I've started my next project, which is to design and print myself custom shoes. I'm gonna start with something simple: pool shoes.
Pool shoes are typically designed to be close-fitting and put as little material around the foot as possible. So they're ideal to iterate through the design of my shoes to find the perfect fit without wasting too much filament. And I need a new pair anyway...
I ordered a bunch of TPU with different Shore hardness and the rolls have arrived. So now I'm designing the shoes.
I have no experience with TPU, but a colleague at work does: he told me he tried to print a remote-control-like flexible sheet of rubber buttons that was 2mm thick, and the sheet readily delaminated when he pulled on it.
I want my pool shoes to be as thin as possible - apart at the front where I need protection. So bearing in mind what my colleague told me, I opted for a wall thickness of 2.5mm. Do you think this is enough?
As for supports, the printer I use at the moment only has one extruder head (it's not mine) so I'll have to print them out of TPU too. Is there a good strategy to limit the amount of material used and maintain structural integrity on such a large print? I'm thinking of drawing the supports myself inside the shoes to control exactly where they will be and limit wastage.
If your coworker's TPU print "delaminated" easily, either by pulling apart at the layer lines or via the individual print lines tearing within a layer, he was either printing it too cold or too fast, or both. Which is a common mistake. People see the print temperatures listed on the side of the spool that are similar to PLA and assume it can just be printed like PLA. This is not the case if you want acceptable results.
You have to print TPU slowly, and if you want the strongest possible part and don't have any tricky overhangs to deal with, you also want to print it towards the upper end of its temperature range. People will be tempted to lower the temperature as far as possible before the stuff just flat out stops coming out of the nozzle in a vain attempt to combat stringing, but this is a fool's errand. You can never get TPU to stop stringing, so don't even bother to try. Just clean up your finished part with a lighter after it's done.
TPU is one of the very few materials -- arguably the only such material -- capable of being printed by a consumer level printer that, when printed correctly, is functionally isotropic. That is, its strength and properties are the same in all directions. Both along and against the layer lines. TPU that's been printed right sticks to itself extremely well.
You have already correctly guessed that this property makes removing supports from it kind of tricky. I've never been able to achieve supports with TPU that can cleanly tear off like with PLA and other more rigid materials. You're going to have to resign yourself to cutting or shaving them off, at least in some capacity. For the ones inside your shoe, this may present some difficulty.
The others are correct about drying the filament, also. Get a filament dryer that can feed directly into your printer. Even a dinky one will do; you don't need high or precise temperatures for TPU drying. I have the $30 OG Sunlu one and it works fine for me.
I've done many TPU prints that were stressed to hell and they hold up amazingly well. Like 2000 lb shock loads on little 2mm walls, hundreds of times, and not tearing or delaminating. Ninjatek Cheetah and Fiberflex 40D mainly, but others seem to hold up really well too.
I printed wheels for my kid's folding wagon 1.5 years ago or so. They have TPU treads that are around 5mm thick with three walls and 20% infill for some sponginess. They've heald up really well. They've been over curbs, rocks, and tons of other surfaces from 2 seasons.
I did tank tracks in TPU - I've since stopped using it, but not because they broke, but because they keep stretching. Removing one element after 10 minutes of play becomes annoying over time. Though I am somewhat curious how long I could continue doing that before something breaks.
I'm starting to wonder if I might be better off printing a mold out of PLA and pouring liquid rubber into it 🙂 I mean technically, the shoes would be 3D printed and it would probably create fewer headaches, because liquid rubber is what I use now with regular molds to create my shoes, and it works perfectly fine.
I'm a bit concerned that this whole endeavor will results in quite stringent requirements in terms of hardware, and difficulty in manufacturing, because ideally I would like my design to be reusable in developing countries for others who have a need for custom footwear like I do but don't have the money. My plan is to turn my FreeCAD design into a configurable OpenSCAD file eventually, that anyone with an el-cheapo printer and some time can use to make cheap bespoke shoes.
So maybe for cheap easy shoes to happen anywhere in the world, maybe a PLA negative and liquid rubber is in fact a better route.
But for now I'll go with the TPU: I have it so I might as well, and I'll learn something.