Simulate Early, Simulate Often... In Rhino
With the advent of 3D printing technology, people have been redefining the way we make things. As small, plastic, oddly shaped objects, rock climbing holds are a seemingly perfect item to 3D print. Hobbyists and companies alike have been trying their hand at 3D printing climbing holds. 3D printing allows for a wide range of shapes and sizes to be made in a one-off fashion, rather than making a large quantity of identical holds from the same mold. Climbing is much more interesting with different shaped and sized holds -- imitating outdoor climbing, where no two rocks are alike.
So why haven’t all climbing-hold-manufacturing companies moved to 3D printing? The answer is, people are hesitant on the strength of 3D printed holds. And rightly so, a 3D printed object is usually not completely solid all the way through. The inside (called the infill) is printed in some pattern with some density, leaving small spaces within the object. Furthermore, since an object is printed in layers, there is some loss of strength as compared to a solid isotropic hold made from casting methods.
While climbers are used to falling, they don’t want the hold breaking to be the reason they didn’t finish a problem. If not expecting a fall, this could also be very dangerous. Therefore, before deciding that 3D printing holds is a good idea, we need to investigate if the hold will fail. Scan&Solve Pro offers the tools necessary to do this. There are not many structural analysis packages out there that can effectively estimate stresses and strains in a 3D printed object, but with Scan&Solve’s ability to analyze orthotropic materials, this goal can be achieved.
The objective of this project is to use Scan&Solve to find the optimal geometry design and best 3D printing parameters for maximum strength. Note that the methods described below can be used for a wide variety of applications, not just climbing holds.
Part 1: Geometry Design (analysis with isotropic materials)
Using Rhino, the hold shown in Figure 1 was created. This hold would offer several different types of hand grips, depending on the orientation chosen when placed on the wall. The goal is to use Scan&Solve Pro to determine the optimal geometry for maximum strength; specifically, if one or two bolt holes are necessary to secure this hold to the wall.
Figure 1: Geometry Design
|Figure 2: sidepull load case, left: force applied on Rhino model, right: technique|
Load Case 1: Sidepull
The first load case chosen simulates the common “sidepull” technique as shown in Figure 2 (right). To simulate someone pulling on the hold towards their body and down, a 75 lbf was placed on the face and direction shown in Figure 2 (left). The faces of the bolt hole were restrained.
Under this load case, the following results (Table 1) were obtained for the 1 hole and 2 hole holds. The max displacement was about 7x higher for the 1 hole hold compared to the 2 hole hold (see Figure 3 for a visual of the displacements scaled by 1000). Additionally, the max Von Mises stress was also much higher -- 191 psi as compared to 94.82 psi for the 2 holes.
|Figure 3: 1000x actual displacement|
Table 1: Values obtained from SnSPro for sidepull load case
|Geometry||Max Displacement||Max Von Mises Stress|
|1 hole||7.34E-4 in||191 psi|
|2 holes||1.23E-4 in||94.82 psi|
Load Case 2: Pocket Pinch
|Figure 4: Pinch load case for pocket hold|
Another possible hand grip--where the climber would pinch the hold with their thumb in the pocket--was simulated by placing a 25 lbf on the pocket face and a 50 lbf on the side face (see Figure 4).
Under these load conditions, SnS produced the results found in Table 2. Similarly to load case 1, the max displacement and max stress were much higher for the hold with only 1 hole as compared to the hold with 2 holes. However, this time there was a 94% increase in max displacement and a 74% increase in max stress.
Table 2: Values obtained from SnSPro for "pocket pinch" load case
|Geometry||Max Displacement||Max Von Mises Stress|
|1 hole||1.76E-3 in||318 psi|
|2 holes||2.87E-4 in||81.5 psi|
In conclusion, for this hold under these loading conditions, 2 holes increases the strength significantly compared to only 1 hole. Therefore, the hold should be produced with 2 holes. Additional simulations can be run to compare holds with varied hole placements, more holds, or altered geometry features.
