A novel method for determination of the filling level within the feed frame of a rotary tablet press

Abstract

Tablets are nowadays produced almost exclusively with rotary tablet presses. These presses usually regulate the compression force by the amount of die filling. Thus, the uniformity of many quality-relevant parameters of tablets depends on the uniformity of die filling which is carried out by special filling systems, the so-called feed frames, in which paddle wheels supply the powder to be compressed as homogeneously as possible to the dies. Because mass variations often occur during tablet production, which limit the production rate and/or lower the quality of the tablets, the functionality and performance of feed frames is the subject of current research. Therefore, the objective of this thesis was to gain a deeper understanding into the processes and phenomena occurring within feed frames, because the die filling process has a significant influence on the quality of tablets. As so far, no suitable methods for the analysis of the powder supply for die filling within feed frames are available, a novel device needed to be developed. Therefore, the thesis mainly focused on the development and application of such a device for the analysis of the powder surface within feed frames. This device is supposed to enable an in-line monitoring of the powder filling level within feed frames and to provide several uniformity parameters for the characterization of the distribution behavior as well as the surface profiles of the powders within feed frames. The developed device consists of an instrumented feed frame, an electronic control system and software for sensor positioning, data acquisition, and evaluation. The studies which were carried out in the present thesis deal among others with the suitability of the device for the monitoring of the filling level and the repeatability of the measurement procedure. Furthermore, studies regarding the effect strengths of different process factors on the distribution behavior of powders within the feed frame and their effect on the tablet masses were conducted. The obtained data provides information on the causes of tablet mass variations in production processes and enables the optimization of tableting processes with regard to the uniformity of the final product. It was demonstrated that the developed device is suitable for quantification of the filling level and the uniformity of the powder supply for die filling during tableting processes. Furthermore, it was shown that the tableting speed, the paddle wheel speed and the powders used for tableting all influence the filling level and the distribution behavior of the powders within the feed frame. In addition, a decrease of the filling level and a sudden refilling effect within the filling chamber of the feed frame have been observed at the end of tableting runs, which both may affect the tablet masses under certain process conditions. Moreover, initial distribution irregularities of the powders within the filling chamber were detected, which persisted through the tableting process and represent a possible cause of tablet mass deviations. In addition, evidence was found that a certain paddle wheel geometry (roundrod) may contribute to an improved inflow of powder into the filling chamber and may promote the mass uniformity of the tablets.