Chapter 4: How Slot Dies Work

The Coating Process

Slot dies operate by dispensing a controlled liquid flow through a precision-engineered slot onto a moving substrate. The fluid is pumped into the die, distributed via the manifold for even pressure, and extruded through the lip gap. The substrate passes beneath at a consistent speed, forming a uniform layer known as the coating bead.

 

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Critical factors include:

• Die Lip Design: Regulates thickness and influences wetting; adjustable lips allow fine-tuning.

• Fluid Delivery System: Controls flow rate via pumps, ensuring steady supply without pulsation.

• Process Speed: Matches substrate movement to avoid defects; higher speeds require optimized viscosity.

• Shim Thickness: Body shims adjust coating thickness by altering the slot gap; offset shims fine-tune lip position for overbite/underbite configurations, which affect bead stability.

 

Slot dies can be oriented in any direction (e.g., vertical for curtain modes) and integrated into R2R lines. The process is pre-metered, meaning coating thickness is determined by flow rate, speed, and width: Thickness = Flow Rate / (Speed × Width).

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Fluid Dynamics Basics

The process relies on principles like shear rate, viscosity, and surface tension. Fluid enters the inlet, flows through the throat to the manifold for even pressure distribution, and exits the lips. For multilayer coating, wedges separate flows to prevent mixing, allowing simultaneous application of different materials.

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​​High shear in the slot can affect shear-sensitive fluids, so designs minimize this. CFD simulations help predict behavior, ensuring uniform exit velocity across the width.

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This mechanism ensures scalability from lab to production where it's noted that proper flow control can achieve sub-micron precision.