Ink and Water Imbalance in Printing, Sharp Increase in Defective Rate
During our company's use of the Heidelberg CD102 printing press for four-color printing, we discovered significant abnormalities in the magenta plate at 50% and 75% dot areas in the text sections (Figure 1), mainly manifested as enlarged and unevenly distributed dots, blurred images and text, noticeable color variations during printing, and a significant increase in water consumption by the equipment. These issues led to obvious fading of printed colors, reduced saturation, loss of midtone gradation, and a notable rise in the defective rate.
Cause of the Fault
The staff first checked the impression cylinder and the feed cylinder, confirming that lubrication was normal and the cylinder pads were clean and free of debris. However, when checking the pressure of the plate-contact inking roller, fluctuations in pressure were observed. Through systematic inspection, the root cause of the fault was ultimately identified: the bearing of the 80mm diameter operator-side plate-contact inking roller had seized due to a lack of lubricant. During machine operation, the bearing sleeve rotated synchronously with the plate-contact inking roller, causing severe wear on the bearing cup (Figure 2). The wear exceeded standard limits, resulting in deep wear on the bearing housing contact surface.
When the equipment was running idle, the 80mm plate-contact inking roller could maintain relatively stable movement within the bearing cup; however, during printing under impression, the roller first contacts the inking roller in one direction, and after impression, it contacts both the inking roller and the printing plate simultaneously. Each rotation of the machine causes the plate-contact inking roller to generate irregular wobbling within the worn bearing cup at the instant of contact with the printing plate. This phenomenon initially damages the printed dots, resulting in the dot blurring and deformation seen in Figure 1. In addition, the wobbling of the plate-contact inking roller when in contact with the printing plate also causes irregular changes in pressure between the inking roller and itself, as well as between the roller and the printing plate, which easily leads to ink streaks during printing, causing an imbalance between ink and water.
Figure 1 Problematic stamp (left) and adjusted stamp (right)
Figure 2 Shaft Bowl Wear Condition
Standardized Maintenance Procedure
Since the form roller mainly relies on the motion of the inking roller and only performs a periodic slight deceleration movement when in contact with the plate, it is necessary to strictly follow the standard in Figure 3 when adjusting the printing pressure. The pressure ratio between the form roller and the inking roller should be 1 mm higher than the pressure on the plate. If the pressure between the form roller and the plate is greater than that with the duct roller, the form roller will decelerate irregularly, leading to uneven ink transfer, excessive water and ink issues, and ultimately unstable ink color.
In actual production, when staff check the form roller pressure daily, they usually focus only on the pressure between the form roller and the plate, neglecting the pressure with the duct roller. The following is the standardized maintenance procedure:
(1) Shutdown Cleaning: Cut off the power and post a lockout tag, then use a specialized cleaning agent to thoroughly remove the ink film on the roller surface and metal debris in the bearing housing.
(2) Component Replacement: Replace the original manufacturer's bearing assembly (including bearing cup/ball/retainer); before installation, perform ultrasonic cleaning and nitrogen blowing to ensure contamination-free installation.
(3) Pressure Calibration: Calibrate using Heidelberg pressure test film (as shown in Figure 3).
(4) Determine Pressure Standards: The ink mark width between the form roller and the inking roller should be 4 ± 1 mm; the contact width with the plate should be set to 4-1 mm.
Figure 3 Printing Pressure Adjustment Standard
Figure 4 Heidelberg stress test film
Maintenance effect
After standardized maintenance, the quality of equipment and printed products has been significantly improved, and all indicators have returned to the normal range:
(1) The 50% outlet increase value of the magenta version fluctuates back and forth from 22%~45% before maintenance to about 18% (in line with the requirements of 17±3% outlet increase).
(2) The gradient smoothness of the gradient level of 75% dot density is increased by 40%, and there is no obvious discoloration during the printing process.
(3) The data monitoring of continuous production for 8 hours shows that the printing water consumption (after the repair of the faulty unit) is reduced by 20%, and the scrap rate is reduced from 22% to 0.2%.
