Coating Matters: Slot Die Coating Fundamentals

Published: July 01, 2011, By By Mark Miller Contributing Editor

Recently, a lot has been asked of coating technology. Products in the printed electronic fields, such as solar panels and battery technology, have pushed the limits of traditional roll-to-roll processes.

For a production facility to have a technical edge, a coating solution must be robust enough to coat thin, precise, and clean. Slot die coating technology provides this accurate and metered technology in a closed process for better control.

Here is a summary of the extent of capability of a slot die (for typical coating fluids):

• Line speeds | 1-600 mpm
• Wet thickness coated | 1-2,000 microns
• Viscosity of fluid | 1-100,000 cP
• Accuracy | ±1%
• Process uses | Dual-sided coating, multiple layer coating, intermittent coating, lane coating

If slot die technology is so great, what is it? Slot dies are defined as premetered coating equipment, because all the fluid you send to the slot die gets coated on the web. But let's start at the beginning.

Let's say you have a special sauce that you want coated on a substrate, but the special sauce is really special. Thousands of dollars special! You can't afford to make enough of your sauce to fill a coating pan (open to the atmosphere) and have a traditional roll coating method slather the sauce onto the web for a trial and then throw away the rest.

What you would prefer — and the slot die technology provides — is a closed system that moves your special sauce from an enclosed tank through a pump to the slot die. The speed of delivery of the pump matches the web speed of the substrate, which allows whatever is sent to the slot die to be deposited on the web to form your product.

When you need to stop making your product, the pump is shut off and the wasted fluid is what remains in the pump, hose, and slot die.

So how much volume is in the slot die? That depends on the internal design or the manifold.

The manifold of the slot die is designed for the rheological family of fluids you will be running through the coating head. In other words, don't expect an adhesive slot die to run anode battery slurry. That would be like asking a smart car to haul cargo like a truck — different design and different results.

The manifold of the slot die is the internal distribution chamber that the fluid uses as a roadmap to distribute correctly onto the web to make your product. This manifold design is one of the five main characteristics that are important in slot die coating. The other four include lip geometry, lip offset, lip adjustability, and manufacturing tolerance.

As you can see, of the five main variables associated with slot die coating, three concern the lip or exit of the slot die. This exit is critical for many reasons but most importantly because the lips of the slot die are the last surface the fluid touches before wetting out onto the web. If the lip surface is not manufactured to an exacting tolerance, the fluid will “remember” the imperfections as defects in coating.

So if your new product development requires coating thin, precise, and clean fluids for value-added product design, I suggest investigating slot die coating. Precision-manufactured equipment provides you with the technology and adaptive coating techniques required for novel product development.

An example of slot die coating equipment includes this Ultracoat V adjustable lip slot die and support system from Extrusion Dies Industries, Chippewa Falls, WI.

Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 612-605-6019; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.coatingtechservice.com.