Understanding Shore Hardness in TPU & TPE

Ask for a flexible part and the first question back is almost always "what hardness?" Shore hardness is the common language of elastomers — quick to measure, easy to quote, and printed at the top of every TPU and TPE data sheet. It is also the most misunderstood number in the selection process, because people treat it as a summary of the whole material when it only describes one thing: resistance to indentation.

This guide explains how the measurement works, what the A, D, and 00 scales cover, why two people can read the same part differently, and why a hardness number alone should never close a material decision.

Editorial note: This is an independent educational guide. Hardness values cited are illustrative of published grades, not a specification. Always confirm hardness and the test method against current supplier documentation.

What Shore hardness actually measures

Shore hardness measures how much a material resists a small indenter being pushed into its surface. That is the entire definition. It tells you about surface stiffness and feel — whether a part is gummy, rubbery, firm, or rigid — and nothing directly about how strong it is, how well it resists wear, or how long it will last.

The number is useful precisely because it is simple. It lets a designer, molder, and buyer agree on roughly how soft a part should feel. The trouble starts when that single number is asked to carry the weight of an entire specification.

How a durometer works

The instrument is called a durometer. A calibrated spring pushes a metal indenter into the material, and a gauge measures how far the indenter penetrates over a travel of zero to 0.100 inches. A reading of 0 means the indenter sank in fully; a reading of 100 means it did not penetrate at all. So a higher number means a harder, more indentation-resistant surface.

Different durometer types use different indenter shapes and spring forces, which is why the scale you use has to match the material. Use the wrong scale and the reading is either crowded at the top of the range or hovering near zero — in both cases, unreliable.

The scales: 00, A, and D

Three Shore scales cover almost everything in the elastomer world:

• Shore 00 — for very soft materials such as gels and soft foam rubbers, too soft to register usefully on the A scale.
• Shore A — a blunt indenter with a moderate spring, for soft and flexible materials. This is the scale most TPEs and softer TPUs live on. It loses accuracy above about 90, where the material barely deflects the indenter.
• Shore D — a sharp indenter with a stronger spring, for hard rubbers and rigid plastics. This is where rigid TPUs and engineering plastics are measured.

Above Shore D, materials are usually measured on Rockwell or other rigid-plastic scales. The practical takeaway: when a soft-material reading climbs past 90A, stop trusting the A scale and move to D.

Which scale for which material

The reference points are rough, everyday comparisons, not measured values — but they help when you are trying to picture what a number like "85A" or "45D" feels like in the hand.

Why two people get two readings

Hardness is one of those measurements that looks foolproof and is not. If two labs report different numbers for the same compound, the cause is almost always procedure, not the material. The things that move the reading:

• Timing. Elastomers relax under load, so an instantaneous reading is always higher than a delayed one. A delay is usually held between 5 and 10 seconds, and it must be consistent. A delayed reading reflects resilience as well as hardness — a weaker, less elastic material creeps more under the indenter and reads lower after the delay.
• Thickness. The sample needs enough material under the indenter — commonly about 0.200 inches — so the bench underneath does not stiffen the reading. Stacking thin parts is a poor substitute.
• Surface. The reading needs a flat, smooth spot. Curved surfaces, textures, and parting lines all distort it.
• Edge distance. Reading too close to an edge lets the material deflect sideways and reads soft.

When a hardness dispute lands on the desk, the first question worth asking is not "whose material is right" but "what delay time and sample thickness did each side use." Nine times out of ten the disagreement is in the method.

Hardness is not performance

This is the part worth tattooing on the inside of the eyelids: two materials at the same Shore hardness can be completely different in service. Held at, say, 85 Shore A, two TPU grades can differ in:

• abrasion resistance;
• tear strength;
• tensile strength;
• compression set and rebound;
• melt flow and processing window;
• chemical and oil resistance;
• adhesion in over-molding;
• hydrolysis resistance, depending on backbone.

A part spec that says only "85A TPU" describes the feel and nothing else. It is the equivalent of ordering a vehicle by its color. For why the chemistry underneath matters just as much, see Polyester vs Polyether TPU.

Hardness, modulus, and strength

Hardness does correlate with other properties, but only loosely and only within a single material family. Within one TPU or TPE family, as hardness goes up, flexural modulus (resistance to bending) generally goes up too — harder usually means stiffer. Tensile strength and creep resistance also tend to track together within a family: a softer grade typically creeps more and carries less tensile strength than a harder one.

The catch is "within a family." Compare across families — a styrenic TPE against a TPU at the same hardness — and those relationships break down, because the underlying chemistry is different. Hardness is a feel comparison, modulus is a bending comparison, and they only line up when you hold the chemistry constant.

Hardness across real TPU grades

Published TPU grades show how wide the family stretches, and how little hardness tells you on its own:

Look at the two softest grades: a 47A and a 68A. The harder one is the weaker one on tensile, and its abrasion loss is nearly ten times higher. If you had specified by hardness alone and picked the 68A "because it sounded tougher," you would have chosen the worse wear material. Hardness did not predict performance — the grade data did.

How to specify hardness properly

Hardness belongs in a specification — it just should not be the whole specification. A useful line looks more like this:

TPU, 85 ± 5 Shore A (3-second delayed reading), polyester backbone, injection-molding grade, black, abrasion resistance prioritized, for a handled part with occasional oil contact. Supplier to provide current data sheet, including abrasion loss and test method.

That version pins down the tolerance, the reading method, the chemistry, the process, and the property that actually matters. It gives a supplier something real to match — and it protects you from receiving a part that hits the hardness target and fails everything else.

Bottom line

Shore hardness is a clear, useful measure of how stiff a material feels at the surface, read with a spring-loaded indenter on the 00, A, or D scale. It is not a measure of strength, wear, or durability, and it varies with timing and sample setup, so it has to be read carefully. Use it as one line in the specification, always paired with the test method, the chemistry, and the properties the part truly depends on.

For more on the measurement scales and durometer practice, the Avient thermoplastic elastomer FAQs cover hardness testing in useful detail.

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