Experimental and numerical analyses of homogeneity over strip width in roll compaction

Abstract

Homogeneity of properties over the width of strips produced by roll compaction of microcrystalline cellulose powder (MCC) has been examined by light transmission through the compact, by measurements of the porosity distributions and by three-dimensional finite element modeling. Light transmission through compacts revealed periodic heterogeneity in the form of alternate dark and light zones. The period seems to be connected to the geometry of the screw and independent on the feed screw velocity which was varied with the roll speed with a constant ratio. Measurements of the porosity of samples cut from the compacted strip show heterogeneity of the density over the width of strips with a higher density in the centre of the strip and a lower density on the sides. These two techniques clearly showed the heterogeneous behavior across the width of the compacted strip of MCC. However, the light zones (respectively dark zones) did not correspond to the lower porosity zones (respectively higher porosity zones). Three-dimensional finite element modeling (FEM) of roll compaction of powders was conducted with two inlet feed conditions: constant feed pressure and constant feed velocity. Results of the simulations using the constant feed pressure show a uniform maximum principal stress and density across the width of the strip. When a constant inlet feed velocity is assumed the maximum principal stress over the strip width was higher at the centre of the strip and decreases to the sides. This profile also corresponds to the density profile over the width of the strip. In this case, the predicted results present a similar tendency to that found by mercury intrusion porosimetery and are in agreement with the measured bulk density of strips produced with different roll speeds.

The Density Distribution in Ribbons from Roll Compaction

“…For the ribbon produced with cheek plates, the region of highest density follows a sinusoidal curve. This fact was described in literature several times and is usually explained as an effect of the rotating tamping screw , . This hypothesis is further confirmed by data displayed in Fig.…”

“…This was the case for all specific compaction forces. These results are in alignment with findings described in literature [9,10,13,16,17]. To quantify the extent of inhomogeneity, two characteristic numbers were derived from the data.…”

Modifications to Johanson's roll compaction model for improved relative density predictions

“…Michrafy et al [17] and Sinha et al [18]. Note that this constitutive model assumes quasistatic behavior and does not include the effects of interstitial air.…”

Investigating the effect of tablet thickness and punch curvature on density distribution using finite elements method

“…It was found that, by using well calibrated models like Drucker-Prager Cap model, it was possible to simulate correctly the process of compaction. Comparison between experimental and simulated results demonstrated that this approach could be well suited to describe the phenomena occurring during powder compaction (Diarra et al, , 2013Han et al, 2008;Michrafy et al, 2011;Sinka et al, 2004).…”