Imbalance in ink painting is not scary! Here's how to deal with dirty printing plates.
The balance of ink and water is not only the foundation of offset printing but also its difficulty. According to the author's understanding, the key to ink-water balance lies in controlling the water. On one hand, it involves the proportion, temperature, and circulation cleanliness of the water; on the other hand, it requires proper adjustment of the water system. Generally, the former is almost static and changes little, while the latter is dynamic with many variable factors, making it difficult to master and often causing dirty prints. What exactly causes dirty prints?Based on practical experience, the author has conducted a simple analysis, which can be roughly divided into: greasy dirty prints, insufficient water dirty prints, light pressure on the water roller causing dirty prints, water roller jumping causing dirty prints, and poor water distribution on the water roller causing dirty prints. It can be said that most of these are due to improper water roller adjustment. Therefore, the author will combine two typical cases recently encountered to share some insights.Dirty prints caused by poor water distribution on the water roller01 Malfunction Phenomenon and SolutionWhile printing the cover of the textbook "Morality and Law," we found a streak-like dirty phenomenon on the magenta plate, resembling fabric texture, evenly distributed, with the entire pattern almost consistently affected.If water is intentionally increased, the dirty marks slightly improve but cannot be completely eliminated. Obviously, the root cause is unrelated to water quantity. Since the cover graphics are sky-blue, and the cyan and magenta layers cover the full plate, printing is relatively difficult. After the press operator initially adjusted the ink-water rollers, the dirty marks did not improve at all. This led everyone to suspect other causes:Could it be unstable quality of alcohol-free dampening solution?Could it be pitted spots on the ink roller?Could it be calcium buildup on the ink roller?A series of question marks left everyone at a loss, and some were even impatient, thinking the product quality was acceptable and there was no need to fuss.As is well known, to improve operational skills, one must learn to continuously analyze and solve problems in daily work, especially difficult problems, without giving up easily.Following this belief, we re-examined the ink rollers one by one and carefully adjusted the form roller, inking roller, and plate, yet no obvious abnormalities were found.Later, we focused on the water system, removed the metering roller, and used the "inking bar" method. We discovered that the form roller's pressure on the distributing roller was less than a 2-millimeter ink bar, which was clearly insufficient. We added pressure at both ends to reach a 7-millimeter ink bar.Upon restarting the press, the malfunction disappeared; the printed magenta dots were clean, even, and the ink color remained stable.02 Fault AnalysisAnalyzing the ink path: the ink repeatedly moving and leveling through the distributing roller cannot form streaks or threads of dirty marks.Analyzing the water path: during printing, if dirty marks are caused by insufficient water, they are typically localized, starting at the chewed edge, and barely affect the trailing end. In such cases, simply increasing the water generally resolves the issue.Clearly, this malfunction was not simply due to insufficient water but had more complex causes. To understand the reasons, it is necessary to carefully analyze the water path transmission (as illustrated below).
The fountain roll (T), metering roll (D), and plate-bearing water roll (A) all supply water to the printing plate in a straight and uniform manner, while only the oscillating water roll (R) moves back and forth in small amplitudes.
The key to the above malfunction lies in the fact that when the pressure between the plate-bearing water roll and the oscillating water roll is significantly low, the operation of the plate-bearing water roll almost relies on the rotation of the printing plate cylinder. This causes the water film on the surface of the plate to not be evenly distributed by the oscillating water roll, resulting in very regular dots in the printed image, which appear as shallow fabric-like streaks throughout, causing contamination.
Contaminated plates caused by water roll vibration
01 Malfunction Symptoms and Solutions
During a certain printing process, it was found that the magenta group often inexplicably had contamination about 10 centimeters from the edge, as shown in the figure below.
After investigation, it was found that this situation had been occurring for several days. We deliberately reduced the machine speed and increased the water volume, and the effect was barely acceptable. However, running the machine so anxiously is not a long-term solution; the cause must be identified as soon as possible.
After careful observation, we found that the shaft seat of the water roller in that color unit was noticeably wobbling. We initially suspected that the pressure of the water roller on the plate might be too high. So, we stopped the machine for inspection, but everything appeared normal.
Could a fastening screw have come loose?
After checking, everything was still normal.
Could the hydraulic cylinder strut have loosened?
By manually pushing the cylinder strut, we discovered it was loose. Normally, when the cylinder is in working condition, it is impossible to move the strut back and forth easily by hand. From this, we concluded that the cylinder's seal was leaking, and it could not ensure the stable operation of the water roller's shaft seat. Therefore, we immediately replaced the cylinder, and the problem was resolved.
02 Fault Analysis
In the above fault, the dirty plate appeared at the upper part of the gripper. When the machine speed was reduced, the amplitude of the water roller's wobble gradually decreased, and the dirty plate phenomenon improved. This is completely different from the dirty plate issue we often encounter due to insufficient water.
Apart from cylinder leakage, there are many reasons that can cause water roller wobbling:
(1) Excessive pressure of the water roller on the plate;
(2) Worn water roller bearings causing wobble;
(3) Poor fit between the water roller shaft end and the shaft seat, causing wobble;
(4) Loose or missing fastening screws;
(5) Poor machining accuracy of the water roller, making it out of round.
In actual production, there are many causes for dirty plates, not necessarily just insufficient water. If the operator mistakenly increases the water supply, it will inevitably cause an imbalance between ink and water, leading to a series of problems. Therefore, only by truly mastering the correct adjustment and use of the offset printing press water system can one truly control the key to water-ink balance and produce better products.