Modifications of the Standard Extruder Screw

Variable pitch extruder screw with reducing pitch 

In this design, the pitch decreases with axial distance as opposed to the screw shown in Fig. 8.32(a). The reducing pitch causes a lateral compression of the material in the screw channel as a result, the normal compression from the reducing channel depth can be reduced or eliminated altogether. In fact, many of these variable reducing pitch screws maintain the same channel depth along the entire length of the screw. 

This screw is more appropriate for a plasticating unit of an injection molding machine. The zero-meter screw is used to reduce the temperature build-up in the material by deepening the depth of the channel in the melt conveying zone of the extruder. The obvious drawback is that the pressure generating capability of the screw will be adversely affected, but this is not a major concern in injection molding applications. In other applications, however, the approach outlined in Section 8.3 is recommended. An extension of the zero-meter screw is the zero-feed zero-meter screw shown in Fig. 8.34. 

This screw essentially consists of only a compression section. This allows a very gradual compression of the material. The screw has been in commercial use for many years and has been successfully used with many polymers, in particular nylon. 

The exact opposite of the zero-feed zero-meter screw is the very rapid compression screw, shown in Fig. 8.35. 

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There are a large number of modifications of the standard extruder screw in use today. It will not be possible to mention all of them, but an effort will be made to discuss the more significant ones. Figure 8.31 shows the standard screw with an additional flight in the feed section. 

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