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Shore A Durometer and Rubber Seal Performance
Apr
Shore A Durometer and Rubber Seal Performance
Shore A durometer is the single most commonly specified rubber property, and one of the most frequently misapplied. Most engineers know that harder rubber resists extrusion and softer rubber conforms to surfaces. Fewer understand how temperature shifts the effective hardness at the point of sealing, why 70 Shore A became the default and when to deviate from it, or why a hardness reading taken from a thin or curved part is not the same as the material’s actual hardness. This article covers what durometer actually controls in a seal, and how to use it correctly in design.
What Shore A Durometer Measures
Shore A hardness is measured per ASTM D2240 using a durometer instrument that presses a truncated cone-shaped indenter into the rubber surface under a defined spring load. The depth of penetration after a standardized dwell time (15 seconds under the standard) gives a reading on the 0 to 100 Shore A scale. Zero represents maximum penetration (extremely soft); 100 represents zero penetration (extremely hard). The scale is linear in indentation depth but not in material stiffness, which matters when comparing values at the extremes.
What durometer measures is resistance to indentation, a static property tested on a flat sheet sample. It correlates with, but is not identical to, the dynamic stiffness of a seal under operating load, the contact pressure the seal generates against a mating surface, or the seal’s ability to recover from compression. These are all influenced by durometer, but also by compound formulation, cure system, cross-section geometry, and temperature. Specifying durometer is a necessary first step; it is not a complete seal specification.
The Shore A Scale in Practical Terms
Why 70 Shore A Is the Default and When to Change It
70 Shore A has become the default specification for o-rings and many molded seals because it represents the center of a favorable property trade-off: soft enough to conform to standard machined surface finishes under typical bolt loads, hard enough to resist extrusion at moderate pressures, and stiff enough to maintain dimensional stability in the gland during assembly and service. For most static sealing applications in the 100 to 1,500 psi (7 to 100 bar) range on a standard machined surface, 70 Shore A is a defensible starting point that has been validated across millions of seal installations.
The correct reason to deviate from 70 Shore A is a specific application condition that the default cannot meet, not a preference for a “premium” harder material or a “softer for better sealing” assumption. The following conditions justify deviation:
The mating surface is rough or irregular and available bolt load is limited. The seal must conform to surface finish variation that a 70A compound cannot fill under the available clamping force. Also appropriate for low-pressure environmental sealing (dust, water ingress) where conformance is the primary requirement and extrusion is not a risk.
System pressure is high enough that a 70A seal extrudes into the diametral clearance gap between the piston/rod and bore. Dynamic reciprocating applications where abrasion wear at 70A is unacceptable. Hydraulic systems above approximately 1,500 psi (100 bar) where extrusion risk is documented. Anti-extrusion back-up rings (PTFE or harder elastomer) are often a more cost-effective solution than specifying 90A primary seals.
When the mating hardware is fragile (thin-walled plastic, glass, or ceramic) and cannot accept the insertion force a 70A seal requires for installation. Medical devices, laboratory instruments, and consumer electronics enclosures often require 40 to 55A seals to minimize the force applied to delicate substrates during assembly.
If the seal must perform at very low temperatures, rubber hardens significantly below its glass transition zone. A 70A compound at -40°F may behave like 90A or harder, losing the flexibility needed to maintain sealing contact. For cold-climate or cryogenic service, specify a softer compound at room temperature so that the effective hardness at minimum operating temperature is still in the functional range. See the temperature section below.
How Temperature Changes Effective Hardness
Shore A hardness is measured at room temperature (23°C / 73°F per ASTM D2240). The material’s actual resistance to indentation changes significantly with temperature, and this is one of the most commonly overlooked aspects of seal design.
At Elevated Temperatures
As temperature rises, rubber softens. A 70 Shore A compound may behave like 55 to 60 Shore A at the maximum operating temperature of a high-temperature application. For most sealing scenarios, softening at elevated temperature improves conformance to the sealing surface and is not a problem. The concern arises in high-pressure dynamic applications where a softer-than-specified seal at operating temperature has inadequate extrusion resistance. Design engineers should calculate extrusion risk at maximum operating temperature, not at room temperature, when the compound will be at its softest.
