Timing relationships among maxima of punch and die-wall stress and punch displacement during compaction of viscoelastic solids.

The times of occurrence of maxima in punch stress, die-wall stress, and punch displacement in instrumented rotary tablet machines are shown to be noncoincidental for compacts exhibiting viscoelastic behavior. Equations are presented which characterize materials behaving as Kelvin solids in either or both distortion and dilation. These equations explicitly relate the timing of stress maxima to punch strain and strain rate. Typical effects are illustrated by data obtained for Avicel PH 101, Klucel, and mannitol. Punch stress maxima are shown to significantly precede the time of maximum punch insertion into the die for viscoelastic materials compacted at rates typical of production. Die-wall stress maxima occur after punch stress maxima due to internal rate-limited processes.