Geometry, extruder screw

Finally, as pointed out earlier, extruder screws have tapered channel sections. The various shaped pressure profiles that can be obtained in simple non-parallel plate geometry explain the experimentally observed pressure profiles in screw extruders. For example, in a common metering screw (with a constant channel depth feed section, followed by a tapered section and a constant channel depth metering section), under normal conditions the pressure profile exhibits a maximum in the tapered section with the pressure dropping in the metering section to the die pressure. 

The obvious difference between the co- and counterrotating extruder is that in the co-rotating geometry both screws rotate in the same direction, either clockwise or coimter clockwise. In the counter-rotating geometry, they rotate in opposite directions. In addition to geometry, this simple difference impacts the way the two screws interact. 

The melt conveying theory discussed in Section 7.4 can be used to determine the optimum screw geometry for melt conveying. This optimum geometry will not normally be used in the metering section of the extruder screw. The optimum channel depth for output can be determined from ... 

The conventional multi-flighted extruder screw has a number of advantages and disadvantages. The basic geometry is shown in Fig. 8.52. 

The melt temperatures and stresses to which the polymer is exposed in the extruder are strongly influenced by the geometry of the extruder screw. The functions of the extruder screw are... 

In the extrusion of medical devices, molecular degradation is often a critical issue. This degradation of the MW can be strongly influenced by the geometry of the extruder screw. Unfortunately, little information on this problem is available in the open literature. Paakinaho et al. [127] published results of a study on the MW reduction along the length of a single screw extruder for three polylactic acid (PLA) resins see Fig. 8.111. 

Most hardfacing materials do not plate well, i.e., a wavy line occurs at the intersection of the welded material. This problem can be eliminated by the use of an inlay, as shown in Fig. 11.14. Here, two basic hardfacing geometries are shown one that has the hardfacing applied to the full width of the flight and the other that has an inlay with hardfacing. Inlays can only be applied to new extruder screws. 

Screw design screw configuration, 00/1D and screw diameter Die geometry / Extrudate properties and solid dispersion performance... 

FIGURE 1.2 Two different extruder screw geometries along with the various geometric factors that describe the characteristics of the screw. (Reprinted by permission of the pnblisher from Middleman, 1977.)... 

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