Self-Wiping Extruders

High speed co-rotating extruders have a closely matching flight profile, as shown in Fig. 10.9. 

There is considerable openness from one channel to the adjacent channel. This is obvious both from the top view of the screws shown in Fig. 10.9 as well as from the cross-section through the intermeshing region, shown in Fig. 10.10. 

Cross-section through the intermeshing region in CSCO extruders 

the open area II is large relative to the obstructed area I. Therefore, there is relatively little tendency for large pressure peaks to form in the intermeshing region. The screws can therefore be designed with relatively small clearances between the 

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  

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Screws of self-wiping extruders consist of one, two, or three thread starts. At an increasing number of thread starts the distance between the screw axes has to increase and as a consequence the maximum channel depth decreases, which in turn influences the maximal throughput per screw rotation. For this reason extruders with four or more thread starts are not... 

Here fp and f s are correction factors for the channel geometry, and Hq is the maximum channel depth. In case of rectangular screw chaimels the correction factors can be calculated analytically [Eqs. (2.6) and (2.7)] for the complex geometry of self-wiping extruders they can be approximated by Eq. (2.32) or they can be calculated numerically (6). The number of parallel channels in a screw with m thread starts equals 2w — 1. This leads to a throughput for a self-wiping twin-screw extruder of (6,7)... 

AP denotes the relevant pressure difference (e.g., over one pitch length for a self-wiping extruder or between two consecutive chambers for a closely intermeshing machine), A and S are constants which are for geometrical similar screws independent of the screw diameter. They are derived from straightforward calculations, similar as given in Chapter 2. 

Figure 13.11 Stability of a 50-mm self-wiping extruder during the polymerization of butylmethacrylate (-I-) stable working point, (—) unstable working point, (dotted pattern) unattainable region.

A closer look at the two types of extruders shows that the closely intermeshing counterrotating twin-screw extruder has a better ability to build up pressures, even in an environment with low-viscosity liquids. This is due to the nearly closed C-shaped chambers, which provide a positive displacement action that enhances stability. Self-wiping extruders, on the other hand, are generally capable of much larger throughputs at comparable residence times. This leads to a preference of counterrotating extruders for... 

An important difference between closely intermeshing and self-wiping twin-screw extruders is the way the screws fit into each other. In self-wiping extruders the screw geometry is such that in the plane through both screw axes there is a very close fit between the two screws (Figure 6.13). This requires a special geometry... 

Figure 6.13 Transport elements in a self-wiping extruder.

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