Slot Die Coating

Slot die coating is one of most popular wet solution coating methods for optical films, adhesives, ceramics, et. al.

Characteristics

Pre-metered method
Coating thickness can be precisely controlled by low rate
Viscosity range: <1000 cps, <500cps preferred
Coating speed range: <60m/min
Typical dry coating thickness range: 0.5um to 50+um
Accuracy (real product) : +/-0.2um for 1-5 um coating; +/-0.5um for 10-20um coating
Shim thickness (frequently used): 75, 100, 125um
DV die with vacuum box
Min. Die Web Gap: 50um

Slot die overview

Slot die solution delivery system and coating zone; Reference: **Anupam Sengupta**, CAFM

Slot die coating head

Fundamentals

Fluid Dynamics in feed slot

Slot die process parameters

Pressure in slot die manifold (Pc); Upstream pressure P0 (or Pvac with vacuum box installed), and downstream pressure Patm
Fluid viscosity (mu) and surface tension (delta)
Web moving speed (V)
Die-Web gap (H0)

Flow types

Fluid flow in slot die is a combination of drag (Couette) type and pressure driven (Poiseuille) type as shown in the picture below. The thinner coating, the more poiseuille flow dominating.

Combination of Couette and Poiseuille flow

Up stream flow dynamics

Downstream dynamics

Combination of downstream and upstream

Maximum coating thickness can be calculated by the above equation.

PROCESS window

Process window in pressure-thickness plane and failure mode.

Process window in Ca-thickness plane, with failure mode out of the window.

Slot die coating system

Coater example

Pump

Comparison

Pump Type	Gear Pump	Diaphragm Pump	Screw Pump
Advantages	•A wide range of fluid viscosity •Very controllable and mathematical •A wide range of speed, smoother and more controllable •Self-priming •Discharge pressure can be very high •Don’t have a BEP or best efficiency point; they are consistently efficient.	•Large displacement volume, e.g. a 2 inch diaphragm pump is able to safely move about 75 Gallons per minute of material •Great Wear Resistance •Variety of Drive Options •Available in Food Grade Designs •Capable of lifting material	•Wide range of flows and pressures •Wide range of liquids and viscosities •High speed capability for freedom of driver selection •Self-priming with good suction characteristics •High tolerance for entrained air and other gases •Low mechanical vibration, pulsation-free flow, and quiet operation •High tolerance to contamination in comparison with other rotary pumps
Disadvantages	•Not recommended for abrasive media or solids •Complicated design and high initial cost. •can be “too noisy” in some environments •limited by their size and unsuitable for operations with large bulk flow rates	•Less consistent in fluid delivery during pumping action, e.g. wink issue •Doesn’t provide high pressure when pumping	•Relatively high cost because of close tolerances and running clearances •Performance characteristics sensitive to viscosity change

Reference 1, 2, 3

Slot die coating types

Slot die techniques	Advantages	Disadvantages
Direct coating (bead)	•Cross-web uniformity predictable •Precision surfaces	•Narrow gaps
Curtain	•Large gap (irregular surfaces) •Increased fluid velocity •Higher line speed	•Air/Vibration •Minimum line speed & flow rate •Neck-in
Tensioned web (Impregnation mode)	•Overcome gap issues •Higher line speed and thin coating	•Fluid/Web/process dependent