Die Filling of Aerated Powders

Author Last Name: 
Chuan-YU (Charley) Wu
Report Type: 
Research Area: 
Powder Flow
Publication Year: 
Publication Month: 
United Kingdom


This report documents the work performed during the last 36 months funded by IFPRI. It covers the following four aspects: 1) suction filling; 2) Rotary die filling; 3) Forced die filling with a paddle wheel; and 4) Segregation during die filling.

For suction filling, a model suction filling system was developed. Die filling efficiency was evaluated in terms of fill ratio, and four different types of pharmaceutical powders were used and. Effects of filling and suction velocities, as well as powder properties, on the efficiency of die filling were systematically investigated. It was observed that for a given die filling system an optimal ratio of filling to suction velocity (i.e. the optimal velocity ratio, v*) could be defined. Therefore, three distinctive phases in suction filling could be identified: i) a filling to suction velocity ratio (vr) below v* would result in piston reaching the specified terminal position before the shoe completes its transit over the die; this is referred to as slow filling ii) when vr = v*, the suction and filling synchronise so that the piston reaches the terminal position precisely when the trailing edge of the shoe starts transiting over the die, which is referred to as the synchronised filling; iii) when vr > v*, the piston reaches the terminal position after the shoe has completed the transit over the die, and this is referred to as fast filling. In slow filling, cohesive and free flowing materials showed two distinctive die filling behaviours: cohesive materials produced a gradual increase of fill ratio, reaching its maximum value at v*, whereas for free flowing materials the fill ratio decreases slightly as the vr increases and at v*, 5-10% reduction in fill ratio was observed. In fast filling, the die filling behaviour is similar to that of gravity filling at high filling velocities, where the fill ratio decreases as vr increases.

For rotary die filling, a model rotary die filling system was developed to mimic the die filling process in a typical rotary tablet press. The system consists of a round die table of 500 mm diameter, equipped with a rectangular die. The die table can rotate at an equivalent translational velocity of up to 1.5 m/s. The filling occurs when the die passes through a stationary shoe positioned above the die table. Using this system, die filling behaviours of 7 commonly used pharmaceutical excipients with various material characteristics (e.g. particle size distribution, sphericity and morphology) and flow properties were examined. The efficiency of die filling is evaluated using the concept of critical filling velocity. It was found that the critical filling velocity is strongly dependent on such properties as cohesion, flowability, average particle size and air sensitivity index. In particular, the critical filling velocity increases proportionally as the mean particle size, flow function, air permeability and air sensitivity index increase, while it decreases with the increase of specific energy and cohesion.

For forced die filling with a paddle wheel, the flow behavior of powders was studied systematically using a model die filling syystem. The fill effects of different turret speed and paddle speed were carried out using a simulated system with four cylindrical dies. Three grades of microcrystalline cellulose powders were examined to explore the effect of particle size and shape on die fill. Flow properties were quantified using the concept of critical fill speed. It was found that critical filling speed and parameter n depend on not only the properties of powders but also the size and shape of die. The ratio of powders entering the die was primarily influenced by turret speed, while paddle speed has a slight impact. High speed video was used to discuss the trait of flow pattern. Mass uniformity was positively correlated with critical fill speed. The powder bed is loosened when the last die through the fill area and consequently it was filled more than others. The researched critical process parameters help optimize powder formulation and process design.

For segregation during die filling, particle size-induced segregation during die filling of binary pharmaceutical blends, consisting of fine and coarse particles in various fractions, was investigated. Coarse fraction was made of milled and sieved acetylsalicylic acid, whereas the fine fraction was mannitol. The die filling process was carried out in gravity filling and suction. The segregation was assessed through determination of the coarse component concentration using UV-Visible spectrophotometry. The obtained values of concentration, determined for ten units of identical volume inside the die, were used to calculate the Segregation Index (SI), which was an indicator of uniformity of the powder blend deposited into the die. It was found that high segregation tendency was generally observed during suction filling at low velocity, due to the effect of air drag, and during gravity filling at low velocity, as it was carried out through three consecutive filling steps. The lowest segregation tendency was observed during suction filling, irrespective of the filling velocity and concentration. The horizontal segregation was mostly observed in the top layers of the die, due to mainly two mechanisms: coarse particles cascading down the heap formed by the powder in the final steps of die filling, which produces higher coarse concentration in the near side of the die, observed at low coarse concentration; or coarse particle cascading down the top surface of the flowing powder stream into the die, which increases the coarse concentration in the far end of the die.