Conversely, if the application requires a specific assembly force (for snap-fit or push-in seals), the force measured at room temperature may be higher than the force at operating temperature. Prototype testing at operating temperature avoids surprises at first installation on production hardware.
At Low Temperatures
Below approximately -20°F (-29°C), most rubber compounds begin to stiffen significantly as they approach their glass transition temperature. At the glass transition, rubber loses its elastic behavior and becomes brittle. The practical consequence for sealing is that a 70 Shore A nitrile seal at -40°F may be effectively as hard as 95 Shore A or higher, so stiff it cannot conform to the sealing surface under available bolt load, and so inflexible that it may crack under assembly stress.
For low-temperature applications, the relevant specification is the compound’s brittle point or TR-10 temperature (the temperature at which the material has retracted 10% from a stretched state), not just the Shore A rating at ambient. Low-temperature grades of nitrile, EPDM, and silicone are formulated to maintain useful flexibility at temperatures where standard grades would stiffen to the point of seal failure. Specifying these grades is not about changing the room-temperature Shore A; it is about choosing a compound whose stiffness vs. temperature curve is suitable for the cold service condition.
Durometer and Extrusion Risk in O-Ring Glands
The most consequential practical relationship between Shore A hardness and seal performance in o-ring applications is the control of extrusion. When system pressure pushes an o-ring against the diametral clearance gap between the rod/piston and the bore, a soft o-ring will flow into the gap and eventually extrude through it, causing nibbling damage that progresses to catastrophic seal failure. A harder o-ring resists this flow.
The extrusion risk for a given o-ring depends on three variables: system pressure, diametral clearance gap, and o-ring hardness. At a given pressure, a tighter clearance gap allows a softer o-ring to be used without extrusion risk. With a larger clearance gap, a harder compound or an anti-extrusion back-up ring is required. The relationship is non-linear: small increases in clearance gap at high pressure can dramatically increase extrusion risk.
| System Pressure | 70 Shore A | 80 to 90 Shore A | Backup Ring Recommended |
|---|---|---|---|
| Below 1,500 psi (100 bar) | Acceptable with standard clearance | Not required | No, for static sealing |
| 1,500 to 3,000 psi (100 to 200 bar) | Acceptable with tight clearance | Preferred with larger gaps | Consider for dynamic sealing |
| Above 3,000 psi (200 bar) | Extrusion risk: not recommended | May still require backup ring | Yes, strongly recommended |
Use the Gland Calculator to verify gland fill percentage for your o-ring cross-section. Gland fill above 85 to 90% at operating temperature increases extrusion risk at clearance gaps, particularly when thermal expansion of the rubber at elevated temperature is also considered.
Durometer and Compression Set
Compression set is the percentage of the original deflection that a rubber material retains permanently after being compressed for a defined time at a defined temperature. A material with 20% compression set has permanently deformed to retain 20% of the original deflection after the load is removed. A material with zero compression set has fully recovered to its original thickness.
Compression set is the primary mechanism by which rubber seals lose sealing force over time. The bolt load in a gasketed joint, or the housing squeeze in an o-ring gland, compresses the rubber to the target deflection at assembly. Over time, as the rubber takes a permanent set, the effective spring-back force against the sealing surfaces decreases. When the sealing force drops below the threshold needed to close the micro-leakage paths in the joint, the seal leaks.
Harder compounds do not inherently have better compression set than softer compounds. Compression set is primarily a function of the cure system and the compound formulation, not the hardness level. A 70 Shore A peroxide-cured EPDM compound typically has significantly better compression set resistance than a 70 Shore A sulfur-cured EPDM compound of the same hardness. Specifying durometer alone does not control compression set behavior.
Compression set increases with temperature. A compound with acceptable compression set at room temperature may develop significant permanent deformation when held at elevated operating temperature. For seals in high-temperature static service, the compression set should be evaluated at the operating temperature, not at the standard ASTM D395 test temperature of 70°C, which may not represent the actual service condition.
When long-term sealing retention is critical, specify both durometer and the maximum acceptable compression set (for example, “compression set not to exceed 20% per ASTM D395 Method B at 100°C for 22 hours”). This gives the manufacturer the compound performance target, not just the hardness, and allows the correct cure system to be selected.
