Gravure printing is known for its deep etched cells, which can reach depths of up to 60 micrometers, leading to a high ink volume on the printed surface. This thick ink layer often causes drying issues, as the ink remains wet even after passing through the long drying tunnel. The printing process involves wet stacking, where subsequent ink layers are applied before the previous ones have fully dried. A thin film forms on the surface of the ink, which can cause color contamination when the next ink is applied.
There are several reasons why gravure prints may not dry properly:
1. **Deep Gravure Cells**: The depth of the cells allows a large amount of ink to be retained, making it difficult for the solvent to evaporate quickly.
2. **Use of Slow-Drying Additives**: To maintain the quality of small, high-gloss dots, slow-drying agents are often added, which can delay the overall drying process.
3. **Balancing Solvent Mixtures**: Achieving the right balance of solvents is challenging, and an excess of slow-drying components can remain in the ink, further hindering drying.
4. **White Ink and Rewinding Issues**: In some cases, white ink is used as the final color, followed by rewinding. If the distance between the drying and rewinding stations is too short, the ink may not dry sufficiently. Additionally, white ink typically requires a thicker layer for better opacity, increasing the drying challenge.
5. **Temperature and Drying Tunnel Design**: The drying system must be carefully controlled. High temperatures or overly long tunnels can cause the film to stretch or deform, affecting print accuracy. Low-temperature, high-airflow drying is often preferred to promote evaporation without damaging the material.
6. **Environmental Conditions**: The rate at which solvents evaporate depends heavily on environmental factors such as air pressure. In low-pressure environments, mechanical ventilation may not be sufficient to control the drying process effectively.
7. **The Boundary Layer Effect**: A boundary layer forms between the ink surface and the surrounding air, consisting of a mixture of hot air and solvent vapor. This layer acts like a "mirror film," trapping solvent vapors and reducing the evaporation rate. As the concentration of solvent vapor increases, the evaporation slows down until it stops completely. Traditional gravure drying methods often overlook this phenomenon, focusing instead on airflow and temperature. While improvements have been made to the drying channel design, the presence of the boundary layer can still leave residual solvents trapped in the ink, leading to incomplete drying.
By understanding these factors and adjusting the printing and drying processes accordingly, it's possible to improve the drying efficiency and quality of gravure-printed materials.
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