Stability of Slot Coating Meniscuses
In slot coating, when the solution leaves the slot die head it passes into one of two constrained channels in the gap between the head and the substrate. The solution then forms a stable meniscus with the upstream and downstream slots in the head – the stability of these menisci is essential for defect free operation.
However, the stability of these menisci is greatly influenced by both the physical properties of the solution and the design of the slot-die head. The stability of the upstream and downstream slots can be manipulated by changing either the gap height, or varying the channel thickness. Link Gbo4d
Typically, the channel length in a slot-die coater is fixed by the length of the feed slot. Changing this channel length is very expensive and requires the re-milling of an entire head, so it is often not possible or practical to do. The channel thickness is much easier to change, as this is generally controlled by a body shim on the top of the slot die head. Since the term is cubic even small changes to this value can have a dramatic effect on the pressure drop.
The problem with adjusting the channel length or the lip height is that it can cause other defects. For example, if the gap is too large it can cause ‘neck-in’ defects where the upstream lip becomes thicker than the downstream lips. This can lead to severe variations in coating thickness and can result in the inability to operate within the stable processing window. Similarly, if the gap is too small it can lead to ‘swelling’ defects where the upstream lip becomes excessively thin and leads to poor edge definition.
A common technique used to reduce these problems is the use of ‘meniscus guides’, a thin protrusion on the bottom of the slot-die head. These act to pin the upstream and downstream menisci to the shims, and stop them from moving along the width of the head. This can reduce the formation of localised defects such as ribbing, but will not eliminate them altogether.
In some cases, the presence of ribs can be due to damage to the slot-die head from mishandling or aggregation of the coating material, and the system may need to be cleaned and reformulated. In this case, re-polishing the slot die head may help to improve the appearance of the ribs and restore good meniscus stability. Alternatively, it may be necessary to investigate the metering system, to ensure that the solution is entering the slot-die head at the correct rate and in the right state. This could include examining the feed system, nozzles or the slot-die head itself. In addition, the nozzle flow can be adjusted to ensure that the metering process is stable, and the viscosity of the solution is consistent. This will also allow the nozzle to work more efficiently, and improve overall coating quality.
