In offset printing, a phenomenon known as "powder on the ink roller" can occur. This happens when powdery particles appear on the ink rollers after the press has been running for some time. These particles can cause ink to deposit in unwanted areas or lead to deinking of the roller, resulting in poor ink transfer. The main reason behind this issue is that the powder particles are either oleophilic (oil-loving) or hydrophilic (water-loving), which disrupts the balance between water and ink.
Water and ink play crucial roles in the offset printing process. The fountain solution provides the necessary water, while the ink is used for image transfer. For high-quality prints, both must be transferred efficiently. However, many unpredictable factors can interfere with this process, making ink control one of the most challenging aspects of offset printing. One common problem is the formation of a powdery coating on the ink roller, which negatively affects ink transfer.
The primary component of the fountain solution is water, and its hardness varies depending on the region. Soft water typically has a total hardness of 0–30 ppm, while hard water exceeds 60 ppm. When using hard water, calcium ions (Ca²âº) can react with the resin in the ink, forming calcium saponification. In the presence of acidic fountain solutions, this reaction may produce substances like calcium citrate or calcium phosphate. Calcium saponification is oil-soluble and can cause ink to accumulate in unwanted areas, leading to crystallization on the rubber roller surface, which hinders both water and ink transfer. On the other hand, calcium salts are water-soluble and can interfere with ink transfer, causing ink to separate from the roller. To prevent this, it's best to use soft water with low calcium content for the fountain solution and choose formulations that minimize calcium buildup.
In offset printing, the materials entering the press include substrates, inks, and fountain solution. Any powdery particles present in the system are usually introduced through these three elements.
If the substrate has a poor surface quality, it can directly introduce powdery materials into the ink system. For example, if the paper contains a lot of dust or fine particles, these can be transferred to the ink system via the printing plate during the impression process, causing the "powder on the ink roller" issue. Replacing the substrate with a cleaner one can resolve this. Even if the paper appears clean, poor coating quality can lead to fluffing or powdering during printing, which can then be transferred to the ink system. This issue can be addressed by reducing press speed, lowering ink viscosity, or using higher-quality paper.
In addition to direct contamination from the substrate, chemical reactions between the fountain solution and the ink can also lead to calcium precipitation. This indirectly introduces powdery particles into the printing system, further complicating the ink transfer process. Proper maintenance, careful selection of materials, and regular cleaning of the press can help mitigate these issues and ensure smoother printing operations.
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