Gravure printing is known for its deep etched cells, which can reach depths of up to 60 micrometers. This results in a high ink volume being transferred onto the substrate, making the drying process particularly challenging. One of the main issues in gravure printing is that the ink layers from previous color stations are not fully dry when the next layer is applied. Instead, a thin film forms on the surface of the ink, which can lead to ink reversal or smudging when subsequent colors are overprinted.
There are several reasons why gravure prints may not dry properly:
1. The deep gravure cells hold a large amount of ink, making it difficult for the solvent to evaporate quickly.
2. To ensure the transfer of fine, high-gloss dots, slow-drying agents are often added, which can hinder the overall drying process.
3. Balancing the mixed solvents used in the ink formulation is complex, and if not properly managed, it can result in excessive amounts of slow-drying components remaining in the ink.
4. In some cases, white ink is printed as the final color before rewinding. If the distance between the drying and rewinding stations is too short, the ink may not have enough time to dry. Additionally, white ink requires a thick layer to provide good opacity, further increasing the drying challenge.
5. During the drying process, the temperature of the dryer should be carefully controlled. Excessive heat can cause the film to stretch or deform, affecting print accuracy and product dimensions. Low-temperature, high-airflow drying methods are commonly used to promote evaporation without damaging the material.
6. The evaporation of solvents is highly dependent on environmental conditions. When ambient air pressure is low, mechanical exhaust systems may not be effective in removing solvent vapors, leading to slower drying times.
7. A key factor often overlooked in traditional gravure drying is the formation of a boundary layer on the surface of the ink. This layer consists of hot air and solvent vapor, creating a barrier that can slow down evaporation. As the concentration of solvent vapor increases in this layer, the evaporation rate decreases. Once the boundary layer becomes saturated, the drying process effectively stops. Most modern drying systems do not account for this phenomenon, resulting in residual solvents remaining trapped within the ink layer. Improvements in drying channel design have helped reduce air resistance and increase airflow, but without addressing the boundary layer issue, some unevaporated solvents may still remain, affecting print quality and drying efficiency.
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