In many recent discussions in the converting industry, I have found myself trying to explain fluid coating technology to people who are familiar with extrusion technology. The resulting conversation feels like an excerpt from Men are from Mars/Women are from Venus!

The control of a turbulent fluid coating onto a substrate requires a different knowledge base than laminar polymer flow regimes. That is why those who play in one world have difficulty playing in the other. The laminar world is more understood, while the turbulent world has many mathematically unsolved and chaotic challenges.

In the laminar world of polymer extrusion, two or more polymers can flow through the same cavity and maintain a distinct layer. The advantages of this phenomenon can be seen in multilayer packaging films that have a printable polymer on one side and an oxygen barrier that can contact food on the other. As long as the geometry of the mold or die is defined, the layers will maintain the shape of that definition (i.e., a rectangular extrusion die will create a flat rectangular sheet with distinct layers).

In the turbulent world of fluid coating, two or more fluids tend to mix on impact, creating a blend of the fluids. So, unlike typical multilayer packaging film, a multilayer fluid coating will end up as a mixture of the fluids introduced together and not as distinct layers. One exception to the typical mixture coating is photographic film. When photographic film is produced, the chemistry, surface energy, and viscosity of the individual layers are controlled to maintain distinct thicknesses and separation.

Now that you can grasp the fluid flow versus extrusion difference, what can be done to keep fluids separate? Outside of a multipass coating operation with multiple curing stations, there are some chemical and physical factors that need to be considered.

While we still all remain on planet Earth, it is true that the polymer extrusion world is a laminar world, while the fluid coating world is a turbulent world. While there are chemical compositions that straddle both, the technology and business of the two technologies do require a different train of thought to process successfully.