Whether coating batteries or adhesive tapes, there are reasons why coating full width is not always optimal. When an operation lends itself to coating wet and dry lanes down web across a substrate, what is required to understand what is involved?
Lane coating is the process of taking a fully formed precision flow of fluid and breaking it into areas of coated and uncoated areas as the substrate moves down web. This appears as lanes of coated fluid with dry lanes in between. This patterned coating can be achieved with any coating technique by placing barriers to the flow path. In slot die coating, this means that the flow area needs to be controlled both inside and outside the flow cavity of the coating head.
Inside the slot die, the flow control can simply be the placement of a seal at the exit point of the slot die. In a fixed lip style slot die, where the slot gap is held open by a body shim, the body shim would simply need to be manufactured with “fingers” that run from the manifold back line to the exit point of the slot die where the fluid would be kept from exiting. The open spaces between the fingers would allow for fluid flow and the full manifold would still be employed to distribute the flow correctly. In a flexible lip style slot die, where the slot gap is controlled by adjustable screws, a metal or plastic deckle would be placed in the front face of the slot die between the upper and lower lip and the adjusting bolt would be clamped down on the deckle. Care needs to be taken with an adjustable lip slot die to make sure that the spacing of the adjusting lip and the deckle size correlate. Adjustable lip bolts should work in groups of 3 across the solid lip assembly and spacing of the adjustable screws are typically 1 inch (25.4 mm) or more. If a narrower spacing is required, an external shim may need to be considered.
The lane dimensions are limited to the internal flow, the external flow and the physical geometry of the shim being developed. Once the shim geometry becomes narrow, a serrated lip may need to be considered. A serrated lip would have the coated and uncoated areas cut and ground into the lip land itself. This permanent feature should only be considered after the experimentation is complete to understand how the fluid reacts with the substrate at operating conditions of production.
When the fluid to be coated is reduced from full width to something less, the manifold is running sub-optimally. With this sub-optimal flow, internal flow analysis will make sure that turbulence and residence time doesn’t become an issue, but experimentation will still be required to understand what is going on outside the coating head. A rule of thumb is to have the manifold run optimally requires that full width coating takes place down to 20% less than full width coating. In lane coating, these uncoated sections can add up and produce flow areas that reduce the flow control of the slot die manifold cavity design.
Computer modeling reduces guess work and provides precise understanding of the internal flow control, but the boundary conditions and mathematical analysis of external flow is limited. The liquid can be understood according to the Navier-Stokes equation, but surface energy and surface tension interaction along with wetting and spreading phenomenon are not described by the math involved. Because of this lack of capability to simulate flow outside the slot die, experiments should be run within the production process conditions to understand how the fluid will react to the substrate after exiting the slot die. This may lead to shim dimension variations and iterative design and manufacturing of flow control features.
The ultimate goal is to reduce converting steps and not have wasted fluid coated onto the substrate of choice. With simulation support and experimental guidance, lane coating and efficient coating can be developed.