Incorrect Hardness Selection

Definition and Context (Shore Hardness, What “Incorrect” Means)

Incorrect hardness selection refers to choosing a hardness for an elastomer or plastic sealing material that does not match the actual operating conditions. In practice, this usually means the Shore hardness. “Incorrect” here does not mean that a Shore value is fundamentally unsuitable, but that the seal yields too much or too little in operation. Both can worsen the sealing function and lead to leakage, wear, or even failure.

The Shore A hardness is an indentation hardness for elastomers. It describes how strongly a standardized indenter penetrates the material under a defined force. Measurement is typically carried out with a durometer according to standards such as ASTM D2240 or ISO 48-4. The measured value is comparable, yet it is only one parameter among many.

In sealing technology, what matters is whether the seal maintains sufficient contact force under pressure, gap dimension, temperature, motion, and over time, without wearing excessively. Therefore, a Shore value alone is no functional guarantee. Two materials with the same Shore hardness can behave very differently in practice, because formulation, fillers, and degree of crosslinking strongly influence long-term properties.

Hardness Is Not the Same as Sealing Function

Shore hardness describes the resistance against local indentation. Yet it says only to a limited extent how well a seal seals over months or years. For sealing performance, compression set and stress relaxation are often at least as important.

Compression set is the permanent deformation after sustained compression. High values mean: the seal does not return and loses geometry. Stress relaxation describes the decay of contact force under constant deformation. This force loss can occur even when the seal still looks geometrically sound. Both effects can lead to loss of sealing despite a fitting Shore hardness.

Typical Failure Patterns: Too Soft vs. Too Hard

An incorrect hardness selection usually shows up in two directions. When the seal is too soft, it deforms excessively under pressure. When it is too hard, it does not conform sufficiently to the mating surface. Which direction is critical depends on whether the application is static or dynamic, how high the pressure is, and how large the extrusion gap is.

Extrusion and Shearing (Too Soft)

Extrusion means that material is pushed through the extrusion gap (gap dimension between components) toward the low-pressure side. With sufficient pressure, the seal can flow into the gap. When this extruded section is subsequently mechanically loaded, shearing occurs. Typical signs are frayed edges, material lugs, or local break-outs.

Whether extrusion occurs often depends on a combination: pressure × gap dimension × temperature × material stiffness. Temperature, in particular, is tricky. Many elastomers soften at higher temperatures, which lowers the effective stiffness and makes extrusion more likely.

Micro-Leakage, Friction, and Assembly Issues (Too Hard)

An overly hard seal can build up high surface pressure under sufficient deformation, yet it bridges micro-unevenness less effectively. This becomes particularly visible at low pressures, because less sealing energy is then available. In practice, this often shows up as micro-leakage — that is, small but continuous leak rates.

In dynamic applications, excessively high hardness frequently raises friction. More friction generates more heat, and heat in turn accelerates aging and wear. In addition, the assembly force rises, which encourages assembly errors and can load components.

Influencing Factors for the Right Hardness Selection (Practical Check Questions)

The right hardness arises from the operating conditions at the sealing point. What matters is what is sealed, where the seal works (static or moving), which pressure applies, including peaks, and how large the actual gap dimension is. Equally important is which temperature is actually present at the sealing point, because it can deviate significantly from the ambient — for example, through frictional heat.

In many cases, a short, structured clarification leads more quickly to the right material than comparing Shore values:

• Does the seal act statically (flange, plug) or dynamically (piston, rod, rotation)?
• How high are the differential pressure and pressure peaks, and are there frequent load cycles?
• How large is the gap dimension under tolerances, eccentricity, and operating pressure?
• Which media are present (oil, water, fuel, solvents), and are there chemical interactions?
• What are roughness and mating-surface hardness, and how critical is leakage for safety or environment?

At high pressures with a relevant gap, the extrusion risk often dominates. Under dynamic operation, the focus shifts toward friction, heat, and wear.

Temperature and Time Effects (Cold, Relaxation, Set)

Shore hardness is usually given at room temperature. In operation, however, the material can behave significantly differently. In the cold, elastomers become stiffer. As they approach the glass transition (the temperature range in which the material strongly hardens), elasticity and conformability decrease. As a result, leakage after motion or after pressure release can be encouraged, because the seal recovers more slowly or incompletely.

Over time, stress relaxation and compression set act. Therefore, contact force can drop even though the seal was tight initially. In the design, what matters is how long the seal must remain tight and which temperature and pressure cycles occur.

Corrective and Preventive Measures (When Hardness Alone Is Not Enough)

When an application sits at the limit, the problem can often be solved through design rather than simply by raising hardness. Against extrusion, controlling the gap dimension usually helps first. Better guidance, tighter tolerances, or a different bearing concept reduce the extrusion gap and lower the load on the seal.

When the gap cannot be made small enough by design, or when pressure is high, a back-up ring is an effective extrusion protection. It supports the elastomer seal on the low-pressure side and limits extrusion into the gap.

For dynamic applications, the robust solution is often a balance: sufficient conformability to the surface together with acceptable friction. In practice, this means combining hardness with the right geometry, surface quality, and, where needed, support or sliding elements. A short consultation with specialized seal experts is sensible when pressure, gap, and temperature vary strongly or when leakage is particularly critical.