TABLETING
Dry granulation: Understanding
the nip angle
This third and final part of the article series about understanding the nip angle by the thin layer model deals with the correlation of nip angle and powder draw-in
I n Part 1[1] the model and the
possibility to estimate nip angles easily is described. It also outlines that a stronger densification
results inevitably in larger nip angles. Part 2[2]
focuses on the interaction
between nip angle and gap, and that a larger gap mandatory requires a larger specific roll force to achieve the same granule properties.
NIP ANGLE, FRICTIONAL FORCES AND POWDER DRAW-IN In addition to its influence on densification, the nip angle also plays an important role for the draw-in of powder by the rolls. The main driver for draw-in is the frictional force between powder and roll surface. This friction is responsible for conveying the powder towards the gap. Figure 1 visualises schematically the forces present during the roller compaction process based on the thin layer model for one roll. Looking at the forces within the
gliding area (Figure 1a): a loosely packed powder layer with a certain density applies a small force Fi onto the roll. By using the parallelogram of forces, Fi can be split into two components, one normal to the roll surface FN and the other one tangential to the roll surface Ft. In principle, this tangential force counteracts the draw-in which can be expressed by the frictional force Ffr. If the tangential force Ft is larger than the frictional force Ffr, the powder
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is not conveyed by the rolls. This is the case within the gliding area, in which the rolls glide under the powder without any transportation. In the right part of Figure 1 the same powder layer is shown at angle α’ located in the nip area. The layer still has the same density and therefore, the same small force Fi acts onto the roll as well as the same frictional force Ffr. But dividing the forces in its normal and tangential component result is a decrease of the tangential component Ft. In general, the smaller
the angle α, the smaller the tangential component becomes. As soon as Ft is smaller than the frictional force Ffr, the powder is drawn in by the rolls. Once the powder is drawn in, the powder layer is densified, and all forces change. The force Fi applied onto the roll increases and therefore also its tangential component. Even, if draw-in conditions for a densified layer would not be fulfilled anymore, it will still move towards the gap, since the layers on top will enforce its downward movement.
The maximum possible angle
α at which powder will be drawn in depends on the extent of the frictional force between powder and roll surface
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