I recently proposed to my long term girlfriend and thought I should find an appropriately nerdy way to mark the occasion. I decided to try and cast a ring. It happened that I had some field's metal (which has a very low melting point) left over from a previous project, so I thought I would try and cast that into a ring using a 3D-printed mould.
I chose a simple Möbius strip design for the ring. A Möbius strip is a simple 2D surface which appears to have two faces, but in fact only has a single face. The 3d version, shown in a video linked below appears to have four faces, but in fact only has two faces, coloured in yellow and green here.
Step 1: Design Your Ring and Then Your Mould
Take any measurements you want from whoever is going to wear the ring, and then create a CAD model of the ring itself. If you are going for a Möbius strip design there is a good video of how to create that design in SolidWorks here.
Once you have the model of your ring, you'll need to create a model of the mould. In SolidWorks this can be done quite easily by creating a suitably sized block, and then doing a "boolean subtract", to remove the ring shape from the mould shape, and then splitting the mould block using a plane.
The slightly complex geometry of the Möbius strip ring meant it was necessary to design a complex splitting plane, so that the ring could be removed from a 2-part split mould without damaging it (i.e. making sure there aren't any over-hung parts of the model).
You'll also need to add a funnel into which you can pour the molten metal, and should also add other riser tubes out of which the air can flow, and when you see molten metal coming out of them you know the mould is full. I forgot to add these riser tubes so had to drill mine out after printing. Video of the mould CAD design here.
Alternatively you can use the STL models which I have generated and included below. They are for a ring with an internal diameter of about 17mm, 4mm wide and 2mm thick. It's probably too large as a ring to wear regularly though (also I forgot to include riser holes as mentioned above, so these will need to be drilled separately).
Step 2: 3D-Print Your Mould
Once you have your CAD design for the mould, save it as an STL file, and 3D-print it (on your own if you're lucky enough to have one, or at one of the many 3D-printing services if not).
The STL files for the mould are included on the previous step, you will need to print two copies of the mould, you can also print a copy of the ring itself for reference, and to see that the ring is a good size before going to the effort of casting.
Step 3: Melt Some Metal and Cast Your Ring
This step involves melting some low-melting point metal, and then handling the molten metal. Please take care, and ask for help if you think there's any chance you need it.
Melt some low-melting point metal, and then pour it into the mould until you see it coming out of the riser/vent holes. Then allow the mould and ring to cool.
The first time I tried this the mould didn't quite seal so I had some leaking metal come out of the side. I found sanding the mating faces of the 3D-printed mould a bit helped. Also putting the mould into a small bowl with ice encouraged the metal to solidify as soon as it came out of the mould (and so self-seal).
Step 4: Remove Ring From Mould and Clean Up
Once it has cooled, separate the two halves of the mould, and remove the ring. I found that I ended up damaging the mould a little when doing so as I had to pry it apart with a small screwdriver (I think I'll only be able to use the mould 3-4 times before having to re-print it).
Clean up any flakes of metal projecting from the ring, and polish to your desired finish. I actually quite like the rough finish from where the casting has settled against the 3D-printed mould, it looks like a wood-grain in places.