The scaleup behavior of blends exhibiting phase inversion during compounding in batch mixers was studied. Similar morphological changes were observed during compounding of polystyrene/polyethylene blends of different batch sizes ranging from 12g to 240g. The time to achieve a continuous phase of the major component, polystyrene, was shown to depend on the scale of the mixing device. Based on visual observation of the morphological changes, a constant nominal-maximum-shear-rate scaleup condition was used. Upon a five-fold increase in batch size the time to phase inversion increased by a factor of 3. This change is explained using a combination of the reduced specific area and reduced mechanical energy input under the experimental conditions. A novel blade design using modular triangular elements was constructed and results from radial and axial scaleup using the new blades are presented. Similarities between the triangular and roller blades are used to highlight the importance of the high-shear region in determining the softening rate of the polystyrene pellets. The flexibility of the blade design was exploited to study the effect of blade configuration on the time to phase inversion. A relative stagger parameter is introduced to explain the observed dependence. Increasing the relative stagger decreased the stress transfer to the batch and increased the time to phase inversion. Implications of these results for mixing in the kneading sections of twin-screw extruders is discussed.