As mentioned previously, when particles possess a platelike morphology, they will tend to align under certain extrusion conditions. The same is true for chopped fibers mixed with the powder (Ref 11, 12, 13, 20, 35, 36, 37, 38). Fibers tend to align parallel to the flow direction when extruded from a converging (i.e., constricting) nozzle, as shown in Fig. 17. When the flow occurs in a diverging nozzle, the fibers tend to align perpendicular to the flow direction. However, for alignment like this to occur, the maximum powder particle size cannot be greater than the edge-to-edge separation between the fibers. The relationship between fiber diameter, fiber volume fraction, and powders size can be derived from a simple geometric model. The maximum allowable powder particle size (A) for fiber alignment is given as (Ref 13):

where D is the fiber diameter and Vf is the fiber volume fraction. Analysis of this equation reveals that smaller powders are required for alignment as the volume fraction of fibers increase. This behavior is illustrated in Fig. 18.

Fig. 17 Fiber alignment during binder-assisted extrusion of short-fiber-reinforced composites. Courtesy of A. Bose
Fig. 18 Maximum powder particle size (A) for fiber alignment in a powder matrix. Source: Ref 13
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