Geometry metering section

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. 

Fig. 9.37 Simulated and measured pressure profiles for an LDPE extruded in a 2.5-in-diameter, 26.5 length-to-diameter ratio extruder, with a metering-type screw having 12.5 feed section with channel depth of 0.37 in and 4.5 turns of metering section of depth of 0.1275. The flow rate is 136 lb/h, the screw speed 60 rpm, and the barrel temperature was set at 300°F. The SBP is shown in Fig. 9.24. The screw geometry is shown at the top of the figure. Simulation was carried out by the first computer simulation package for plasticating extrusion developed by the Western Electric Princeton Engineering Research Center team (17). [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

A a three-zone screw with feed section of constant flight depth, a compression section (it is the geometry and position of the compression section that are designed to match the melting or softening characteristics of the polymer), and a metering section ... 

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 material in the metering zone is a melt. As such, the treatment combines flow and heat transfer. The metering section, while still complex, lends itself more readily to technical analysis than the other two zones. Before undertaking the technical analysis at each of the zones, it is appropriate to consider the various important aspects of screw extruder geometry. Figure 7-3 is a schematic of a screw and a barrel (double-flighted screw with two flow channels in parallel). 

Fig. 12.14 shows the geometry of a typical metering section of a single-screw extruder and shows... 

We are now in position to develop a model for the metering section of a single-screw extruder. In practice, the screw is rotated inside the barrel and helical or cylindrical coordinates are needed to describe the geometry. However, because the channel depth is usually small relative to the radius of the barrel, we can treat the flow using rectangular Cartesian coordinates as shown in Figure 8.18. It is customary to attach the axes to the rotating screw, which makes it appear... 

---