Bypass Vented Extruder Screw

Another method to control the flow rate into the extraction section is to use a bypass system. This system was proposed by Willert of Egan Machinery Company [14]. Maddock and Matzuk [15] discussed the principles of the bypass vented extruder and described actual experiments with different screw geometries. The bypass vented extruder is shown schematically in Fig. 8.42. 

The polymer melt is forced from the metering section into a bypass flow channel by incorporating a multi-flighted screw section with reversed pitch and shallow channels between the metering section and the extraction section. The bypass channel has one or more adjustable restrictions to control the rate of flow into the extraction section. The polymer flows from the bypass channel into the beginning of the extraction section. Maddock and Matzuk concluded that bypass venting allows a wider range of operation, is less susceptible to instabilities, and is less operator-sensitive. 

A different bypass venting system was described by Anders [16] it is shown in Fig. 8.43. 

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