Measurement Accuracy: When Hardness Readings Are Unreliable
Shore A hardness readings can be significantly inaccurate if the measurement conditions don’t meet ASTM D2240 requirements. The most common sources of error on incoming inspection:
- Thin samples: ASTM D2240 requires a minimum specimen thickness of 6 mm (0.25 in.) for Shore A testing. Thinner samples allow the indenter force to be partially transmitted through the rubber to the backing surface, producing an artificially high hardness reading. O-rings tested on a flat surface give falsely high readings if the cross-section is below 6 mm. Use a test block conforming to the o-ring curvature, or test on stacked samples of sufficient total thickness.
- Curved surfaces: Testing directly on a curved surface (o-ring, extruded profile, or tubing) produces lower readings than the true material hardness because the indenter contacts a curved surface rather than a flat one, reducing effective contact force. Readings from curved parts should be corrected or compared to flat sheet samples of the same compound.
- Instantaneous vs. dwell readings: Shore A hardness decreases over the first 15 seconds after the indenter contacts the rubber as the material creeps. Instantaneous readings (read immediately at contact) are higher than 15-second dwell readings. ASTM D2240 specifies the 15-second dwell value. Inspectors using hand durometers who read the value immediately upon contact will report a higher hardness than the standard-compliant value. Specify whether your acceptance criterion is based on instantaneous or 15-second readings to avoid inconsistency between your lab and your supplier’s.
- Temperature of the sample: Hardness is temperature-dependent. Samples tested immediately after removal from cold storage or after exposure to heat will give readings that don’t represent room-temperature material properties. ASTM D2240 requires conditioning at 23°C ±2°C for at least three hours before testing.
- Single-point vs. average: Hardness varies across a molded part due to compound flow and cure uniformity during molding. A single reading may not represent the average hardness of the part. For acceptance testing, take a minimum of three readings at different locations and report the average.
How to Correctly Specify Durometer in Your Design
A complete durometer specification for a rubber seal should include the following:
- Shore A value and tolerance: specify the nominal hardness and an acceptable tolerance band. Standard tolerance for most rubber compounds is ±5 Shore A. Tighter tolerances (±3 Shore A) are achievable but add cost and should only be required when the application genuinely requires it.
- Test method: ASTM D2240 is the US standard; ISO 7619-1 is the international equivalent. Specify which standard applies to your acceptance testing to avoid disputes between results.
- Dwell time: specify 15 seconds for standard testing. If your application requires the instantaneous value for some reason, state this explicitly.
- Sample condition: specify “cured sheet” or “finished part with test block” for o-rings and curved profiles. Accepting readings from unsupported curved parts without this callout allows inaccurate readings to pass inspection.
- Compression set requirement where applicable: for any application where long-term sealing retention matters, specify maximum acceptable compression set per ASTM D395 Method B at the relevant test temperature. This is a separate specification from hardness but directly related to the seal’s service life.
Summary
Shore A durometer controls a seal’s resistance to indentation, which directly affects conformance to sealing surfaces, extrusion resistance, assembly force, and as a downstream consequence, compression set behavior over the service life. The 70 Shore A default is well-supported for most static sealing applications at moderate pressures on standard machined surfaces. Deviating from it requires a specific engineering reason: surface finish limitations, pressure-driven extrusion risk, assembly force constraints, or temperature-adjusted stiffness requirements.
Durometer is a necessary specification but not a sufficient one. Specifying 70 Shore A without a cure system callout, compression set requirement, or test method leaves the most important long-term sealing performance variable uncontrolled. The full seal specification addresses hardness, compound, cure system, compression set, and dimensional tolerances together.
Wayne Rubber’s engineering team can review your application conditions and recommend the right hardness, compound, and cure system for your seal. For o-ring gland design, the Gland Calculator covers fill percentage, squeeze, and clearance gap recommendations for all AS568 cross-section series.
Discuss Seal Durometer with Wayne Rubber’s Engineering Team
Send us your application conditions and we will recommend the right Shore A specification, compound, and cure system for your seal.