Hardness Is Not a Spec: How to Select Rubber Hardness for Sealing (Without Guesswork)

If you design the groove, specify the seal, or troubleshoot leakage—this is for you.

Shore A hardness is a useful control parameter, but by itself it is not a sealing strategy.

Seals don’t fail because “55 Shore A was wrong.” They fail because the joint did not generate and maintain the required contact pressure across tolerances, temperature, time, movement, and assembly realities.

Hardness is one input. Sealing is a system outcome: design × compound × cure × installation × environment.

1) What hardness actually controls (and what it doesn’t)

Hardness influences

• Contact pressure at a given squeeze (harder → higher contact pressure for the same compression, up to a point)
• Conformability (softer → fills micro-gaps better; tolerates rough/uneven surfaces)
• Assembly force & friction tendency (hardness affects it, but geometry and surface often dominate)
• Stability in the groove (harder carriers resist creep and pull-out better)

Hardness does not reliably predict

• Compression set (a 70A compound can have poor CS; a 55A can be excellent)
• Long-term sealing retention (stress relaxation + joint design dominate)
• Leak tightness by itself (geometry + squeeze + tolerance variation matter more)

2) The 4 variables that should drive hardness selection

A) Squeeze % (the first “real” spec)

Squeeze = how much the seal is compressed in the joint.

• Low squeeze (5–15%) → needs high conformability or geometry that concentrates contact pressure
• Medium squeeze (15–30%) → most seals live here
• High squeeze (30–45%+) → risk of high assembly force + long-term set if not designed carefully

Key point: For a given geometry, hardness is your dial to reach target contact pressure within your squeeze window.

B) Contact pressure requirement (what you’re really buying)

A seal leaks when contact pressure is lower than the forces that open micro-leak paths:

• internal fluid/air pressure
• joint roughness, parting lines, tolerance gaps
• deformation under service load

This is why 70A can leak: if squeeze is limited or the geometry doesn’t load correctly, you get inadequate contact pressure where it matters.

C) Friction & assembly force (the hidden failure mode)

A seal that seals in theory but cannot be assembled consistently becomes a production failure:

• seals are stretched, twisted, installed dry, or damaged
• seating is incomplete (and leakage appears “unexplained”)

Hardness influences force, but surface tack, lubrication strategy, and groove geometry often dominate.

D) Tolerance stack-up & movement (static vs dynamic)

• High gap variation → softer seals maintain contact better
• Dynamic sliding → too soft can increase wear and frictional heat
• Pressure + extrusion gap risk → may require higher hardness and/or anti-extrusion support

3) Why 70A can leak and 55A can seal better (and vice versa)

Case 1: 70A leaks, 55A seals better

Typical when:

• squeeze is limited
• tolerance variation is high
• surfaces are rough/uneven
• geometry spreads load rather than concentrating it

Result: 70A may not conform enough to close micro-gaps at low squeeze; 55A can.

Case 2: 55A leaks, 70A seals better

Typical when:

• higher internal pressure or pressure peaks exist
• extrusion gaps are present
• seal must resist deformation to maintain contact pressure

Result: 55A deforms or relaxes too much; 70A holds shape and pressure.

Takeaway: Hardness is not “better or worse.” It is “matched or mismatched.”

4) The 40 Shore A zone: when soft seals are the right answer (and when they fail)

A) 40A solid O-cords / rings (static sealing, high conformity)

Works well when

• low-to-moderate pressure static sealing
• imperfect alignment, surface irregularities, slight ovality
• low assembly force + high conformity are required

Can fail when

• an extrusion gap exists (clearance + pressure)
• long-term compressive load drives stress relaxation and loss of contact pressure
• high stretch during fitment demands excellent elastic recovery

Practical guidance

• Use 40A when extrusion is controlled by design
• Validate sealing after dwell (24–72 hours), not only on day one

B) 40A hollow EPDM seals with splice joints (easy-fit / roll-fit)

Common pitfalls

• high friction/tack → difficult sliding/rolling → installation damage
• splice weakness due to localized strain
• compression drift when squeeze varies or stress relaxation is high

What to control

• friction/tack behavior (functional surface requirement)
• stress relaxation at service temperature
• splice design + cure matching

5) When one hardness can’t do two jobs: co-extruded seals (hard carrier + soft bulb)

Typical architecture

• Hard carrier (~65–75A, dense): retention, groove stability, pull-out resistance
• Soft bulb (~35–45, often sponge/foam): tolerance take-up, sealing at low force

Important note: For sponge, density + compression behavior often matter more than Shore A.
Define: density range + load-deflection at specified squeeze + recovery after cycling.

Validate: interface peel/tear strength, especially at corners/radii and after cycling.