Die Filling of Aerated Powders

This report summarizes the work performed during the last 12 month primarily by the project student, Mr. El Hebieshy. It covers the following three topics:

1) The effect of die shape and orientation on flow behavior of aerated powders during die filling.

This is a natural continuation of the DEM work reported in our first IFPRI report, in which a systematic numerical analysis of the effect of die shape and orientation was presented. In order to validate the DEM results and provide physical insights into the effect of die shape and orientation on die filling behavior, we performed an extensive experimental investigation using 10 powders of distinctive powder characteristics and various shaped dies as used in our DEM simulations reported last year. We found that powder flow during die filling was mainly determined by two primary powder characteristics: the true density and the median particle size (d50), which could adequately distinguish die filling behavior of most powders considered in this study. In addition, for non-axis-symmetrical dies (e.g. oval and rectangular), the die orientation affects the die filling performance, which also depends on the aspect ratio of the die openings. It was shown that the effect of die shape on die filling performance was not significant.

2) Theoretical modeling of die filling of aerated powders.

A theoretical model was developed to predict the mass flow rate during die filling with various powders considered in this study. In order to validate the theoretical model, two sets of experiments were performed: i) the open die experiments in which the effect of entrapped air is eliminated, and ii) the closed die experiments in which the entrapped air could have a significant impact on the flow behavior of air sensitive powders. It was found that the developed theoretical model was capable of predicting the mass flow rate and the critical filling speed, and could capture the influence of air sensitivity of the powders during die filling.

3) Size-induced segregation during die filling.

A preliminary experimental study was performed using a mixture of two powders with similar density but different particle sizes. Segregation tendency at various filling speeds was examined and it was found that the segregation tendency decreased with an increase in filling speed.

Based upon current research and the tasks proposed in the original IFPRI proposal, the following future project plan is proposed:

• Segregation during die filling. A systematic study on segregation induced by density and size difference (i.e. density- and size-induced segregation) will be performed, as well as airflow induced segregation.

• Modelling die filling with aerated powders. The theoretical model developed recently will be extended to analyse die filling with aerated powders, especially by considering build up of air pressures inside the die during die filling. A thorough experimental study on pressure gradient induced by the entrapped air will also be carried out, for which an instrumented die with air pressure measurement during die filling processes will be designed. The experimental study will be used to validate and refine the model.

• Flow behavior of aerated powders during rotary die filling. Most die filling studies focused on translational motion of the shoe or the die, while in practical tableting processes, die filling was performed in a rotary tableting machine. To mimic the real die filling process, we plan to design and build a rotary die filling system. Using this system, how rotational angular speed and powder characteristics on die filling performance of aerated powders will be experimentally explored for the first time.