Gravure printing relies heavily on the solubility of inks, as it directly affects the formation of knife wire defects. One key factor is the short ink path in gravure systems—the distance from the doctor blade to the impression point is typically less than a quarter of the cylinder's circumference. This short path makes it easier for ink impurities and large particles to become lodged in the doctor blade, leading to streaks on the printed material. When mixed solvents are added according to specifications, solvent volatility and balance become critical. If the evaporation rates of different solvents in the mixture are imbalanced, the slow-drying components may remain in the ink while the fast-drying ones evaporate, worsening the problem.
In gravure inks, if the solubility parameter of the slow-drying agent closely matches that of the resin, the true solvent remains effective, ensuring good ink solubility. However, if the quick-drying agent has a similar solubility parameter to the resin, it acts as a pseudo-solvent, causing instability and particle re-aggregation during printing. This can lead to various issues, including the formation of knife wires—a common defect in gravure printing.
To address this, one traditional method involves installing a metal screen or using nylon stockings (one or two layers) on the ink return tube to filter out impurities. However, due to the nature of the ink and the 20% mixed solvent added post-opening, these measures often fail to prevent particle re-agglomeration effectively.
The short ink path in gravure printing also makes knife wire formation almost inevitable. The ink flows from the engraved cells through a very short path to the printing substrate, under the pressure of the rubber roller. This short path increases the likelihood of particles getting trapped between the doctor blade and the cylinder, resulting in visible lines on the final print.
It’s worth noting that flexographic printing avoids such issues because its ink path is significantly longer—roughly half the anilox roller’s circumference plus half the plate roller’s. This allows any fine lines caused by particles to be quickly leveled out before reaching the printed material.
Toluene-based inks offer better solubility due to the high compatibility of toluene with other solvents. As long as the mixed solvent follows the solubility parameters correctly, ink stability improves, greatly reducing the risk of knife wire defects during printing.
On the other hand, benzene-free and ketone-free inks tend to cause more frequent knife wire issues. Initially, the problem might not be obvious, but over time, it becomes more pronounced. These inks are prone to sticking to the doctor blade, and even after cleaning, the issue tends to recur. If left unattended, the buildup on the blade can quickly lead to visible lines on the print.
Compared to toluene-based inks, benzene-free and ketone-free inks still lack full solubility performance. Their instability poses a major challenge in practical applications, leading to quality losses during the printing process. This makes them less reliable, especially in high-speed or high-precision gravure operations.
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