Venting through the Screw

An interesting deveiopment in devolatilizing extrusion was described by Bernhardt in 1956 [20]. In this extruder, the volatiles are removed through the screw instead of through a vent port in the barrel. The screw has a hollow core connecting with a lateral hole in the extraction section of the screw see Fig. 8.49(a). 

The volatiles are withdrawn through a rotary union at the rear of the screw. This venting was tested in practice on acrylic and was found to perform reliably for extensive periods of time. This process is also used for processing PET powder with conventional two-stage screws. This concept has also been applied to barrier screws. 

Eastman Kodak Company received a patent (U,S. Patent 6,164,810) on a barrier screw with a vent hole located within about two diameters from the end of the feed section, between a main flight and a barrier flight. The vent hole is located such that there is little chance of polymer plugging the vent hole. The volatiles are vented out to the back of the screw through an axial bore. This screw design can be used to process PET powder into film. 

In the past barrier screws were used only with PET pellets because pellets are not susceptible to air entrapment. Powders, however, are susceptible to air entrapment and do not process well on conventional barrier screws. The internally vented barrier screw developed by Eastman achieves higher throughputs of PET powder. This screw design can also be used for other hygroscopic resins like ABS. 

---