Why Is Hardness Not Enough When Selecting Silicone Rubber for Long-Term Sealing?

When selecting silicone rubber for seals, hardness is often one of the first specifications discussed.

A customer may ask:

“Do you have 50 or 60 Shore A?”

Hardness matters, but it does not tell the complete story when a seal remains compressed for a long period.

Compression set is another important consideration.

1. What Is Compression Set?

After a rubber seal remains compressed for a period of time, it may not fully return to its original shape when the load is removed.

This permanent deformation can affect its ability to maintain sealing force.

Therefore, a seal may perform well initially but gradually lose recovery after extended service.

2. Can Two Rubbers with the Same Hardness Behave Differently?

Yes.

Long-term sealing behavior can also depend on:

• Base elastomer
• Compound formulation
• Curing system
• Operating temperature
• Compression ratio
• Exposure time
• Contact media
• Temperature cycling

Two materials rated at 60 Shore A should therefore not automatically be considered equivalent.

3. What If the Seal Is Exposed to Fuel or Oil?

For seals exposed to fuel, oil or other demanding media, compression set is only part of the evaluation.

Material selection should also consider:

media compatibility, volume change, mechanical-property retention and recovery after long-term compression.

This is where specialty elastomers such as FVMQ may be worth evaluating.

4. When Can MY-FSR Be Considered?

MY-FSR Low Compression Set Fluorosilicone Compound can be evaluated for specialty sealing applications where fuel/oil exposure and long-term sealing behavior are important.

Final material selection, however, should be based on the actual operating environment and finished-part testing.

5. What Information Should Be Provided?

For a more useful recommendation, provide:

• Operating temperature range
• Contact media
• Required hardness
• Seal type
• Compression conditions
• Continuous or intermittent operation
• Current material and existing problem

FAQ

Does the same hardness mean the same compression set?
No. They describe different material characteristics.

Is lower compression set always better?
It can be important for long-term sealing, but it should not be evaluated independently of other requirements.

When should FVMQ be considered?
It may be evaluated when the application combines demanding sealing requirements with exposure to fuel, oil or related media.

Can MY-FSR directly replace an existing rubber compound?
A direct replacement should not be assumed. Application and finished-part testing are required.

What information is most useful for selection?
Temperature, media, hardness, seal design, compression conditions and the current failure mode.

Related Product:
MY-FSR Low Compression Set Fluorosilicone Compound