Browsing by Subject "Operating window"

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    Flow limits in slide coating
    (2009-12) Tjiptowidjojo, Kristianto
    Slide coating is the leading method for simultaneous multilayer coating. The thickness of the coated layer is set solely by the flow rate and speed, and independent from other process conditions. The uniformity of the layer, however, can strongly depend on the other process conditions and uniform coating can only be achieved in certain ranges of operating conditions. The parameter space in operating conditions where coating is feasible is referred as coating window. Slide coating window, even for single-layer case, is not well understood as the slot coating window, another popular premetered coating method. The goals of this thesis are to uncover the mechanisms of coating bead breakdown at the edges of the windows and to predict critical operating conditions corresponding to the bead breakup. The study was initiated by developing a one-dimensional viscocapillary model of slide coating. The viscocapillary model are found to be accurate only at low capillary numbers and small gap-to-thickness ratio. The accuracy of the viscocapillary model is then improved by augmenting it with full 2-D Navier-Stokes theory at the coating bead region, where the flow is fully two-dimensional. The hybrid model is as accurate as the full 2-D model, providing that the matching conditions are assigned properly and their locations are far enough such that the approximations employed in deriving the viscocapillary model remain valid there. These tools are employed with the addition of flow visualization experiments for uncovering bead breakup mechanisms at low vacuum, high vacuum, and low flow limits. In addition to uncovering bead breakup mechanisms, theoretical modeling also yields to predictions of the critical operating conditions that mark the edges of coating windows. The predicted coating window agrees qualitatively with the flow visualization experiments.