Grippy, Hollow, Silicone Tires for Robots and RC

3D printers are commonly available, and they allow the creating of hard plastic parts. However, even with TPU and flexible filaments, the minimum hardness materials that can be printed is still too stiff to make good tires (you may end up with floppy sidewalls, but the material isn't "sticky"). Fortunately, there is a material available at nearly every hardware store for just a couple dollars that is fantastic for this: Silicone. It's sold for sealing bathrooms, but it's also a fantastic material for DIY casting.

When you need a grippy tire for your robot, it's a reasonably quick and easy process to 3D print a mold, and cast a tire. Several other instructables cover this. Here are a few:

• https://www.instructables.com/id/Grippy-robot-whee...
• https://www.instructables.com/id/high-grip-rubber-...
• https://www.instructables.com/id/Casting-Grippy-Si...

These are all solid wheels, and as a result the tires are stiff and heavy. In this instructable I'll present a method for casting tires that are hollowed out. This means they are lower weight and higher flexibility. This should mean the tires perform better in off road and high grip situations. If the tire is too flexible, and the weight of the vehicle deforms the tire too much, you can use foams in the same way RC car tires do.

This tire manufacturing method is suitable for micro-scale rock crawlers, all terrain robots or if you just want accurate scale tires that you can't buy. I wouldn't rate these tires for particularly high speed, so probably don't go putting them on racing vehicles.

In most silicone casting instructions, people mix glycerin or cornflower with the silicone to help it to set. This is because silicone sets slowly (3mm per 24 hours). Because we're only casting sidewalls of ~1mm thickness, you can do this process without any additives - although yes, it does take a bit longer to set. I've only tested this process on small-scale (~3cm) wheels, but there's no reason why it couldn't be scaled up or down. I've found that when sidewalls are lower than 1mm or so, it starts being hard to mount onto the rims, so there is a practical lower limit of about 1.5-cm diameter tires.

I used a similar process to create small tracks if tank-style robots are more your style. The design and molding process for tracks may be the topic of a future ible.

• Hardware Store Silicone Sealant
• 3D printer + filament
• Blender3D and a computer if you want to design your own tires/tread patterns
• Tools to demold such as blunt tweezers, a wooden skewer etc.

The first step is to design the tire and make the mold. I'm not going to teach any modeling software here as you guys probably already have your favorites. My process looks like:

1. Model the overall shape
2. Make a single "lug" of the tread
3. Wrap the tread around the tire, and boolean "union" it so that it's a single object
4. Subtract the tire from a cylinder to form a two mold-halves. The tire should be split around it's circumference. There should be a hole in the cylinder. The profile of the tire will be determined by the diameter of this hole
5. Model a "scraper" This defines the inner profile. It's major features are:
   1. Protruding lugs at the width of the mold so that it can be held aligned while sliding it around the mold
   2. An inner "tongue" that will define the inner profile and wall thickness of the tire
   3. A handle sticking out one side. This makes it easy to grab and slide around the mold.
   4. A bevel on the tongue to help force silicone into the tread and against the sidewalls.
6. Model the rim it will fit onto. It's hard to make one that's easily printable, but most printers can do overhangs of about 1mm without too much issue.

It's probably easier to look at the pictures in this step.

When designing the tire, the more rugged the tread the harder it will be to remove from the mold. That said, you can go pretty chunky, and because it is a two part mold you can put details on the sidewalls of the tire. Try to stay above 1mm or so for the sidewalls. Thinner than that and it will be hard to mount onto the rims.

I don't use alignment pins on the mold because it's hard enough to get the tire out when you can use relative motion between the two mold halves. If there's something locking them together it'd be even harder to remove. Instead I use alignment markers and use tape to hold the mold halves together.

If you don't want to model your own, I've attached the model above both as STL's and as a .blend file (blender 2.8)

Grab your favorite slicer, lay out the parts, and hit print.

The cast retains a lot of detail, so higher resolution prints will give better looking tires. However, I'm impatient so I printed at 0.3mm layer height.