Part 2: Optimize 3d print parameters for high strength low cost
Currently, FEA of 3D printed objects is challenging as the material properties of the 3D printed part depend on many factors and are also orthotropic. First of all, there are multiple types of 3D printing technologies--FDM, SLA, DPL, etc. While they are all in the realm of additive manufacturing, each type prints in a slightly different way, resulting in different material properties of the final print. For simplicity, and since it is probably the most widely used and inexpensive technology, this blog post will focus on FDM printing.
|Figure 5: Shell thickness and infill %, Left: 20% infill, 0.8mm shell. Right: 80% infill, 3.2mm shell|
In addition to the printing technology, there are many print parameters that affect the material properties. Shell thickness, infill percentage, infill type, and layer height are a few of these parameters. For this study, the layer height and infill type will be held constant, and the infill percentage and shell thickness will be the parameters of interest. The infill percentage is a measure of the material density within the part. Two infill percentages are shown in Figure 5. A lower percentage is desired as it saves material and also reduces weight and print time. The outer shell is printed as a solid material and its thickness can be varied. Again, a thinner shell is desired, as it reduces print time and material cost.
Intuitively, increasing shell thickness or infill percentage should increase the strength of the part; both add material in some way and make the object stiffer. The goal of this study is to use tools available in Rhino and Scan&Solve to estimate the max stress and displacement of a climbing hold printed with varying infill percentage and shell thickness. Using the results, an ideal combination of printing parameters for this object should present itself.
Simulation Setup (For 3D Printed Parts)
To obtain acceptable representations of the stress-infill % and stress-shell thickness trends, 3 different shell thicknesses (0.8mm, 1.6mm, and 3.2mm) and 3 different infill percentages (20%, 50% and 80%) will be used for a total of 9 scenarios.
Step 1: Prepare Rhino Model
Since the shell and infill have different material properties, the rhino model of the climbing hold must be broken into separate components. This was accomplished through the following procedure:
Use OffsetSrf (direction: inside, thickness: desired shell thickness) to create inner polysurface (infill)
Use Shell (thickness: desired shell thickness, select bottom surface as surface to remove) on original polysurface to create shell
Use DupFaceBorder on base of infill polysurface
Use the Surface from planar curves icon on the Surface Tools tab to create a surface from the curves just duplicated from the border
Use ExtrudeSrf to extrude newly created surface by the shell thickness
Join bottom surface and extruded thickness surface and use the cap command to create a closed polysurface
This process was repeated 3 times on separate models to create holds with 0.8mm, 1.6mm, and 3.2mm shell thicknesses. See Figure 6 for an illustration of the rhino geometry for a hold with 1.6mm shell.
Figure 6: Rhino model of hold with 1.6mm shell. Infill (Red), Shell (Yellow)
Step 2: Determine Material Properties
The next, and most challenging step, was to determine material properties. Since the object is printed layer by layer, the 3D printed material can be considered as quasi-isotropic -- the material properties in the in-plane directions are the same, but the out-of-plane (thickness direction) material properties are different. To import a new quasi-isotropic material into Scan&Solve, you must have the following material properties in each primary direction: elastic modulus, Poisson ratio, and shear modulus. The “in-plane” elastic modulus (E11, E22) was found from Optimatter, an online database containing experimentally found material properties based on 3D print parameters, including infill percentage. The “out-of-plane” elastic modulus was estimated by assuming the material is around 20% weaker in this direction . Poisson ratios were assumed to be v12=0.25, v13=v23=0.2 (based on UT El Paso study on tensile properties of PLA additive manufactured parts ). Shear moduli were estimated using the isotropic relationship between shear modulus and elastic modulus . Using these methods, the following values were obtained and imported into SnS.
Table 3: Material Properties
|Infill %||Density (kg/m^3)||E11=E22 (Pa)||E33 (Pa)||v12||v13=v23||G12 (Pa)||G13=G23 (Pa)|
Figure 7: Forces applied to pinch hold
Step 3: Setup simulation in SnS Pro
Once the materials are in SnS, the simulations can be set up and run. All components were added as orthotropic materials using the specify option (see documentation on Using Orthotropic Materials for more info) with the thickness direction from the base to the top of the hold (direction of print). The shells were always added as solid PLA (100% infill) while the interior polysurfaces were added as the desired infill percentage for that run. Restraints were placed on holds. Forces added were 60lbf placed on the right face and 15lbf placed on the left face in the directions shown in Figure 7.
