The flow and segregation behaviours of the binary mixtures with different particle sizes during die filling in air and in a vacuum were investigated using a coupled discrete element method (DEM) and computational fluid dynamics (CFD) in three dimensions (3D). Two types of computational setups were considered: i) a fully 3D model with real wall boundaries and ii) a thin sliced model with a slice of the full domain using two parallel periodic boundaries. Die filling with a stationary shoe and a moving shoe were simulated. It was found that the flow and segregation results obtained using the fully 3D and thin sliced models were essentially identical, implying that the thin sliced model can be used to simulate the die filling with a reduced computational time and good accuracy. It was also observed that, for die filling with a stationary shoe, the powder flow rate was reduced by the entrapped air in the die. For the die filling in a vacuum, vertical segregation occurred with a high concentration of fine particles at the bottom of the die as the fine particles can sift through the voids between coarse particles. The presence of air reduced the extent of this segregation by suppressing the percolation of fines through the voids. For die filling with a moving shoe, the effect of air on powder flow and segregation behaviours is negligible when the process is dominated by nose flow as considered in this study, since the air can readily escape from the die cavity. Segregation was also induced during die filling with a moving shoe, due to the free surface segregation and the filtration of fines through the voids between the coarse particles. Consequently, a high overall concentration of fines in the die was obtained, while the concentration of fines at the far end of the die was much lower than that at the near end. (c) 2010 Elsevier B.V. All rights reserved.