Flexible filament (TPU)

Every material so far has been a rigid plastic that you'd rather didn't bend. TPU is the opposite: a rubber you can print. It stretches, squashes, springs back and grips, and it opens up a whole class of parts the rigid filaments can't make — straps, gaskets, seals, phone bumpers, tool grips, machine feet, and flexures that flex thousands of times without cracking. Designing with it means thinking about a property the others didn't have: how soft it is.

Hardness is the number that matters

Rigid filaments you choose by stiffness and strength. Flexible ones you choose by Shore hardness — a scale that says how firmly the material resists being pressed. Most printable TPU sits in the Shore A range: 95A is common and relatively firm (a shopping-cart wheel), 85A is noticeably softer and more rubbery (a flexible shoe sole). Lower numbers are softer and more elastic; higher numbers are firmer and easier to print. The higher end (95A and up) prints far more reliably, so unless a part genuinely needs to be squishy, start firm.

Hardness sets the material's feel, but the part's stiffness is something you design. A 95A strap printed thick and solid barely bends; the same filament printed thin, or with a flexure geometry, is floppy. So you get two knobs: pick the Shore hardness for the base feel, then tune the actual stiffness with wall thickness, infill and shape. That's why the same filament can make both a rigid-ish bumper and a soft gasket.

Printing it: slow, and fed short

TPU prints differently because it's floppy on the way in. Push a soft, elastic strand through a long gap and it buckles like a rope you're trying to push instead of pull, so it jams. The practical consequences are two: print slowly — flexible filament punishes speed more than anything else — and it strongly prefers an extruder that grips the filament right at the melt zone rather than pushing it down a long tube. (Which extruder your printer has is covered in the printer-anatomy section; the takeaway here is that very soft TPU may simply not print well on some machines.) Retraction, stringing and oozing all get fussier too, because you can't yank an elastic strand back cleanly.

Design the flex into the geometry

The reason to reach for TPU is almost always motion — something that has to deflect and recover — and that's a design job as much as a material one. A living hinge, a snap that flexes on every use, a spring that stores energy: TPU gives these a fatigue life that rigid plastics can't, because it bends elastically instead of accumulating cracks along layer lines. The geometry that makes flex work — thinning a hinge, shaping a cantilever, laying out a compliant spring — is the same whether the material is a semi-rigid PP or a soft TPU, and it's covered where those mechanisms live: Living hinges and flexures here in the printing path, and the springs and flexures in the Joints path.

One reminder that carries over from every material: orientation still rules strength. Even a flexible part is weaker across its layer lines than along them, so a strap or hinge that's pulled in service should have its layers running along the pull, not stacked across it. A flexible part torn along a layer line fails as surely as a rigid one.

TPU covers the flexible corner of the material world. The stiff, dimensionally-demanding, wear-hard corner is the opposite extreme — fibre-filled and engineering filaments: Filled and specialty filaments.