DIE Filling Process Simulation Using Discrete Element Method (DEM)

Powder compaction and sintering are important techniques for the mass production of geometrically complex parts. Powder is poured from a reservoir into the feeding shoe, which then passes the cavity one or more times thereby delivering powder into it. The powder is then compressed to create a relatively brittle green body. Finally, the green body is ejected from the cavity and sintered in a furnace where thermal activation below the melting point produces a fully dense structure. Necks form and grow between adjacent grains thereby eliminating the porosity of the part. In general, a consistent and uniform die filling process is always desirable. Heterogeneity during die filling can propagate through the subsequent processes and finally lead to serious product defects, such as cracking, low strength, distortion and shrinkage. An approach using discrete element method (DEM) simulation is proposed to reproduce die filling process and investigate process characteristics that affect final sand cake shape and may lead to in-homogeneities in powder during the filling process. An experimental apparatus was set-up to reproduce the filling process in a cylindrical cavity and subsequent compaction step. In the frame of this work metallic power was substituted by wet sand in the experiments to test and validate the numerical models. The final shape of the cake is evaluated and used to validate the numerical model. The LIGGGHTS code was used for the DEM simulations and a coarse grain model was implemented to reduce computational efforts. Cohesive forces are also considered and a capillary model was implemented to correctly account for liquid bridge forces. After successful calibration, numerical model can be used to predict final shape of the cake. Differences in the formed cake could be observed. Small amounts of water have considerable influence in cohesive properties of sand and therefore in the cake shape and powder distribution.