Durometer is how hard a rubber part is — the first number most drawings specify, and the one most often specified by habit rather than by requirement. Move the scale below to see what each hardness feels like, what it is normally used for, and what it costs you.
Shore A scale
60Shore A±5
Feels like
A car tire tread, or a pattern eraser
Typical parts
- General-purpose gaskets and static seals
- Bumpers and end stops
- Isolation mounts carrying real load
- Rubber-to-metal bonded parts
What you gain
The broadest working band in custom molding — balances sealing, resilience and wear.
What you give up
Nothing in particular, which is why most parts start here.
Positions are typical commercial ranges. A durometer on its own never specifies a compound — it is one line of an ASTM D2000 callout, alongside polymer family, temperature class and media resistance.
Reading a durometer, and the ±5 rule
±5
Standard commercial tolerance on a molded rubber part, per ASTM D2240. A part called out at 70 Shore A and measuring 66 is in specification. Tighter than ±5 is possible but has to be agreed up front, and it narrows the compounds that can hold it.
6.4mm
Minimum thickness for a valid reading. Thinner sections read hard because the indentor feels the surface underneath. Stacking plies is allowed for lab specimens; on a finished part, thin sections and tight curves cannot be measured reliably at all.
15sec
Rubber relaxes under the indentor, so the reading falls as it sits. CRC's accredited lab reports a timed 15-second reading unless a specification calls for instantaneous. Comparing an instantaneous number to a timed one will show a difference that isn't there.
23°C
Rubber stiffens as it cools, so hardness is specified at a standard temperature. A 60 Shore A part will read considerably harder at −20 °C, and that stiffening — not the room-temperature number — is what governs how it behaves in cold service.
Which scale applies
Shore OO
Foams, sponges, gels
Open- and closed-cell sponge, and gel-like elastomers too soft to register on Shore A.
Shore A
20 – 95
Almost all molded rubber. CRC's work is specified here unless the part is a sponge or a hard urethane.
Shore D
Above 95 Shore A
Hard rubbers, urethanes and plastics. A part reading 95A+ should carry a Shore D callout.
Typical hardness by part
| Part | Typical range | What moves it up or down |
|---|---|---|
| Anti-vibration mount | 40 – 60 A | Softer isolates more; the mount series and load set the number. |
| Bumper / end stop | 50 – 70 A | Higher for repeated impact, lower for quiet contact. |
| Static gasket, flat flange | 50 – 70 A | Drop to 40 A if the flange is rough or bolt load is light. |
| Static gasket, rough surface | 30 – 50 A | Conformance matters more than strength. |
| O-ring / face seal | 70 – 80 A | 70 A is the commercial default; higher resists extrusion. |
| Dynamic shaft seal | 70 – 85 A | Needs to hold its lip geometry against rotation. |
| Diaphragm / bladder | 40 – 60 A | Must flex through millions of cycles without hardening. |
| Bellows / boot | 35 – 55 A | Low durometer keeps articulation forces down. |
| Grommet | 40 – 60 A | Softer to install by hand; firmer to resist pull-through. |
| Bushing | 60 – 80 A | Sets the torsional rate as much as the geometry does. |
| Wiper / scraper blade | 60 – 80 A | Firm enough to scrape, compliant enough not to score. |
| Roller / drive wheel | 75 – 95 A | Higher for load and abrasion, lower for grip. |
| Abrasion liner | 40 – 60 A | Counter-intuitive: softer rubber often outwears harder rubber. |
| Rubber-to-metal bonded pad | 50 – 75 A | Chosen from the spring rate the assembly needs. |
What durometer does not tell you
- It is not strength. Two compounds at 70 Shore A can differ by a factor of two in tensile strength and tear resistance. Hardness says how far the surface deflects under a standard indentor, nothing more.
- It is not the same across polymers. A 60 Shore A EPDM and a 60 Shore A FKM behave differently in a gland, because modulus, resilience and compression set differ even when hardness matches.
- It does not predict compression set. How much of the squeeze a seal keeps after months at temperature is a separate measured property, and it is usually the one that decides whether a seal survives.
- It is not a spring rate. For mounts and bushings, the load-deflection curve is what matters. Durometer is an input to that curve, not a substitute for it.
Not sure which number your part needs?
Send the application rather than a durometer — load, temperature, media, surface finish and duty cycle. CRC's engineers pick the hardness from the requirement, and the accredited lab verifies it on the compound that ships.