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Cai Chengji: What is the relationship between intaglio knife lines and ink?
For the gravure printing process, the solubility of the ink is primarily related to the formation of blade marks. Because the ink path in gravure printing is short, the distance from the doctor blade to the impression point is generally less than 1/4 of the circumference of the plate cylinder. Sometimes, to prevent the ink from drying too early on high-gloss small dots, the doctor blade is intentionally brought closer to the impression point, further shortening the distance. If there is a problem with the solubility of the ink, impurities and large particles may likely embed in the doctor blade, resulting in streaks being drawn on the gravure and transferred to the substrate, forming blade marks.
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For the gravure printing process, the solubility of the ink is primarily related to the formation of blade marks. Because the gravure ink path is short, the distance from the doctor blade to the impression point is generally less than 1/4 of the circumference of the plate roller. Sometimes, to prevent the ink from drying too early on high-gloss small dots, the doctor blade is intentionally moved closer to the impression point, further shortening the distance. If there is a problem with the ink's solubility, impurities and large particles may embed in the doctor blade, resulting in streaks on the gravure and transferring to the substrate to form blade marks. Especially after adding a mixed solvent in the specified proportion, there will be issues with solvent evaporation and balance, meaning that the ratio of the added mixed solvent and the evaporation rates of each solvent during the evaporation process are severely asymmetric, which will directly lead to the retention of slow-drying agents in the mixed solvent and the escape of fast-drying agents.
In gravure inks, if the solubility parameter of the slow-drying agent is close to that of the resin, the retained solvent is real, and the ink will not have issues with solubility deterioration. However, if the solubility parameter of the fast-drying agent is close to that of the resin, the retained solvent will be false, leading to a series of ink solubility problems. At this point, the originally uniformly dispersed ink will experience particle re-aggregation during the printing process, with blade marks being one of the typical issues in gravure.
One classic method to solve blade mark issues in gravure production is to install a metal mesh on the ink return pipe of the ink tray or to cover it with women's glass stockings (one or two layers) to filter the ink online. However, due to the inherent solubility issues of the ink and the approximately 20% mixed solvent added after opening the ink barrel, which affects the solubility of the resin, the gravure process cannot effectively suppress the re-coagulation of ink particles.
Moreover, due to the short ink path characteristic of gravure, blade mark issues are unavoidable. The ink path in gravure is very short; from scraping off the excess ink on the gravure surface with the doctor blade to transferring the ink from the gravure mesh holes to the substrate under the action of the rubber impression roller, the ink path does not exceed 1/4 of the circumference of the plate roller.
This is because the gravure process uses a forward doctor blade, which bears the impact of the ink in the tangential direction of the plate roller's rotation. The gravure doctor blade must move axially left and right to prevent particle embedding. When printing high-gloss small dots, the doctor blade holder needs to be adjusted upward and inward, bringing the blade head closer to the center of the plate roller, which further shortens the ink path. The shorter the ink path, the greater the risk of blade marks forming due to particles embedding between the doctor blade and the roller. It is also necessary to explain why flexo printing does not have blade mark issues: the doctor blade on the anilox roller in flexo printing can also produce fine lines due to particle embedding caused by ink solubility issues, but since the ink path in flexo printing is much longer, being 1/2 of the anilox roller circumference plus 1/2 of the plate roller circumference, the fine lines drawn on the anilox roller are covered by the rapid leveling of the ink before they can be transferred to the substrate.
The advantage of toluene-based inks in terms of solubility is largely due to the influence of toluene solvent, which has good compatibility with other solvents. As long as the mixed solvent combined with toluene meets the aforementioned three principles of solubility, the solubility of gravure ink can be guaranteed, significantly reducing the probability of blade marks during the printing process.
Benzene-free and ketone-free inks are prone to blade mark issues during printing. At the beginning of printing, the faults are not obvious, but after a while, it is found that blade mark issues increase, and these blade marks are often caused by unidentified particles embedding in the doctor blade. Additionally, these inks tend to stick to the doctor blade, and after stopping for cleaning, there may be some improvement, but after a while, ink will accumulate on the doctor blade again, and if not addressed, blade marks will quickly form.
Compared to toluene-based inks, the solubility of benzene-free and ketone-free inks has not yet fully reached the corresponding indicators of toluene-based inks, and stability issues have become their main application dilemma, leading to losses during the printing process, which has become their Achilles' heel.
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