose. When the cylinders are engaged under pressure, the gripper edge yields and vibrates, producing sliding friction that forms bars. Especially when the load is abnormal or lubrication is poor, the wear between the cylinder shaft end and the bearing sleeve intensifies, and bars are most easily produced near the gripper edge.

Solution: The gripper-edge portion of the rubber underlay can be cut into a trapezoid to reduce the impact force when engaging under pressure; when the cylinder bearing is seriously worn, the bearing should be replaced in time.

Wear of the water and ink roller bearings.

Foreign matter hidden in or wear of the bearings, bearing seats, and bearing locking parts of the water and ink rollers can also cause obvious axial play and radial runout of the water and ink rollers during movement, thereby producing bars. In particular, when the bushing or transmission gear of the oscillating roller is worn, the vibration generated by the meshing of the cylinder gears affects the normal water distribution, ink distribution, and transfer of the oscillating roller, triggering the appearance of bars.

3 Wear of water and ink rollers

After a period of use, due to cleaning dirt particles, paper fuzz, ink skin, and other debris, or improper installation, uneven pressure adjustment, and other factors, the water and ink rollers are worn; the surface of the ink roller is uneven or the axis line is not straight, and in severe cases, the ink roller deforms to be thick at both ends and thin in the middle, or thick in the middle and thin at both ends.

When the offset press is working, this will cause the ink roller to jump, aggravating local friction and producing intermittent band-shaped bars.

For the water roller of the water unit, if the cloth cover has an incomplete stitched seam, or the inner diameter size is inappropriate, or the cloth cover becomes loose and wrinkled after long use, or the cloth cover of a new water roller sheds hair severely and forms a pile of fluff, affecting normal water delivery, bars can also be triggered.

4 Improper pressure adjustment

The cylinders of an offset press complete printing through relative extrusion. During the rolling process of the cylinders, the two corresponding cylinders repeatedly shift from the working surface (image-text contact surface) to the gap, and then from the gap to the working surface. These two transitions cause sudden load changes and produce impact. This impact causes instantaneous pressure changes and relative slip at the printing image-text contact surface, thereby producing bars.

Improper pressure between the ink form rollers and the plate cylinder.

Although the layout of the four ink form rollers of an offset press is thick at the front and back and thin in the middle, using different diameters to avoid overlapping bars, because the four ink form rollers form two groups that contact the plate separately, every two ink form rollers contact one oscillating roller at the same time. When the first ink form roller contacts the gap (gripper position) of the plate cylinder, the impact force generated pushes the oscillating roller up and then it drops; slip occurs between the oscillating roller and the ink form roller, affecting the change in ink volume transferred by the first ink form roller to the plate surface, thereby producing bars, and at the same time affecting the uniformity of the ink film of the second ink form roller; when the second ink form roller contacts the gap of the plate cylinder, the oscillating roller also experiences the process of being pushed up and then dropping. At this time, the first ink form roller is inside the plate, and the dropping oscillating roller impacts the first ink form roller and produces bars. Similarly, the third and fourth ink form rollers also produce bars on the plate, so the entire plate surface shows a group of bars that are sometimes light and sometimes heavy. Solution: Frequently check the diameter and axis tolerance accuracy of each ink roller to ensure the normal working state of the ink rollers. Accurately adjust the pressure between each roller and the plate according to the standard sequence; generally it can be adjusted so that the impression widths are 5mm, 4mm, and 3mm in sequence, and ensure that the axial play of the ink form rollers is less than 0.3mm.

Improper pressure between the oscillating roller and each ink roller.

The ink rollers in the inking section of an offset press basically adopt a soft-and-hard matching mode. When the pressure between rollers is adjusted too high, severe extrusion occurs between the hard roller (oscillating roller) and the soft rollers (ink form rollers, ink distributing rollers). Under pressure, the contact surface of the soft roller sinks and bulges slightly on both sides, affecting ink transfer and thus producing bars; or when the reversing time of the oscillating roller does not match the operation of the cylinders, the axial movement of the oscillating roller affects the ink volume on the ink rollers and produces bars.

Solution: Install each roller in its working position according to the standard sequence, accurately adjust the pressure between each roller and the oscillating roller; the impression width is about 3-5mm, and each reversal of the oscillating roller should be completed within the gap time of the cylinder.

Improper adjustment of water roller pressure.

When the pressure between the water roller and the plate cylinder is too high, the impact force generated when the plate cylinder switches between the working surface and the gap causes the water roller to vibrate, and water is applied unevenly, producing bars. Or because the friction between the water roller and the plate cylinder intensifies, the residual ink layer on the dots on the plate is destroyed, causing the dot base at that place to be destroyed, and insufficient ink absorption causes bars.

Solution: When adjusting the pressure between the water roller and the plate cylinder, insert a 0.15—0.2mm feeler gauge between them to test the pressure; it is appropriate that a frictional resistance can be felt by hand but the gauge can still be pulled out smoothly. The pressure of the upper water roller should be greater than that of the lower water roller, and the pressure at both ends should be kept consistent.

5 Improper cylinder packing

In actual work, the blanket cylinder is an elastic body and has slight errors when bearing pressure. A cylinder with thick packing is not in a pure rolling state, but has sliding within a certain range, which is prone to vibration and produces bars; when the packing is too thick, the compressible blanket cylinder, under pressure, produces a forward or backward bulge on the surface of the blanket, which excessively rubs the plate surface and causes ink bars; if the replaced blanket is not stretched tight or an old blanket becomes loose, under printing pressure the blanket slips on the cylinder surface and also leads to bars.

Sometimes, because the imprint at the gripper portion is too light, paper is locally padded, which increases the impact force in the cylinder gap cycle and can also cause bars. Cylinder packing is generally available in soft, medium, and hard types. Soft packing generally does not easily produce bars, but because it is highly elastic, the printing effect is not ideal, so it is rarely used; printing with hard packing gives clear dots, but the packing has little elasticity, and bars easily appear on offset presses with low precision.

Solution: The cylinder packing must be determined by precise calculation under the premise of ensuring the minimum suitable printing pressure. Under the premise of ensuring clear imprints and small dot deformation, choose medium-hard packing as much as possible. It is best to use air-cushion blankets.

6 Vibration caused by transfer in the transmission mechanism

The cylinder grippers of an offset press generally rely on strong springs to close the paper grippers and grip the paper for printing. The roller at the end of the gripper shaft rotates with the cylinder. When it reaches the fixed cam opener plate on the impact wall plate, it squeezes the spring to open the gripper; when the roller leaves the opener plate, the force acting on the spring disappears and the gripper closes. During the movement, the gripper needs to open and close twice to complete the process from receiving paper from the transfer gripper for printing to handing it over to the delivery gripper after completion. Because the opening and closing produce a relatively large impact, it affects the stability of the impression cylinder's operation, causing slip between the impression cylinders, and in severe cases bars appear. Similarly, the cylinder on-off pressure and the operation of the cam drive mechanism also cause vibration and produce bars.

Basically, these are the problems that can produce bars. If the bars are formed by improper installation, that is not something a master can solve.