Wall friction and its effects on the density distribution in the compaction of pharmaceutical excipients

International audienceThe effect of powder-die wall friction during the compaction of pharmaceutical excipients has been investigated for three modes of lubrication: lubricated die, non-lubricated die and with the lubricant mixed with the powder. Coulomb friction is assumed and the wall friction coefficient was evaluated from the transmission ratio (applied pressure/transmitted pressure), the transfer ratio (radial pressure/axial pressure) and the aspect ratio (height/diameter of tablet). The friction coefficient of three pharmaceutical excipients was measured with respect to the relative density of the tablet by means of an instrumented press. It was found that the behaviour of the friction depends on the powder and the lubrication mode. Mixing the powder with a lubricant reduces the friction with respect to that of the lubricated die, but the evolution of the friction coefficient with the densification is different. The effect of the wall friction on the axial density distribution in the tablet was investigated by experiment and by modelling. The model was based on Janssen-Walker analysis coupled with the Heckel equation. For comparison, only the single action compaction in a non-lubricated die was considered. It was found that the measured and predicted axial density decrease from the top to the bottom of the tablet. Moreover, the predicted and measured density had the same tendency, but different values. However, the analysis should not be applied to the compaction of the powder mixed with lubricant because no physical parameter for this mode of lubrication is taken into account in the model

The effects of ambient temperature on the compaction of pharmaceutical powders

International audienceThis article presents an experimental study of the effects of raised ambient temperature in dies and punches on the compaction of pharmaceutical powders. The experiments use an instrumented hydraulic press having a temperature-controlled enclosure allowing the ambient temperature of die and punch to be varied from 20 to 57 degrees C. A pharmaceutical powder was compacted at temperatures in this range and mechanical parameters, such as stress transfer ratio, stress transmission ratio, and die-wall friction, were analysed to examine the effects of heat transfer between tools and powder. In particular, it is shown that increasing the environmental temperature of die and punch increases the transfer ratio and the die-wall friction. The radial pressure is also slightly increased at the first stages of the compaction. However, the stress transmission is reduced by increasing the temperature. This may indicate an increase of shear stress. It is also observed that the particles undergoing compaction are `softened' by increase of the temperature. This softening is certainly due to rise in temperature of the powder generated by the compaction and by the heat flux transfer between the die and the tablet. It is suggested that these effects could be important in industrial tablet production installations without air conditioning and thus subject to variations in ambient temperature

Modelling of pharmaceutical powder compaction and its sensitivity to powder properties and tablet curvature

National audienc

Compression des poudres pharmaceutiques : analyse expérimentale et numérique de la distribution de densité

National audienc

Pharmaceutical powders compaction: Experimental and numerical analysis of the density distribution

Issu de : 4th French Meeting on Powder Science and Technology, Compiègne, FRANCE, May 4-6, 2004International audienceThis paper investigates the axial density distribution during the single action compaction of pharmaceutical powders in rigid dies. Experimental results of the compaction of microcrystalline cellulose are compared to those predicted with an analytical model and with numerical analysis. The axial density distribution within tablets compacted at different loads are measured using a mercury porosimeter of slices from tablets. The analytical model based on the Janssen–Walker analysis coupled with the Heckel equation is applied to determine the axial density distribution. Representing the mechanical behaviour of the powder compaction with the Drucker-Prager/Cap model and by measuring the die wall friction, computation of the axial density within the tablet is also obtained by solving the boundary problem with finite element method for the loading–unloading cycle.Results of the axial density obtained with the analytical model and numerical method at end of the loading step show a qualitative agreement with the measured data. However, the measurements are better predicted with the numerical results obtained with the simulation of the loading–unloading phases