Shore Hardness for Urethane: How to Specify It on a Drawing

Shore hardness for urethane is a number from a durometer that tells you how far a standard indenter sinks into the material under a fixed load. Most urethane parts are specified on the Shore A scale, usually between 40A and 95A. Harder grades move to Shore D. The number is only useful if the drawing also states the scale, the test method, the sample thickness and the reading time. Without those, two labs can report two different values for the same part. For cast urethane prototypes, a complete callout saves a quote revision and a failed first article.

We make cast urethane parts through vacuum casting, so this guide covers what an engineer or buyer needs to write on a print and what to check when parts arrive.

Key Takeaways

• Write the scale with the number (70A, not 70). Shore A, Shore D and Shore OO use different indenters and loads, so the values are not interchangeable.
• Use Shore A for soft to firm elastomers and Shore D for hard elastomers and plastics. Switch scales when a Shore A reading goes above 90 or a Shore D reading falls below 20.
• Never convert A to D with a chart for acceptance. Conversion tables are rough estimates, and ASTM D2240 defines no official equivalence.
• A hardness callout should include ASTM D2240, sample thickness of at least 6 mm, 23 °C conditioning and the reading time.
• A UL 94 flame rating belongs to a material at a stated thickness, not to your finished part. Specify it separately from hardness.

Table of Contents

• What Shore hardness measures in urethane
• Shore OO, Shore A and Shore D: which scale to use
• Why you cannot convert Shore A to Shore D
• How urethane hardness is tested and what changes the result
• How to pick a hardness for a part
• Flame retardant urethane and UL 94 ratings
• A drawing callout that holds up
• Where cast urethane fits in a development program
• Our perspective
• Get cast urethane parts quoted
• FAQ

What Shore hardness measures in urethane

Shore hardness is the resistance of a surface to indentation. Boedeker’s polyurethane technical guide defines it as the relative resistance to a specified indenter under a specified load. A durometer presses a spring-loaded pin into the surface and reads depth on a 0 to 100 dial, where a higher number means harder.

That makes it a surface test of one property. It does not measure tensile strength, tear strength, abrasion resistance or rebound. Two urethanes at 80A can behave very differently in a wheel, a gasket or a bumper. Treat hardness as the first filter and the other properties as the next ones.

Shore OO, Shore A and Shore D: which scale to use

The scales differ in indenter shape and spring force. Dynisco’s testing reference describes the Shore A indenter as a truncated 35° cone with a 0.79 mm blunt tip. The Shore D indenter is a sharper 30° cone with a 0.1 mm radius tip, and it uses a stiffer spring. According to Accu’s guide, Shore D applies a 4,536 gf load against 822 gf for Shore A.

Accu’s guide describes the Shore A range this way: below 20A is gel-like, 20A to 40A is soft flexible rubber, and 80A to 95A is hard and approaching semi-rigid. Above 95A, Shore D is the better scale. Millathane’s data sheet notes that millable urethanes can be compounded from 30 to 99 Shore A and up to 65 Shore D, with most useful products in the 50 to 80 Shore A range.

The switching rule

Readings near the ends of any scale are the least reliable. The usual practice under ASTM D2240 is to move to Shore D when a Shore A reading exceeds 90, and to Shore A when a Shore D reading falls below 20. If your target is 92A, ask yourself whether the part is really a Shore D part and say so on the drawing.

Why you cannot convert Shore A to Shore D

Every engineer eventually sees a chart that maps 95A to roughly 45D. Those tables are approximations. Qualitest’s comparison shows 95A near 46D and 50A near 12D, and states that such charts are for estimation only and should not be used in specifications. Boedeker’s ASTM testing overview goes further and says the two scales do not correlate and cannot be compared.

In practice that means three rules:

1. Specify the scale you will inspect on, and inspect on that same scale.
2. If you move a design from an A-scale material to a D-scale material, treat it as a new material, not a conversion.
3. Compare materials only within one scale.

How urethane hardness is tested and what changes the result

Hardness is a method result, not an absolute property. The ASTM D2240 standard text sets out the conditions that matter:

• Thickness: at least 6.0 mm for A and D types. A thinner sample gives a falsely high reading because the indenter feels the hard surface beneath.
• Distance from edges: at least 12.0 mm from any edge, with flat, parallel, smooth faces.
• Temperature: conditioning at 23.0 ± 2.0 °C per Practice D618, with the instrument in the same atmosphere for 12 hours.
• Readings: five readings, with the median reported.
• Reading time: ASTM calls for a reading within one second of firm contact. ISO methods for rubber often specify a longer dwell, such as 15 seconds, according to Dynisco.

Reading time deserves attention because urethane creeps under the indenter. An instant reading is always higher than a timed reading on the same compound, so quote the dwell time or your supplier and your incoming inspection may disagree by a few points without either being wrong.

A worked example

Suppose you design a handheld controller with a 3 mm overmold grip and call out “70A.” A cast sample 3 mm thick is below the 6 mm minimum, so a durometer reading on the finished grip will read higher than the material really is. The correct plan is to cast a 6 mm or thicker test button from the same batch, read it per D2240, and treat any reading on the thin grip as an indication only.

How to pick a hardness for a part

Start from the function, then confirm with a short range of samples rather than a single value.

These ranges are starting points for a design review, not guarantees for any one resin. The harder the grade, the less it flexes and the more load it carries, but harder is not always better. A stiffer urethane can crack where a softer one would flex, and a very hard grade may lose the grip or sealing action you wanted.

