Extrudates flow pattern

On the basis of the simplified view of the flow patterns just described, a model for predicting mass transfer rates can be developed using penetration theory and the fact that mass is transferred simultaneously from both the nip and the wiped film. We can therefore write that the total molar mass transfer rate from an element of fluid over a length dk in the extruder is... 

A. Berlis, E. Broyer, C. Mund, and Z. Tadmor, Flow Patterns in Aprtially Filled Extruder Screw, Plastics Polym., 41, 145-148 (June 1973). 

Li and Manas-Zloczower (31) used the CFM commercial FIDAP FEM package to simulate the three-dimensional isothermal flow patterns and distributive mixing in three consecutive filled, closed C-shaped chambers of fully intermeshing, counterrotating extruders, having the dimensions of Leistritz 30.34 (30 denotes the centerline distance and 34 the barrel diameter in mm units). An equal pressure drop per C-shaped chamber was applied for the calculations. The melt was assumed to be Power Law above y() and Newtonian below it. The design, process, and material variables are given by the authors. 

T. Ishikawa, T. Amano, S. I. Kihara, and K. Funatsu, Flow Patterns and Mixing Mechanisms in the Screw Mixing Element of a Corotating Twin-screw Extruder, Polym. Eng. Sci., 42, 925-939 (2002). 

Fig. 12.15 The ratio of entrance pressure drop to shear stress at the capillary wall versus Newtonian wall shear rate, T. . PP , PS O, LDPE +, HDPE , 2.5% polyisohutylene (PIB) in mineral oil x, 10% PIB in decalin A, NBS-OB oil. [Reprinted by permission from J. L. White, Critique on Flow Patterns in Polymer Fluids at the Entrance of a Die and Instabilities Leading to Extrudate Distortion, Appl. Polym. Symp., No. 20, 155 (1973).]...

Fig. 12.18 Flow patterns above the oscillating region of HDPE. (a) Microtome cut along the cylinder axis of HDPE solidified inside a capillary die showing that the flow patterns are formed at the die entrance, (b) Microtome cut of the HDPE extrudate resulting under the same conditions as in (a). [Reprinted by permission from N. Bergem, Visualization Studies of Polymer Melt Flow Anomalies in Extruders, Proceedings of the Seventh International Congress on Rheology, Gothenberg, Sweden, 1976, p. 50.]...

The site of the sharkskin distortion is again the die exit, and so is the screw thread pattern. The site of, and the mechanism for the gross extrudate distortion are problems that have no clear answers. The work of White and Ballenger, Oyanagi, den Otter, and Bergem clearly demonstrates that some instability in the entrance flow patterns is involved in HDPE melt fracture. Clear evidence for this can be found in Fig. 12.18. Slip at the capillary wall, to quote den Otter, does not appear to be essential for the instability region, although it may occasionally accompany it. ... 

This study serves to illustrate the methodology of the lamellar model approach and indicate its complexity and limitations. Extension to other mixer-reactors with complicated flow patterns, e.g., static mixers or twin screw extruders is possible with very little conceptual modification. 

In the purpose of comparisons between theory emd experiments, this short review points out the need for significant experimental studies related to flow patterns, pressure drops and extrudate swell ratios for polymer melts characterized in shear and elongation. Streamlines, velocity components and stress fields related to flow birefringence studies should be used to validate numerical simulations of entry and exit flows. Progress in flow computations in domains involving the upstream and downstream channels together with the exit region are also necessary. 

The flow pattern of the powder mixture through the extruder influences the quality of the shaped article. The rheology of concentrated ceramic suspensions can be divided into four classes ideal plastic, Bingham, shear thinning, and shear thickening (dilant). 

White, J. L. (1973). Critique on flow patterns in polymer fluids at the entrance of a die and instabilities leading to extrudate distortion. Appl. Polym. Symp, 20, 155-174. 

In the twin-screw extruder, the screw system is open in the axial direction. It is actually open in the longitudinal direction of the screw channel with passage from the inlet to the outlet of the machine. Figure 5.2 illustrates the flow pattern of material from the feed section in a co-rotating compounding extruder. 

Figure 5.2 Flow pattern of material from the feed section in a co-rotating compounding extruder...

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