Recall that the goal of this study was to determine the optimal print parameters for max strength. While we want to ensure the climbing hold will not break, we also need to consider the print time and material cost. A part that is very strong but takes much longer to print and costs more than traditional climbing hold manufacturing methods would not be worth it. Cura -- the free slicing software made for Ultimaker 3D printers -- provides estimates for the print time and amount of material used for the selected settings. Results from Scan&Solve and Cura are tabulated below.
Table 4: Results from SnSPro and Cura
|Shell thickness||Infill %||Max Displacement (in)||Max Stress (psi)||Print time||Amount of material (g)||Material cost ($0.03/g)|
As expected, material cost increases the most with higher infill percentages. A similar climbing hold made by traditional methods would retail for around $15. Therefore, since our goal is to drastically decrease the price of a hold, all 80% infill holds do not meet this goal. At around $13 for just material cost, these holds do not provide a significant enough discount to be worth it.
Similarly to material cost, the print time increases quite a bit as the shell thickness and infill percentage increase. The low range print time of 8hrs is still quite long. Depending on the speed settings and type of printer you have, this time may be drastically reduced. For now, let’s just compare times between hold types. Print time increases with shell thickness and infill percentage, as expected.
Turning now to the most important characteristic, strength, it is apparent from Table 4 and Figure 8 that 1.6mm shell thickness significantly reduces the max stress in the hold as compared to a 0.8mm shell thickness. Another important observation is that the 3.2mm shell holds actually had slightly higher max stress than the 1.6mm shell holds. This is counterintuitive, however, it could be due to the limited cross sectional area of the infill. Additionally, as infill percentage increases, the max stress decreases. This is another hypothesis that has been confirmed by Scan&Solve.
|Figure 8: Max Stress Study|
Now that we have analyzed the important characteristics individually, let’s compare between them and search for the best printing parameters that yield high strength, low cost, and short print time. As discussed earlier, all holds with 80% infill are out since they are too expensive. Additionally, all holds with a 3.2mm shell are out since they are not stronger than the holds with 1.6mm shells. Although the short print time and low cost are tempting for the 20%infill/0.8 mm shell hold, the max stress is too high for comfort. That leaves the 0.8mm shell with 50% infill and the 1.6mm shell with 20% infill and 50% infill. Although printing with any of these setting combinations will produce a good hold, let’s increase the force applied to the hold and compare max stresses obtained from SnS to yield stresses from Optimatter.
The new load applied is 300lbf to the right face and 200lbf to the left face (in the same directions). These forces are much higher than you’d expect to see in a climbing hold, but a robust and safe design should have no problem with these loads. The results are tabulated below.
Table 5: Extreme load study
|Infill %||Shell thickness||Max Stress (psi)||Yield Stress from Optimatter (psi)|
Under this extreme load case, a hold with 20% infill and 1.6 mm shell thickness would break (compared to the yield stress of a 20%infill material). A hold with 50% infill and 0.8mm shell thickness has a max stress of 890 psi, which is below the yield stress of 1059 psi. While a hold printed with these parameters would most likely not fail, it is better to be safe and print with the higher shell thickness of 1.6 mm. From this analysis, the optimal print settings for this hold are found to be 1.6mm shell thickness with 50% infill. At $8.76/hold, this is almost half the price of a store bought hold!
In conclusion, Scan&Solve Pro is a great tool for validating your 3D print design. With isotropic materials, you can determine the best geometry for maximum strength. Once you have validated your geometry design, use Scan&Solve’s orthotropic material analysis to determine the best 3D print settings. Note that all results in this blog should be treated in a qualitative matter, as they are all estimates based on assumed material properties. However, even a quick comparison study such as this one can provide quite a bit of insight.