Effect of Zinc Stearate on Apparent Density, Mixing, and Compaction/Ejection of Iron Powder Compacts

The behaviour of zinc stéarate (0–4 wt-%) during mixing with iron powder has been studied by monitoring changes of apparent density and comparison with powder prepared by depositing zinc stéarate from solution onto the iron particles. Apparent density was increased by the development of thin surface films at low lubricant contents but was decreased by the formation of thick layers and free lubricant at high contents. Pressure/density data were obtained from floating-die compaction of cylindrical iron powder compacts containing deposited zinc stéarate with and without die-wall lubrication to determine the relative significance of interparticle friction, particle/die-wall friction, and compaction inhibition. An optimum lubricant addition minimized interparticle friction during compaction. This was greater than the optimum observed for loose packing owing to the more arduous conditions prevalent in compaction. Compaction pressure losses associated with interparticle friction were lower than those caused by die-wall friction. The latter was only minimized by lubricant additions which also caused undesirable compaction inhibition. The forces required to overcome particle/die-wall friction were similar for both compaction and ejection.