Flow anomalies

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

Yamano, M., Uyeda, S., Foucher, J.P. and Sibuet, J.C. (1981) Heat flow anomaly in the middle Okinawa Trough. Tectonophy.sic.s, 159, 307-318. 

Chapman, D. Rybach, L. 1985. Heat flow anomalies and their interpretation. Journal of Geodynamics, 4, 3-37. 

To date, most wall-slip studies have concentrated on either eliminating or quantifying slip effects in laboratory rheometry. Slip effects have been traditionally treated as flow anomalies, which make the measurement of material properties a challenging task. There is less evidence on how to utilize this flow anomaly to industry s advantage. A comprehensive understanding of the slip phenomenon as it relates to food texture and food processing is of crucial importance to the food industry, where the goal is to produce quality products at reduced cost... 

Fig. 12.17 Scanning electron micrograph of HDPE extruded at a shear rate slightly lower than the oscillation region, showing sharkskin. [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.]...

N. Bergem, Visualization Studies of Polymer Melt Flow Anomalies in Extrusion, Proc 7th Int. Congr. on Rheology, Gothenburg, Sweden, 1976, p. 50. 

Polymer melt rheologists constantly applied the notion of wall slip in describing a variety of polymer melt flow anomalies [ 10,11,14]. However, no explicit knowledge of a molecular mechanism for and microscopic origin of wall slip had been available. Therefore, for 40 years no explanation was given about why wall slip was pertinent and how it produced such melt flow anomalies as flow oscillations and sharkskin-like extrudate distortion. One objective of current research around the world is to explore the molecular nature of wall slip and establish a correlation between wall slip and some of the anomalous melt flow phenomena. 

Since the Navier s slip hypothesis of the last century, most experiments have failed to obtain positive evidence for a slip boundary condition on macroscopic scales in low molar mass liquids. However, Navier s notion of slip turns out to be extremely useful and convenient for the latest description of flow anomalies of highly entangled polymer melts including linear polyethylenes (LPE). The ability of a melt/solid interface to possess two profoundly different states as shown by Fig. 4a,b clearly reveals the potential role of interfacial slip in governing various melt flow phenomena in high pressure extrusion. Before reviewing recent experimental studies that have elucidated the molecular origins of different flow... 

Extrudate distortion has been viewed as explicit evidence for melt flow instabilities or melt fracture. This is another calamitously misleading assertion in the massive literature of over 3000 papers on the subject. Because extrudate distortion was also observed even without any signature of slip, it was concluded by Blyler and Hart [32] that the slip process is not an essential part of the flow instability. This dilemma, that the flow anomalies including flow oscillation cannot be accounted for in terms of either a constitutive instability or interfacial slip mechanism, has persisted until very recently. Denn coined this plight the paradox [10b]. 

Jones M. Q. W. (1992) Heat flow anomaly in Lesotho implications for the southern boundary of the Kaapvaal craton. Geophys. Res. Lett. 19, 2031-2034. 

Wissbrun earlier observed a very long relaxation time and high elasticity for anisotropic melts of aromatic polyesters, as well as several other types of flow anomalies. Unfortunately, in most of these earlier studies, the rheological behavior of liquid crystal melts of polymers could not be directly compared with that of the isotropic phase of the same polymers because of their high clearing temperatures. 

AlfSldi, L., Gdlfl, J. and P. Liebe, 1985. Heat flow anomalies caused by water circulation. Journal of Geodynamics 4, pp. 199-217... 

The high temperatures required by mantle plumes are not supported by the expected positive heat flow anomalies. 

Experience has shown structured fluids to be more difficult to manufacture, due to the complexity of their rheological profiles. In addition to elasticity, dilatancy, and rheopexy, certain structured fluid compositions may exhibit solid-like properties in the quiescent state and other flow anomalies under specific flow conditions. For emulsions and solid particulate dispersions, near the maximum packing volume fraction of the dispersed phase, for example, yield stresses may be excessive, severely limiting or prohibiting flow under gravity, demanding special consideration in nearly all unit operations. Such fluids pose problems in... 

The orifice angle of the cone can signiflcantly influence the flow anomalies that are frequently found with high molecular polyethylene (PE). In such cases, changing the melt temperature has little effect. Computer programs are available to calculate the distribution of shear rate in the extrusion flow channel. It is important to ensure that the critical shear rate occurs near the die exit resp. is confined to as narrow a region as possible. With the accumulator head, the effects of flow anomahes can be influenced by the extrusion velocity. To achieve the shortest cycle times with continuous extrusion, the plastic usually exits from the orifice at close to critical shear rate. 

So far, we have considered fractures, which have continuous pressures and tangential velocities but discontinuous normal velocities. Shales, by contrast, have discontinuous pressures and tangential velocities but continuous normal velocities. Special flow anomalies can be constructed by superposing linear combinations of the two. Sometimes a third class of flow arises, namely, rapid flows through mineralized faults and streaks which are not open to production. These are responsible for velocities tangent to the fracture but are different from... 

One tool that can help solve these challenges is to employ Computational Fluid Dynamics simulation. This gives an insight into the inside of dosing and mixing plants. The processes involved can be studied in quasi slow motion and this provides valuable information about the various flow anomalies which are constantly observed in practice. 

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