This guide will be for casting polyurethane wheels. The process will likely work for other tread materials, but may need some adjustments. This is a very common way to make wheels in the combat robotics community. There are other guides for this like the Just Cuz Robotics one, but these will incorporate our personal experiences in casting many wheels and provide additional tips for success.

What are polyurethane wheels

Polyurethane wheels are a custom type of wheel that you can make yourself. They are typically cast urethane around a 3D printed hub. They are substantially denser than foam wheels and have improved grip.

Advantages of Urethane wheels

Good traction relative to other wheel types
Durable

I have had my wheels take direct hits from steel horizontal blades. They have been entirely fine 90% of the time. The only damage typical is a small nick that can be glued back down. These wheels barely wear down on steel floors. Woodfloors do make the wheels wear down much more from driving around.

Customizable

Diameter
Hub
Durometer
Color
Other (Tread patterns, cool text, flap wheels)

Disadvantages

Heavy (~1g/cm^3 of polyurethane)
High time and effort investment
Takes 16 hrs to cure (no day of wheels!)
If you don't mix the urethane correctly, it won't cure and will remain a mess.

Materials Needed

Polyurethane (suggesting vita flex and Reynolds Plastic for locals)
A wheel mold (Suggesting TPU printed)
A wheel hub (Suggesting TPU printed)
A scale
Mixing Cups
A mixing tool (popsicle sticks, toothpick, etc.)
Mica powders
Gloves
Ventilation

Wheel Design

Parts of a wheel

(Motor mounting) Hub (This is what attaches the wheel to the motor)

Wheel Hub
Tread (The polyurethane)

The terminology for wheels is slightly strange with there being two different parts called the hub. Throughout this guide, we will be referring to the part of the wheel that the tread is cast around as the hub or wheel hub. When we discuss the part that attaches the wheel to the motor, we will be calling it the Motor Mounting Hub.

Motor Mounting Hub:

The key things to think about for wheel design are: what motor mounting hub are you going to use, what diameter wheels do you need and how you are keeping the polyurethane on the wheel. The diameter you choose is heavily dependent on your design. I really like these Repeat gold screw hubs because of how adaptable they are. The Just Cuz ones are also good.

Wheel hub and tread:

Almost all cast wheels you’ll see have hooks built into the wheel hubs. This is to provide mechanical connection between the cast material and the wheel hub. In theory you could make a wheel hub just a circle, but the wheel would be entirely dependent on the adhesion between the wheel hub and cast material. Some people, like our very own Caleb Hecht, also integrate a channel down the middle of the wheel to keep the tread from being able to slide sideways relative to the wheel. There are other methods to create a mechanical interface for the tread to stick to. Some people create wheel hubs with the infill exposed so that polyurethane can flow into it. Any method will probably work well. But the hooks definitely will.

It is typically a good idea to make the wheel hub the majority of the wheel, with the tread very thin. Thin treads have a lot of advantages. First, the tread will be solid, which makes it much heavier than the printed hub. Second, thin treads also save cast material, which saves money. Last, it allows you to adjust how squishy the wheel ends up being by varying the print settings.

Caleb’s Wheel Hub and Tread

*This wheel is an example of a hub with hooks and a channel to keep the tread from sliding off the wheel.*

Mold Design

Mold design can be fairly simple. You need the walls of the mold to be the shape of the final wheel and you need a way of centering the wheel in the hub so the wheel is concentric. You can do this with just a peg in the middle of the hub you press the wheel onto. Some people like to use the mounting holes in the motor mounting hub to bolt it to the mold to keep the wheel from floating. Because TPU is slightly less dense than polyurethane, it will float during casting. I prefer the peg method because you don't have to make holes in the mold which urethane can leak out of during the casting process. As long as the peg is a tight enough fit, friction and the layer lines of the prints should keep the wheel from floating. A small weight can also be used to keep the wheel from floating.

*The mold centers the wheel hub via a peg in the center.*

I also like to have a small catch tray built into the molds to catch any extra polyurethane that may spill from the mold if I accidentally overfill it.

