The design-of-experiments analysis showed that the three responses were significantly affected by water content and extrusion
type. The usable yield fraction also was significantly affected by the extrusion speed. Pellet size and usable yield were
more influenced by the extrusion system, whereas pellet dispersion was more influenced by the water quantity (see Figure 4).
The pellet size was greatest in the axial system, moderate in the dome system, and smallest in the radial system. The dome
system allowed the highest usable yield and the narrowest pellet dispersion, compared with the other systems. Increasing the
water content increased the particle size and the usable yield, and decreased the pellet distribution. Other studies observed
the favorable effect of water quantity on other extrusion systems, notably the pellet-size increase by agglomeration, the
pellet-dispersion decrease, and the usable-yield increase (2, 5, 9, 24, 28, 29, 32). Moreover, extrusion speed increased the
yield (14). These results can be explained by the previous morphological observations. At lower water quantities, the rods
observed after axial extrusion did not pass through a 2000-μm sieve, and the fine pellets observed after radial extrusion
passed through the 1400-μm sieve, which in both cases decreased the usable yield fraction. A significant interaction between
the extrusion system and the water quantity showed that water quantity had less influence on pellet size in the axial system
(but great influence in the radial system), and little influence on pellet distribution and usable yield in the dome system.
Presenting a compromise between the three responses, the dome system offered the best characteristics in terms of pellet-size
dispersion and usable yield fraction. At high water quantities, the three extrusion systems' results were more similar.
The following equation obtained for elongation is shown in Figure 4:
Pellet elongation was influenced significantly by water quantity and, to a lesser extent, the extrusion system. An increase
in water quantity improved pellet morphology by decreasing elongation. In the same way, some studies showed that increased
water quantity yielded better roundness (2, 9, 11). A significant interaction between the extrusion system and water quantity
showed that dome extrusion gave the best characteristics in terms of the morphology of the granules at lower water quantities.
Water quantity had less influence on pellet elongation for this system. For the axial system, the elongation (manifested by
rods) decreased as the water quantity increased. For the radial system, the elongation at the smallest water quantity (manifested
by nonspherical pellets) also decreased with an increase in water content. Finally, the extrusion dome produced the least
elongation at the lowest water quantity and was not affected by an increase in the water quantity. At high water quantities,
the three extrusion systems provided pellets with similar morphology.
Pellet yield fraction.
The following equation obtained for friability is shown on bar graphs in Figure 4:
Pellet friability was influenced significantly by the extrusion system. The axial system produced the least fragile pellets,
followed by the dome system. The radial extrusion produced friable pellets. The water quantity did not affect the friability
significantly, but the authors observed an interaction between the extrusion system and water quantity. The increase in water
quantity decreased the friability of pellets obtained after radial extrusion, did not influence the dome and axial systems.
Figure 4: The effects of factors and interactions on various responses (Y1–Y8).
The following equation obtained for diametral crushing force is represented on bar graphs in Figure 4:
The pellets' diametral crushing force was influenced significantly by the water quantity and, to a lesser extent, the extrusion
system. An increase in the water quantity improved pellets' resistance by increasing strong bonds, as other authors showed
(24, 28). The axial extrusion system obtained the most resistant pellets, followed by the extrusion dome. The radial system
yielded the least resistant pellets. These results can be explained by the differences in extrudate and pellet densification
between the three extrusion systems. Combined with friability analysis, these results indicated that the axial system produced
pellets with the best strength properties, followed by dome extrusion. The likely explanation is that extrudates and pellets
obtained through axial extrusion are denser than those obtained with the other systems.
Figure 5: Response values according to extrusion method and water quantity for various responses (Y1–Y8).