Conventional Vented Extruder Screw

For the same reason, the helix angle is often increased from the conventional 17.66° (square pitch) to as high as 40° [12]. 

As discussed in Section 8.5.1, the optimum pump ratio ranges from 1.5 to 2.0. In practice, the pump ratio is often selected in the range from 1.2 to 1.4. However, these lower pump ratios make the extruder more susceptible to vent flow. Sometimes vent flow is avoided by starve feeding the extruder. However, if starve feeding 

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Vented extruders are significantly different from non-vented extruders In design and in functional capabilities. A vented extruder is equipped with one or more openings (vent ports) in the extruder barrel, through which volatiles can escape. Thus, the vented extruder can extract volatiles from the polymer in a continuous fashion. This devolatilization adds a functional capability not present in non-vented extruders. Instead of the extraction of volatiles, one can use the vent port to add certain components to the polymer, such as additives, fillers, reactive components, etc. This clearly adds to the versatility of vented extruders, with the additional benefit that the extruder can be operated as a conventional non-vented extruder by simply plugging the vent port and, possibly, changing the screw geometry. 

The extruders were of a similar size, but the one used for the HDPE/filler trials was longer to accommodate a vent zone and had an alternative conventional screw. The PBT/pigment trials used a screw with a mixing element following the barrier section. As the two investigations used screws of similar diameters and together covered a range of screw features, the essential details are compared in Table 13.1. 

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