Robert Bradely, Quinn Burzynski, Asher Elmquist, Mitchell Langer
Overview of Current Progress
To continue investigating the stiffness of 3D printed parts, additional samples were printed using FormLabs elastic material. This introduces an additional direction of discussion specifically focused on these same properties when compared within a highly flexible material.
Based on feedback given by the teaching assistant in response to the first project update, addition samples made from Tough material were printed, but tested prior to the post-curing operation. This was suggested to understand the layer properties and how these may affect the part as a function of the post-curing process. Uncured flexible parts were also printed and tested. The results are discussed below.
Table 1. Samples tested for three separate materials. For the Elastic and Flexible material, the vertical print orientation is not possible with the equipment available.
Experiments Run/Parts Tested
The uncured Tough material samples are shown in Fig. 1 below after testing was performed. For parts that split on failure, the cracking occurred along the interior cross geometry and resulted in a more ductile failure than the fully cured Tough samples. Where the cured parts failed and displayed cracking between the print layers, the uncured parts did not have any visible cracking between the print layers.
Figure 1 – Uncured Tough material samples after compression test.
The post curing in the SLA print process serves, in part, to reduce the part anisotropy. As a result, it would be expected that parts tested without this post-curing process would show an increased effect of print orientation on sample stiffness. These results are shown in Fig. 2 where each print orientation for cured samples show no difference in part stiffness. For uncured samples (blue), the part printed horizontally shows the largest stiffness whereas the vertical part shows the smallest stiffness.
Figure 2 – Print orientation affects the part stiffness only for the uncured samples.
The second material that was printed and tested was the Flexible SLA material at the MakerSpace (Fig. 3). Figure 4 shows the stiffness curves for two print orientations and three different cross section geometries. Note that vertical printing was unachievable for this material. While the uncured Tough material demonstrates a strong correlation between print orientation and part stiffness, the uncured flexible samples show a smaller effect. For both materials, the horizontal print resulted in the stiffest parts. It is also clear that the flexible material behaves very nonlinear, meaning that the stiffness changes as function of displacement even in the elastic region. One of the most interesting takeaways from this test is that the print direction influenced the ultimate strength of the part. For the large and small cross geometry, the diagonal print orientation withstood a higher force before complete failure.
Figure 3 – Printed Flexible material showing print orientation and cross section geometries.
Figure 4 – Flexible material dependence on print orientation and cross section geometry
The stiffness of the flexible material appears to have a nonlinear stiffness even for small deflections. One interesting test that was completed was the repeatability of the part deformation for the solid cross section printed in the diagonal orientation. The part was compressed to approximately 8000N in five trials as seen in Fig. 5. The part went through an extreme amount of nonlinear deformation, yet remained almost completely elastic. Designing and modeling spring systems with this kind of material nonlinearity can be very challenging but also can provide desired characteristics for certain applications. Material nonlinearity can help dissipate energy at low amplitudes and provide high stiffness at high amplitudes. This source of information is especially useful in the aerospace industry in the design of wings and fuselage components.
Figure 5 – Repeatability investigation was completed to investigate the behavior of nonlinear stiffnesses.
While the diagonally printed part experienced no failure during the five trials, the sample which was printed horizontally failed before 8000N. The failure experienced was a splitting along the direction of print. Figure 6 shows the failed part (left) and the flexible part after 5 compression trials. To understand if this is systematic and the result of the print direction, or simply a part defect, additional parts with the same parameters will be printed and tested. This behavior will be commented on further in the final report once additional experiments are conducted.
Figure 6 – The horizontally printed flexible sample (left) failed after a single compression test. The part printed diagonally shows a repeatable nonlinear stiffness across five trials without failure. The repeatability and effect of print orientation will be tested further and commented on in the final report.
The third material that was printed and tested was the Elastic SLA material at the MakerSpace. Figure 7 shows the stiffness curves for two print orientations and three different cross section geometries. Note that vertical printing was unachievable for this material. There appears to be a strong difference in elastic stiffnesses due to the print direction. It is also clear that the flexible material behaves very nonlinear, meaning that the stiffness changes as function of displacement even in the elastic region. These properties are consistent with what was discussed above for the Flexible material. As discussed for the Flexible material, ne of the most interesting takeaways from this test is that the print direction influenced the ultimate strength of the part. This statement is consistent with the elastic material, however, an opposite result occurs. The horizontal print orientation withstood a higher force before complete failure. The reason for this is still unknown but it will be something worth investigating further for the final report.
Figure 7 – Elastic material dependence on print orientation and cross section geometry
Per the first project update, two coil springs were created: one with SLA Tough material, the other with black ABS using an FFF process. The springs are shown in Fig. 8, with their corresponding stiffness shown in Fig. 9. As these comparisons do not show specific features, and given that the same material cannot be used between the two processes, no further comparisons between FFF and SLA will be made in relation to this project.
Figure 8 – Black ABS FFF (left) and Tough SLA (right) printed coil springs
Figure 9 – Force deflection curve showing stiffness of two additively manufactured coil springs