In order to simulate an extrusion process or design a screw, the mathematical
description of the screw geometry must be understood. This section provides the
basic details that describe a screw and the complex mathematics that describe the
channels.
The single-screw extruder screw can be single flighted or multiple flighted. A
conventional single-flighted screw is shown in Fig. 1.3. This screw has a single
helix wound around the screw root or core. Multiple-flighted screws with two or
more helixes started on the core are very common on high-performance screws
and on large-diameter melt-fed machines. For example, barrier melting sections
have a secondary barrier flight that is located a fraction of a turn downstream from
the primary flight, creating two flow channels: a solids melting channel and a meltconveying
channel. Moreover, many high-performance screws have two or more
flights in the metering section of the screw. Barrier screws and other high-performance
screws will be presented in Chapter 14. Multiple flights are very common
on larger-diameter extruder screws, because this creates a narrower channel for
the polymer melt to flow through, leading to less pressure variation due to the rotation
of the screw. In addition, the multiple flights spread the bearing forces between the flight tip and the barrel wall. Melt-fed extrusion processes will be discussed in
detail in Chapter 15. The screw is rotated by the shank using either specially
designed splines or by keys with rectangular cross sections. The mathematical
zero position of the screw is set at the pocket where the screw helix starts. Most
extruder manufacturers rotate the screw in a counterclockwise direction for viewers
positioned on the shank and looking towards the tip. This rotation convention,
however, is not standard.