Coating Matters | Designing Equipment for Fluid Flow

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In premetered coating, a slot die is the control valve that turns fluid flow through a pipe into a precise, flat, and even coating on a continuous substrate. How does a chunk of metal do this? I like to think of a slot die as a series of compression and relaxation zones, or gaps. These gaps need to shape the fluid into the form required for final distribution at the precise pressure and consistent velocity as it is deposited onto the substrate. You could pour the fluid on with a hose, paint it on with a brush, or spray it on as a mist, but if precision is your concern, then slot dies are your method.

The manifold cavity is a chamber that the fluid is introduced to from the pump. This manifold has a shape dictated by the molecular character of the fluid itself. This character is a function of the rheology (dynamic viscosity) and the viscoelasticity. These two characteristics define the shape of the manifold with two main variations:

  • A Newtonian fluid dictates an infinite (same cross section from center to end) cavity design.
  • A polymeric fluid dictates a coat hanger (curved) cavity design.

To feed the manifold cavity properly, you have to consider the pressure drop across the path traveled from the pump through the slot die and out the slot end of the die, all while maintaining properly velocity throughout the system. Pressure drop along the cavity is the driving force for flow across the width of the slot die as fluid is introduced to the slot die from a central entrance. If the pressure gradient within the manifold is not even, the flow also will be uneven and present as an uneven coat weight across the substrate.

With polymeric fluid, flow control at the exit of the slot die may be required to “shape” the flow. This is usually presented in the form of lip adjusting bolts, with the lip of the slot die being the metal tips of the upper and lower body of the slot to be formed.

Some slot dies are designed with a secondary manifold prior to the slot exit to allow for a smaller relaxation zone. This secondary manifold provides for corrections in flow when a smaller primary manifold is required for initial flow distribution, but fails to optimize flow speed prior to slot die coating.

After distribution within the manifold cavity chamber (a relaxation zone), the fluid is compressed through a slot (a compression zone) that has a height dictated by the viscosity of the fluid. The higher the viscosity, the larger the opening; the lower the viscosity, the smaller the opening. The idea is to have the fluid move at the same velocity as it exits the slot die, therefore the fluid forces control the distribution onto the substrate.

Another way to view the coating phenomenon of slot die technology is to consider that the precision is dependent on uniform volumetric movement per unit width across the width of the substrate. But be careful! The slot opening is a very powerful parameter—it affects coating speed to the power of three. For every unit of slot height change, the speed is affected in triplicate.

In the end, a premetered slot die coating system should be optimally designed based off the liquid properties and operating conditions. Too much fluid or process condition change, and the slot die will be ineffective. Design the coating method based off known limits of production, and you will be successful.

Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.

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