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ABSTRACT:The objective of the present study is to examine the validity of the theoretical predictions on the operating windows of slot die coating. The operating window is defined as a domain inside which stable and uniform coating is possible; different types of coating defects are found outside the window. A flow visualization technique was applied to observe the coating bead, particularly the positions and shapes of the downstream and upstream menisci, just before and after coating defects appear at high coating speeds. Special features of coating bead shapes, which lead to onsets of ribbing and air entrainment, were identified. The two-dimensional flow in the coating bead region was computed by the commercial software package FLOW-3D ® . Critical features observed experimentally for the onset of coating defects were used to judge whether the coating flow was within the operating window. The theoretically evaluated operating windows were found to be much larger than

An experimental study was carried out to examine how uniform rimming flow is established for a very small volume fraction of aqueous Newtonian solutions in a partially filled rotating horizontal cylinder. There exists a certain critical volume fraction (Vc) for each solution, where the rotational speed required to achieve uniform rimming flow takes a minimum value. Counterintuitively, it takes greater rotation speeds for both larger and smaller volume fractions than this. Axial instabilities are observed for liquid volume fractions above or below this critical value. For V>Vc the defects are mainly of shark-teeth and turbulent types, while for V<Vc, fingers and rings are formed. The critical volume fraction increases with increasing fluid viscosity. Reducing surface tension increases the minimum rotational speed for V>Vc, but has very little effect for V<Vc. The lowest volume fraction for rimming flow found in the present study is 0.25%. The dimensionless minimum rotational speed Ω to achieve rimming flow is presented as a function of the dimensionless liquid volume fraction ϕ. The competing effects of fluid inertia and viscous force on rimming flow are demonstrated from a dimensionless plot of Ω versus ϕ.

Tensioned‐web slot coating (TWSC) technology was developed in the early 1980s and was considered an efficient method for thin liquid film coating. An experimental study was carried out to investigate the effects of several key parameters on the minimum wet thickness of TWSC. The experiment was performed on a pilot coater with dilute Newtonian poly(vinyl) alcohol (PVA) solutions as test fluids coating on polyethylene‐terephthalate (PET) substrates. It was found that the minimum wet thickness for TWSC was between 0.5 and 1.5 μm, which is much lower than that obtained using conventional slot die coating. The minimum wet thickness was proportional to the tension number TN, which is defined as the ratio of fluid viscous force over web tension. Furthermore, on the basis of the experimental data, two additional dimensionless groups: dimensionless pressure PN and dimensionless surface tension LN were found to be relevant to the performance of TWSC. A universal correlation involving these dimensionless groups as well as the die geometry was used to predict the minimum wet thickness of TWSC. POLYM. ENG. SCI., 47:841–851, 2007. © 2007 Society of Plastics Engineers

A simple model based on the lubrication approximation has been developed to analyze heat transfer of power‐law fluids inside an extrusion die. This model can effectively predict the influence of temperature variations and other production alterations on flow uniformity. The predictions of this model are compared with those based on three‐dimensional finite element simulation, and the agreement is satisfactory. The optimal design concept can be incorporated into this model so that a specially designed choker bar can be inserted into the extrusion die to reduce flow nonuniformities. The shape of the choker bar is initially suggested by the lubrication model; minor corrections through the three‐dimensional finite element simulation are then introduced so that a choker bar can be properly constructed to eliminate flow nonuniformities.