Knife wire is a common and challenging quality issue in gravure printing. Due to the short ink path in this process—typically less than 1/4 of the plate roller’s circumference—the distance between the doctor blade and the impression point is minimal. If the blade is positioned closer to the imprinting area for technical reasons, the ink path becomes even shorter. This short path increases the risk: if foreign particles become embedded in the doctor blade, the thin lines created by these particles cannot be smoothed out by the ink's natural wetting properties. As a result, these lines are transferred to the printed material, causing visible knife wires.
One major cause of knife wire is solvent-based gravure ink. These inks inherently carry a higher risk of this defect. Besides contamination from environmental factors, another key factor is the fluctuation in solubility of the mixed solvents used. Solvent-based inks typically use a blend of 2–3 solvents, designed to dissolve the resin effectively. The solubility of the resin depends on three main principles: polarity similarity, solubility parameter compatibility, and solvation. A well-formulated solvent mixture usually satisfies all three. However, as the solvents evaporate, their volume ratios change, leading to shifts in the overall solubility parameters. If this fluctuation exceeds ±15% to 20%, the resin may re-agglomerate, forming insoluble particles that can get stuck in the doctor blade, resulting in knife wire defects.
During the initial phase of gravure printing, one common method to evaluate whether an ink is prone to knife wire is to let the plate roller idle for over 30 minutes before reprinting. This test checks for potential particle aggregation. While this method is practical, it lacks a strong theoretical foundation when it comes to understanding the solubility behavior of mixed solvents. Many practitioners might not consider the underlying chemistry behind such issues.
Water-based gravure inks, on the other hand, do not rely on mixed solvents, which theoretically reduces the risk of solubility fluctuations due to evaporation or mixing ratio inconsistencies. However, knife wire still occurs in water-based printing. This is because other factors contribute to solubility instability. First, changes in alkaline substances can alter the pH of the ink, affecting its solubility. Second, the combination of water-based emulsions with color pastes may lead to poor dispersion or even insolubility of the resin.
The compatibility between the water-based emulsion and the color paste directly affects the performance of the ink. Some water-based inks have a shelf life of only six months; beyond that, they may separate or precipitate, making them difficult to remix. This is essentially a solubility issue, often caused by an incompatible mix of the emulsion and the color paste. Therefore, even though water-based inks avoid some solvent-related problems, they are not immune to knife wire defects.
Interestingly, when only a water-based solution is used without an emulsion, the occurrence of knife wire is significantly reduced. This suggests that the problem lies in the mismatch between the aqueous solution and the emulsion. However, in film gravure printing, the adhesion and ink film resistance depend heavily on the emulsion, making it impractical to exclude it entirely. Thus, manufacturers face a challenge in ensuring the stability of water-based inks after adding emulsions, which remains a critical issue in the industry.
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