Sunday, 4 October 2020

Casting Silicon Tyres.



I'm definitely into developing robot stuff at the moment and after the MTV robot project, where I built everything apart from the chassis, I wanted to build some chassis ideas for a smaller platform. The first attempt is above, a CNC'd folded aluminium chassis with 3d printed wheels on which are riding DIY glow in the dark silicon tyres!

I wanted to share my process, the first thing to know is that I've used some really cheap silicon, the type of silicon used in bathrooms as seen above. The important thing is that it is pure silicon with the acetic acid smell as it cures. If you open the tube and it smells very vinegary you have the right stuff!

I've been using about this much silicon per batch, it's about a golf ball size blob and is plenty for a set of 4 small tyres. To it I am adding a pea size amount of acrylic paint, and I happened in these first batches to use some glow in the dark acrylic paint. It doesn't seem (even with different colours) to matter if you put more in than a pea size amount as the tiniest blob seems to colour the silicon well. I'm also adding a few drops of glycerin, the glycerin seems to thin the silicon slightly but certainly seems to make it flow a little easier.
I simply CAD'd some moulds, they are in 2 parts and the small pip and hole correspond to ensure that the mould and the core are centralised creating an even tyre. In the photo above you can see that I've packed the mould with some of the mixed silicon before squeezing the mould core in. 


Squeezing the two parts together you get a lot of material pushed out of the sides, which you can scrape off and put into the next mould. Make sure that the moulds are fully closed so the walls of the tyre stay nice and vertical. 

Above you can see the filled moulds, I left the first set for around 12 hours before turning them out but later sets have been happily de-moulded after around 3 hours. At 3 hours the tyres are strong enough to be removed and handled but still smelt quite strongly indicating that they aren't fully cured. 

Pulling out the mould core they seperate cleanly and the tyres are nicely formed. Having made about 12 of these now I haven't had any with any air bubbles or defects. They need a little cleaning as they tend to have a little "flash" silicon around the edges but it's easily removed. The tyres are strong yet very soft and grippy. I'm planning to experiment with fitted tyres and also treaded tyres in the future but for now these tyres fix to wheels very well using a thing film of super glue. They glow nicely as well! :) 

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