New Test Part
The test part for this project is from one of our member’s senior design project. Our original proposal was to test the fixed hook end. Developments in the design project showed that the moving hook piece is the most critical part for strength. So instead of testing the fixed end, the moving hook will be tested. Print time was approximately 14 hours for two parts.
First Prints
The first prints have been completed. Two copies of the moving hook were printed from tough PLA. Both had extra shell layers and top/bottom layers. This should provide a good baseline strength for future tests. These pieces will be destructively tested and the results will inform our next steps.
Strength Testing
To test the strength of the parts, they will be loaded similarly to their actual use case. The parts have three forces acting on them. There is a pivot reaction force at the base, a cable pulling through a hole, and a clamping force on the end of the hook. The pivot will be mounted so that the part can rotate, the end of the hook will push against a ridgid stop, and the amount of tension on the cable will be measured. The target strength comes from a person’s maximum grip strength and the kinematics of the handle.
Strength Target
The average adult male has a maximum grip strength of roughly 500 N[1]. The handle has a mechanical advantage of 5:1, so the target strength is 2500 N(or 562 lbs.), so that a normal person cannot break the handle even with maximum effort.
Potential Improvements
One way to increase the strength in 3D printed parts is by using stronger materials. While PLA was used for this iteration, we may consider testing various materials to optimize strength characteristics. Additionally, we plan to vary the infill pattern to improve the part’s performance. Preliminary printed parts had infill patterns set based on slightly modified default printer settings in the slicing software. Extra shell, top, and bottom layers were added to improve the baseline strength. While increasing the infill density will improve strength in the part, more material is needed making the part more expensive, more time consuming to produce, and ultimately more potentially wasted material. Infill is also not ideally shaped to improve strength in the desired directions, so extra material may be better spent elsewhere. As with any optimizing exercise, the aim is to use the least material necessary while providing the maximum strength. Orientation for the first test was chosen to orient shell layers such that continuous filaments run the length of the part. Additive manufactured parts are anisotropic due to their layer-wise printing method, orientation of the material beads is critical to predicting and minimizing failure in our project.
Post Process Annealing
FDM printed parts can be annealed post printing to improve strength. The process of annealing involves gradually heating the part to its glass transition temperature, at which point the polymer chains begin to move slightly and release internal stresses in the part. One source found an increase in ultimate tensile strength of 30%.[2] This technique works for PLA and several other printing materials. We plan to investigate annealing further and use it for future prints.
Future Plans
The test stand must be printed as well as several more test samples with various settings. Additional samples printed in nylon will be compared. An important step moving forward is determining our load sensing apparatus. Something easy to use and accurate would be ideal. We will move forward with annealing on future prints. And we will continue to look out for other potential changes to improve strength.
References
[1] Hand Grip Strength: age and gender stratified normative data in a population-based study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3101655/
[2]Slavkovi?, Vukašin & Grujovi?, Nenad & Disic, Aleksandar & Radovanovi?, Andreja. (2017). Influence of Annealing and Printing Directions on Mechanical Properties of PLA Shape Memory Polymer Produced by Fused Deposition Modeling.
[3] How to Anneal Your 3d Prints for Strength
https://rigid.ink/blogs/news/how-to-anneal-your-3d-prints-for-strength