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Roll coating is a widely used process for depositing a very thin liquid
layer onto a continuous web. Typical applications include the coating
of magnetic media, adhesive tape, films, foils, and paper. Computational
fluid dynamics (CFD) can be used to model the fluid transfer region between
two rolls in a forward or reverse roll coater and can predict the film
split ratio (amount of fluid transferred to each roll), coating uniformity,
and operability limits.
In roll coating systems, one or more of the rolls is usually rubber-backed.
This helps to avoid the clashing of rolls, reduces the sensitivity to
roll runout and enables thinner films to be coated. NEKTON 3.1 now models
these deformable rolls and deformable substrates, using the linear spring
model of Carvalho and Scriven. The model has been validated with the numerical
results of Carvalho.
The deformation of the roll affects the actual distance between roll surfaces,
the flow rate metered through the gap and the meniscus position. Since
deformation of the roll also affects the pressure gradient at the meniscus,
it will have an effect on the stability of the flow. Carvalho has shown
that roll deformation delays the onset of the ribbing instability.
Pressure contours in a forward roll coater with flooded inlet and a
deformable lower roll. The roll deformation causes increased flow through
the nip and pushes the meniscus to the right when compared with the rigid
roll case.
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