Devolatilization in Extruders

FIGURE 8.33 Degassing in a single-screw extruder. Zone 1 consists of a typical plasticating screw section zone 2 is a deep channel section with a vacuum port zone 3 is a metering section. The channels in zone 2 are partially full to facilitate degassing. 

FIGURE 8.34 Cross-sectional views of (a) a double-flighted twin-screw extruder and (b) a triple-flighted single-screw extruder. Each system exhibits three melt pools which are available for degassing. 

The few models available concerned with devolatilization (DV) in single-screw extruders are based on diffusion theory (Biesenberger and Sebastian, 1983). Experimental data available suggest that the process is more rapid than can be accounted for by diffusion alone. However, a summary of the model is useful, because at least the most important variables are brought out. 

FIGURE 8.35 Model for devolatilization in extruders. Gas removal occurs from the melt film on the barrel wall and from the rotating melt pool. 

The evaporation rate of A from the film is obtained from a mass balance on the film, which is 

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