Analysis of gravure printing ink and knife wire failure

Knife wire is a common and challenging quality issue in gravure printing. Due to the short ink path in this process, the distance between the doctor blade and the imprinting point usually doesn't exceed 1/4 of the plate roller's circumference. If the scraper position is raised closer to the imprinting point for technical reasons, the ink path becomes even shorter. This short distance increases the risk that any foreign object embedded in the blade can create a thin line that isn’t smoothed out by the ink’s own wettability. As a result, this thin line is transferred to the printed material, causing a knife wire defect. One major cause of knife wires in solvent-based gravure inks is the fluctuation in solubility of the mixed solvents used. These inks typically consist of 2–3 solvents, with the formula designed to ensure proper solubility of the resin. The solubility of resins depends on three key factors: polarity similarity, solubility parameter matching, and solvation. While a well-designed solvent mix meets these principles, the solubility parameters can change as the solvent mixture evaporates, leading to shifts in the volume ratio of each component. If the solubility parameter fluctuates beyond ±15% to 20%, the resin may re-agglomerate, forming particles that can become trapped in the blade and cause knife wires. In production, one common method to assess whether an ink is prone to knife wires is to let the plate roller idle for over 30 minutes before reprinting. This helps identify potential particle aggregation. However, while this approach is practical, it lacks a solid theoretical foundation when considering the solubility behavior of mixed solvents. Water-based gravure inks, although not using mixed solvents, still face knife wire issues. Despite the absence of solvent volatility, other factors like pH changes due to alkaline substances or incompatibility between the aqueous solution and emulsion can lead to solubility problems. The compatibility between the water-based emulsion and color paste significantly affects the ink’s solubility and dispersibility. Some water-based inks have a shelf life of only six months; after that, they may separate or precipitate, making them difficult to restore even after remixing. Interestingly, when using only water-based solutions without emulsions, knife wires are significantly reduced. This suggests that the instability comes from the mismatch between the solution and the emulsion. However, in film gravure, the use of emulsions is necessary for adhesion and ink film resistance. Therefore, it's not feasible to avoid them entirely. As a result, water-based inks must include emulsions, yet their solubility stability remains a challenge for manufacturers.

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