Work of friction and net work during compaction

Series of tablets were compressed in a reciprocating tablet machine with a gradually increasing die wall friction. The force needed on the upper punch to maintain the tablet dimensions constant increased with the die wall friction while the lower punch force decreased. The change in punch forces due to differences in die wall friction had no effect on the tablet strength. The net work of compaction should be constant under these circumstances. The net work calculated by subtracting the work of friction and the expansion work from the gross work input was constant when the frictional work was calculated according to one of the two equations proposed in the literature (Järvinen & Juslin 1974) while the other appears to give an overestimation of the work of friction.