A friend of mine and I both have a similar project in mind, the manufacture of custom footwear with our hackerspace’s shiny new multi-material 3D printer. It seems like a match made in heaven, a machine that can seamlessly integrate components made with widely differing materials into a complex three-dimensional structure. As is so often the case though, there are limits to what can be done with the tool in hand, and here I’ve met one of them.
I can’t get a good range of footwear for my significantly oversized feet, and I want a set of extra grippy soles for a particular sporting application. For that the best material is a rubber, yet the types of rubber that are best for the job can unfortunately not be 3D printed. In understanding why that is the case I’ve followed a fascinating path which has taught me stuff about 3D printing that I certainly didn’t know.
A friend of mine from way back is a petrochemist, so I asked him about the melting points of various rubbers to see if I could find an appropriate filament His answer, predictably, was that it’s not that simple, because rubbers don’t behave in the same way as the polymers I am used to. With a conventional 3D printer filament, as the polymer is fed into the extruder and heated up, it turns to liquid and flows out of the nozzle to the print. It ‘s then hot enough to fuse with the layer below as it solidifies, which is how our 3D prints retain their shape. This property is where we get the term “plastic” from, which loosely means “Able to be moulded”.
My problem is that rubber doesn’t behave that way. As any casual glance at a motor vehicle will tell you, rubber can be moulded, but it doesn’t neatly liquefy and flow in the way my PLA or PET does. It’s a non-Newtonian fluid, a term which I was familiar with from such things as non-drip paint, tomato ketchup, or oobleck, but had never as an electronic engineer directly encountered in something I am working on.
A Newtonian fluid has a linear relationship between shear stress and shear rate. That’s dry language for saying that when you press it, it moves, if you press it more, it moves more, and the readiness with which it moves, or its viscosity, is the same across all pressures.
I’m used to viscosity, having run all manner of dodgy old cars I’m particularly familiar with selecting the correct oil by viscosity figure. A non-Newtonian fluid doesn’t have this linear relationship, and its viscosity changes with pressure. For example the non-drip paint has a high viscosity until you press it with a paint brush, at which point its viscosity falls and it becomes liquid enough to spread around. Rubber does this too, and were I to attempt to squeeze rubber filament through my extruder, it would become very viscous and block it up. The closest thing to a rubber I could reasonably use is TPU, or Thermoplastic PolyUrethane, but as you might guess from its name, it’s not a rubber in the same sense as the rubbers I’m looking at, even though it’s what many people use for shoes. It’s flexible, but not grippy.
So if rubber is non-Newtonian and I can’t print with it, how do they mould it? An online search finds specialist plants for rubber extrusion and moulding so it’s possible, but in fact those rubber moulded items you’re familiar with won’t be made with liquid rubber. Instead they press shredded rubber into a mould and heat it so that it fuses, resulting in a moulded shape. I was fascinated to find that the process doesn’t require excessive temperatures, though whether that makes it achievable in a hackerspace is yet to be determined. Has anyone out there experimented with real rubber? Meanwhile I have those multi-material uppers to work on.
Ya, i dont know about all that. If you ever burned a tire it drips as it burns.
That drip is not melted rubber, it is pyrolytic oil. Vulcanization, the process by which rubber crosslinks are formed curing it into a solid, is not a reversible process. Rubber hardens by a chemical process, not a thermal state transition.
In other words, it’s like comparing polyethylene to epoxy cement when it comes to plastics.
Can you 3D print with SBS or SEBS plastic? Lego uses those plastics for its tyres and they are pretty flexible.
Rubber typically hardens with heat. When you heat rubber extrusion the process is more like aluminum extrusion than a 3d printer. Goo goes in, and a solid is pushed out of a heated die in the desired shape.
If you want to form actual rubber your best bet is to first print your part with a flexible filament, then create a mold with a heat durable material like plaster, fill the mold with cushion gum and vulcanize,
3d printers are cool, but not every problem is solved by a hammer.
Might be a good time to review thermoplastic vs thermoset polymers, cross-linking, vulcanization etc.
Oversimplified, thermoplastics are the ones that you can heat up and melt, and they turn into the original material when cooled. Thermosets “cure” in some way, just like how you can’t unbake a cake.
Normal 3D printing uses thermoplastics, while most rubber-like material is thermoset, except for “thermoplastic” elastomers (TPE), one type of which is thermoplastic urethane (TPU).
This.
I don’t think tires are made of rubber crumb like the article claims. Rather, rubber crumb is made of tires.
Real rubber grows on treesin the form of latex, and was famously first vulcanized with heat and sulphur on Charles Goodyear ‘s wife’s stove. Almost all of what you call rubber is something else.
Maybe a filament with some thermoplastic with a lower melting point then rubber with rubber particles mixed in might have qualities that you’re looking for in terms of traction. I vaguely recall some experiments with actual footwear where they tried mixing carborundum powder into the rubber for shoe soles to increase grip on icy surfaces and for where properties, but it turned out that it wore flooring out like crazy.
Didn’t Nike founder Bill Bowerman, Phil Knight’s former track coach, pour rubber into his wife’s waffle iron at first?
Actually I looked it up, and they used urethane
Laser sintering thermoplastic polyurethane powder would be one way, or easier yet, printing a mold and pouring two part PU into it.
The ever fabulous public library of Lebanon Ohio has this wonderful video on YouTube
https://youtube.com/shorts/LNL5tyuhlmw
Rubber doesn’t melt. It’s not a thermoplastic or did I get something wrong?
Maybe Santoprene ?
What about silicone? I know prusa have been working on something there. Theres also PEBA which is grippy.
FilX in Cape Town, South Africa produce a most interesting “rubber” filament called TPR. I have used this with great success. https://www.filx.co.za/tpr/