Fault Analysis of High-Speed Rotogravure Printing
High-speed rotogravure printing operates at high speeds, with long paper paths. Once a fault occurs, whether during shutdown for adjustments or startup, the waste generated can be enormous. The causes of such faults are manifold: one is the impact of physical and chemical changes in the ink; another is the effect of the ink's interaction with the anilox roller and doctor blade, as well as variations in plate depth; third, the physical and chemical properties of the paper and the static ink distribution system can affect the process. Additionally, operator errors and changes in the printing environment also play a significant role.
1. Poor Ink Transfer in Fine Text on the Gravure Plate, Resulting in Missing Strokes and Gaps
On a hot summer night, with the temperature and humidity in the workshop high, and relative humidity approaching 100%, a paper break occurred during the shift change, leading to a machine stop. After reloading paper and restarting, the machine operator noticed that the black plate’s fine text had faint ink coverage and significant gaps. The operator reduced the machine speed and asked the assistant to wipe the black plate thoroughly. After increasing the speed again, the operator checked the sample and was still unsatisfied—the fine text was completely "blocked."
Generally, fine text on the black plate has a small surface area and low ink consumption, meaning it doesn't require frequent ink replenishment. As the solvents in the ink evaporate over time, the solubility of the ink decreases, causing plate blocking. The extended stop time during shift change caused the ink in the plate’s cells to dry. When wiping with solvent, the solvent evaporated rapidly, taking away significant heat, causing moisture in the air to condense on the plate. This effect was particularly noticeable due to the high humidity and temperature. The ink, now diluted with water and with reduced solubility, needed to be fully removed. After cleaning with degreasing powder, the plate was thoroughly wiped with a dry cloth, and a new ink formula was prepared, adding a small amount of butyl acetate. Once the machine was restarted, the speed was quickly raised above 100 meters per minute, and the fine text on the black plate printed correctly.
2. Poor Ink Transfer of Fine Dots in Process Printing Plates
In process printing, proper ink transfer of fine dots is crucial for print quality. On the second printing run of a product, the paper used was 230g/m² white cardboard. After setting up the machine and starting printing, the operator noticed that the fine dots on the cyan plate were severely missing, with too many white spots. Adjusting the ink viscosity didn’t improve the situation. The operator then examined the paper and found its surface to be somewhat rough. When the paper’s surface roughness reaches a certain level, it becomes difficult for the static ink distribution system to transfer ink from the cells onto the paper, resulting in increased white spots in the highlights.
Next, the operator checked the cyan plate’s printing unit and realized that the anilox roller wasn’t equipped with a static ink distribution system. The entire static ink distribution system was then thoroughly inspected. The brush was found to be clogged, and in some places, the brush was not in contact with the anilox roller. After cleaning the brush, the static generator was checked for functionality, and the non-static anilox roller was replaced. Additionally, the paper was changed, ink viscosity was adjusted, and after restarting the machine, the speed was increased to over 100 meters per minute, with the fine dots transferring correctly and the white spots disappearing.
3. Sudden Plate Misalignment During Registration
The stability of print registration relies on the tension control system and the registration tracking system. The registration marks play a key role in maintaining registration stability. In addition, printing pressure, drying temperature, and environmental factors like temperature and humidity also significantly impact registration.
During a transitional period between summer and autumn, the weather was hot during the day and cool at night. After a cold front, the humidity dropped dramatically. The printing job involved wide-format 80g/m² coated paper for beer labels, with fine registration lines of only 0.15mm. After the shift change, the operator noticed that the middle printing unit had a registration misalignment. Attempts to correct the deviation had no effect. Upon inspecting the tension control system, no issues were found with the unwinding or rewinding tension or the inter-unit tension.
The operator then examined the registration mark and discovered that the registration mark on the middle printing unit had insufficient ink transfer, and the front half of the mark was partially dried. This caused the registration sensor to read the wrong signals, which led to subsequent errors in the following sensors and misalignment. The final check revealed that the floating roller had moved to the top, causing it to not respond to the registration deviation. Furthermore, changes in the workshop temperature and humidity had altered the paper’s dimensional stability, making the registration misalignment more likely.
To fix the issue, the operator adjusted the ink viscosity to ensure clear registration marks. Then, the drying temperature for each printing unit was fine-tuned, followed by adjusting the printing pressure and drying temperature to account for the paper’s expansion. After the registration mark was correctly read, the registration deviation was adjusted, and the print speed was slightly altered to accommodate the paper's behavior, ensuring proper registration.
4. Challenges with Color Adjustment After Changing Plates
Sometimes, the surface of the paper contains hard particles, which, if not properly removed during cleaning, can damage the printing plate when entering the press, causing plate crushing. After 500,000 sheets of purple-red base ink were printed, a new plate was changed, but the purple-red color was noticeably different, with a thicker ink layer and a deeper color. Adjusting ink viscosity didn’t solve the problem, requiring a technician to adjust the ink composition by altering its color strength. To do this, a reducer was added to adjust the ink’s color strength, while small amounts of red and yellow inks were used to adjust the cyan and magenta components of the ink. This necessitated reworking the ink formula, which was frustrating for the press operator. Besides impacting the shift’s production output, the color adjustment consumed extra ink and print samples.
5. Ink Contamination on Cooling Rolls
Gold ink is relatively adaptable to printing conditions, but improper handling can still lead to unexpected problems. This issue occurred during a night shift, after printing a yellow base with gold lines. The gold ink, which adheres poorly, tended to stick to the cooling rolls. The press operator had increased the printing speed, thinking it was an easier job. The yellow base ink consumed more ink and had a better printing adaptability, while the gold ink, due to excessive ink layers, increased the peeling force. If the drying process was insufficient or if the cooling water temperature was too high, it affected the ink’s adherence.
Upon inspecting the printed samples, the operator noticed that the cooling rolls had accumulated gold ink. The drying air temperature in the oven was too low, preventing the gold ink from drying properly. The operator stopped the machine, cleaned the cooling rolls thoroughly, adjusted the drying temperature, and set the machine speed to an appropriate level. After these corrections, the issue was resolved.
6. Static Electricity Issues
Static electricity is a common problem during the winter and spring months, when dry weather leads to static buildup from the friction between high-speed running paper and metal rollers. When the static charge reaches a certain level, it discharges, causing issues with the printed samples, particularly fine lines that appear as streaks. Initially, the operator mistook this for contamination caused by the doctor blade, but replacing the blade didn’t fix the problem. More seriously, the accumulated static charge on the paper made it difficult to separate sheets during cutting, potentially leading to paper jams. In extreme cases, the accumulated charge could even cause sparks, which, when combined with solvent vapors, could lead to explosions.
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