The most important thing for a mold is going to be a draft angle. Draft angle is the small angle between the demolding direction and the sides of the mold. Having a draft angle allows the mold item. If there is one thing you follow from this guide in your mold designs, make sure to include a draft angle on all vertical walls. I use a 1-2° draft angle and it works great.

Draft angle and catch tray

This small angled gap between the side of the tread and the mold is the draft angle. It makes removing the wheel much easier. Outside the casting area, I’ve included a catch tray to keep any overflow from going onto my table.

How to make molds that you won’t have to spend two hours destroying when you inevitably forget to put mold release on one.

You should always use mold release before casting. But we all forget at some point. Some of us more than others. Especially, when it’s the night before the event and you’re exhausted from a day of working on robots. To make a mold that you can still remold after goofing up is not too hard. I only make my molds 1mm thick for all the walls. This does make the mold prone to changing shape somewhat if they are handled roughly. But if you are careful, it makes demolding wheels much easier. Then you can carefully slide a hobby knife in between the mold and the tread. Slowly work the knife around the circumference of the wheel. You shouldn’t be cutting, you’re just using the knife to wedge apart the mold and tread. Then flex the mold away from the bottom of the wheel hub. Now you’ve freed your wheel.

Tips for casting

Mold Release

Vaseline as an alternative

A note on PPE and staying safe

Polyurethane is safe once it has cured, but the wheel casting process can have some hazards involved. It can be harmful to get uncured polyurethane on your skin. The fumes from polyurethane can also be unpleasant to breathe. We strongly recommend wearing nitrile gloves and casting in a well ventilated area. Please be safe.

The Casting Process

Design hub and mold
Print hub and mold
Add mold release to the mold (dont put it on the hub!)

If using a spray on mold release

Make sure to follow the instructions on the can. In general, you don’t want to spray too much. The mold release should not be pooling or running in the mold.

If using vaseline

Scoop some vaseline onto a paper towel. Rub it into the mold making sure to cover the surfaces. Take a clean paper towel and wipe out any excess. The mold should have a very thin layer of vaseline.

Mount wheel hub to the mold
According to the instructions on the urethane, mix enough urethane to fill the mold

You can determine how much urethane you need by inputting the density of vitaflex polyurethane, 1 g/cm^3, as the material the CAD. I typically like to mix ~10-15 extra grams of polyurethane. This extra material can be used if you spill any. If you end up with extra urethane, you can leave it in the cup you used to mix it. It will be helpful later.

You can add it 1-2% weight worth of mica powders now if you want pretty wheels instead of the flesh tone that the urethane will normally cure in.
Pouring the urethane

When you pour, it is best to pour the urethane in one place and let it spread out, this makes air bubbles less likely.
MAKE SURE NOT TO SCRAPE THE SIDES OF THE MIXING CUP. Unmixed polyurethane may be on the edges of the mixing cup. If this makes it into your wheel mold, it will not cure. Instead it will remain sticky and gross.
Don't leave your mixing tool in the leftover urethane if you want to use it again, it will get stuck when it cures

Wait 16 hours for the urethane to cure. Put it somewhere where nobody will bump it in the meantime. The urethane should be cured after 16 hours, but it may still be tacky. I like to make my wheels before I go to bed. Then I will come back a couple of hours after those 16 hours are up to demold. Its during this step that the extra urethane you mixed will come in handy. You can use this urethane to check how cured the batch is. It should deform but spring back when you poke it with your mixing tool.
When you’re ready to pull the wheels out, they should simply pop out with a lil pushing on the bottom of the mold.
If they don’t you can slide a hobby knife in the space between the edge of the wheel and the mold to gently free the wheel wherever it is sticking. This process should also work if you forgot mold release and made your molds the way described above.

FAQ

FAQ: Why not just 3D print TPU treads?

TPU is a rubber like plastic you can 3D print. Looking at it, it seems like a great candidate for the tread of a wheel. Unfortunately, despite being rubbery, it is very slippery.

FAQ: How do I cast Nonwheel-Wheels?

You don’t just have to use urethane for wheels. You can also make treads for shufflers! Its essentially the same process, just not round.

Polyurethane Wheel Casting Guide