An evalution of mixing quality of three different types of mixing elements

Abstract

In this paper, the performance of three different mixing elements on color dispersion in HDPE and LLDPE polymer melt stream during extrusion is studied. Two similarly designed Maddock mixers and a Stratablend II mixer are used as the last part of a general purpose single screw. Moreover, an in-line melt camera is used for quantification of mixing quality by visualization of grayscale of the color dispersion and thus mixing. The Stratablend II mixer produces the lowest and most uniform standard deviation, which is the best tool to characterize its excellent mixing performance. Both Maddock mixers showed the same trend but higher values of standard deviation. All results are then compared with a full 3D finite element method simulation. Simulations clearly indicate that Stratablend II mixer has the the best mixing abilities and that these are mainly given by its unique design with high average value of shear stress. The role of elongational stress does not appear to have a high influence on mixing for these mixers. The results also suggest that the key factor for achieving better mixing is the frequency with which a large fraction of the material passes through the high shear stress regions of the mixer.