Effect of deposition conditions and in-nozzle geometry on alignment of fiber additives in fused filament fabrication

Incorporating low concentrations of fiber-type additives into thermoplastic feedstocks for polymer additive manufacturing processes such as fused filament fabrication (FFF) can enhance mechanical properties of printed parts, thereby enabling superior performance. However, controlling the alignment of such fibers remains very challenging due to the complex interactions between internal nozzle flow and external conditions after filament extrusion. This work investigates a strategy for actively promoting perpendicular fiber alignment by using embedded orifice structures. Flow visualization experiments were carried out using optically transparent filaments in order to investigate this strategy. A suspension of ball-milled carbon fibers in uncured polydimethylsiloxane was extruded onto a heated bed, replicating realistic FFF processing conditions. Real-time fiber orientation angles were measured during extrusion and within the solidified filament. It was shown that the extent of squeeze flow can be controlled by altering the gap between the nozzle tip and the printing bed. In addition, two nozzle configurations were evaluated—a straight channel nozzle and an orifice embedded nozzle. Experimental results were compared with computational fluid dynamics simulations to characterize fiber rotation dynamics. A key finding of practical interest is that the alignment of fibers in the printed part can be controlled by adjusting the in-nozzle channel geometry and the gap distance. The findings provide valuable guidance into optimizing FFF parameters for producing high-performance polymer composites with fiber additives.