Flexible filament is often sold with a number such as 95A, 90A or 85A after the letters TPU. That number is not a recommended nozzle temperature or a product generation: it describes Shore hardness, a measure of how the material resists indentation under a defined test.
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For many shoppers, a lower Shore A number suggests a softer, more compliant filament; a higher number suggests a firmer one. But the feel of a finished 3D print depends on much more than the spool label. A thick solid 90A block may resist a finger much more than a thin, hollow 95A bellows. Wall count, infill, geometry, temperature and the particular TPU chemistry all influence the final part.
What does Shore A mean?
The Shore hardness scales use specified indenters and methods. The A scale is widely used for softer rubber-like polymers, while other scales such as Shore D suit harder plastics. A reading of 95A is not "95% hard" and does not mean the material has 95% of any standard modulus or strength.
You also cannot subtract two Shore readings and assume the difference is linear. A five-point difference near one end of a scale is not guaranteed to feel like a five-point difference elsewhere. The measured hardness is a single characteristic of a standard specimen under particular conditions, not a full mechanical profile.
Polymaker's flexible-filament guidance explains the usual ordering of its 90A, 95A and higher-flow firmer grades, while explicitly noting that infill, wall count, layers and other settings change perceived flexibility.[1]
| Label commonly seen | General feel | Practical printing consideration |
|---|---|---|
| Around 85A | Relatively soft and easy to bend | Can buckle or deform in long or poorly constrained feed paths |
| Around 90A | Flexible, often suited to soft grips or compliant features | Usually more demanding than firmer TPU |
| Around 95A | Firmly flexible, a frequent general-purpose starting point | Often easier to feed than softer grades |
| Around 98A or Shore D-labelled flex | Firmer and less rubber-like in many designs | May feel stiff in solid walls despite being sold as "flexible" |
These are qualitative guideposts, not a guarantee of every brand's material response. Some products marketed at the same hardness differ markedly in tensile strength, resilience, friction, surface finish and feeding behaviour. Read the grade-specific technical data sheet.
Why a 95A print can feel rigid
Consider a closed box with four thick walls and 100% infill. Even if the underlying TPU is compliant, the shape offers little opportunity to flex. Now consider a thin web, curved spring, perforated lattice or hollow bumper printed from the same spool: geometry permits much greater deformation.
Printed parts have an effective stiffness determined by polymer and structural design. Their response can also be direction-dependent because extrusion paths and layer interfaces are not the same as a solid injection-moulded specimen. An infill percentage is therefore not a substitute for specifying the actual intended force-deflection relationship.
For a soft cushion, increasing hollow space or designing slender walls can have a larger effect than changing from one hardness grade to another. For a grip requiring dimensional stability, firmer TPU may help, but a better cross-section could deliver the required compliance with less change in process difficulty.
The softness–printability trade-off
Flexible filament must be pushed into a hotend. Unlike rigid PLA, soft TPU can compress, bend and find gaps between drive gears and the filament guide. A long Bowden path can make this worse because the filament behaves like a spring inside the tube. Short, constrained direct-drive paths are often advantageous for softer grades.
Prusa's flexible-material guide observes that softer materials are generally more difficult to print.[2] Polymaker likewise distinguishes the feed-path requirements of its 90A and 95A products.[3] These statements do not prove that every direct-drive printer can handle 85A, nor that every Bowden printer fails with 95A. The exact extruder, guide geometry and material flow behaviour decide compatibility.
Before ordering an especially soft grade, check the machine maker's specified Shore range, the spool maker's profile and whether a supported external-spool or special feed path is needed. Automated multi-material feeders can be more restrictive than the hotend itself.
What Shore hardness does not tell you
Tensile strength: Two grades with the same Shore number can break at different loads.
Elongation at break: One may stretch farther before fracture.
Compression set: A bumper held compressed for a long time may not fully recover its original shape.
Abrasion resistance: Useful for wheels, pads and wear surfaces, but not uniquely specified by indentation hardness.
Chemical resistance and UV stability: These depend on TPU formulation and additives.
Printability: Melt flow, moisture sensitivity and filament diameter consistency matter alongside softness.
Polymaker's family technical information reports different modulus and elongation figures across TPU grades, demonstrating why Shore number alone cannot describe all mechanical behaviour.[4] The published figures apply to their specified samples and test methods, not automatically to a printed consumer part.
How to choose hardness for a real object
Phone or handheld grip: A firmer flexible TPU may provide resilience without feeling excessively floppy. Consider texture, wall thickness and the grip's fit around the object.
Soft bumper: More compliant grades or a hollow geometry may absorb deformation, but the design must be validated for repeated impact and mounting retention.
RC tyre or flexible wheel: Hardness influences grip and deformation, yet surface texture, tread geometry, wear, friction and load are equally important. There is no universal best Shore number.
Gasket: Compression behaviour and the sealing surface are critical. A printed gasket's layer lines may leak; a Shore label does not establish sealing performance or chemical suitability.
Hinge or strap: Fatigue and elongation matter as much as initial bend force. Avoid certifying a printed strap for safety or load-bearing use without relevant testing.
Flexible decorative figure: A 95A grade may be adequate if the walls are thin and the printability advantage matters more than maximum softness.
Settings still matter
Printer profiles for TPU often use controlled speeds, material-specific temperatures and different retraction from rigid plastics. Soft filament may buckle under unnecessary high pressure, especially if the extruder tries to push it faster than it melts. An excessive idler force can deform it; too little can slip.
Moisture can cause roughness, bubbles and inconsistent flow. Prusa's flexible-material guide emphasises storage and drying concerns.[2] Use the exact supplier's drying instructions and do not assume one temperature works for all TPU grades or spool materials.
For build-plate adhesion, some flexible grades bond aggressively with smooth surfaces; a manufacturer-approved separator may be needed. Never assess print success by how strongly a TPU part refuses to come off the plate. Safe removal is part of a good process.
How we would test flexibility fairly
An honest comparison would print multiple specimens of the same geometry from controlled batches of 85A, 90A and 95A TPU, using separate optimised profiles. We would measure force versus deflection, recovery after repeated bending, dimensional fit and any feeding failures. Solid blocks and thin-wall samples would be assessed separately to show the role of geometry.
We have not completed those controlled prints. The qualitative descriptions above come from supplier documentation and material mechanics, not fabricated measurements.
The short answer
Start around 95A if you need a generally printable flexible part and your printer explicitly supports it. Look toward lower Shore A values when the application genuinely requires more compliance—and expect the feed path and settings to become more demanding. Design geometry alongside material choice; a well-designed 95A lattice may flex more usefully than a poorly designed solid 85A object.
Sources
[1] Polymaker — PolyFlex TPU95 and Shore hardness explained
[2] Prusa — Flexible materials guide
[3] Polymaker — Flexible TPU printing and feed path guidance