Material Anisotropy
Material Anisotropy is the phenomenon that a material will not have the same mechanical properties in every direction[2]. This is especially evident in 3D printed materials, where the part will be much stronger in the direction of printing (shown by red arrows in figure 1), because of this the part will fail much sooner in the direction perpendicular to direction of printing (shown by the yellow arrows in figure 1). Due to this phenomenon, it is imperative that the parts be printed in the direction of the applied force. This way the hook does not fail even under the maximum force that can be applied by the grip force of the average males hand (~500N)[4].
Figure 1: Anisotropy due to print direction
Annealing and Warpage
Since plastic is a poor conductor, different parts of the part cool at different rates causing the part to create stress within the part[3]. In FDM printing, as the filament is laid down, it cools and shrinks. This leaves stress in the direction of the filament, which is detrimental to the overall strength. The internal tensile stress means that in bending, one side is already closer to its yield stress and the part will fail at a smaller load.
Figure 2: How internal stresses are generated in FDM parts
A workaround to this problem is the post-process known as annealing. When a part is inserted into an oven and slowly heated to the glass transition temperature (Tg) the residual stresses in the part are relieved, increasing the ultimate stress of the part[3]. As the stresses are relieved, the filaments contract axially, and expand radially. This causes the part to expand in the direction normal to the layers, and contract in the plane of the layers.
Part and Test Fixture Design
The part chosen for the strength analysis and optimization is the moving hook part. The hook needs to apply a large reaction force at the end, but the pull cable has a very low mechanical advantage. This places large stresses on the area around the pull cable hole. An early test part broke under relatively low load, which prompted investigation the strength of the part.
The parts will be tested under a similar loading to the use case. A reaction force will be applied at the end of the hook, a pivot will allow the hook to rotate, and a pull cable will apply force until the ultimate strength is reached. Figure 3, shown below, depicts the assembly of the testing mechanism.
Figure 3. Early model of a testing setup. The holes in the base are for bolting to a stationary surface.
Figure 4. Sketch of testing apparatus. Force is manually applied and measured by a gauge until the ultimate strength is reached.
Figure 5. Printed test stand with test part. Note the test part is shorter than the original part. The area removed was not limiting the strength, and a smaller part saved plastic on prints.
Figure 6. The test stand (black) and the first round of prints (green).
| Batch 1 | Batch 2 | Batch 2A | Batch 3 | |
| Infill% | 40 | 40 | 40 | 40 |
| Walls | 8 | 8 | 10 | |
| Top/bottom Layers | 14 | 14 | 14 | 18 |
| Material | Tough PLA | Tough PLA | Tough PLA | Tough PLA |
| Mass(g) | 26 | 15 | 15 | 17 |
| Print time(hours per part) | 7 | 3.5 | 3.5 | 4 |
Table 1. Test part parameters
Annealing Results
The Form cure machine was used to heat the part to its glass transition temperature (62C) for one hour. After completing this process, the parts were removed and shrinkage was observed. The shrinking phenomenon can be seen in the figures below. The original part height was 1.03 inches and the final height is 1.06 inches, an expansion of 3.0%. The original length (longest dimension) was 3.25 inches and the final length is 2.97 inches, a contraction of 8.7%. Further tests will be needed to determine if this shrinkage is consistent enough to correct for and maintain tolerances.
Figures 7 and 8. 3D printed part before and after annealing. 2A was annealed for 1 hour at 62C.
Testing Plans
The parts will be tested the week of 4/22 and with those results, we can begin drawing conclusions and possible testing additional prints if time permits. We are currently tracking down a proper force sensor for the tests.
Bibliography
[1] Mechanicalc. “Stresses & Deflections in Beams” https://mechanicalc.com/static/img/Beam/Theory/Optimized/bending-stress-beam.png
[2]
Fabb. “Why Are Isotropic 3D Printed Parts Important?” Fabbaloo, Fabbaloo, 29 June 2017, www.fabbaloo.com/blog/2017/6/29/why-are-isotropic-3d-printed-parts-important.
[3]
“How Annealing Makes Your 3D Prints Better.” Fargo 3D Printing, 9 Nov. 2017, www.fargo3dprinting.com/annealing-makes-3d-prints-better/.
[4] Hand Grip Strength: age and gender stratified normative data in a population-based study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3101655/