(4) The equipment stability monitoring found that during 72 hours of continuous operation, the infrared thermal imager showed that the temperature of the plate roller bearing was stable in the normal range of 40±2°C, and there was no abnormal peak in the vibration spectrum analysis.
Problem analysis
01/ Defects in maintenance mechanisms
One of the core causes of this failure is that there are obvious defects in the equipment maintenance mechanism, and the relevant operations violate the requirements of the equipment maintenance manual, which stipulates that the ink roller needs to be disassembled every month, the bearing condition should be checked, lubricating oil added and the ink roller pressure should be adjusted in a standardized manner. In addition, during the previous maintenance, ordinary feeler gauges were mistakenly used instead of Heidelberg's special ink trace width detection film, resulting in a gap setting deviation of 2.2mm, and the pressure of the ink roller was mistakenly set to 6.2mm (55% exceeding the standard value), resulting in bearing overload.
When the normal indentation of the rubber roller is 4mm, the pressure is 18N; When the indentation reaches 6mm, the pressure increases to 63.5N, and when the indentation reaches 8mm, the pressure is as high as 149N. The larger the indentation, the faster the wear of the equipment, the higher the consumption of ink and dampening fluid, and the corresponding increase in electricity consumption. At the same time, excessive pressure will lead to obvious heat accumulation of the rubber roller, higher temperature and wide indentation, which will increase the actual pressure in the printing process nonlinearly, accelerating the hardening of the rubber roller. Although the hardness of the rubber roller can enhance the corrosion resistance to a certain extent, it will significantly reduce the water and ink transfer capacity, increase the ink loss of the printing equipment and the wear of the printing plate graphics. Therefore, without affecting the printing quality, the pressure of the rubber roller should be reduced as much as possible, and the minimum value specified by the manufacturer should be kept as optimal.
02/ Do a good job of rubber roller maintenance
The daily maintenance of rubber rollers needs to form a standardized process, and the pressure of the plate needs to be checked during the daily handover; During weekly maintenance, it is necessary to focus on checking the pressure of the plate roller and the ink roller, and do a good job in the maintenance of the plate ink roller to ensure that it is always maintained in a relatively stable state during production.
The surface of the new rubber roller should be uneven, and it feels a little astringent to the touch (Figure 5), so as to ensure the uniform transfer of water film and ink. In the process of long-term use, due to the constant contact with ink, dampening fluid and various solvents every day, coupled with the influence of the external environment, the hardness of the new rubber roller will usually increase after a period of use, which is a normal state. If the shrinkage and hardening of the rubber roller are light, the ink transfer characteristics can be maintained by adjusting the pressure appropriately.
The increase of the hardness of the rubber roller also means that the paper wool paper powder and the oxidized substances in the dampening solution have been deposited on the surface of the roller, forming a hardened film. This hardened film must be removed periodically (Figure 5) to prevent it from continuing to harden and glaze, which can cause instability in ink transfer and water transfer. When the surface hardens, the dirty glazing substance should be removed with calcium removal paste during maintenance.
The choice of cleaning agent is the most critical factor in determining the life of the rubber roller. Although highly volatile detergents are easy to remove hardened sediments on the surface, they will also cause the shrinkage of the rubber rollers. Improper use of cleaning agents will also lead to the expansion, contraction and deformation of the rubber rollers, affect the stability of the pressure between the rubber rollers, and even corrode the pipelines, sealing rings, machine paint and even rubber blankets. Therefore, semi-water-based, low-VOCs car wash water should be chosen. After maintenance, the rubber roller needs to be restored to the feel of the new rubber roller to be considered in place. There are no shortcuts to maintenance, and it must be persistent.
Figure 5 Surface of a new rubber roller (left) and surface of a glazed rubber roller (right)
The key to the quality of printed products lies in the balance between water and ink, and the plate water roller and ink roller play a crucial role in maintaining this balance. In daily production, various quality problems caused by improper adjustment or wear of the rubber roller are common. Therefore, it is essential to regularly inspect and test the rubber rollers, and to perform regular maintenance and adjustment of the roller surface, diameter, and pressure to ensure that each roller is in an appropriate condition. When adjusting the equipment, the process should be data-driven and standardized, contributing to the high-quality development of the industry.