SLOT COATING

The Slot Die Group

This group includes several methods that use a similar geometry "slot" die from which the coating transfer flow bridging to the web issues directly from the exit of the die manifold's slot.

Three commercially important versions are discussed here and a fourth, slot curtain coating, is described in Curtain Coating.

Slot Die section view displaying internal manifold

Transfer flow of slot-bead coating

Slot-bead Coating with applied vacuum.

Slot-Bead Coating (a.k.a. "slot coating", "die coating")

Possibly the most commonly applied liquid film coating method.
Process Features:

• tight die lip-to-web clearance (50 µm - to - 1000 µm)
• small 2-D transfer flow ("coating bead") stabilized by viscous and/or capillary interaction with die lip surfaces and moving web
• coating zone geometry has strong influence on operating latitude and can influence attainable coating uniformity
• applied low-level vacuum extends operating window under certain conditions

Commercial Features:

• extensive range of industrially-important coating solutions and suspensions
• wide range of coating thicknesses (typically 10 µm - to - 500 µm).
• frugal productions of both small and large volumes because of wide operating window, minimal edge waste, efficient starts and stops
• pattern coatings (stripe, "patch") with specialized slot-bead coating systems

Applications
Slot bead coating is extensively used for coating suspensions (slurries, emulsions) and/or "dilute" polymer solutions (i.e., where viscoelasticity effects are not significant).
Some application examples:

• PSA tapes, labels and sheets (also including Si release films) that are used in a variety of industries such as electronics, packaging, construction, and medical
• medical/pharmaceutical - transdermal drug delivery patches, blood testing strips
• electronics - flexible printed wiring boards, battery electrodes, ceramic capacitors
• graphic/display films including LCD color filters
• packaging including barrier layers of various functions (grease, moisture, oxygen, odor)
• construction/automotive including films with protective coatings (e.g., "hardcoat")

Extrusion Coating

Extrusion Coating

Process features:

• substantial clearance (up to 100 X the final coated thickness) with elongated bead or liquid "bridge" stabilized by viscosity
• can coat higher viscosity liquids that may also exhibit substantial elasticity (e.g. polymer melts, "concentrated" polymer solutions)
• negligible, avoided or inconsequential liquid interaction with die lips

Applications:

• polymer melt sheet casting
• hot melt adhesive coating
• membrane casting (polymer solution)
• hot wax coating for packaging
• casting of flexible polymer belts and sheets

Tensioned-Web Slot Coating

Tensioned-Web Slot Coating

Process Features:

• die positioned so that lips intersect nominal web path in a span (no immediate coating roll present)
• transfer bead essentially a lubrication flow between moving tensioned-web and die lips achieving thinly-coated layer and preventing direct lip-web contact
• extremely thin coatings (down to 1 µm) at commercially useful speed and precision
• operating window dependent upon lip geometry and also critically sensitive to web characteristics and dynamics (e.g., wrap angle, tension, and web stiffness)
• coating uniformity critically dependent on uniformity of web characteristics and dynamics

Applications:

TWSC is employed where extremely thin coatings are required; i.e., thinner than possible with acceptable operability and/or uniformity from slot-bead coating.

• magnetic media (magnetic tape and cards)
• thin barrier layers
• subbing layers for other functional coatings