Knowledge Base Fact Sheet
What is Slot Die Coating?
What is Slot Die Coating and how does it work? (IKB-073)
Slot die coating is a pre-metred coating process that has been widely used in industrial settings for the precise manufacture of thin films. Slot die coating is a highly scalable technique that enables the uniform coating of thin films with low material waste and operational cost.
Slot die coating was invented for the production of photographic films by the Eastman Kodak Company in the early 1950s. Since its inception, slot die coating has then been adopted and deployed in a wide range of manufacturing processes including pharmaceuticals, packaging and electronics.
The coating method as shown in the image below, works by dissolving the material to be dispensed into a solution with a suitable viscosity (1). This solution is then pumped into the slot die head (2), where it is dispensed across the width of the substrate. The substrate then moves (3) beneath the slot die coat head and a smooth film is produced on the surface of the substrate (4). This process results in a continuous thin film, which can easily be optimised so that near 100% material efficiency can be reached.
Advantages and benefits:
There are a number of benefits to using a slot die coating process to produce functional thin films. Slot die coating is a well-established industrial technique that has long been used on roll-to-roll and sheet-to-sheet production lines, so the technique is readily scalable when moving a process from R&D to large scale production. Slot die coating is a continuous process that once optimised is capable of coating up to hundreds of metres per minute.
A pre-metred coating process, is one in which the final film thickness is dependent upon the rate of at which the solution is dispensed. In a pre-metered coating process the entire volume that is dispensed from the print head is coated onto the surface of the substrate, as such there is minimal material wastage. This can significantly reduce operating costs if the material to be coated is a high cost component.
Additionally, slot die coating offers a great level of flexibility in the materials used. The process window of suitable coating materials is wide as the functional material needs only to be dissolved in a suitable solvent. The slot die head is gentle on the material solution throughout the coating process and so is suitable for coating polymers and functional organic materials in energy storage applications such as batteries and solar cells.
What controls the coated thickness?
As already described in slot die coating the entire volume dispensed from the print head is used to produce the thin film. This allows us to say that the volume dispensed (Vd) and volume of the thin film (Vf) are equal, i.e. Vd = Vf. The volume dispensed can be described by:
Vd = Pump rate * time
The volume of the thin film as shown in the diagram is:
Vf = length * thickness * width
As the substrate moves during the coating process the length coated is, length = substrate speed * time. Putting all of the above together gives:
Pump rate * time = substrate speed * time * hickness * width
This can then be rearranged to give an equation for film thickness (t):
Where Pr is the pump rate, Ss is the substrate speed and w is the width. The thickness calculated above represents the thickness of the wet film immediately after the material is dispensed. As the film dries the solvent used to disperse the functional material evaporates and the film becomes thinner. The dry film thickness is then dependent on the concentration of the solution and the density of the material when dry. The dry film thickness can then be calculated using the equation below:
Where C is the concentration of the solution and ρ is the density of the dry material. The equation shows that the coated thickness is independent of both the temperature of the substrate and the height of the slot die head above the substrate. Additionally it illustrates the number of parameters available to the operator to control the film thickness. Of these the concentration of the solution is the most easily and readily controlled when preparing the solution or ink to be coated.
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08 July 2020
IKB073 Rev. 1