Effect of Infill Ratio on the Tensile and Flexural Properties of Unreinforced and Carbon Fiber-Reinforced Polylactic Acid Manufactured by Fused Deposition Modeling

Parts printed by fused deposition modeling (FDM) are used as tool aids and functional prototypes, and it is necessary to improve their mechanical properties to broaden their application. In this study, the influence of the infill ratio on mechanical properties was investigated using polylactic acid (PLA) and 15 wt.% carbon fiber-reinforced PLA (CFPLA). The FDM process was used to produce tensile and flexural samples with infill ratios of 0%, 20%, 40%, 60%, 80% and 100%. The results show that the mechanical properties of both materials were enhanced gradually as the infill ratio increased, and this effect was more pronounced for PLA. The ultimate tensile strength (UTS) was higher for PLA than for CFPLA for all infill ratios; a maximum UTS of 35.65 MPa (100% infill ratio) was obtained for PLA. Conversely, a lower UTS (18.78 MPa) was recorded for CFPLA with a same infill ratio due to (i) higher porosity, (ii) weak interface bonding and (iii) low adhesion of CF/matrix. In general, PLA samples were stiff, while CFPLA was ductile for all infill ratios. On the other hand, PLA was more resistant to bending; with an infill ratio of 100%, PLA reached a maximum value of 78.32 MPa, and it was 43.34 MPa stronger than CFPLA. The analyses of specific tensile strength and specific flexural strength showed that the infill ratio of 50% produced poor mechanical properties for all samples. The infill ratio parameter should be considered when designing FDM parts, especially when the strength-to-weight ratio is important.