TPU Hardness Explained: 85A, 95A, and 68D

Who this guide is for

This guide is for anyone reading a TPU label and wondering what "85A," "95A," or "68D" actually tells them. It covers what the Shore scales measure, why you can't compare an A number with a D number directly, what hardness means for your extruder and automatic material system, and what it means for the finished part. For the hardware side in more depth, see the TPU compatibility guide.

TPU: what to check before you buy a spool ↗

What does a Shore hardness number measure?

Shore hardness is an indentation test. A gauge called a durometer presses a spring-loaded indenter into the material, and the reading shows how far it goes in. ASTM D2240, the US standard, says the test "is based on the penetration of a specific type of indentor when forced into the material under specified conditions." Recreus, a maker of flexible filament, describes it as "a measure of the resistance of a material to penetration by a needle under a defined spring force." Readings run from 0 to 100, and a higher number means a harder material.

The filament makers in this guide cite two standards:

• ASTM D2240. NinjaTek's NinjaFlex data sheet lists "85 Shore A" measured to ASTM D2240. The standard covers twelve durometer types, from A and D to softer and specialist types such as OO and M.
• ISO 7619-1. Polymaker's PolyFlex TPU95 data sheet lists 95A measured to ISO 7619-1. That standard covers the A scale "for rubbers in the normal-hardness range," the D scale "for rubbers in the high-hardness range," and two more scales: AO for low-hardness and cellular rubbers, and AM for thin test pieces. ISO 7619-1 has since been withdrawn and replaced by ISO 48-4:2018, which uses the same four scales. Data sheets often still cite the older number.

Is 68D TPU harder than 95A?

The letter tells you which durometer was used, and each type has its own indenter shape and spring force. The standards assign the A scale to the normal-hardness range and the D scale to the high-hardness range. The letter alone doesn't settle every comparison, because the ranges overlap (more on that below). For the TPUs in this guide, though, the makers' own data agree: Bambu's AMS lite FAQ ranks 55D and 77D TPU as harder than 95A, and Bambu's data sheets list its 68D TPU as far stiffer than its 95A one. A 68D TPU sits well above a 95A TPU, not below it.

You can't convert one scale to the other with a simple formula. ASTM D2240 says the indenter geometry and force mean "no simple relationship exists between the measurements obtained with one type of durometer and those obtained with another type." Recreus notes that the A scale is used for flexible types and the D scale for rigid types, but that "the ranges overlap." We don't give a conversion table in this guide for that reason. Treat any A-to-D chart as approximate, and don't use one to compare two spools.

Bambu Lab makes the difference concrete because it publishes both kinds of TPU, tested to the same standard with similar printed specimens:

Young's modulus measures stiffness, and in X-Y the 68D grade is listed at more than a hundred times the 95A figure. Both still stretch a long way before breaking in X-Y, which is why both are sold as TPU. Our side-by-side comparison of these two Bambu filaments goes through the rest of the data.

Does the same Shore number mean the same stiffness?

ASTM D2240 also says "no simple relationship exists between indentation hardness determined by this test method and any fundamental property of the material tested." NinjaTek's technical specs table shows why that matters. It lists:

• Its Armadillo filament at 75D, with a tensile modulus of 396 MPa.
• PLA, also at 75D, with a tensile modulus of 1,669 MPa.

Same Shore reading, very different stiffness. Hardness is a useful label for sorting TPUs into soft, medium, and firm, but it doesn't predict every property. Read the modulus, elongation, and other figures on the data sheet when they matter for your part. Compare figures only within one maker's sheets, too. Polymaker measures its PolyFlex TPU95 modulus to ISO 37 (33.6 MPa in X-Y), while Bambu uses ISO 527, so those two numbers aren't directly comparable.

Common grades, from softer to firmer

These are verified products from our catalog, with each hardness taken from the maker's own page or data sheet:

Bambu Lab's A2L TPU guide orders its own A-scale grades "from hardest to softest" as 95A HF, then 90A, then 85A, which matches how the scale reads. D-scale TPU like Bambu's TPU for AMS is a different kind of product: firm and tough rather than squishy.

To weigh two of these spools against each other, see Siraya Tech Flex TPU 85A vs NinjaFlex 85A, PolyFlex TPU90 vs PolyFlex TPU95 and NinjaTek Cheetah vs Bambu TPU 95A HF.

What hardness means for printing

Is softer TPU harder to print?

Prusa's flexible materials article says it directly: "Generally speaking, the softer the filament is, the harder it is to print." The reason is mechanical. An extruder pushes filament from behind, and a soft strand bends and buckles instead of transmitting that push. NinjaTek's technical specs table shows the stiffness gap. On its printed test specimens, NinjaFlex (85A) has a tensile modulus of 12 MPa against 26 MPa for Cheetah (95A).

Makers' speed advice follows hardness. NinjaTek's printing guidelines give these ranges:

The softer grade gets the slower settings. These are NinjaTek's numbers for its own filaments, so use the data sheet for whichever spool you buy. Polymaker's PolyFlex TPU95 data sheet, for example, gives 30–50 mm/s.

Direct drive vs Bowden

On a direct-drive printer the extruder sits right above the hotend, so the filament has very little room to buckle. On a Bowden printer it has to travel through a long tube first. NinjaTek's guide to FDM printing says direct drive "allows for better, more stable material control, especially when using flexible and specialty filaments." It adds that "some TPUs can be tricky to print on a Bowden," and that its firmer Cheetah (95A) and Armadillo (75D) "are known to feed well" on that kind of printer. NinjaTek's NinjaFlex data sheet describes prints in the softer NinjaFlex as "ideal for direct-drive extruders."

