Mechanistic Analysis of Air-Induced Segregation during Powder Flow into a Confined Space

Die filling is a typical process step in the manufacturing of a wide range of particulate products (e.g. pharmaceutical tablets, catalyst pellets and powder metallurgical components) in various industries, which normally takes place in the presence of air. Segregation of powder mixtures during die filling due to the buildup of air pressure in the confined space can have a detrimental effect on the quality of the end product. In this study, segregation mechanisms of powder mixtures during die filling were experimentally and numerically investigated. Binary mixtures of powders with either similar particle density but different mean particle sizes or similar particle size but different particle densities were considered to investigate segregation due to size or density differences. Using a segmented shoe and die, the size induced segregation and density induced segregation during die filling was examined quantitatively. The results indicated that, during die filling, depending on the shoe speed, different segregation mechanisms came into effect; the sieving segregation mechanism often occurring in hoppers was observed during all condition where air was able to freely escape and the fluidization segregation mechanism was observed when the air inside the die, while being displaced by the powder was only able to escape by permeating through the powder bulk in the shoe It is also found that the composition ratio of the mixtures had only a minor effect on the segregation patterns for the materials considered in this study. Numerical analysis was also performed using a coupled DEM-CFD, from which the air-induced segregation mechanisms were explored.