Practical implications of theoretical consideration of capsule filling by the dosator nozzle system

Abstract

Eight lactose size fractions with mean particle sizes ranging from 15.6 to 155.2 μm were characterized by their failure properties using a Jenike shear cell. The effective angle of internal friction was found to be constant for all size fractions, with a mean value of 36.2°. Jenike flow factors could only be obtained for the two most cohesive size fractions presumably due to limitations of the shear cell. Angles of wall friction, ϕ, were determined for all size fractions on face ground and turned stainless steel surfaces. These decreased with increasing particle size up to around 40 μm, above which they became effectively constant for both surfaces. The rougher turned plate gave consistently higher values of ϕ for each particle size. Simple retention experiments with a dosator nozzle and a range of powder bed bulk densities showed good retention was possible only up to a particle size of around 40 μm. Retention was difficult or impossible above this size. Values of ϕ were applied to equations derived in the theoretical approach described previously (Jolliffe et al 1980). This showed that the strength required within a powder to ensure arching increases with increasing particle size up to around 40 μm. Above this size, this strength requirement becomes constant. This is related to the powder retention observations. Finally, the failure data was used to calculate the minimum compressive stresses required to ensure powder retention within the dosator nozzle, by employing the equations described by Jollife et al (1980). This suggested that, as powders became more free flowing, a larger compressive stress is necessary and that the angle of wall friction should be lower to ensure stress is transmitted to the arching zone.