Effects of the metal type and the roughness of the die wall on the expended work for tablet ejection.
The aim of the present work was to obtain an impression about the consequences for the tableting process when dies of different quality are used. Two hard metal dies and one die made from tool steel, each with a distinct roughness of the die wall, were compared by compacting dry-blended powder mixtures on an eccentric tablet press. The feasibility of the tableting process was assessed by quantifying the ejection force and the work expended by the lower punch during a first phase of the ejection process. With decreasing amounts of lubricant, abrupt permanent impacts were observed leading to conditions at which an ejection was no longer possible. This observation did not depend on the roughness of the die wall used or on the compaction pressure, but strongly depended on the metal type of the die used and on the tested formulation. For the tool steel no difference was found in this respect between the two tested formulations (0.5% versus 2% silica aerogel). In either case, a concentration of 0.3% magnesium stearate was sufficient; however, a concentration of 0.2% magnesium stearate was not sufficient. For both hard metal dies, concentrations of 0.3% (0.5% silica aerogel) and 0.5% magnesium stearate (2% silica aerogel) were definitely not sufficient. Within the ranges above a minimum lubrication, the ejection force and the ejection work increased with the degree of the die wall roughness on a scale comparable to that of the tested formulation factors. In particular, if changes of the die tooling are likely to occur in the life cycle of a product, it is highly recommended that the quality of the dies be considered in the development phase.