I recently played a game of disc golf with a friend of mine from out of town. I've only played maybe half a dozen games of the sport, but this time my friend gave me some pointers that made me stop and think.

Some Basics

In disc golf, there are multiple types of flying disc to use in different situations; the main three are the 'driver' for going long distances, the 'putter' for the last few meters to the post, and the 'mid-range' for problems in between. I was having trouble getting my drivers to fly far and straight, which my friend told me had to do a lot with speed.

He told me that in some cases, a slowly-thrown mid-range disk would fly further than a driver that wasn't thrown fast enough. I hadn't really thought about the aerodynamics of the discs all too much. I wasn't an enthusiast so to me they were all just 'frisbees' which I realize now is an assertion that could get me crucified in some circles. Each type of disc has it's own special aerofoil shape, tuned for different use cases. Knowing absolutely nothing about disc design, I thought it seemed like it could be an interesting 3D printing challenge.

Goal-Oriented Design

I knew that I was not good at throwing the high-speed drivers, as an untrained disc golfer, but that using a regular mid-range disc to drive with could be difficult. Since I didn't know how to design a proper disc, or how to perform CFD on my models, I attempted to design a hybrid disc somewhere in between a mid-range and a driver. I went off memory of the disks my friend had lent me and tried to make some judgement calls based on my rudimentary grasp of aerofoil geometry.

I think I really just got lucky because what I ended up with wasn't half bad! My disc is essentially a fancy revolved solid, with some Boolean geometry in the platter. My first design took several basic constraints into consideration.

Requirements

• durable and impact-resistant
• ridged enough not to deform easily
• flexible and soft enough to take a beating
• printable within a 256 x 256 x 256mm build volume
• capable of flight
• not terrible

Material Choices: TPU

For my primary material, I chose a standard Thermo-Polymer Urethane (TPU) filament since I wanted something durable and impact-resistant. TPUs tout famously good stats in both categories, with very high layer adhesion as well. They are also notoriously difficult to print with a filament changer like the Bambu Lab AMS I have, so that would limit me to a single-material print. This posed a problem, as I suspected that the TPU I had would be too flimsy to prevent deformation during use of the disc; I wanted to do a multi-material print to include features made from something stiffer (in this case, PETG) to increase rigidity.

Reinforcement with PETG

Since I couldn't print PETG and TPU simultaneously using my filament changer, I decided to design my disc model with two separate parts. I printed the internal "skeleton" part of PETG, ahead of time. I designed the TPU aerofoil with a hollow void in it nearly the exact shape of the skeleton; I had the printer stop halfway though the print so that I could insert the PETG part into the void and then continue printing over it, permanently sealing it inside.

This added additional constraints. The internal part would need to be designed to be insertable mid-print, meaning at least one surface needed to be planar, lest it intersect with the print head's path of travel after the print resumed. The inserted part also could not interfere with layer adhesion too much, since TPU is known not to fuse with PETG during multi-material prints; there would need to be vias through the internal solid feature to allow for continuous structural features to connect the top and bottom of the TPU outer layers and prevent de-lamination along layer lines. This was my first time trying the technique and the results went surprisingly well!

Modeled Geometry as Infill

Another technique I have picked up that I find rather useful is modeling infill directly into the model itself. This can help a lot in circumstances where extra strength is desired in some areas of a part. in this case, I wanted to make sure the part was well balanced, but I also wanted to reduce material consumption. I hollowed out the rim (which would have used far too much material if printed solid) and added an angled spar to support the top surface of the model during printing.

With this method I was able to increase wall thickness and decrease infill, so material generally went more where I wanted and less where I didn't. The final print was 156g total weight.

Some Light CFA

A friend at work was kind enough to perform some Computational Fluid Analysis (CFA) on the profile of the aerofoil formed by my disc model and these were the results:

Overall, more drag than lift. I'm not sure if this is par for the course, or if I just made a really bad disk. It would be useful to know these numbers for a similar product on the market so I can know if the numbers I'm getting here are any good at all. While the results were interesting, unfortunately I did not have enough time on this project to conduct a thorough analysis to compare theoretical performance to actual performance. It might also have been interesting to take a 3D scan of a professionally manufactured disk and then compare CFA simulation results between the two models for a theoretical performance comparison analysis. I did not have the tools for this, however.

Results

Final Thoughts

All in all, this was an interesting project that I may pursue further in the future, though I would attempt to use some other design techniques, and make different materials decisions. It turns out the standard disk is already constructed of pure TPU (no stiffening members required) so I may not need to engineer an over-complicated multi-material print. I want to also figure out how I could print smoother transitions between the top surface and the edge of the aerofoil. I was limited by the maximum acceptable overhang angle printing TPU on my printer with the orientation I chose, so I may experiment with other orientations.

Thanks for reading,
~Joseph

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