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Offset printing technology: NO is not allowed for ghosting and overprinting
In offset printing, three major issues commonly occur: ghosting, overprinting errors, and bar defects. These problems can lead to blurred images, distorted tonal ranges, and an overall loss of clarity in the final print. As a result, it's crucial for printers to implement preventive measures during production. However, identifying the exact cause of these faults is essential for effective troubleshooting. Among these, ghosting and overprinting errors often appear similar, which can lead to misdiagnosis and unresolved issues. Therefore, accurately distinguishing between ghosting and overprinting plays a key role in maintaining high-quality output.
Ghosting refers to faint lines or side shadows that appear next to printed lines or dots. These shadows are lighter than the main elements and can manifest as double ghosts—where duplicate images or light lines appear close to the original. Ghosting typically reduces the sharpness of edges, affects color and tone reproduction, and causes blurriness in the image. It can be classified into three types based on direction: vertical, horizontal, and A/B ghosting. Vertical ghosting appears above or below the original dot, while horizontal ghosting occurs on the left or right side. A/B ghosting is specific to multi-color presses and involves mirrored shadows on opposite sides of the sheet.
When ghosting occurs in fine text, it can make the entire layout unreadable, resulting in waste. In color prints, severe ghosting can distort the image’s mid-tones, making it difficult to achieve accurate color balance and sharpness.
Overprinting, on the other hand, refers to the process where each color layer is applied on top of the previous one, aligning precisely with the intended position. Accurate overprinting ensures that all color layers overlap perfectly, creating a clear and consistent image. If this alignment fails, it results in overprinting inaccuracies. These inaccuracies can be partial (like a fan-shaped distortion at the edge) or affect the entire sheet due to misalignment in positioning.
The causes of overprinting errors include mechanical issues, improper machine adjustments, process inefficiencies, and material quality. For example, worn parts, incorrect paper feeding, or uneven ink and water distribution can all contribute to misregistration. Similarly, paper moisture content and flatness also play a role in overprinting accuracy.
Although both ghosting and overprinting errors are directional and can impact image quality, they differ in their causes and manifestations. Ghosting typically occurs in single-color printing and involves repeated, lighter images next to the main ones. Overprinting, however, requires multiple layers and focuses on the alignment of different color plates. Understanding these differences is essential for effective problem-solving and maintaining print quality.