Engineering Answers / Material & Selection
A durometer number tells you how hard the material feels. It does not tell you whether the seal will hold at temperature — and that gap causes most of the wrong specifications we see.
Shore A measures resistance to indentation, which is a proxy for stiffness. On a seal, higher hardness gives more sealing force but conforms less to a rough or warped surface and needs more compression load. On a consumer part, hardness controls whether it feels soft and grippy or firm and structural. Our stock range is Shore 20A to 80A in ten-point increments, and the choice should come from the sealing or feel requirement, not from a competitor's data sheet.
Shore A is an indentation test: a spring-loaded conical indentor is pressed into the material and the depth of penetration is read on a scale from 0 to 100. Soft rubber reads low, hard rubber reads high. It says nothing directly about tensile strength, tear resistance, elongation or compression set.
That matters because buyers often use hardness as a shorthand for "quality" or "durability". It is neither. A 40A compound can easily outperform a 60A compound in tear and in compression set, depending on the polymer, the filler loading and the cure.
What hardness genuinely controls is stiffness, and stiffness is what the application feels:
A seal works by being compressed. The compression generates contact stress, and contact stress is what stops the leak. Harder material generates more stress for the same compression — but it also needs more force to achieve that compression, and it conforms less to a rough or warped mating surface.
That trade produces a real design rule:
| Service condition | Hardness direction | Reason |
|---|---|---|
| Rough or warped mating surface | Softer | More conformability, more contact area |
| High internal pressure, narrow gap | Harder | Resists extrusion into the gap |
| Deep groove, high compression | Harder | Maintains force without over-compressing |
| Assembly by hand, low clamp load | Softer | Reaches sealing stress with less force |
| High temperature, long dwell | Harder plus better set | Resists softening and permanent deformation |
The failure modes are mirror images. Too soft, and the seal extrudes into the gap under pressure or takes a permanent set and stops sealing after a few cycles. Too hard, and it never conforms enough to seal a surface that is not perfectly machined, so it leaks from day one at a pressure the drawing said it should hold.
Hardness is the single biggest lever on how a moulded silicone part feels, and consumer programmes usually have a target feel before they have a target number. A useful translation:
Two things complicate the translation. First, wall thickness changes perceived softness as much as hardness does — a 1 mm wall at 60A feels softer than a 3 mm wall at 40A. Second, surface finish changes grip: a matte texture at 50A feels grippier than a polished 50A.
The range we hold is Shore 20A to 80A in ten-point increments, and the ends behave differently.
Very soft (20–30A). Harder to demould, more prone to distortion during ejection, and more likely to tear at the parting line. In compression moulding the charge placement becomes critical because the material does not flow readily. Zero-draft and minor undercuts are still feasible thanks to silicone's low elastic modulus and elongation at break of 300%–800%, but the ejection force has to be gentle.
Very hard (above 70A). Flows poorly in LSR injection and needs more injection pressure, which increases flash risk at the shut-offs. It also feels less like silicone and more like a stiff rubber, which is usually not what the product wanted.
A complete callout is more than a number. Ours would read: Shore A 50 ±5, measured on the finished part per ASTM D2240, 15-second reading, service temperature 120 °C.
Four things belong on the drawing or in the RFQ:
Ask "what is this hardness meant to achieve" before quoting. If the answer is "the seal must hold 0.5 bar at 120 °C", the conversation can move to compression set and groove design — which is where the leak will actually be decided. If the answer is "it should feel like the competitor's", the answer is a prototype in two hardnesses, not a specification.
Shore A is stiffness, not strength. On a seal it sets the balance between sealing force and conformity; on a consumer part it sets the feel. Specify it with a tolerance, a measurement method and the service temperature, and check it against the function rather than a competitor's data sheet. If the part has to seal at temperature, hardness is only half the story — compression set is the other half.
No. Shore A measures indentation resistance, and different elastomer families reach the same number with different modulus, elongation and compression set. A 50A EPDM, a 50A nitrile and a 50A silicone will feel similar under a durometer and behave very differently in a seal at 120 °C. The number is a starting point for the material family, not a cross-family equivalence.
±5 Shore A is the normal commercial tolerance, and tighter than that starts to cost money without improving function, because durometer readings on a curved or thin part already carry several points of uncertainty. What matters more is specifying the measurement method — on a flat plaque versus on the finished part — and the conditioning time before reading.
Yes, and this is where specifications fail. Silicone stiffens as it cools and softens as it heats; a seal specified at 50A room temperature is not a 50A seal at 150 °C. The design should be checked at the service temperature, not at the bench. The same applies to low-temperature applications, where a soft grade can become stiff enough to leak on first actuation.
Below roughly 20A the material becomes difficult to demould and prone to distortion, and thin sections lose dimensional stability. Very soft grades also tear easily at the parting line during ejection. If the application genuinely needs a 15A feel, the usual answer is a foamed or gel-like construction rather than a solid moulding, or a geometry change that increases the perceived softness.