1. Project Outline
The primary objective of this project is to explore the mechanical anisotropy in parts fabricated using Fused Filament Fabrication (FFF) Printing. This investigation focuses on understanding how the material type and the printing process parameters influence the mechanical properties of the printed parts. Our approach encompasses two main activities: printing sample parts with varied parameters and conducting tensile testing on these samples. Utilizing the FUSED FORM minilab printer at the Polymer Engineering Center, we have successfully completed the preparation of Polylactic Acid (PLA) samples. So we finally did the Tensile Test and analyzed the results. The tensile tests were performed on each sample from the two batches using the tensile testing machine available at the Discovery Building (Figure 1).
Figure 1. a) Tensile Testing Platform b) Samples
2. Tensile Test and Discussion
The tensile tests were performed on an Instron 5967 universal testing machine with a 30 kN load cell at 5mm/min extension rate. Type V specimens with a thickness of 2 mm. According to ASTM D638 standard, we used Type V specimens with a thickness of 2 mm [1]. However, the actual thickness depends on the printing quality, varying from 2.1 mm to 2.4 mm. For the first batches of samples, the measured tensile properties are two orders of magnitude lower than those of the PLA filament, and there is no significant effect of any printing parameter on the mechanical properties of the printed specimens (Table 1 and Figure 2). The reason would be attributed to the setup of the universal testing machine.
Table 1. Tensile Test Results: 1st Batch of Samples
Figure 2. Tensile Test Results: 1st Batch of Samples
After that, we printed 2 more batches of samples and did tensile tests. Figure 3 shows the stress-strain curve of the 2nd batch of samples, where the solid line and dotted line correspond to 0° and 90° printed samples, respectively. The mechanical properties derived from the stress-strain curves are consistent with the mechanical properties of the filament. It is obvious that 90° printed samples generally have better mechanical properties, such as tensile modulus and ultimate tensile strength (UTS), than 0° printed samples, confirming our hypothesis that the direction of voids would affect the mechanical anisotropy.
Figure 3. Tensile Test Results: 2st Batch of Samples
Table 2 summarizes the overall mechanical properties of PLA specimens, and the details are also presented in the bar charts as shown in Figure 4. For specimens with 0° printing angle, the tensile modulus significantly increase with layer width (p < 0.05) and layer height has a significant effect on elongation ratio (p < 0.05). Both layer height and width have a significant effect on UTS (p < 0.05) and UTS increases with increasing layer height and layer width (Figure 4b).
Table 2. Overall Mechanical Properties of PLA Specimens
Figure 4. Analysis of Effect of Printing Parameters on the Mechanical Properties of Specimens
It is counterintuitive that increasing printing resolution decreases the mechanical properties of the specimen. However, we find that the increase in tensile modulus and UTS is a consequence of printing defects inside the specimens. Defects play a key role during failure of the specimens. If the radius of curvature of a defect increases, the UTS and modulus of the sample will increase because stress is less concentrated in the sharp region, and crack initiation will require a larger energy and higher K1c value. Decreasing printing resolution increases the radius of curvature of voids, so larger modulus and UTS are obtained, which can also be confirmed by our fracture surface analysis. It should be noted that the direction of the defects or voids are important because we did not find a similar increase in modulus and UTS for 90° printed samples (Figure 4c). For 0° printed samples, the defects are perpendicular to the tensile direction, while for 90° printed samples, the defects are parallel to the tensile direction. For 0° printed samples, none of the printing parameters has a significant effect on the mechanical properties (p > 0.05), probably because the quality of the narrow section of the sample varies a lot for different batches of samples (Figure 4c).
The mechanical anisotropy of these specimens is investigated in Figure 4d. These three anisotropic ratios are generally larger than 1, except for A?. It is expected that if PLA chains are aligned along the shear direction, the elongation ratio will decrease, whereas tensile modulus and UTS will increase. Nevertheless, the qualities of the samples vary a lot among each batch for 90° printing angle, so it is difficult to validate our assumption. Additionally, due to the impact of defects, layer height shows a significant effect on A? (p < 0.05). In coincidence with our expectation, the increase in printing resolution also increases the anisotropic ratio of strength. For AE and A?, none of the printing parameters contribute to a significant effect.
In addition, the fracture cross-section under tension also reflects the anisotropy of the specimen. As shown in Figure 5, the fracture cross-section of the 0° printed sample (top) is approximately 90° to the loading direction, which is similar to isotropic materials (Figure 6) . The fracture surface of isotropic materials usually forms along the plane of minimum principal stress, meaning that the fracture surface typically aligns perpendicular to the direction of force, forming an angle of approximately 90°. However, when we observe the fracture cross-section of the 90° printed sample (bottom), we find that the fracture surface is approximately 45 degrees to the loading direction, indicating that the specimen’s load-bearing capacity in the 90° direction is lower than in the 0° direction.
Figure 5. Fracture surfaces from tensile testing of 0° printed samples (top) and 90° printed samples (bottom).
Figure 6. Analysis of fracture surfaces from tensile testing of 0° printed samples
3. Conclusions
Our investigation into the mechanical anisotropy of 3D printed components using polylactic acid has provided significant insights into the influence of various printing parameters on mechanical properties. By systematically varying the build orientation, layer height, and feed rate, our study has revealed that layer width plays a crucial role in enhancing the tensile modulus, confirming that increased layer width leads to a higher tensile strength. Additionally, our results indicate that layer height is pivotal in affecting the elongation ratio, showing that with an increase in layer height, there is a corresponding increase in elongation.
Moreover, the ultimate tensile strength was found to increase with a decrease in printing resolution, which involves an increase in both layer height and layer width. The defects and their geometric characteristics significantly impacted the failure modes of the printed components. Particularly, an increase in the radius of curvature of defects was associated with an increase in ultimate tensile strength, underscoring the importance of defect management in optimizing part strength.
While none of the printing parameters showed a significant impact on the mechanical properties under a 90° printing direction, this could be attributed to variations in sample quality across different batches. Additionally, our findings suggest a potential interaction between layer width and layer height that may influence the elongation ratio, although this evidence is not robust.
In conclusion, our study underscores the critical importance of understanding and controlling the printing parameters and defect characteristics to optimize the mechanical performance of PLA components produced via Fused Filament Fabrication. These insights pave the way for transitioning 3D printed parts from mere prototypes to functional end-use products capable of meeting specified mechanical requirements.