Experimental evaluation of friction effects on the unloading curve during powder compression

Powder compression plays a critical role in pharmaceutical tablet manufacturing, where understanding the mechanical behaviour of powders during compaction and unloading is essential for optimizing process efficiency and product quality. One key factor influencing the unloading curve is friction between the powder and the die wall, which affects stress distribution, elastic recovery, and overall tablet integrity. This study aims to experimentally assess the effect of friction on the unloading curve during powder compression and evaluate the impact of external lubrication on this process. Four powders—ibuprofen (IBU), microcrystalline cellulose (MCC), mannitol, and sodium chloride (NaCl)—were selected based on their distinct deformation mechanisms. Compression tests were conducted using a universal testing machine, with and without external lubrication (magnesium stearate in ethanol), while wall friction measurements were obtained using a powder rheometer. The results demonstrated that lubrication significantly reduced friction coefficients and altered unloading behaviour, but the extent of this effect varied between materials. MCC exhibited the most pronounced improvement, with reduced elastic recovery and a smoother unloading curve. In contrast, NaCl and mannitol, displayed less predictable effects, suggesting that lubrication alone does not fully stabilize their decompression behaviour. Notably, IBU showed minimal response to lubrication, indicating that its elastic recovery is primarily dictated by intrinsic material properties rather than die-wall friction. These findings highlight the necessity of tailoring lubrication strategies based on powder-specific properties to optimize tableting performance. Future research should explore the combined effects of internal and external lubrication to further enhance process control and tablet quality.