Benefits of die-wall instrumentation for research and development in tabletting.

Instrumented presses used in tabletting research and development are normally equipped to measure punch force and displacement. Die-wall monitoring is rare, probably because instrumentation and calibration are quite difficult. The authors critically examine the tenets of radial pressure measurement in compression physics. The theoretical background concerning axial to radial stress transmission during the different phases of the compression cycle is presented. The literature reporting on the use of radial stress measurement to assess the self-lubricating properties of materials or the effect of lubricants is reviewed. Examples of interpretation of radial pressure cycles to define the basic material behaviour are given. The influence of particle size and shape as well as that of process and formulation variables on die-wall response are also discussed. Substantial inconsistencies can be seen in the literature with respect to the interpretation of experimental data, often because of the poor reliability of results and mostly because powders are essentially not solid, isotropic bodies. There is also a distinct lack of complementary tabletting parameters that would help understanding their comparative benefits. For this reason, original data on 13 model compounds are presented together with a classification of the materials encountered in pharmaceutical tabletting, based on selected parameters. In conclusion, none of the determined parameters, including those derived from radial pressure measurement, is able, alone, to predict the material behaviour under compression. Although die-wall instrumentation contributes little to the development of improved tablet formulations, it is valuable for characterising the mechanical properties of the materials. This is particularly advantageous given that the mechanical properties account for variations in tabletting performance to a much greater extent than the magnitude of the interparticulate attractions. Nevertheless, because of the peculiar nature of powders compared to solid, isotropic bodies, there is a need to develop new models for analysing their behaviour and to put more emphasis on examination of time-dependent deformation in the later stage of the compression cycle.