For most custom silicone seals, the best Shore hardness range is 40-60 Shore A. Choose 30-40 Shore A for low-pressure, high-conformity seals; 50-60 Shore A for general industrial and dynamic applications; and 60-70 Shore A for higher pressure or structural support. The final choice depends on compression available in the groove, operating temperature, media exposure, and whether the seal is static or dynamic. FromRubber tests every lot to ASTM D2240 and can formulate from 10 Shore A to 90 Shore A for custom compounds.
Shore hardness is often misunderstood as a "feel" property, but for seals it is a functional specification. Too soft and the seal extrudes or loses shape; too hard and it cannot conform to surface imperfections, causing leakage. Selecting the right value requires balancing four engineering factors, not guessing.
How application requirements drive Shore A selection
Use this decision framework instead of defaulting to "70 Shore A because it is standard":
▪Sealing force and groove compression. A static flange gasket usually needs 20-30% compression. Softer silicone (40-50 Shore A) achieves this at lower bolt load. Harder material requires more clamping force and may not seal against machined surface roughness.
▪Pressure and extrusion risk. At pressures above 5 bar (72 psi) or with large clearance gaps, specify 60-70 Shore A or add a backup ring. Soft seals extrude into gaps under pressure and lose sealing force.
▪Dynamic vs static service. Reciprocating or rotary seals need enough hardness to resist abrasion but enough softness to follow the mating surface. Most dynamic silicone seals are specified at 50-60 Shore A.
▪Temperature effects. Silicone stiffens at low temperature and softens at high temperature. A seal specified at 50 Shore A at 23°C may behave like 58-60 Shore A at -40°C. Specify hardness for the actual operating temperature, not room temperature.
Case study: hardness mismatch in a food-processing gasket
A food-equipment OEM specified 70 Shore A silicone for a clamp-style pipe gasket because the engineering team believed harder material meant longer life. After installation, the gasket leaked at multiple joints during the first pressure test.
FromRubber reviewed the design and found that the tri-clamp fittings provided only 15% available compression. At 70 Shore A, the gasket could not deflect enough to fill minor surface imperfections on the stainless-steel ferrule. We recommended switching to 50 Shore A FDA silicone and tightening the cross-section tolerance to ±0.1 mm.
The revised gasket passed hydrostatic testing at 10 bar and sanitary clean-in-place (CIP) cycling. The customer later standardized on 50 Shore A for all tri-clamp gaskets up to 150°C, reducing both leak reports and assembly torque complaints.
Shore A hardness selection guide by application
| Shore A range |
Feel / behavior |
Best applications |
Watch out for |
| 20-30 |
Very soft, high conformability |
Light-contact seals, damping pads, membranes |
Extrusion under pressure; poor dimensional stability |
| 40-50 |
Soft, excellent sealing at low force |
Static gaskets, flanges, food/medical seals |
Not for high-pressure or large clearance gaps |
| 50-60 |
Balanced elasticity and stability |
General O-rings, dynamic seals, industrial gaskets |
May need backup ring above 5-7 bar |
| 60-70 |
Firm, good gap resistance |
Higher-pressure seals, automotive, large sections |
Requires higher assembly force; less conforming |
| 70-90 |
Hard, structural |
Stops, guides, load-bearing components |
Generally unsuitable for sealing |
Always specify hardness with a test standard (ASTM D2240 or ISO 7619) and tolerance. Industrial tolerance is typically ±5 Shore A per batch; tighten this only when the design is sensitive to small hardness changes.
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