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

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. Homogeneity across the width of strips produced by roll compaction of microcrystalline cellulose powder (MCC) has been examined by experiments and by three-dimensional finite element modeling. Both measurements and modeling demonstrated the non-homogeneous properties across the strip width, which results from the feeding system, the mechanisms of powder-roll friction and the cheek plates on roll sides. Periodic behavior of compacted strip of MCC.