3D printing of short fiber reinforced composites via material extrusion: Fiber breakage

Material extrusion (MEX) is a promising method to manufacture short fiber reinforced composites. However, the dynamic fiber flow process has not been well understood. This paper presents quantitative statistical analysis of the fiber breakage and orientation change during the MEX process and develops a simplified CFD model to understand the experimental phenomena. The PA66 polymer filaments reinforced with 20 wt% chopped carbon fibers, whose average length and diameter are 81 mu m and 5.5 mu m, are used in this experiment. X-ray mu CT scans are used to detect the fibers inside the polymer matrix. Fiber tracing method based on three-dimensional images is used to extract the fibers' length and orientation information. The results show that nozzle diameter has a great influence on fiber breakage, as a smaller nozzle outlet diameter induces higher shear rates and larger volume fraction of high shear rate regions inside the nozzle. A larger nozzle leads to less fiber breakage and better mechanical properties. When the layer height is set to be 0.15 mm, the average fiber length of the bead fabricated using 1.0 mm nozzle is 73.85 mu m, and that of the bead fabricated with 0.4 mm nozzle is only 63.55 mu m. The corresponding tensile strength and Young's modulus of them are 88.0 MPa, 2.53 GPa and 82.5 MPa, 2.36 GPa. The fibers with different lengths have different breakage ratios in different printing stages (extrusion and deposition), and the lengths of the fibers after breakage are mostly 30-50 mu m. The orientation of the fibers inside the deposited bead is related to the layer height. A larger layer height leads to better alignment in the nozzle moving direction but lower tensile strength and Young's modulus due to the higher porosity and less bonding area between layers. When using 1.0 mm nozzle, the average fiber tensors in the nozzle moving direction of the bead fabricated with layer height of 0.10 and 0.15 mm is 0.934, and that of the bead fabricated with layer height above 0.20 mm is about 0.960. The tensile strength and Young's modulus of the specimens fabricated with layer height of 0.10 mm are 89.3 MPa and 2.67 GPa, while that of the specimens fabricated with layer height of 0.30 mm can only reach 73.4 MPa and 2.08 GPa.