It certainly is. While it may not be the coating of choice for many applications, it is a serious contender in a significant minority of cases. It is inexpensive to implement and maintain and it lends itself to rapid, inexpensive product changes.

Precision air knife coating—as opposed to what I refer to as the air squeegee process—is routinely capable of controlling coating weight within ±5 percent, and can often be controlled to within half that range with reasonable care. The typical operating range for precision air knife coating is:

• coating speed: 40 to 400 fpm
• coating thickness: 1 to 200µm
• solution viscosity: 1 to 50 cp
• air knife pressure0.2 to 10 in.-water
• air knife opening: 0.02 to 0.07 in.
• air knife to web gap: 0.05 to 0.20 in.

In precision air knife coating, the fluid below, or upstream of, the air knife remains on the web in a thick layer. In the air squeegee process, much of that fluid is blown off the web, usually as a spray or a stream. Air knife pressures in precision air knife coating are typically well below 1 psi, often several inches of water. Air squeegee pressures run one or two orders of magnitude higher.

In precision air knife coating, coating weight decreases with increasing air knife pressure and increases with increasing solution viscosity and web speed. Coating weight is controlled with air knife pressure. The application device, typically a skim pan or crude extrusion die, has no major effect on coating weight despite our instinctive feelings that it ought to. The basis for this apparent anomaly leads to the strengths and weaknesses of this coating process.

Any coating flow on a moving web has an alternative flow regime that satisfies the required force balance, has the same net flow per unit width, is a magnitude thicker, and is often a bit unstable. All laminar flows on a moving web have a parabolic velocity profile with the fluid at the web surface traveling at web speed.

In coating flows, the fluid at the air interface is just a bit slower than web speed. The alternate or thicker regime, typically 15 to 25 times thicker, has a much steeper velocity profile and the flow at the air interface is opposite the web. The volume flow up exceeds the volume flow down by the same net flow as in the coating flow.

This thicker flow forms just below the air knife impingement zone (often called the meniscus) because once the air knife allows the thinner coating flow to pass, the excess fluid collected below the meniscus will not flow down until the lower layer grows thick enough to balance the upward shear force from the web.

All of this would just be some fascinating fluid-flow trivia, but flow disturbances in the lower layer often upset the meniscus and cause small fluctuations in the coating flow above the air knife. Regular periodic waves in the lower layer upset the meniscus and form chatter. The flow below the air knife is also prone to rivulets which again upset the meniscus in the impingement zone and form streaks.

These would appear to be fatal flaws but they are not. If the coating fluid smoothes well after coating—like a good varnish or enamel—the coating is smooth. If not, the coating suffers streaks and chatter. In most cases the proper surfactants can be found to promote post coating smoothing without ruining wetting and adhesion.

Do not discount this old process if you need an inexpensive flexible coating process.

Neil I. Steinberg

Somerset Engineering

864/244-8829

steinbni@aol.com