Even on a direct-drive machine, extra drag hurts soft TPU. Bambu's A2L TPU guide warns that "excessive resistance (e.g., from an overly long PTFE tube) can easily lead to extruder gear slipping, filament wear, or even feeding interruptions." If your printer isn't known for flexibles, start with a 95A grade before trying 85A.

Automatic material systems: hardness is the rule

Bambu Lab is the clearest example of hardness deciding what an AMS can take:

• Bambu's filament guide lists filament that is "too soft (e.g., TPU 95A/85A/83A/80A, damp PVA or BVOH)" as not recommended, because it "cannot pass smoothly through the AMS / AMS lite channel."
• The AMS lite FAQ is more specific. TPU that "can be used with AMS lite: 77D, 55D, or higher hardness." TPU that "cannot be used with AMS lite: 95A, 85A, 83A, 80A, or lower hardness."
• Bambu's store page for TPU 95A HF says "All AMS Series NOT Compatible." Its page for the 68D TPU for AMS says "All AMS Series Compatible."

One newer exception exists. Bambu's A2L TPU guide lists TPU 95A HF and TPU 90A as "only compatible with AMS HT" on that printer, fed through the AMS HT's dedicated TPU outlet, while TPU for AMS is "compatible with all AMS series." That's one specific setup with its own loading steps, not a general rule. Our A2L filament page covers that printer's TPU options. For anything else, treat A-scale TPU as an external-spool filament. Our AMS filament compatibility guide covers Bambu's rules in full, and other makers' automatic systems have their own lists.

What hardness means for the part

Flex and feel

Lower hardness generally means a softer, more compliant part. Bambu describes TPU 95A HF as "an exceptionally soft and flexible filament with extraordinary impact resistance." It describes the 68D TPU for AMS as combining "exceptional toughness with the flexibility and durability of TPU" and suggests it for "protective cases, phone covers, door stoppers, sports equipment," and similar parts. If you need a part that squashes and springs back, such as a grip, bumper, or seal, an A-scale grade is the usual choice. If you need a tough part that resists impact but holds its shape, a D-scale TPU fits better.

Your slicer settings change the feel as much as the filament does. Bambu's TPU for AMS page notes that "the higher the wall loops or sparse infill density, the lower the elasticity, and vice versa." A 95A part printed solid can feel firmer than a 95A part with sparse infill.

Layer direction

Firm TPU can behave differently across its layers. Bambu's data sheets list elongation at break in Z, across the layers, of 31% for TPU for AMS against more than 480% for TPU 95A HF. Z-direction impact strength is 9.6 kJ/m² against 86.3 kJ/m². If a 68D part will be bent or struck across its layers, orient it so the load runs along them.

Abrasion and grip

Prusa says flexible filaments "are usually very resistant to wear (scratching)." Where makers publish abrasion figures, firmer grades don't always lose more. NinjaTek's technical specs table lists abrasion mass loss of 0.08 g for NinjaFlex (85A), 0.06 g for Cheetah (95A), and 0.03 g for Armadillo (75D). The NinjaFlex data sheet gives the method: ASTM D4060, 10,000 cycles. Lower is better. NinjaTek also says Cheetah's abrasion resistance is "40% better than ABS and 76% better than PLA." These figures come from one maker's tests, and most TPU data sheets we checked don't publish abrasion at all, so don't compare them across brands.

None of the sources we checked publishes a grip or friction figure by hardness. Softer grades are often chosen for grip, but we don't have maker data to quantify it.

Quick reference

Quick answers

Is 95A TPU softer than 85A? No. Both are on the Shore A scale, where a lower number means a softer material, so 85A is the softer of the two.

Can you convert Shore A to Shore D? Not with a simple formula. ASTM D2240 says "no simple relationship exists" between readings from different durometer types, so treat any A-to-D chart as approximate.

Can 95A TPU go in a Bambu AMS? Bambu's AMS lite FAQ lists 95A and softer as TPU that "cannot be used with AMS lite," and its store page for TPU 95A HF says "All AMS Series NOT Compatible." Bambu's A2L TPU guide lists one exception: TPU 95A HF fed through the AMS HT's dedicated TPU outlet on the A2L.

Which TPU hardness is easiest to print? Prusa says "the softer the filament is, the harder it is to print." If your printer isn't known for flexibles, start with a 95A grade before trying 85A.

Before you pick a hardness

• Check the letter as well as the number. A D-scale TPU like Bambu's 68D TPU for AMS is much firmer than the A-scale grades in this guide.
• Don't convert between A and D with a chart. Compare like with like.
• Match the grade to your extruder. If your printer isn't known for flexibles, start at 95A.
• If you use an automatic material system, check its maker's hardness rules before buying.
• Decide what the part needs: squash and grip (lower A numbers) or toughness that holds its shape (D scale).
• Read the data sheet for modulus, elongation, and print settings, and compare figures only within one maker's sheets.

Ready to choose a spool? The TPU filament shortlist sorts verified TPUs from 85A to 68D by hardness and feed path. If you're deciding between two specific 95A filaments, see PolyFlex TPU95 vs Bambu TPU 95A HF.