Wheel Casting
There are a few guides out there on casting your own wheels for combat robotics or other projects, but they can be hard to find, skip steps, or are otherwise difficult to approach. This guide will go through the full process from CAD to complete wheel.
Before we begin, a note on safety: There’s a wide range of materials out there that can be used for molds, inserts, and treads. Check the safety sheets for the materials you’re using to make sure you’re working with adequate safety gear and ventilation.
Start with your wheel core. For mine, I’m using printed TPU. The critical design aspect for these is the core should mechanically lock to the cast material. Without a feature like this it is likely the cast tread will detach while in use.
With the wheel complete now you can begin creating the mold.
IMPORTANT NOTE: Because this wheel design is 30A urethane cast onto 95A TPU, both of which are flexible materials, it’s not too much effort to get the wheel out even with no draft angle. If you’re using stiffer materials you may need to design a draft angle into your tread profile or increase the complexity of the mold to allow flat surfaces
Create a generic volume for your mold and insert the wheel assembly into it 
As with making the wheel tread use the cavity feature to remove the wheel volume from the mold body 
Add a cutout to your mold blank that will allow you to interact with a portion of the wheel core when it’s in the mold. This will help with post cure removal.
At this point you can work on either the top or bottom of the mold. For this example we’ll start with the bottom.
Cut the mold blank to create the core structure for the bottom half of the mold. Make sure to trim down any protruding alignment features to ensure that when clamped for casting the clamping surface is able to make full contact 
Add an alignment feature to the bottom half of the mold to ensure concentricity during casting. You may also want to add filleted or chamfered features to aid in assembly
The same mold blank can be used to create the top half of the mold. Using the same base file after cutting the wheel volume from it ensures that the dimensions are consistent across parts and the reference points are all the same.
Use the same trim points as you used for the bottom half, but retain the opposite part 
Cut back the top half to include a matching centering feature. The exact geometry here may need to be adjusted based on the tolerances of your 3D printer. In my case a gap of 0.003″ was sufficient for easy fit up 
Adding prying features to your mold will aid in disassembly after curing 
Add cutouts to allow your casting material to flow into the tread area of your mold. I suggest having a reasonable volume above the tread to allow a larger amount of material to be added at one time
Next up is an optional element that will simplify mold clamping and should significantly reduce flashing.
Adding bolt clearance holes to the top of the mold will give you an easy option for clamping your mold halves together 
Add additional bolt clearance holes that line up with your top half clearance holes then add cutouts for a threaded insert/feature. In my case I used small hex nuts and added a matching hex cutout 
When fully assembled this is the cross section of the mold including the TPU insert and tread
At this point your CAD work is done and you can begin printing your wheel cores and molds. You should be able to use CAD to determine how much casting material you’ll need per wheel. Use that to determine how much casting material to use for a batch based on the number of cores and molds you intend to use during a single casting session.
Note: This mold design is using the exact geometries of the wheel components to create the mold cavity. If you want to increase compression on your insert to reduce flashing adding a thin rim to the top and/or bottom portion that is aligned with the outer edge of the insert to give a bit of extra squeeze on your insert will help significantly with that. If you’re using printed molds you’ll likely need to do these in steps of your printed layer height or you may get unexpected compression. For the specific stuff I’m using the flashing cleanup went pretty fast so I didn’t bother including those features.
For anyone wanting to take a closer look at the CAD a .STEP file of the assembly can be downloaded below:
Note: Make sure to add margin to your material calculations to account for overflow and other material use inefficiencies. With small volumes you will likely want to have at least 25% margin, if not more.
Once the wheel cores and molds are printed you can move on to assembling the molds and mixing your casting material. Be sure to follow the instructions on mixing closely. While many materials will cure without perfectly following the instructions the further you get from the intended process the higher the chances of something going wrong. Typically you’ll want to apply a mold release to your mold to reduce the degree to which the cast material sticks to it. Whether or not you’re making a wheel with a draft angle the mold release will help make your cast wheel material stick to just the parts you want it to.
Once you’ve printed your wheel molds and inserted your threaded inserts/nuts you can coat them in mold release 
The wheel cores can now be installed. The stringing here shouldn’t negatively impact the casting as the TPU fibers will be fully within the cast tread 
Attach the tops of the wheel molds applying enough torque to securely seat the top mold section
Once you’ve got your mixing done you’ll want to make sure you can cleanly and accurately fill your molds.
For molds that are small enough where an open pour is impractical a disposable syringe makes a great option for filling them 
I prefer to fill from one side of the mold. This has two benefits – The flow of urethane will meet at a single point reducing the chances of trapped air in the mold and you can easily see when the mold is close to full to moderate how much additional urethane is added 
As the mold approaches full the level of urethane slowly fills the unused openings 
When the unused sides have urethane in them you can stop adding additional urethane to the fill sides 
As the urethane settles the level across the four openings evens up. If you’re impatient or prone to poking at things but don’t want to risk opening the molds too early you can pour some leftover urethane on an easily cleaned surface to give you something to check when it’s time to demold
With the molds filled it’s time to wait for the material to cure. In the case of the Vytaflex 30 I used the recommended minimum wait time is 16 hours.
One quick time skip later:
Once you’ve reached cure time (you can check this by testing your small blob and making sure it’s solid) you can then remove the screws holding the mold parts together 
I like to run a small pointed object along the interface between the urethane and printed mold. This makes it easier to pull the wheel free when demolding as the urethane can squeeze through the gaps. You could also cut the overflow off if you prefer 
The bottom portion of the mold can now be removed and the wheels can be extracted 
With the screws clamping the mold halves together the flashing was thin enough that running a fingernail along the edge of the wheel fully separated the tread from the flashing
With that done, now it’s time to pry the wheel out of the other half of the mold.
Now for the finishing touches, trimming and flashing cleanup
I prefer to use high quality snips to trim off the excess. With this wheel design you can have one side of the snips ride along the TPU core and quickly trim off the leftover urethane 
A few snips later and the wheel is nearly ready to mount 
A crisp edged tool can be drug across the interface between the tread and core to separate any remaining flashing 
With the flashing cleaned up the wheel is ready to use 
With the flashing cleaned up the wheel is ready to use
Hopefully this was helpful. Details on the specific tools/materials used are below-
- 3D Printer: Bambu P1S and A1 Mini
- TPU: Overture High Speed TPU
- Molds: Bambu Matte PLA
- Urethane: Smooth-On VytaFlex 30
- Urethane Colorant: Smooth-On UVO
- Mold Release: Smooth-On Universal Mold Release Spray
- Snips: Engineer NS-04 Precision Side Cutters