Han Guanhua Zhang Pifu: Talking about Printing AB Ghosting

In the article "Revisiting the Issue of Ghosting in Offset Presses," we briefly introduced AB ghosting. Now, let’s dive deeper into its causes and solutions. AB ghosting typically occurs under three key conditions: first, when a press uses double-diameter impression or paper transfer cylinders equipped with two sets of gripper teeth; second, during wet-on-wet printing processes; and third, when there are issues with the gears or tooth rows on these double-diameter components. These factors can lead to misalignment and result in AB ghosting. AB ghosting is one of the most common issues in multi-color offset presses that use double-diameter cylinders for both impression and paper transfer. These presses feature two sets of grippers on each cylinder—usually referred to as A set and B set. Normally, the printed image from the previous color should perfectly align with the next one. However, if the manufacturing or installation accuracy of the gripper teeth is poor, or if the gears or tooth rows are worn, the images may not overlap correctly. This misalignment creates ghost images either before or after the main print, repeating across the sheet and forming what is known as an AB ghost. The presence of AB ghosts can significantly affect print quality, reducing image clarity, tone gradation, and sharpness. In severe cases, it can even lead to defective products, causing financial loss and damaging a printer’s reputation. Therefore, regular maintenance and prompt troubleshooting are essential to prevent such issues. To address AB ghosting, there are generally two approaches. First, ensuring the precision of the gripper teeth by cleaning the gripper seats and lubricating the bearings. Second, adjusting the tension of the double-diameter gears, although this is usually done only in specific situations like after a jam or mechanical failure. When AB ghosting appears, it’s important not to act blindly. Start by asking the operator about the circumstances—was the issue gradual, sudden, or related to a previous accident? Observing the pattern of the ghost images using a magnifying glass can also help. For example, are the ghosts appearing on both sides, only on the leading or trailing edge, or in specific areas? Based on these observations, you can narrow down the cause and develop a targeted solution. Determining the exact source of the problem requires experience and technical knowledge. Techniques such as elimination, replacement, and testing can be used. For instance, if a ghost appears only when all four color groups are pressed together, it might indicate an issue with the gear alignment of one particular cylinder. Let’s take a real-world example. A ghosting issue occurred after a major machine jam. Upon inspection, the printed dots showed ghost images above and below them, consistently aligned. The fourth color group had no ghosts, but the others did. By testing different combinations of color groups, it was determined that the third pass was responsible. Adjusting the gear position resolved the issue. In conclusion, the root cause of AB ghosting lies in misalignment due to worn or improperly adjusted components like gears, grippers, or bearings. The goal is to minimize the impact of these errors and ensure accurate paper transfer. While AB ghosting may seem complex at first, with proper training and experience, it is a manageable issue that can be quickly resolved.

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