Title:
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Tri-Helical direct Gravure coating
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Tri-helical, gravure roll coating, operated in reverse mode, is investigated via a combined experimental,
theoretical and computational approach.
New experiments were conducted on a pilot coating rig, designed to simulate the industrial process.
Flow visualisations reveal the underlying flow structure within the roll-to-web transfer region
and highlight when loss of coating stability (streaking) occurs. The latter is found to be influenced
by the depth of the tri-helical grooves and the capillary number. Experiments show that as the
web-to-roll speed ratio is increased, so too is fluid pickout from the grooves, although the coated
film thickness may decrease.
A key feature of the present investigation is the formulation of a novel complementary mathematical
model. By starting with a simplified form of the coating process and progressively adding
complexity a set of models are developed, first for simple zero pitch angled rectangular grooves
then for grooves of arbitrary shape and groove pitch. A further extension to the model is the inclusion
of a non-Newtonian model for the fluid (specifically a shear thinning power law formulation).
Analysis of the application of shear thinning fluids to the moving substrate is also conducted.
The base model developed is that for rectangular grooves of zero pitch, which takes the form of
an analytical solution of the flow equation (a Poisson equation) along a groove. An extension to
this model is made by solving the Poisson equation for non-rectangular grooves using the finite
element method. Simple meniscus models were applied to make the problem tractable. Agreement
between experimental data and predictions from the model is seen to be good for the range of
operating conditions considered up to the onset of streaking.
A final extension to the model considers grooves at non-zero pitch angles, representative of the
industrial coating process. The limitations of this model, when compared to experimental data are
examined and a physical explanation is postulated for the breakdown of the model at steep pitch
angles. Under conditions of breakdown, the high pressures within the groove are consistent with
the idea that elastohydrodynamics is an important mechanism in the transfer of fluid within the
coating bead for discrete cell gravure coating.
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