Chapter 7: Slot Die Coating Procedures

General Starting Points

Begin with material preparation: Filter fluids to remove particles, which can cause streaks or blockages—aim for particles <1/10 of slot gap. Set pump rates based on desired thickness using the formula: Flow Rate = Thickness × Speed × Width. Monitor solids % as it influences rheology.

Steps:

1. Prime the die to eliminate air, using low flow to avoid splatter.

2. Start substrate at low speed (5-20 m/min) to form the bead.

3. Adjust flow and monitor uniformity with gauges; increase speed gradually.

4. Ramp up to production speed, watching for defects like ribbing from high speed/low viscosity mismatches.

5. For multi-layers, sequence flows to prevent bleeding, adjusting for differing viscosities.

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Handling High-Viscosity Fluids

For viscosities over 10,000 cP (e.g., battery slurries), use heated dies or cored holes for temperature control, reducing viscosity for better flow. High solids % exacerbates this, so preheat to 40-60°C. Particle size matters—larger particles in high-viscosity fluids can settle, requiring agitation.

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Shims and Pumps Usage

• Shims: Stainless steel or poly; use for lip gap (0-508 microns) and offset adjustments. Thicker shims for wider gaps suit low-viscosity; thinner for precision in high-viscosity to control shear.

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• Pumps: Positive displacement types are preferred for precision in slot die coating, as they provide constant flow independent of pressure.

  • Peristaltic Pumps: Use squeezing action on tubing; no fluid contact, ideal for shear-sensitive or abrasive fluids. Best for low to medium viscosity (1-10,000 cP) and particle-laden slurries (up to 1mm particles). Pros: Easy cleaning, low pulsation; Cons: Limited pressure (up to 10 bar), tube wear. Vs. Progressive Cavity: Peristaltic better for hygienic/clean-in-place; progressive for continuous high-volume.

  • Progressive Cavity Pumps: Helical rotor in stator creates cavities; excellent for high viscosity (up to 1,000,000 cP) and solids (up to 50%). Provides steady, low-pulsation flow. Pros: Handles abrasives well; Cons: Harder to clean, potential shear damage to sensitive fluids. Preferred over peristaltic for very thick materials like pastes.

  • Gear Pumps: Intermeshing gears; good for medium to high viscosity (100-50,000 cP), precise metering. Best for clean, non-abrasive fluids; leaks with low viscosity.

  • Diaphragm Pumps: Pulsation-dampened versions for low viscosity; handle corrosives but less precise for high solids.

  • Syringe Pumps: For lab-scale; precise for low volumes, any viscosity, but not for production.

   

• For low viscosity (<100 cP), gear or diaphragm minimize leaks; for high (>10,000 cP), progressive cavity or peristaltic ensure flow without excessive pressure. Particle size affects choice—progressive cavity tolerates larger particles better.

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This procedure aligns with University of Minnesota's premetered coating teachings, emphasizing pump selection for stable bead formation.

Chapter 6: Setting Up Slot Dies in a Coating Line

Chapter 8: Applications and Industries