Residence Time Distribution Curves of Screw Elements in Twin-Srew Extruder

Abstract

Abstract The additiveness of equivalent residence times method of deconvolution/convolution was used to determine the RTD curves of individual screw elements in the melt region of a twin-screw extruder. Initially RTD curves were obtained injecting a tracer (TiO2) at two different ports in the extruder barrel, using several screw configurations and operation conditions. Both ports are downstream, after the melt seal, well inside the molten polymer screw section. Then using the deconvolution method the RTD curve referent to the portion between the two ports was calculated. Finally, using again the deconvolution method and the data from several screw configurations the RTD of each individual screw element was obtained. The independence of screw element positions was also evaluated, where specially designed screws (e. g. changing the position and type of screw elements) were tested, showing no significant effect in the RTD curves. Kneading elements of 45 degrees (KB45) showed an important contribution of conveying, while kneading elements of 90 degrees (KB90) practically do not show any conveying, being characterized mainly by a pressure flow. With RTD curves of individual elements is possible, using the convolution method, predict the RTD curve of any screw configuration. Experimental measurements using screw configurations specially designed were used to check this analytical method. Finally it was applied to data published in the literature, obtained using other deconvolution methods and the results compared and discussed.