Application Note
Why “Correct Hardness” Still Fails in TPU / TPE Applications
In many TPU and TPE projects, material selection often begins with a single number: hardness.
While hardness is an important descriptor, it is frequently treated as a decision variable rather than a result.
This oversimplification is one of the most common reasons why applications fail even when the “correct” hardness has been selected.
While hardness is an important descriptor, it is frequently treated as a decision variable rather than a result.
This oversimplification is one of the most common reasons why applications fail even when the “correct” hardness has been selected.
Application Context
Hardness-based selection is common across extrusion, injection molding, overmolding, and soft-touch applications.
In early project stages, materials with similar Shore hardness values are often considered interchangeable.
However, field performance frequently diverges despite identical nominal hardness.
In early project stages, materials with similar Shore hardness values are often considered interchangeable.
However, field performance frequently diverges despite identical nominal hardness.
Key Constraints That Hardness Does Not Capture
- Material family:
TPU (polyether vs polyester), SEBS-based TPE, and other elastomer systems behave fundamentally differently at the same hardness. - Processing behavior:
Flow stability, shear sensitivity, and melt strength are not reflected in a hardness value. - Time-dependent performance:
Aging, compression set, migration, and long-term elasticity evolve beyond initial testing.
Material Selection Logic
Hardness should be viewed as an outcome of formulation and structure, not the primary selection criterion.
Two materials at the same Shore A value may differ significantly in molecular architecture, soft segment chemistry,
and additive systems, leading to different real-world behavior.
Two materials at the same Shore A value may differ significantly in molecular architecture, soft segment chemistry,
and additive systems, leading to different real-world behavior.
In practice, hardness is often the easiest parameter to compare, but it rarely governs long-term stability,
process robustness, or field reliability.
process robustness, or field reliability.
What Is Often Overlooked at Early Stage
- Short-term sample approval does not represent long-term service behavior.
- Hardness equivalence does not imply processing equivalence.
- Adjustments made to meet hardness targets may compromise stability elsewhere.
Where This Consideration Fits in Our Portfolio
Applications where hardness appears to be the primary requirement are typically addressed under standard TPU and TPE categories.
When hardness must be balanced with processing stability, aging resistance, or multiple performance constraints,
a broader material review is often required.
When hardness must be balanced with processing stability, aging resistance, or multiple performance constraints,
a broader material review is often required.
For applications involving long-term stability, processing consistency, or multiple constraints,
material selection often requires evaluation beyond hardness alone.
material selection often requires evaluation beyond hardness alone.
Post time: Jan-07-2026