The flow behaviour of powders during the process of die fill was examined. Gravity and suction fill experiments were carried out using a model shoe-die system. Five grades of microcrystalline cellulose were studied to identify the effect of particle size and density on flow. Flowability was quantified using the concept of critical velocity. Under gravity fill, the critical velocity was one order of magnitude higher for powders with large particle size compared to smaller particles. Under suction fill conditions, the critical velocity increased significantly compared to gravity fill, showed no consistent relationship with particle size, and the powders performed more similar to one another. Using high speed video, the gravity and suction fill mechanisms were discussed in the context of air flow and pressure build-up. The effect of shoe velocity, suction velocity and height of the powder in the shoe was explored in more detail. It was shown that one can identify individual contributions from material properties and process parameters to the flow behaviour during die fill; however, the flow performance depends on the inter-relationships between powder characteristics and process parameters. The die fill mechanisms described can be used to assist the optimisation of powder formulation and process design.