Meaning
Industrial technique for the continuous application of liquid chemicals to a fast-moving roll of flexible material. Manufacturers utilize high speed web coating to produce items like adhesive tapes, solar films, flexible sensors and battery electrodes. The method requires precise synchronization between the fluid delivery system and the mechanical drive of the web.
Hydrodynamic Stability
Maintenance of a steady liquid film at high velocity depends on the balance of inertial and viscous forces. During high speed web coating, the air dragged along by the moving substrate can interfere with the wetting of the fluid. This phenomenon, known as air entrainment, causes voids and patches of uncoated material.
Increasing the pressure at the coating gap helps to displace the air and maintain a continuous layer.
Dynamic Wetting
Interaction between the fluid and the moving surface occurs in a fraction of a second. At the speeds required for profitable production, the dynamic contact angle of the liquid can increase to the point where the fluid no longer spreads. Optimizing the surfactant package allows for faster wetting without the formation of foam.
Successful high speed web coating depends on reaching an equilibrium where the fluid covers the substrate before the drying process begins.
Operational Capability
Demonstrated throughput of a production line is measured by the volume of acceptable material generated per hour. While a pilot setup might achieve quality at ten meters per minute, high speed web coating often targets speeds exceeding several hundred meters per minute. Reaching these rates without a rise in defect density requires a deep understanding of the mechanical vibrations and fluid fluctuations in the system.
The cost of a failure at these speeds is the immediate loss of a large quantity of expensive substrate.
