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, we will go into greater detail about its causes and solutions. AB ghosting typically occurs under three specific conditions: first, when double-diameter imprinting or double-diameter paper transfer cylinders are used, meaning each cylinder has two sets of gripper teeth; second, during wet-on-wet printing processes; and third, when there is a problem with the gears or tooth rows on these double-diameter cylinders, which can lead to AB ghosting. AB ghosting is one of the most common issues in multi-color offset presses that use double-diameter impression and paper transfer cylinders. These presses have two sets of grippers on each cylinder—known as A set and B set. Normally, the printed image from the first color should align perfectly with the image from the second color on the blanket. However, if the manufacturing or installation accuracy of the grippers is poor, or if the gears or gripper mechanisms are worn, the images may not overlap correctly. This misalignment results in ghost images appearing either before or after the actual print, creating the so-called AB ghost effect. This type of ghosting can significantly impact print quality, reducing image resolution, tone levels, and sharpness. In severe cases, it can even result in defective prints, leading to financial losses and reputational damage for the printing company. Therefore, regular maintenance and prompt troubleshooting are essential to prevent such issues. To address AB ghosting, there are generally two main approaches: ensuring the precision of the gripper teeth and adjusting the tension of the double-diameter gears. It’s important not to rush into solutions without understanding the root cause. First, consult the operator to determine whether the ghosting appeared gradually, suddenly, or after an incident like a paper jam. Gathering this information helps identify the underlying issue. Next, analyze the ghosting pattern using a magnifying glass. Look for whether the ghosting appears across the entire print, only on one side, or in specific areas. Based on this, you can develop a targeted solution. Finally, determine which part of the machine is responsible by using methods like elimination, replacement, or testing. For example, suppose a ghosting issue arises after a serious paper jam. Upon inspection, you notice ghost images above and below the printed dots, repeating consistently. Further investigation shows that the ghosting only occurs when all four color groups are printed together, suggesting a misalignment in the third color group’s transfer cylinder. By adjusting the gear position according to theoretical guidelines, the issue can be resolved. The root cause of AB ghosting often lies in misaligned components such as gears, grippers, bearings, or swing rods. These misalignments cause adjacent grippers to pass through the same area at different times during the cycle. The goal is to minimize these errors and ensure accurate paper transfer. Although AB ghosting may seem complex at first, it becomes manageable once you understand the methods and theory behind it. With the right approach, it's a solvable issue that doesn't need to be overwhelming.

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