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Views: 0 Author: Site Editor Publish Time: 2026-09-22 Origin: Site
An EPDM foam gasket can feel soft in your hand and still require too much force to close an enclosure.
Another material may compress easily when it is new but fail to recover well after remaining compressed for a long period.
These are different material-selection problems.
For gasket buyers and design engineers, EPDM foam compression deflection, compression set and stress relaxation describe different parts of the material's behavior. Looking at only one number can lead to the wrong foam being selected.
A useful comparison should therefore answer three separate questions:
Property | What it tells you | Typical design question |
|---|---|---|
Compression deflection | Force or pressure required to compress the foam | Can the enclosure close without excessive force? |
Compression set | Permanent deformation remaining after compression and recovery | Will the gasket recover after the load is removed? |
Stress relaxation | Loss of sealing force while held at a fixed compression | Will the gasket maintain contact pressure over time? |
None of these values should be evaluated without the test conditions.
Compression deflection describes how much pressure is required to compress foam by a specified percentage of its original thickness.
For example, a supplier may report compression stress at:
10% compression
25% compression
30% compression
50% compression
The percentage matters.
A material requiring relatively little pressure at 25% compression may behave very differently when compressed to 50%.
This is especially important when the gasket is installed between two surfaces with an uneven gap.
Imagine an enclosure where a foam gasket sits between a lid and a metal housing.
If the foam is too firm, closing the lid may require excessive force.
Possible results include:
Panel distortion
Plastic housing deformation
Excessive fastener load
Difficult assembly
Inconsistent closure
High stress around hinges or clips
If the foam is too soft, the opposite problem may occur.
It may not generate enough contact force against the mating surfaces to accommodate irregularities or maintain the intended seal.
The goal is therefore not simply to choose the softest foam.
The goal is to identify an appropriate compression range for the actual assembly.
Suppose a hypothetical foam reports a compression stress of:
5 kPa
at the intended compression.
If the nominal compressed gasket area is:
0.01 m²
a simplified calculation gives:
5,000 N/m² × 0.01 m² = approximately 50 N
This can provide a first estimate of the foam compression load.
However, it is not the same as the measured force required to close the finished enclosure.
Actual assembly force may also be affected by:
Gasket geometry
Corner design
Friction
Adhesive
Uneven gaps
Housing stiffness
Fastener positions
Local over-compression
For important projects, evaluate the foam in the actual enclosure rather than relying only on a laboratory value.
A common quotation may show only:
Compression deflection at 25%
That number is useful, but it may not describe the customer's real installation.
Suppose the gasket thickness is 10 mm.
Because of manufacturing tolerances, the installed gap may vary between 6 mm and 8.5 mm.
The foam could therefore experience very different compression levels across the same enclosure.
In this situation, it is more useful to request compression data across the expected operating range.
A practical request would be:
Please provide compression stress at the minimum, nominal and maximum installed compression, together with the test method, sample thickness and conditioning conditions.
That gives the supplier a real engineering question instead of simply asking for a “soft EPDM foam.”
Compression set evaluates how well a material recovers after being compressed for a specified period and then released.
It is particularly relevant when the gasket:
Remains compressed for long periods
Is opened and closed repeatedly
Experiences elevated temperatures
Must recover after maintenance or servicing
A lower compression-set result generally indicates less permanent deformation under the stated test conditions.
But the test conditions are essential.
A compression-set value should ideally be accompanied by:
Compression percentage
Test duration
Test temperature
Specimen thickness
Recovery time
Test method
Without these details, two percentages may not be directly comparable.
For example, a result measured after room-temperature compression should not automatically be compared with another result measured after prolonged exposure at elevated temperature.
These two properties are often confused.
Compression set looks at shape recovery after the load is removed.
Stress relaxation looks at force retention while the foam remains compressed.
Consider a gasket that is installed and never opened.
Even if the foam remains physically compressed between the two surfaces, the pushback force it applies can gradually decrease.
That loss of force is stress relaxation.
For a long-term static seal, this behavior may be important because maintaining contact pressure can be as important as dimensional recovery.
For critical sealing applications, ask whether relevant force-retention or stress-relaxation data are available instead of relying on compression set alone.
A simple way to remember the difference is:
Compression deflection:
How difficult is the foam to compress?
Compression set:
How well does it recover after compression?
