If you’ve ever injured yourself, whether you’ve broken an arm or a leg — you are probably familiar with the astounding inconvenience that comes with having a plaster cast. Wrapping it in plastic whenever you shower, accumulating a pallet worth of stains and dirt on the surface and that one itch you can never get.
Making casts this way seem pretty irrational until you consider their redeeming quality that makes them significantly better than their alternatives — their customizability. Wrapping a broken leg in plaster, for instance is much easier than trying to fit any other kind of durable material around it.
However, let’s pretend for a second that you could somehow image your leg. Then well, you could print a cast in the exact shape of your leg using whatever material you’d like.
The material could be water resistant, perforated, glow-in-the-dark — really whatever you want it to be.
How? Using 3D printing!
3D printing is a kind of additive manufacturing that has to do with creating an object by slicing it into many cross sections and printing these cross sections one on top of the other.
In this article, we are going to cover how to design a 3D printed leg cast (although, without the fancy imaging part).
To design the cast, I used a program called TinkerCAD which is a free online CAD (computer assisted design) platform which you can use to design 3D printable objects.
TinkerCAD has a library with a bunch of different shapes, numbers and letters which you can use to build basically anything.
But since they don’t have a leg brace just lying around we’ll have to use segmentation — i.e breaking the larger cast up into smaller shapes and then putting them all together.
Step 1: The Outline
The outline of the cast can be broken down into two paraboloids and a cylinder, like so.
- The long paraboloid has a diameter of 73 mm and a height of 190 mm.
- The shorter paraboloid has a length of 53 mm, a width of 73 mm and a height of 68 mm
- The cylinder has a diameter of 49 mm and a height of 121 mm
Then, we merge all the shapes.
Now, these two shapes are independent and since we want them to be one collective shape, we’ll select both of them and press the group function. This basically sticks them together so if you move one, you move the other.
Step 2: The Interior
Now, of course, we need the cast to be hollow and have no bottom. So, we’ll duplicate the shape and decrease it’s length and width by 4 mm (two on each side). We’ll then make it a hole and merge it into the body of the solid shape.
A hole is essentially negative space, so whenever it is merged with another object, it will make a hole of that shape in the object.
Kind of like how when you walk through a snowbank, your boot makes a hole which is shaped like your foot in the snow.
Step 3: The Bottom Surface
After the rim of the cast flairs out, we want it to curve a little bit, so we’ll add the outline of an oval to the bottom.
To do this, we’ll take a cylinder, shrink it down to a height of 5 mm, make its length 52 mm and it’s width 76 mm . We’ll then duplicate it, decrease it’s length and width by 4 mm, make it a hold and merge it with the original one. Then, of course, group the two objects.
We’ll then lift the remainder of the cast up by by 5 mm — and place it on the outline.
Step 4: The Smaller Holes
Now in order to fasten the cast around the leg, we need holes which the straps can be threaded through. We are going to make three holes, each with a length and height of 40 mm.
To do this, we’ll take a cube, change it’s length and height to 40 mm and increase its width to 80 cm. Turn it into a hole and duplicate it twice, so there are three identical holes. Then, we’ll put these holes through the back of the cast so that they poke out of each side and group everything.
Next, we’ll cut the cast down the middle to separate it into two halves. To do this, we’ll make a rectangle with length and width and height, make it a hold and group it through the center of the cast.
Finally, we’ll add a little hole in the front shaped like a rectangle. We’ll take a cube, change it’s length and width to 8 mm and 16 mm and its height to 20 mm. Rotate it by 30 degrees (or so it’s roughly parallel with the surface of the cast), make it a hole, merge it with the cast and group everything.
Now, of course, this cast won’t fit perfectly on everyone’s leg, but it is a basic template for a cast that can be customizable to anyone’s leg. And, depending on what materials you print it in, it might solve all the inconveniences you get with your current cast!