Influence of Small Die Width on Filling and Compacting Densities

Abstract

Experiments conducted to try to establish the influence of small die openings on the actual filling density are reported. A special apparatus, consisting of rectangular dies of various widths, has been utilized with copper and mixed elemental bronze powders. The results show that filling densities decrease with decreasing die opening. The angle between sense of motion in filling and main die axis also influences the filling density, through rebounds during powder fall; these rebounds play a positive role. The changing quantity of powder entering various dies can be explained by assuming that a boundary layer is present on the die surfaces. This boundary layer, with density tending to zero when approaching the walls from the bulk of the powder, is dependent on grain shape and sieve analysis. A simple theory, including a hydraulic radius of ‘compacting’ area, has been proposed and the corresponding formulae for different die geometries derived. The theoretical lines are in good agreement with the results of experimental tests. The boundary layer widths depend on powder type, lubricant, and angle between filling motion and main die axis. This work enables the design engineers involved in tooling to estimate the possible density excursions on PM parts of varying thicknesses, while the require- ment of completely uniform density necessarily demands ‘tricks’ with tools, filling devices, and common practice. PM/0448