What every filament on this shelf has in common is that it is elastic at room temperature and still melts like a plastic, which ordinary vulcanised rubber cannot do. The hardness printed on the label is a consequence of the internal ratio of rigid to flexible segments, and moving along that ratio is the only thing that separates one grade from the next.
So this family has a single dimension, and choosing within it means choosing a number.
Reading the Shore scale, because the numbers mislead
Hardness is quoted on the Shore A scale, which stops at 100. Crucially the scale is not linear in how a part feels: the top of it is crowded, so a ten-point drop near 95A changes the character of a part far more than the same ten points lower down. Assume the difference between adjacent grades is bigger than the numbers imply, in both directions.
| Material | Feel | Choose it for |
|---|---|---|
| TPU 98A | barely rubbery, springy | parts that must hold shape while absorbing knocks, and the firmest starting point for a difficult extruder |
| TPU 95A | firm rubber | the default flexible: cases, gaskets, tyres, dampers |
| TPU 85A | genuinely squashy | seals, liners and grips that must conform |
| TPE | varies enormously | soft tacky parts at low cost, and polypropylene compatibility |
Most people choose too soft. Ask what the part must do rather than how it should feel, and the answer usually moves up the table by one row.
Diagnose failures upstream of the nozzle
The most useful orienting fact about this shelf: when a flexible print goes wrong, the cause is almost never in the hotend. Melting these polymers is easy. Getting them to the melt zone is the difficulty, and it gets harder in step with softness — which is why the family's difficulty ratings run from 2 out of 5 at the firm end to 5 out of 5 at the soft end while the temperatures barely move.
The practical consequences are set out in detail on the individual pages, since the thresholds differ by grade. At family level, three rules of thumb:
- Softer means slower, and the relationship is steeper than most people expect.
- A machine that handles one grade may refuse the next one down, and the cause is usually hardware rather than profile.
- Retraction settings that work on rigid filament will not transfer here at all.
If you are choosing a first flexible spool for an existing machine, choose the firmest grade that will do the job. That advice is worth more than any setting on this page.
Geometry sets the softness as much as the spool does
This is the insight that saves the most money in this family. A printed part is a lattice, not a solid, so wall count and infill density change how soft it feels more than a hardness grade does. Two walls and sparse infill in a firm filament feels softer than four walls and dense infill in a soft one.
Before buying a softer spool because a case felt rigid, look at how nearly solid the case was printed.
What the whole family shares
Moisture sensitivity. Every member bubbles and prints furry when damp, and every member is fixed by four to six hours in a dryer at 55 to 60 °C. This is the most common flexible-filament complaint and the most easily solved.
Over-adhesion. They stick to plates readily and sometimes stubbornly; a textured sheet is the safe default.
Poor bridges and overhangs, and supports that weld themselves to a soft surface. Support removal on a flexible part is slow hand work, and on a commercial job it is a labour line rather than a rounding error.
Dimensional drift. Nothing here settles at the moment it comes off the plate, so a scale factor derived from a published shrinkage figure is not worth applying. Test pieces are the method across the whole shelf.
Matching a grade to a job
First flexible spool, or a Bowden machine? 98A. Phone case, gasket, damper, RC tyre? 95A. Something that has to squash, conform or seal? 85A, and plan for a slow print. Want soft and tacky cheaply, or need to bond to polypropylene? TPE, and treat every new spool as an unknown material.