Powder die filling under gravity and suction fill mechanisms

In pharmaceutical tablet manufacturing, the powder formulation is filled into a die and compacted into a tablet using rigid punches. Die fill is important because it limits the productivity of tablet presses and determines key quality attributes of tablets including weight and content uniformity. Die fill occurs due to gravity and suction fill mechanisms. A model linear shoe-die filling system has been instrumented with pressure measurement devices for detailed characterisation of air pressure evolution as the powder mass is delivered in the die. Systematic experiments were carried out using a range of microcrystalline cellulose powders to explore the role of powder properties (such as particle size and bulk density) and operating parameters (such as shoe and die geometry, shoe and suction punch kinematics and powder filling level) on powder delivery. Existing models were found inadequate to describe the mass flow rate of powders under a diversity of gravity and suction filling conditions. The pressure measurements enabled the development of a new die fill model using the Buckingham theorem. The model includes separate terms for the contribution of the mass of powder delivered under gravity and suction fill mechanisms. The experimental procedures required to extract the parameters of the model are described. The model is applicable to the handling and dosing of fine and cohesive powders where small differences in air pressure have a significant impact on the powder flow process. The practical application of the model for predicting die filling behaviour in a high-speed rotary tablet press is demonstrated by assuming operating conditions of a typical rotary tablet press. This approach can be adapted to assist formulation design and process development for operations involving handling and dosing of fine and cohesive powders.