Flight geometries

P. Y. McCormick [Chem. Png. Prog., 58(6), 57 (1962)] compared all available data. The comparisons showed that flight geometry and shell speed should be accounted for in the value of K. He suggested that shell rotational speed and flight number and shape must affect the overall balance however, data for evaluating these variables separately are not available. Also, it is not beheved that the effect of gas velocity... 

McCormick (1962) reworked the data of Miller et al. (1942), Friedman and Marshall (1949), and Saeman and Mitchell (1954) with a view to obtaining a single correlation of the form of Eq. (12-101) for the volumetric heat-transfer coefficient. He demonstrated that all the data could be correlated with values of the exponent n from 0.46 to 0.67. Although the evidence was far from conclusive, he believed that a value of 0.67 for the exponent n was most reliable. Individual values of the constant K were obtained from the results of each of the workers cited above. He found that it was a function of the solids properties, the flight geometry, the rotational speed, and the dryer holdup, but that there was insufficient evidence available to relate K to these parameters. 

The constant k depends upon the flight geometry and its value for different design-loaded flights are given in Table 7.1 [19]. 

Equations 7.57(a) and (b) assume a zero radius of curvature between the flight flank and the screw root. If the radius is taken into account, the screw contact area is reduced by Rc(4 -ti). From this point of view one would like to use a large radius i.e., R. = H. The resulting flight geometry is shown in Fig. 7.13 and also in... 

Another method to reduce the screw contact area is to use flat slanted flight flanks, i.e., a trapezoidal flight geometry see also Fig. 8.4(b). If the flight flank angle is 45°, the screw contact area in the flat plate model becomes ... 

When the channel width is about ten times the channel depth, the screw contact area will be reduced by about 10%. However, the cross-sectional area of the screw channel will be reduced by the same percentage. Thus, the net effect will be less beneficial than the curved flight geometry. 

Solids conveying rate versus channel depth for 75-mnn screw showing both a single- and a double-flighted geometry results based on model with curvature... 

Dekker [3] studied the effect of various flight geometries on solids conveying performance. He proposed that many extrusion instabilities might be due to internal deformation of the solid bed. Internal deformation is more likely to occur when the internal coefficient of friction of the polymer particles is low. Spherical particles tend to have a lower internal coefficient of friction than non-spherical (e.g. cylindrical) particles and are, therefore, more susceptible to internal solid bed deformation. This may explain the often observed difference in extrusion behavior between strand pelletized and die-face pelletized material. 

The Ingen Housz screw combines a barrier geometry with multi-flighted geometry to obtain significant improvements in melting. A picture of the barrier section geometry is shown in Fig. 8.62. 

The barrier section geometry is shown with the screw channel unrolled onto a flat plane. The solid bed is divided into several parallel solid channels. The melt is collected in several parallel melt channels. It is possible to achieve this multi-flighted geometry by a significant increase in the helix angle. The total melting length can be expressed as [27] ... 

Flight geometries to create elongational flow (the arrows indicate the movement of the screw flight relative to the barrel)... 

Multiple passes through the HSRs can be achieved by using a multi-flighted geometry combined with a generous flight clearance. A possible geometry is shown in Fig. 8.83. 

Note that the material is displaced by an axial distance of three times the pitch when it reenters the screw. Thus, in a double-flighted geometry, there are three more or less independent down-channel flows. When the number of parallel flights is p, the number of independent down-channel flows Oj is ... 

One of the implications of this constraint is that the centerline distance for triple-flighted or trilobal screws, p = 3, has to be quite large. The smallest centerline distance for a triple-flighted geometry is Cl/D = 0.5V3 (= 0.866) see Eq. 10.3 and the maximum channel depth 0.134 D. As a result, the channel depth of the screw becomes relatively small and so does the channel volume. Consequently, the throughput capability of triple-flighted screws is limited. 

Lai Took and Worth [29] proposed modified flight geometries to increase the centering force on the screw in order to reduce metal-to-metal contact. The two flight geometries they proposed based on theoretical calculations are shown in Fig. 11.10. 

Flight geometry to reduce the chance of metal-to-metal wear... 

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