Effect of Paddle Shape On Flow of Powder Through Feed Frames On Rotary Tablet Presses

Effect of paddle shape on flow of powder through feed frames on rotary tablet presses

G. Singh*, J. Givand**, P. Rajniak**, R. Chern**, S.D. Reynolds** and I.C. Sinka*

*Department of Engineering, University of Leicester, University Road, Leicester, LE1 7RH, UK
**Merck, Sharp & Dohme, Sumneytown Pike, West Point, PA19486, USA

A commercial scale high speed rotary tablet press can produce in excess of 0.5 million pharmaceutical tablets per hour. This involves the same number of die fill operations, which must be executed consistently to ensure weight and content uniformity from tablet to tablet. High productivity can only be achieved if the formulation of the powder and the selection of process parameters are optimised.

Die fill is a complex and dynamic process and consists of a number of distinct stages that present unique features: delivery of the powder to the press (in batches or continuously), powder flow from hopper to feed frame, powder flow through feed frame to die opening, and die fill. In this paper we examine the flow of powders through a lab scale rotary feeder which was developed at the University of Leicester to explore the mechanisms present in the feed frames of rotary tablet presses. The feeder pictured below consists of an inlet hopper, a disk shaped frame housing a paddle wheel and a powder outlet.

The flow regimes were examined for a poorly flowing placebo formulation and the following parameters were considered:

·        Paddle wheel speed: 25, 50, …, 400 rpm

·        Outlet diameter: 5, 10, 15, 20 mm

·        Shape of paddle wheel spoke cross section: round, square and inclined

It is shown that the powder delivery is not always as continuous as the figure below implies and that poor flowing materials can perform surprisingly well under the conditions imparted by the rotary feeder. The results are quantified in terms of mass flow rate through the feeder, which increases and saturates as the freed frame speed is increased. It was interesting to note, however, that the mass flow rate per unit area of the outlet can increase or decrease as the outlet diameter is reduced, depending on powder. For all cases a significant effect of the shape of the paddle cross section was observed.

Experimental set-up of lab scale rotary feeder