Shore Hardness Guide: How to Select the Right Rubber Hardness for Sealing
Introduction
A lot of sealing failures trace back to one overlooked spec: hardness. Engineers often default to whatever value worked on the last project, or assume "harder is more durable." Neither assumption holds up once you look at how a seal actually behaves under pressure, motion, and temperature. This article covers what Shore hardness means in practice, how it affects sealing performance, and how to narrow down the right range for your application.
Shore Hardness Specifications and Material Performance Differences
Sealing applications almost always use the Shore A scale, running roughly from 20A (soft, almost gel-like) to 90A (firm, close to rigid plastic). Harder materials move onto the Shore D scale. To make the numbers easier to picture, here's roughly how different Shore A ranges feel by hand, and where they typically show up:
| Shore A Range | What It Feels Like | Typical Applications |
| 30A–40A (very soft) | Like jelly or soft putty — deforms easily under light pressure | Anti-slip mats, cushioning feet, skin-contact seals |
| 50A–60A (soft) | Like a rubber shoe sole — some give, mild rebound | General-purpose O-rings, conveyor belting, bridge bearing pads |
| 65A–75A (medium) | Like a car tire — balances flexibility and support | Sealing rings, industrial roller coatings, door seals |
| 80A–90A (firm) | Like a bicycle tire — only dents under real pressure | Wear plates, load-bearing blocks, industrial gaskets |
| 90A+ (hard) | Close to rigid plastic — minimal give |
Mold liners, hard rubber components, wiper blade edges |
These are general reference points, not fixed rules — actual feel and performance still depend on the specific compound. Hardness is one data point, not the whole picture. Compression set, temperature range, and chemical compatibility matter just as much — sometimes more. Two compounds can carry the same Shore A number and still behave very differently once you factor in how much they permanently deform after repeated compression, or how they hold up against a specific oil or cleaning agent. A 70A silicone (SIL), a 70A NBR, and a 70A EPDM will all feel similar off the shelf, but they diverge quickly once heat, oil, or ozone exposure enters the picture — silicone holds up better across temperature extremes, NBR resists oil more effectively, and EPDM handles weathering and ozone better. FKM sits in its own category, generally firmer (60A and up) and valued mainly for chemical and high-temperature resistance rather than flexibility. That's why hardness should be treated as a starting filter, not the final answer — the material family matters just as much as the number.
Where Hardness Actually Matters for Sealing
Compression and sealing force. Softer compounds fill in surface irregularities more easily, which is why low-pressure static seals often lean soft (30A–50A). Push that too far under high pressure, though, and the material extrudes into the gap it's supposed to seal. Harder compounds hold their shape under pressure but need more compressive force to seat properly — under-compress a 90A seal and it may not seal at all.
Wear and service life. Dynamic seals — reciprocating pistons, rotating shafts — take continuous friction. Soft materials wear down faster in that environment. Most dynamic seals land somewhere in the 60A–80A range, trading a bit of conformability for longer service life.
How hard it is to actually install. This gets overlooked. Very soft O-rings twist or pinch during assembly, which shows up later as scrap or field failures. Very hard seals don't flex enough to seat cleanly and can scratch the mating surface on the way in. Worth checking against your actual assembly process, not just the spec sheet.
Temperature and media exposure. Hardness isn't a fixed number once the part is in service. Cold stiffens rubber and reduces elasticity; heat softens it and speeds up compression set. Prolonged exposure to oils or solvents can also soften a compound below its rated hardness. Datasheet hardness is typically measured at room temperature — if your application runs well outside that range, or involves aggressive media, it's worth checking hardness and compression set data at your actual operating temperature rather than relying on the standard spec alone.
Picking Hardness by Application
For static seals — flange gaskets, O-rings that don't move — conformability usually wins, so 40A–70A is a reasonable starting range. For dynamic seals — anything reciprocating or rotating — you're balancing wear resistance against flexibility, and 60A–80A is the more common range. A few industry patterns worth knowing:
- Automotive seals exposed to heat and oil (engine, fluid systems) often run 70A–90A.
- Medical device seals in contact with the body or fluids tend toward 30A–60A, prioritizing softness and biocompatibility, on top of whatever certifications apply.
- Electronics and waterproofing gaskets usually use soft silicone, 20A–50A, since the seal relies on gentle, consistent compression rather than resisting pressure. If you're starting from scratch: nail down the operating conditions first — static or dynamic, pressure, temperature, media — then pick the material family, then fine-tune hardness within that family based on how the part gets assembled and compressed. It's also worth checking whether the groove or housing design was engineered around a specific hardness assumption; swapping in a softer or harder compound without adjusting compression ratio or groove fill can undo the benefit you were trying to get in the first place.
A Few Mistakes Worth Avoiding
Assuming harder always seals better. It doesn't — sealing performance comes from compression, fit, and rebound working together, not hardness in isolation. Treating the same hardness number as equivalent across materials. A 70A silicone and a 70A FKM will behave quite differently under load or heat, even with the same rating — the same goes for NBR versus EPDM at matching hardness, since one is built for oil resistance and the other for weathering. Selecting purely off the datasheet. Room-temperature hardness is a reference point. What the seal actually experiences in service — heat, cold, chemical exposure — can shift its real-world behavior meaningfully.
FAQ
1.Is higher Shore hardness always more durable?
Not automatically. It resists extrusion better, but conformability drops, and durability really comes down to the specific application — pressure, motion, and what the seal is exposed to.
2.What hardness works best for O-rings?
70A is a reasonable general-purpose default. Low-pressure static seals sometimes do better softer, around 50A–60A, while high-pressure dynamic seals may need 80A–90A.
3.Does a seal's hardness change over its service life?
Yes. Compression set, heat aging, and chemical exposure all shift a seal's effective hardness over time, which is part of why sealing force gradually weakens.
4.Can two materials with identical hardness numbers behave differently?
Definitely. Hardness is one property among several. NBR, EPDM, silicone, and FKM can all be produced at the same Shore A number, yet differ significantly in elasticity, compression set, and chemical resistance — the material family often matters more than the hardness digit itself.
5.How do I figure out the right hardness for my project?
Share your pressure range, temperature range, media type, and whether the seal is static or dynamic. That's usually enough for an engineer to narrow the range and suggest a sample for testing.
Conclusion
Hardness selection should follow from how the seal is actually used — static or dynamic, under what pressure, installed how — rather than a default assumption that harder is better. The same hardness number doesn't mean the same performance across different compounds, and real operating conditions will shift a seal's behavior away from its datasheet value. Build in margin accordingly.
Hardness rarely gets chosen in isolation — it works alongside compound selection, mold design, and how the part gets assembled. At LT Rubber, our team reviews your working pressure, temperature, and media conditions to recommend a material and hardness range, and we can produce small-batch samples for validation before you commit to full production. Not sure which hardness fits your application? Reach out to our engineering team for a free material consultation.