Other properties to put on the drawing next to hardness:

• Operating temperature range, since hardness shifts with temperature
• Wear mode: sliding wear and impact wear favor different formulations
• Contact with oils, fuels or cleaning agents
• Color and surface finish
• Whether the part will be overmolded or bonded to a rigid substrate

For the wider material decision, our guides on material selection for product development teams and material selection in aerospace and automotive prototyping go through the trade-offs. If you plan to combine a soft grip with a rigid body in production, see our notes on overmolding material rules and trials.

Flame retardant urethane and UL 94 ratings

If your part is an enclosure, a housing or a component in electrical equipment, you will often see a UL 94 requirement alongside hardness. They are separate properties: a hard grade is not automatically flame retardant, and a flame-retardant grade says nothing about Shore hardness.

UL describes the ratings for enclosure and structural plastics as 5VA, 5VB, V-0, V-1, V-2 and HB. In plain terms:

• HB means a horizontal specimen burned below a specified maximum rate. It is the baseline rating.
• V-0 means a vertical specimen burns no more than 10 seconds after each flame application, no more than 50 seconds in total across 10 flame applications, and drips nothing that ignites cotton below.
• V-1 and V-2 loosen those limits. V-2 allows flaming drips.

So V-0 is the stricter and more commonly requested rating for electronics housings, while HB is a much weaker claim.

What a UL 94 rating does and does not tell you

Three points prevent the most common mistakes:

1. The rating belongs to a material at a stated thickness. Think Plastics gives the example of a grade that is V-0 at 1.5 mm but only V-2 or HB at a thinner section. A rating with no thickness is incomplete.
2. It is not a product certification. A V-0 material does not make your finished product UL listed. The product is evaluated against its own standard.
3. It is limited in scope. UL’s own explainer says the small-scale tests apply to plastic materials in devices and appliances, not to materials over 13.0 mm thick or building construction materials.

Flame-retardant formulations also change the base polymer. Millathane’s data sheet notes that antimony-oxide and halogenated compounds can reach V-0, while halogen-free options use alumina trihydrate or magnesium hydroxide. Adding a flame retardant to a standard grade usually means new testing, because the rating belongs to the tested formulation. Put the required rating, the thickness and any halogen-free requirement in your request at the start, not after the first samples arrive.

A drawing callout that holds up

Here is a hardness note you can adapt:

Material: cast polyurethane, 70 Shore A ± 5, per ASTM D2240, 6 mm minimum test button, conditioned at 23 °C, one-second reading, median of five.

Add these items when relevant:

• A tolerance, since real batches vary by a few points. Asking for exactly 70A with no tolerance invites disputes.
• The place and the sample the hardness applies to: a test button cast with the part, or the part itself.
• The UL 94 rating, tested thickness and color, if flame performance matters.
• Who runs the incoming inspection and with which instrument, so the same method is used both sides.

Where cast urethane fits in a development program

Vacuum casting suits the stage where you need functional parts in an elastomer or rigid urethane without paying for steel tooling. You can try 60A, 70A and 80A versions of the same grip, compare them in hand and in test fixtures, and lock the winning grade before you commit to a production process such as injection molding. Our list of injection molding materials helps when you plan that move, and our longer piece on sourcing urethane by Shore hardness from China covers the buyer’s side.

Our perspective

Most hardness disputes we see in a project review come from an incomplete callout, not from a bad part. A drawing that says “70” leaves the scale, the thickness and the method open, and every one of those moves the number.

Our advice is to treat hardness as a range, test it on a proper button, and decide between two or three grades with parts in your hands. Vacuum casting, with no minimum order quantity, makes that practical for a startup with a single design and for an R&D team running a comparison.

We are ISO certified, and a dedicated project manager works with you on the specification, so questions about scale, tolerance or flame rating are settled before casting, not after.

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Get cast urethane parts quoted

If you have a CAD file and a target hardness, you can upload it for an instant quote on our vacuum casting service. We also run CNC machining, 3D printing and injection molding, so a design can move from cast prototype to the next stage with the same team. Include the Shore scale, tolerance and any UL 94 requirement in your request.

FAQ

What is the Shore hardness of polyurethane?

It spans a wide range. Millable urethanes can be compounded from 30 to 99 Shore A and up to 65 Shore D, and most useful products sit between 50 and 80 Shore A. Cast urethane parts for gaskets, grips and bumpers commonly fall in the 40A to 90A band. The right value depends on the function of the part.

What is the difference between Shore A and Shore D?

Shore A uses a blunt 35° cone and a lighter spring for soft to firm elastomers. Shore D uses a sharper 30° cone and a stiffer spring for hard elastomers and plastics. Use Shore D when Shore A readings rise above 90, and Shore A when Shore D readings drop below 20. The two scales cannot be reliably converted.

What does 90A urethane feel like?

At 90A, urethane is hard, close to semi-rigid. Think of a firm wheel or roller rather than a soft grip. Readings above 90A start to lose accuracy on the A scale, so specify Shore D if you want to inspect a harder part.

What is the difference between UL 94 HB and V-0?

HB is a horizontal burn test where the material burns below a specified rate. V-0 is a vertical test, with burning stopping within 10 seconds per flame application, 50 seconds in total, and no flaming drips that ignite cotton. V-0 is the stricter rating, and both ratings apply to a material at a stated thickness.

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