Orientation and bending/breakage of short fibers in fused deposition modeling additive manufacturing: formation mechanism and control principle

In fused deposition modeling (FDM) of short fiber reinforced composites, the fiber orientation and breakage play an important role in the properties of fabricated part. Here, a multiphase flow simulation verified by XCT test is employed to investigate the extrusion process of short fiber reinforced polymer in nozzle during the FDM, focusing on the flowing, rotating, and bending of fibers. During the extrusion process, overall, short fibers gradually rotate and align with the nozzle axis and then the printing direction with a decreasing orientation angle, meanwhile accompanied with fiber bending. The melt flow in nozzle is axisymmetric and featured as a radially shearing flow. If the isovelocity circle of shearing flow is tangent to the extension line of a fiber, the highest speed locates at one end of the fiber, leading to fiber rotation and alignment with the nozzle axis. If the isovelocity circle is directly tangent to a fiber, the highest speed locates on the fiber between two endpoints, leading to the fiber bending and even breaking. The probability of fiber bending increases as fibers approach the nozzle axis or longer fibers are employed. Additionally, the increase of velocity gradient of shearing flow improves both the fiber rotating and bending. Finally, the principles to control the fiber orientation and bending/breakage are proposed and verified by tests with different nozzle sizes. This work is beneficial for understanding dynamic behaviors of short fibers during the FDM and then controlling the fiber orientation and breakage in fabricated part.