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Analysis of Gravure Ink and Blade Wire Failures
Knife marks are one of the common quality defects that are difficult to avoid in the gravure printing process. Due to the short ink path in gravure printing, the distance from the doctor blade to the impression point generally does not exceed 1/4 of the circumference of the printing cylinder. If, for process considerations, the position of the doctor blade is raised closer to the impression point, the distance will be even shorter. The risk of this short ink path is that if there are foreign objects embedded in the doctor blade, the ink's own wettability will not be able to pull out the fine lines created by the foreign objects, and these fine lines will inevitably transfer to the substrate, forming knife marks.
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Knife lines are one of the common quality defects that are difficult to avoid in gravure printing. Due to the short ink path in gravure printing, the distance from the doctor blade to the impression point generally does not exceed 1/4 of the circumference of the printing cylinder. If, for process considerations, the position of the doctor blade is raised closer to the impression point, the distance will be even shorter. The risk of this short ink path is that if foreign objects are embedded in the doctor blade, the ink's own wettability will not be able to pull out the fine lines, and the fine lines will inevitably transfer to the substrate, forming knife lines.
1. Solvent-based gravure ink
When using solvent-based inks in gravure printing, the reasons for the unavoidable knife lines, in addition to foreign objects embedded in the doctor blade due to environmental factors, mainly lie in the fluctuations in the solubility of the mixed solvents used in solvent-based inks.
The mixed solvents for gravure printing generally consist of 2 to 3 types of solvents, and the core of the formulation must first ensure the solubility of the mixed solvent for the resin (solute). The three key elements of resin solubility include: the principle of similar and close polarity, the principle of similar solubility parameters, and the solvation principle. A good mixed solvent formulation generally conforms to these three principles. However, since the calculation of the solubility parameters of mixed solvents is related to the volume parameters of the solvents, after the mixed solvent has evaporated for a period of time, the volume ratio of each solvent will change significantly, and the solubility parameters of the mixed solvent will also change greatly. As long as the fluctuation of the solubility parameters is controlled within ±(15% to 20%), the solubility of the resin is still guaranteed. However, if this limit is exceeded, the dissolved resin may re-aggregate into larger particles, forming solubility defects. Under these conditions, if particles are embedded in the doctor blade, knife line defects are inevitable.
In the production line of gravure printing, when evaluating whether a new ink is prone to knife lines, the method often used is to let the printing cylinder run idle for more than half an hour and then reprint. The intention is to assess the risk of ink particle aggregation when the mixing ratio of the solvent-based ink is severely inconsistent with the evaporation ratio. This method is very scientific, but it is questionable how many people consider this from the theoretical perspective of mixed solvent solubility while using this method.
2. Water-based gravure ink
Water-based gravure inks do not use mixed solvents, so theoretically, there is no risk of solubility fluctuations caused by inconsistencies in mixing and evaporation ratios. However, the objective reality is that knife lines are still unavoidable in the printing of water-based gravure inks. In fact, the reasons for the solubility fluctuations in water-based gravure inks still exist: first, changes in the alkaline substances in water-based gravure inks cause pH changes, leading to changes in solubility; second, the matching of aqueous solutions and water-based emulsions in water-based gravure inks, whether their mixture will cause resin insolubility.
The compatibility of the solubility and drying properties of water-based emulsions with color pastes directly affects the solubility and dispersibility of water-based inks. Some brands of water-based inks have a safe storage period of only 6 months; beyond 6 months, precipitation and layering may occur, and even after re-stirring, it is difficult to regenerate. This is actually a solubility issue, specifically the compatibility issue between the color paste and the water-based emulsion. When water-based inks experience solubility issues, especially those using water-based inks in gravure transfer mechanisms, knife line defects are bound to occur.
There is a noteworthy phenomenon: if only water-based solutions are used to form water-based inks for gravure printing, the occurrence of knife lines during the printing process will be significantly reduced. This phenomenon contradicts the solubility fluctuations of water-based inks formed by the mismatch between water-based solutions and water-based emulsions. However, in film gravure printing, the adhesion of the ink and the resistance of the ink film cannot be separated from water-based emulsions, so it is unrealistic to avoid adding water-based emulsions to prevent knife lines. Water-based emulsions must be added to water-based inks, but after adding water-based emulsions, the stability of solubility cannot be ensured, which creates a dilemma for current water-based ink manufacturers.
1. Solvent-based gravure ink
When using solvent-based inks in gravure printing, the reasons for the unavoidable knife lines, in addition to foreign objects embedded in the doctor blade due to environmental factors, mainly lie in the fluctuations in the solubility of the mixed solvents used in solvent-based inks.
The mixed solvents for gravure printing generally consist of 2 to 3 types of solvents, and the core of the formulation must first ensure the solubility of the mixed solvent for the resin (solute). The three key elements of resin solubility include: the principle of similar and close polarity, the principle of similar solubility parameters, and the solvation principle. A good mixed solvent formulation generally conforms to these three principles. However, since the calculation of the solubility parameters of mixed solvents is related to the volume parameters of the solvents, after the mixed solvent has evaporated for a period of time, the volume ratio of each solvent will change significantly, and the solubility parameters of the mixed solvent will also change greatly. As long as the fluctuation of the solubility parameters is controlled within ±(15% to 20%), the solubility of the resin is still guaranteed. However, if this limit is exceeded, the dissolved resin may re-aggregate into larger particles, forming solubility defects. Under these conditions, if particles are embedded in the doctor blade, knife line defects are inevitable.
In the production line of gravure printing, when evaluating whether a new ink is prone to knife lines, the method often used is to let the printing cylinder run idle for more than half an hour and then reprint. The intention is to assess the risk of ink particle aggregation when the mixing ratio of the solvent-based ink is severely inconsistent with the evaporation ratio. This method is very scientific, but it is questionable how many people consider this from the theoretical perspective of mixed solvent solubility while using this method.
2. Water-based gravure ink
Water-based gravure inks do not use mixed solvents, so theoretically, there is no risk of solubility fluctuations caused by inconsistencies in mixing and evaporation ratios. However, the objective reality is that knife lines are still unavoidable in the printing of water-based gravure inks. In fact, the reasons for the solubility fluctuations in water-based gravure inks still exist: first, changes in the alkaline substances in water-based gravure inks cause pH changes, leading to changes in solubility; second, the matching of aqueous solutions and water-based emulsions in water-based gravure inks, whether their mixture will cause resin insolubility.
The compatibility of the solubility and drying properties of water-based emulsions with color pastes directly affects the solubility and dispersibility of water-based inks. Some brands of water-based inks have a safe storage period of only 6 months; beyond 6 months, precipitation and layering may occur, and even after re-stirring, it is difficult to regenerate. This is actually a solubility issue, specifically the compatibility issue between the color paste and the water-based emulsion. When water-based inks experience solubility issues, especially those using water-based inks in gravure transfer mechanisms, knife line defects are bound to occur.
There is a noteworthy phenomenon: if only water-based solutions are used to form water-based inks for gravure printing, the occurrence of knife lines during the printing process will be significantly reduced. This phenomenon contradicts the solubility fluctuations of water-based inks formed by the mismatch between water-based solutions and water-based emulsions. However, in film gravure printing, the adhesion of the ink and the resistance of the ink film cannot be separated from water-based emulsions, so it is unrealistic to avoid adding water-based emulsions to prevent knife lines. Water-based emulsions must be added to water-based inks, but after adding water-based emulsions, the stability of solubility cannot be ensured, which creates a dilemma for current water-based ink manufacturers.
Key words:
Knife silk,Intaglio
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