Squeeze Flow Induced Fiber Alignment in Fused Filament Fabrication of Carbon Fiber and PDMS Mixture

Fused Filament Fabrication (FFF) represents a significant segment of additive manufacturing processes, mainly known for its ability to fabricate complex structures using various materials. Among these, polymer composites integrated with short carbon fibers have gained immense attention due to their potential to exhibit augmented mechanical and thermal properties, positioning them as ideal candidates for high-performance applications in the aerospace and automotive industries. The optimization of these enhanced properties is intrinsically connected to the alignment of carbon fibers within the extruded filament. As current research indicates, this alignment is not a mere consequence of the filament extrusion but is significantly influenced by many factors. Primary among these are the configuration of the printing nozzle and the interactions occurring at the deposition bed. Despite widespread recognition of the merits of these composites, a conspicuous knowledge gap persists regarding the dynamics of fiber rotation during deposition. This paper seeks to bridge this aspect by incorporating experimental and computational methodologies.<br/>Through flow visualization experiments, we observed fiber orientation in multiple focal planes during the extrusion process. This facilitated an in-depth assessment of the alignment angle, indicative of fiber rotations during extrusion. Initially, we examined the fiber rotations as exiting the nozzle, capturing their trajectory and orientation. This was followed by mapping the alignment angles as the extruded filament after the solidification, offering insights into the end-state orientation of the fibers. To foster a comparative analysis, we further performed a numerical flow simulation inside the nozzle and on the deposition bed. The computational calculation was done in a 2-dimensional incompressible Newtonian flow model, which provided velocity information within the filament. The flow field calculation paired with the Advani-Tucker orientation tensors was compared to the fiber orientation obtained from the flow visualization experiment.<br/>We also varied configurations of the nozzle and the deposition bed design configurations. Specifically, by adjusting the gap between the nozzle tip and deposition bed, we introduced different squeeze flows, allowing us to delineate their impact on fiber rotation. Additionally, we varied in-nozzle flow geometries. By comparing the "straight channel" against the "orifice-embedded" nozzles, we manipulated flow fields, and each impacted fiber alignment during the extrusion and deposition process. Nozzle geometry significantly influenced the alignment of carbon fibers in the printed filament. The utilization of a "straight channel" nozzle led to fibers aligning parallel to the heating bed, while the "orifice-embedded" nozzle resulted in fibers aligning perpendicular to it. Moreover, the computational calculation revealed that squeeze flow introduced an additional factor to fiber alignment during the deposition process. In conclusion, our findings regarding nozzle gap, in-nozzle geometry, and squeeze-flow phases play a significant role in the fiber alignment within the printed filament. This study provides insights into the mechanics of fiber alignment in FFF and highlights advancements in customizing polymer composite materials, signaling a significant shift in additive manufacturing material science.