I recently decided to take on my first ever DIY electric skateboard build which you can see here https://www.instructables.com/id/FIRST-DIY-ELECTRI...
The build was a huge success so i thought id try and go a little further and cast my own wheels. So i thought id create an instructable for both the board and the wheels in case anyone is looking to do the same.
The first step was to decide what size wheels i wanted to cast and look into the best way to do it without spending a lot of money.
I looked into having aluminium molds made but that would have cost a lot of money and with this being some what of an experiment i didnt want to spend that. I have worked on a few projects in the past where i have cast my own parts so i decided to go down the 2 part silicone mold route. But first i needed a wheel to mold so i decided to make one using plaster.
I got a 1kg bag of plaster from hobbycraft and a 110 inside diameter pipe which i believe is used for drains.
Im lucky enough to own a laser machine so i used a piece of 3mm ply and cut a circle the exact same size as the pipe with a circle dead centre to ensure i got the 3d printed insert in the middle of the cast.
The reason behind the 3d printed part i wanted the wheels to be shaped like general wheels where the outside slopes in towards where the bearing is inside the wheel. So as you will see from the picture i created the 3d printed part to give me that when casting.
I then fixed the 3d printed part to the board using hot glue and made sure it was dead centre using the circle id cut. The did the same with the pip around the edge.
I then mixed the plaster and poured it into the mold. When designing the 3d printed part i made sure that the height of the part was exactly half of the overall thickness i wanted the wheel to be so i knew to stop right at the top.
Quick note that spraying the mold with mold release did help with removing the part afterwards.
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Step 1: Plaster Cast Wheel
So once the plaster had set i removed the plaster wheel half from the pipe mold and repeated the process until i had both halves.
As you can see from the pictures they did need some cleaning up which was expected but the good thing about plaster is that its so easily sanded down and smoothed out.
I did a little bit of sanding on both halves before fixing them together, I wasnt too sure how i was going to fix them together but it turned out that just ordinary super glue worked really well so i just used that. Once i had them both fixed together i sanded the whole wheel so it was nice a smooth and then added some filler so fill in any air bubble holes, let that dry and then sanded again until i got a really nice smooth finish.
There were some small air holes still visible but that wasnt too much of an issue.
Step 2: Creating the 2 Part Mold
This is the fun part.
I cut a square piece of foam core leaving around an inch around the outside of the wheel and then used modellers clay to pack out the bottom and up to half way up the wheel. Once id done this and it was smooth i then added the foam core sides to the box. Once i had the box finished i used a pen to create some registration marks top help the two halves of the mold fit together perfectly every time i use them.
Once sealed with hot glue i mixed the silicone. The silicone i used is CS25 Condensation cure silicone rubber from EasyComposites (linked below). This stuff is so easy to use and is a 100:5 ratio by weight, meaning for every 100grams of silicone you add 5grams of the Catalyst that come with it.
This stuff is amazing, it dries fairly quickly, gives a decent pot life and even without a degassing machine it doesnt leave too many bubbles.
I will point out that is is very easy to underestimate how much you need so always buy more than you think you will need. If you do run out though its not the end of the world as you can just add more once its cured as silicone will bond together as if it was one piece.
I dont have a degassing machine so once id mixed the silicone i poured it into the mold from a decent height trying to keep the flow of silicone as small as i could to prevent bubbles and then tapped the sides and the bottom of the mold after id finished pouring to try and dislodge as many bubbles as i could.
Once i had done the first half i removed the foam core and the clay and was left with the wheel and the silicone mold half as one. Add the foam core back to the mold but not the clay.
Adding mold release is key here as if you dont add any the silicone you pour in now will just fix the current silicone and you will have to cut you mold to get your piece out. I used a spray release agent but you can get many different types.
So add a good amount of mold release to all the surfaces and then mix and pour your next batch of the silicone to make the second half of the mold. And let that cure.
Once it has cured you can remove the mold from the foam core box, pull the halves apart and remove your piece.
Silicone can be found here:
Step 3: Time to Cast a Wheel
I needed to find out how much resin i was going to need so i used a trick i learnt a while ago which is to fill the mold half with dry rice, empty it into the cup i was going to mix the resin in and times that by 3. This will give you enough for the whole mold plus a little extra. Better to have too much than not enough.
Before closing the mold ready for the resin i used a stanley knife to cut away some of the silicone to create a funnel for me to pour the resin in and also a small channel for air to be able to escape so i didnt get a huge bubble hole in the final cast.
I also wanted to add something into the mold before i poured the resin to both give the centre of the wheel strength and to give me direction of where to drill the wholes to mount chain sprockets. I had these 3d printed and inserted them into the mold before closing it.
Once i knew how much resin i would need, the 3d printed insert was added and the two halves were fixed together i got to mixing the resin.
I used Xencast PX90 Hard Flexible Polyurethane Casting Resin (linked below) for the wheels as this gives a nice surface and isnt too hard when fully cured.
Like the silicone this stuff is super easy to use as it is a 1:1 ratio by either weight OR volume so essentially mix exactly the same amount of part A with part B.
If you want to add colour which i did with one of the wheels i poured part B into the cup and added the black pigment (also from easy composites). Mixed really well and then added part A. Once id mixed them both really well i began to pour slowly into the mold. The resin doesnt have a huge pot life meaning it will start curing pretty soon after mixing both parts so make sure you have everything set up and ready to before mixing the two parts.
The resin gets HOT and cures from the thickest part to the thinnest part. Always wear gloves and just be careful as they are chemicals your working with after all.
Resin can be found here https://www.easycomposites.co.uk/#!/resin-gel-sili...
I did this process twice, once with black pigment and once without. The casts came out really really well and im so happy with the final parts. I then simply added the bearing and they were done and ready to be ridden.
If you are going to attempt to cast your own wheels or cast anything for that matter i cant emphasise how helpful the guys at easy composites were and cant recommend them enough.
Any questions give me a shout.