TPU and Flexible Filaments

What flexible filament is

Flexible filaments are thermoplastic elastomers (TPE). Prusa explains that TPU is a subtype of TPE and that the main practical difference between flex filaments is Shore hardness, a scale for how soft a rubbery material is: the lower the number before the "A", the softer it is. Prusa gives about 60A to 90A Rule of thumb as the usual range for TPU, and spools are also sold harder than that; Polymaker's PolyFlex TPU95, for example, is rated 95A Maker spec. Prusa's key rule: the softer the filament, the harder it is to print.

Strengths and limits

Prusa lists flexibility and toughness with excellent tensile strength, little to no shrinking or warping, good layer adhesion, high mechanical and chemical resistance, wear resistance and flexibility in the cold. Polymaker says PolyFlex TPU95 can stretch more than three times its original length; its data sheet gives elongation at break of about 550 percent Maker spec. Typical uses from Prusa: spacing washers, stamps, RC tires, shoe soles and belts.

The limits, also from Prusa: high print difficulty, low print speed, very poor bridging and overhangs, hard-to-remove supports, stringing, and moisture absorption.

Two makers, two sets of numbers

Setting	Prusa knowledge base (flex in general)	Polymaker PolyFlex TPU95 TDS
Nozzle	230 to 245 °C, by hardness and brand Starting point	210 to 230 °C Maker spec
Bed	60 to 75 °C, hotter for big parts Starting point	25 to 60 °C Maker spec
Part cooling	Off Starting point	On Maker spec
Speed	About 20 mm/s typical, 30 to 40 mm/s maximum Starting point	30 to 50 mm/s Maker spec
Retraction	Off to start, then raise it if needed Starting point	3 to 6 mm Maker spec
Drying	Prusament TPU: 60 °C for 4 to 6 hours Maker spec	70 °C for 8 hours Maker spec

These disagree on almost every line, including whether to run the part fan. That is not an error: they describe different products. Follow the data sheet for the spool you have and test. A temperature tower printed slowly is a good first check.

Feeding: where flex prints fail

Prusa lists filament tangling in the extruder gears and nozzle clogs among the common problems, and advises:

• Print slowly. Prusa's typical speed is about 20 mm/s Starting point.
• Keep retraction low. Lower retraction reduces the risk of clogging and tangling. Prusa suggests turning retraction off, then raising it gradually while checking results. See Retraction and the retraction test.
• Set the idler carefully. If the gears press too hard, the filament can tangle.
• Watch long filament paths. Prusa notes that its XL's long filament path needs special attention when loading flex, and that automatic filament change may not work properly with flexible filaments. Extruder layout matters here; see Direct Drive versus Bowden.

Prusa notes that a cold pull can clear an obstructed nozzle. For other blockages, see Clogged Nozzle.

Bed adhesion

Prusa recommends a textured powder-coated sheet, which needs no separator, or a separator such as glue stick or Kapton tape on smooth or satin sheets. It warns that very soft flex may bond too strongly to the bed. See Build Plates and Surfaces.

Supports and overhangs

Flex bridges and overhangs poorly, and its supports are hard to remove. Prusa suggests a Z gap between support and part of about 0.3 mm and an XY separation of at least 100 percent Starting point. See Support Settings, and orient the model to need as little support as possible; see Print Orientation.

Moisture, storage and drying

Prusa lists TPU among the materials that most often need drying, and says wet flexible filament is hard to print. Prusa's general wet-filament signs are a poor surface, more stringing, blobs, bubbling and weak layers. Store TPU sealed with desiccant and dry it to the maker's figures; see Drying and Storing Filament.

Post-processing

Prusa's flexible materials guide gives no post-processing advice, and this guide adds none. Expect support removal to be the main cleanup job: Prusa lists difficult support removal among TPU's limits.

Enclosure and ventilation

Polymaker's PolyFlex TPU95 data sheet lists a closed chamber as not needed Maker spec, and Prusa describes TPU as having little to no warping. Ventilate as you would for any FDM printing; see FDM Emissions and Ventilation.

Common failures

What you see	Usual cause	Where to go
Filament balled up around the gears	Too much retraction, idler too tight, too fast	Slow down; retraction off, then tune
Strings across the part	Wet filament, retraction, temperature	Stringing
Extrusion stops mid-print	Clog or tangle	Clogged Nozzle
Saggy bridges and overhangs	Normal for flex	Overhang Droop