The mixing efficiency of an eccentric-disc kneading zone in intermeshing co- and counter-rotating twin-screw extruders

The distributive mixing efficiency of a twin-screw extruder kneading zone consisting of eccentric disc elements was measured using an online video technique. Both co- and counter-rotation were examined. Viscous Newtonian silicone oil was used as model liquid and black iron oxide pigment served as tracer substance. Under isoviscous, creeping flow and non-diffusive conditions and for a fixed flow rate ratio of the colored and uncolored feed streams, the intensity of segregation S is only a function of the kinematic parameter Lambda(the ratio of the imposed extruder throughput and the throughput at zero axial pressure gradient). The measured dependency of S on Lambda is in qualitative agreement with the results of Pawlowski for a single screw extruder. The data was also plotted against the dimensionless speed of rotation, i.e. the product of the screw speed and the average residence time within the mixing section. This brings the abscissa ranges for mixers with different conveying capacity closer together, and differences in mixing efficiency between the tested configurations can be better interpreted. The energetic efficiency of the mixers investigated is compared by applying the concept of specific action. Ibis helps to decide which mixer geometry and operating conditions produce a given homogeneity with the lowest amount of work done by viscous forces.