Some Rheological Observations in Powder Compaction Process

Abstract

Constructing a packed bed of polystyrene beads in a test die, the axial compaction stress (σ₁) and radial stress to a die wall (σ₂) were measured over compaction procedure. After the piston was fixed on the way to compact, the stress relaxations of σ₁ and σ₂ were also investigated. These two kinds of experiments were performed under a loosely packed bed (experiment 1) and a pre-compacted closely packed bed (experiment 2). The measured temperature range was 25°C (beads behaved as an elastic body) to 95°C (beads behaved as a viscoelastic body).
In the experiment 1, at low temperature, a small amount of stress relaxation was observed due to rearranging mechanisms of powder, because polystyrene beads behaved as an elastic body. However, at high temperature where a viscoelastic behavior was predominated enough, a conspicuous stress relaxation was observed. When the temperature was higher and the compaction speed was slower, the radial stress (σ₂) was observed to be greater, which is recognized as the result of viscoelastic deformation of plastic beads.
In the experiment 2, on the other hand, when the pre-compacted closely packed beds was investigated, higher ralaxation modulus was observed. In this case also, the decrease of axial stress (σ₁) was observed during the relaxation period; however, the decreasing tendency of the radial stress (σ₂) was not so remarkable in this case. At high temperature, the increase of the radial stress was observed due to the rearrangement accompanied viscoelastic deformation of contact parts of particles. However, after increasing period, σ₂ decreased gradually and approached σ₁.