Shore hardness

Shore hardness is a 0-100 rating of how much a material resists being pressed in by a standard probe, so higher means harder.

Why it matters

Shore hardness is how flexible filaments are labelled. You will see it on TPU spools as a number followed by a letter, such as 95A. Higher numbers feel stiffer, lower numbers feel softer and rubberier.

Shore A hardness scale with everyday examplesA vertical Shore A scale from 0, softest, to 100, hardest. A rubber band is about 25A, car tyre tread about 70A, and a common TPU grade 95A.HarderShore A0–100TPU 95ATyre tread 70ARubber band 25ASofter
On the Shore A scale, 95A, a common TPU grade, sits near the hard end, far firmer than a rubber band. Example values from Wikipedia's Shore durometer article; 95A from Polymaker and Ultimaker data sheets.

The letter is the scale. Shore A is used for softer materials, with a truncated cone probe, and Shore D for harder ones, with a sharper probe. The two scales are not interchangeable, so a figure in A cannot be compared directly with one in D.

Brands publish this in their technical data sheets. Polymaker lists its PolyFlex TPU95 as 95A under ISO 7619-1, and Ultimaker gives a Shore A hardness of 95 for its TPU 95A. The same number from two brands may still print differently.

Print settings depend on the grade, so check the hardness before you buy. Flexible filament is also prone to stringing and to absorbing moisture.

Sources