Coating Matters | Down-Web Uniformity

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Sometimes there is a complex, multivariable solution to a coating defect and sometimes…well, let me get the issue in front of you first, then we’ll discover the solution.

When coating a substrate with a fluid, the liquid has two cross-sectional areas of concern:

1. the cross-web uniformity
2. the down-web uniformity

In cross-sectional uniformity, the control comes from the design and implementation of the technology of the coating head. It can seem as though we spend all our time making adjustments to equipment to reduce the overall variation across the web. Most coating enthusiasts are familiar with the factors for cross-web coat weight control:

• Coating system geometry
• Web speed
• Roll speed
• Web tension
• Rheology of the fluid
• Tuning of the gaps at the point the fluid interfaces the substrate

But what are the factors that affect down-web variation? There has been more than one project in which I have seen coaters chasing a cross-web coating variation only to find that the down-web variation was so large that, until the variation was reduced, you would simply be chasing your tail. There are two main classes of down-web coating control factors:

1. Roll coating—in which coating system geometry, fluid rheology, and roll speed control down-web variation; and
2. Pre-metered coating—in which pump rate and web speed control down-web variation.

Okay, for roll coating, the key is to track and realize when speed control is varying. This can be automatic or manual, but it has to be done before (and during) a coating run. Tracking the web speed will identify repeat variations and sinusoidal down-web issues.

What is the main difference between roll coating and pre-metered coating, such as slot die? This is when I put down my soap box and preach about flow meters. Since variation in pre-metered coating is a simple two-function equation, and you are already verifying the web speed, the next prong is to verify fluid flow from the pump. I don’t know how many times I have fought to check the flow rate before starting the trial with a stopwatch, only to have the hose hooked up to the pre-metered coating head and the flow rate for the next production run go unverified. Okay, now I’ll step off the soapbox.

So when it comes to precision roll-to-roll coating, make sure the manufacturing precision is high for the surfaces the fluid is touching, but don’t stop there! This will only help you with cross-web variation. Invest in web speed tracking and a fluid flow meter. Also, remember to check cross-web and down-web coating caliper variation before signing off on good product or you will be spinning your wheels when quality control flunks the product.

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.