Stress relaxation:
How much sealing force does it retain while remaining compressed?
These three measurements answer different questions.
A foam with good compression-set performance is not automatically the easiest foam to compress.
Likewise, a very soft foam is not automatically the best long-term sealing material.
Foam buyers often use density as a shortcut for hardness.
For example:
90 kg/m³ must be firmer than 65 kg/m³.
That assumption is not always reliable.
Density measures mass per unit volume.
Compression deflection measures the material's resistance to compression.
Formulation, cell structure, cross-linking and manufacturing conditions can allow materials with similar density to have different compression responses.
Likewise, two materials with different densities can sometimes produce similar compression forces.
For gasket selection, an EPDM foam material should therefore be evaluated using actual compression data rather than density alone.
Cell structure is another important selection factor.
Open-cell EPDM foam may be considered when the project requires:
Soft conformability
Low closure force
Gap filling
Acoustic absorption
Air permeability in appropriate applications
For example, an open-cell EPDM foam sample can be tested when low compression force and conformability are priorities.
Closed-cell EPDM may be considered when the application requires a different sealing structure, moisture resistance or lower permeability.
However, choosing a closed-cell material does not automatically guarantee that the finished enclosure is waterproof or dustproof.
Performance also depends on:
Gasket geometry
Compression level
Joints
Corners
Surface flatness
Fasteners
Adhesive
Assembly design
The completed enclosure should be validated at system level when a specific sealing rating is required.
Instead of sending only:
Need soft EPDM foam, 10 mm thick.
provide the working conditions.
Useful information includes:
Nominal gasket thickness
Minimum and maximum installation gap
Gasket width
Gasket shape or drawing
Available closure force
Expected compression percentage
Contact surface
Operating temperature
Water, oil, UV or chemical exposure
Whether the joint opens repeatedly
Required service life
Required test standard
Adhesive-backing requirement
Annual quantity
For an enclosure with variable gaps, a useful enquiry might say:
The closed gap varies across the enclosure. We need an EPDM foam gasket that remains in contact at the largest gap without creating excessive closure force at the smallest gap. Please recommend sample grades and provide comparable compression data.
This gives the supplier enough information to propose relevant materials.
Use the same test conditions whenever possible.
A useful comparison table may include:
Item | Sample A | Sample B |
|---|---|---|
Material type | ||
Cell structure | ||
Density | ||
Thickness | ||
Compression stress at 10% | ||
Compression stress at 25% | ||
Compression stress at 50% | ||
Compression set | ||
Test temperature | ||
Test duration | ||
Recovery period | ||
Stress relaxation data | ||
Assembly trial result |
If suppliers use different methods or test conditions, keep the results separate rather than treating them as directly equivalent.
No.
Lower compression force can be useful when an enclosure is lightweight or has limited fastening force. However, the gasket still needs enough contact pressure to accommodate surface irregularities and meet the application's sealing requirement.
The best value depends on the assembly.
There is no single percentage that is correct for every application.
A result must be evaluated together with the compression level, temperature, duration, recovery time and test method.
A specification should therefore define both the target result and the conditions under which it is measured.
No.
Density is useful for identifying a material grade, but it does not uniquely determine compression force, recovery or long-term sealing behavior.
Compression data should be evaluated separately.
Request data at compression levels relevant to the actual installation.
If the gasket may operate from approximately 15% to 40% compression because of gap variation, data only at 50% compression may have limited value for the design.
No.
Compression set indicates recovery after compression and release.
A long-term seal may also depend on stress relaxation, environmental aging, gasket geometry, adhesive performance and assembly design.
For gasket projects, “soft,” “medium” and “hard” are not precise engineering specifications.
The most useful information is the actual working range:
minimum gap → nominal gap → maximum gap
combined with the available closure force and required sealing performance.
This makes it possible to compare materials based on how they will operate in the finished product instead of selecting them by touch or density alone.
Send AITO Foam your gasket drawing, gap range and operating conditions to discuss suitable EPDM foam samples.
The final material should be approved in the actual assembly, and the agreed test conditions should be retained with the purchase specification so future production batches can be compared consistently.
ASTM D1056 — Flexible Cellular Materials: Sponge or Expanded Rubber
ASTM D395 — Rubber Property: Compression Set
ISO 3384 — Rubber, vulcanized or thermoplastic: determination of stress relaxation in compression