If you haven't found the "horizontal size compensation" or "horizontal expansion" option in your slicer, go looking for it. It means you don't have to put a dimensional tolerance on everything. It means that I can have the scraper be the exact size in the CAD software, and I know it will be a fairly tight (but still movable) fit once printed out.

PLA doesn't stick to silicone, so technically no mold release is needed. That said, because of the shape of a tire tread, adding some lightweight oil can make demolding significantly easier. After applying release agent (or not) all that we need to do to prepare is to hold the two halves of the mold together. For this I used electrical insulation tape because it's easily removable when the mold has set. Make sure the alignment markers are aligned

Then you need to put a bunch of silicone into the mold. I do this with a wooden popsicle stick. I find that trying to be conservative on the silicone is more likely to result in a failed molding where the silicone doesn't get into all the corners, so make sure you add enough.

Finally run the scraper around the inside of the mold, removing excess silicone when necessary. Make sure that the silicone has gotten into the tread as well as you can, and inspect the bead of the tire to check that it will be able to sit on the rim. If too much silicone has come off, you can put some on the scraper and smear it around again.

After a few minutes exposure to the air a thin layer will form on the silicone surface. When this happens, further scraping will peel the layer and it will stick on the scaper. Then when you scrape, the solidified silicone will scratch the interior of the tire and can result in thin and missing patches. To avoid this, work quickly.

If you're in a hurry for the part, you can mix glycerine, cornflour or one of several other additives to the silicone. (as described in other instructables). This reduces the amount of time you have to use the scraper but means you can produce a complete tire in half an hour or so. I found that using accelerants altered the texture of the silicone and resulted in a stiffer compound. Because of the thin walls of these tires, an accelerant is completely optional. If you're casting thicker parts you probably want to use an accelerant.

If you want to color the tire you can mix in various paints. In the past I've used some super-cheap water-colors, and others online suggest acrylics. For this, just put some silicone into a mixing cup with the paint and stir it well.

My tube of silicone claims to cure at 3mm per day. 3D prints are not airtight, so this is 3mm from both sides per day. The thickest silicone on my tire is 2mm thick, so as a result I should let the silicone dry for 8 hours (overnight). If you used an accelerant it could be done in as little as half an hour, but it never hurts to wait extra time. If you rush it, parts of the tread will stay in the mold and you'll have to cast a new tire entirely, so it's worth waiting until it's definitely dry. If you're using real silicone, it will smell of acetic acid while curing. When it stops smelling, it's probably mostly dry.

If you have a chunky tread pattern it will be quite difficult to remove the tire. The tools I used for removing the tire are:

• Blunt-nose tweezers
• A wooden kebab skewer. The sharp end is both pointy and somewhat soft.
• A popsicle stick

Start by freeing up the sidewall. Then stretch the two halves of the mold apart and push in between them to begin freeing up the tread area. Continue working the tire lose. There's no magic here, just patience. Silicone is reasonably tough, but you should still be careful and take your time.
If the silicone separates into an inner and an outer layer, or leaves tread lugs behind, then you didn't wait long enough before de-molding.

Overall I'm very happy with the quality of the resulting tires. While I haven't completed a vehicle with them yet, in my (unscientific) tests, they seem to do quite nicely. Definitely better than anything else I've produced at home before. The detail captured by silicone from the hold is higher than I expected. It's possible that with a high resolution print you could put scale details on the sidewalls.

With this mold design the bead where the tire mounts onto the rim is the same thickness as the sidewall, so you should glue the tire onto the rim to prevent it coming off on lateral forces. With the open mold and scraper method, I don't think that there is a way to have a thicker bead. Perhaps there is another mold design? I did look at how RC car tires are produced, and they have an interior mold. When demolding, compressed air is used to balloon the tire over the inside mold. Perhaps something similar could be done? I'm not sure if hardware-store silicone is stretchy enough to do this, and I'm not convinced I could remove the exterior mold without being able to squish the tire.