Engineering / Rubber Properties / Compression Set

Compression Set

Compression Set (CS)

The Compression Set (CS) of soft materials, such as elastomers, is the permanent deformation that remains when an applied force is subsequently removed. As elastomers are compressed over time, they lose the ability to return to their original thickness. Compression Set Testing measures the ability of a material to return to its original thickness after a compression force is applied at a set deflection and temperature.


Compression set measurements are important when determining the best elastomeric materials to be used. For example, in the case of a shock isolation pad, the ability to protect an accidentally dropped unit may be compromised as the CS measurement becomes more significant. Compression set is critical for the sealing functionality, so the better the CS, the better the long term sealing.

Compression Set Measurement

In practical terms, a material that has a zero compression set is one that returns to 100% of its original thickness, while a material that has a 100% compression set is one that does not return any of its original thickness.

The compression set (ASTM D395) of a material is determined using one of two testing methods, Compression Set A and Compression Set B.

Test Method A

Compression Set Under Constant Force in Air applies a 1.8kN force to the specimen for a set time and temperature.

Test Method B

Compression Set Under Constant Deflection in Air applies a force necessary to the specimen to compress it to 25% of its original height for a set time and temperature.

Compression sets A and B are defined as the percentage of original specimen thickness after it has been left in normal conditions for 30 minutes.

O-ring compression set testing is performed using the similar ASTM D1414 B Method.

Calculating Compression Set

Calculating the compression set for a material is accomplished using formulae that are specific to each of the test methods.

Test Method A

CA, the compression set A, is given by CA = [(to – ti) / to] * 100 where to is the original specimen thickness, and ti is the specimen thickness after testing.

Test Method B

CB, the compression set B, is given by CB = [(to – ti) / (to – tn)] * 100 where to is the original specimen thickness, ti is the specimen thickness after testing, and tn is the spacer thickness or the specimen thickness during the test.


Testing compression = (t0 – ti) = 0.25cm – 0.177cm = 0.073cm (29% compression)

Permanent compression = (t0 – tn) = 0.221cm – 0.177cm = 0.044cm

CB = {(0.25 – 0.221)/(0.25 – 0.177)}/100 = 39.72%

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