Learn more about the curtain coating technique for slot die coating by Mark Miller of Coating Tech Slot Dies including advantages, disadvantages, technical equations to be aware of, and multi-layer coating applications.

1. CURTAIN COATING TECHNOLOGY

2. CURTAIN COATING
• Advantages
o Large gap
o Reduced lines and streaks
o Increased fluid velocity
o Higher line speed
• Disadvantages
o Air
o Vibration
o Minimum line speed & flow rate

3. CURTAIN COATING
CURTAIN COATING
• Advantages
o Large gap (irregular surfaces)
Adjustable slot die height from substrate (250 mm)

4. CURTAIN COATING
CURTAIN COATING
• Advantages
o Reduced lines and streaks (slot die to substrate gap)
Air entrainment

5. CURTAIN COATING
U
curtain
coatingθD
CURTAIN COATING
• Advantages
o Increased fluid velocity (hydrodynamic assist)
Hydrodynamic assist of curtain enables high coating speeds (0.5
cm3/s)

6. CURTAIN COATING
CURTAIN COATING
• Advantages
o Higher line speed (minimum line speed)
Weber number
We = ρQV/σ
We = Weber number
ρ = density
Q = volumetric flow rate
V = impingement velocity
σ = surface tension

7. CURTAIN COATING
CURTAIN COATING
• Advantages
o Multilayer coating (slide die)
24 layers

8. CURTAIN COATING
• Multi-layer coating
o 2-3 layers in typical slot die design
o Slide die design accommodates more layers
o Separate fluid delivery system for each layer
o Combined outside the slot exit
σtop < σbottom (surface energy stability)

9. CURTAIN COATING
• Multi-layer coating
o Challenges
 Contact interface
 Downstream separation
 Viscosity difference between layers
 Surface layer differences
Invasion of fluid from one layer to another occurs when
the layer thickness is less than 1/3 of the coating gap

10. CURTAIN COATING
• Disadvantages
o Air
o Vibration
o Minimum line speed & flow rate

11. CURTAIN COATING
CURTAIN COATING
• Disadvantages
o Air (entrainment at dynamic wetting line)
3D environmental air disturbances (air shields)

12. CURTAIN COATING
CURTAIN COATING
• Disadvantages
o Vibration
Edge guides (fluid affinity)
Material of construction
Overcoat and in-bound coating
Solvent feed along edge
3D design to carry fluid by mechanical means

13. CURTAIN COATING
Re = ρQ/μ
Re = Reynolds number
ρ = density
Q = volumetric flow rate
μ = viscosity
CURTAIN COATING
• Disadvantages
o Minimum line speed & flow rate
We = ρQV/σ
We = Weber number
ρ = density
Q = volumetric flow rate
V = impingement velocity
σ = surface tension U/V
U = web speed
V = impingement velocity

14. CURTAIN COATING

15. CURTAIN COATING

16. CURTAIN COATING

17. CURTAIN COATING
• Advantages
o Large gap
o Reduced lines and streaks
o Increased fluid velocity
o Higher line speed
• Disadvantages
o Air
o Vibration
o Minimum line speed & flow rate

18. CURTAIN COATING
• Balance of liquid momentum and surface tension
• Uniform pressure across fluid flow between edge
guides
• If the coating thickness is smaller, then the web
velocity has to be higher
• Sharp lip edge geometry
V = U * h > Vmin = 2σ/ρVcurtain
Vcurtain = (2gh)0.5 free fall estimation