There are many ways to coat a substrate. These are characterized as ‘premetered’ or ‘self-metered’ [1]. Premetering means a pump dispenses the exact amount of liquid needed: 100% goes on the substrate. Self-metered means the thickness is determined by the fluid mechanics at the coating head (geometric details and fluid properties). However, premetering does not guarantee that the thickness will be uniform across the web. Here, we discuss thickness profile control for some common methods used in battery coating: slot die, lip and comma coating.

Determining coating thickness

With knife coating, the substrate passes through a bath of liquid before a rigid ‘knife’ scrapes off the excess liquid. Knives come in many shapes. The comma bar shown in Figure 1a is one common shape. In the central portion of the coating, the wet thickness is determined by the gap between the substrate and the bar. Thus, the uniformity of a comma coater is determined by the uniformity of the gap, which is determined by the straightness of the comma bar face and variations in substrate thickness. Substrate variations can be eliminated using the reverse comma configuration, as shown in Figure 1b. Edge dams must be used at the sides of the bath to confine the liquid, and these dams reduce the flow at the edges, minimizing edge bead formation.

There are several slot die configurations [2]. Most dies use the pressure drop from flow through the slot to distribute the coating liquid across the width, as seen in Figure 2a. Adjustable lip dies can achieve uniformity within +1% to 2%. However, substrate thickness variations can be problematic, and the gap often is maximized to minimize this effect [3]. Unfortunately, maximizing the gap minimizes variations in the center of the web but tends to maximize the capillary effects that lead to edge bead formation. There are ways to reduce edge bead formation while preserving premetering (e.g., chamfering the shim near the exit); however, some run their dies as ‘wiping dies’ [4] (see Figure 2b), turning the die into more of a knife coater – losing the ability to premeter. A more economical alternative to a wiping die is a lip coater (see Figure 2c) that does not need what is shown as the horizontal faces to be machined as precisely as a typical slot die would require.

Conclusion

There are two main problems in profile control: uniformity in the center of the web and edge bead formation. Premetered coating offers operational advantages and can produce a very uniform profile in the center of the sheet; however, edge bead control with a slot die requires care. Self-metered methods can be very effective and produce lower edge beads but often have slightly more variation in the center of the web than slot die coating. 

References

1. Ruschak, Kenneth J., Coating flows. Annual Review of Fluid Mechanics 17.1 (1985): 65-89.
2. Lightfoot, E. J., Toward a better understanding of the regimes of slot die coating. Converting Quarterly 3Q (2024) 58-60.
3. Schweizer, Peter M., Premetered Coating Methods: Attractiveness and Limitations, Springer Verlag (2022).
4. Telep, David., Lithium-Ion Battery Cell R2R Manufacturing (2017) AIMCAL Fall R2R Conference Plenary presentation.

E.J. (Ted) Lightfoot, Ph.D.
716-449-4455; ejl@tedlightfoot.com
ARC Member; ARC TV Presenter;
ARC R2R Presenter