Casting Grippy Silicone Tyres

Over the past year, I have been designing and building antweight combat robots. They are a category of combat robot which have to weigh under 150g and must fit into a 4 inch cube. For both the previous robots I built control was one of my biggest issues. I found it really difficult to steer my robot. Either it was way too responsive, or just had no grip and was skidding all over the place.

For my next robot, I decided to build a 4 wheel drive pusher, and to make sure it had as much grip as possible, I wanted it to have silicone tyres. In my previous robots I had either resorted to buying premade wheels , or printing my own and using cross-sections of a bike inner tube as grip. Both these solutions turned out not to be ideal. The premade ones were expensive and only came in standard sizes, and the bike inner tube wasn't at all grippy.

Some other friends in the antweight community had shown me some of the tyres they had made themselves using silicone, and I thought this would be ideal for my next robot. In this Instructable I'll go through how I designed, cast, and built the wheels for my most recent robot with a 3D printed mould and everyday supplies. In particular, I'll be using a silicone sealant and cornstarch mixture for the tyres, popularised by this instructable. It is also possible to use binary mould making silicone to achieve impressive long lasting results.

This process can also be used to make any kind of grippy wheels for any project you might encounter, however in this instructable, I'll be using my antweight robot weels as an example.

Making these Tyres is surprisingly simple, and the most complicated thing you'll need is access to a 3D printer.

Apart from the printer, all you will need is:

• Some clear silicone sealant that can be bought from any hardware store
• Cornstarch
• Some kind of mixing vessel, I used an old piece of Tupperware
• A small spatula or mixing tool, I used a plastic knife.

The first step of the process is to design the mould for the tyre. I did this in Onshape, a browser-based 3D CAD tool, however, you could also do this in Fusion 360 or any other CAD software.

To begin I designed the rest of my robot and worked out how big the wheels were going to need to be. I started a new part studio and made a basic sketch of the rim and tyre that I would revolve around a central axis to create the new parts. I then gave the outer edges of the rim a fillet and extruded an appropriately shaped hole out of the centre for the shaft of the motor. I added treads to the tyre by making a single sketch of a triangle, extruding that from the tyre, and then using the circular pattern tool to repeat the extrude feature around the central axis of the tyre.

To make the mould I copied the tyre to another space in my part studio, and started a new sketch on one of the faces. I used the project tool and drew a smaller and a larger circle for the two pieces of the mould. By then extruding different areas of the sketch in different directions, I created two pieces which would fit together leaving the space for the moulded tyre.

These pieces including the rims were then printed, ready to start the casting process.

To make the silicone, squeeze out about the amount of silicone you think you'll need to fill the mould, you can either do this by pushing on the back of the tube with your hands or the handle of some tool, or by using a caulking gun. Mix in about half as much cornstarch by volume as silicone into it. I have found that you don't need to be very precise about this. The cornstarch adds moisture into the mix and speeds up the curing time of the silicone, but the more cornstarch the less grippy the tyres will be. As long as there's some though, the silicone will cure properly.

Mix this up using the mixing tool until there aren't any pockets of cornstarch and you are happy with the consistency. You should now have a decent amount of silicone putty that can be used to fill the mould.

Using the spatula or mixing tool, plaster the silicone putty against the treaded walls of the base piece of the mold.

Then after making sure there's enough putty, insert the top pice of the mould and squeeze tightly. You could use a vice for this step. The tighter you squeeze the putty, the less cleanup will be needed in the next step.

Ater waiting for about an hour the silicone should have cured enough to remove the tyre from the mould. This can be done by either pressing the slightly extended part of the mould against a flat surface, or by prying it open with a utility knife. It is then possible to pull the part straight out using your hands.

After coming out of the mould there will be some flash (remaining silicone squeezed out of the mould)stuck to the part. You can either remove this using a utility knife, or if it is only thin, by tearing it off with your fingers.

You now have a successfully cast tyre, which can be fitted onto one of the 3D printed rims and used in your next robot. You can reuse the mould as many times as you want or reiterate upon the design for whichever project you may have.

If you liked this instructable please consider liking it and voting for it in the casting contest.