What is Compression Set?

Unit of Measure	%
Test Method	JIS K6767-1976

Compression set testing is used to determine the ability of foam materials to maintain elastic properties after prolonged compressive stress.  The test measures the somewhat permanent deformation of the specimen after it has been exposed to compressive stress for a set time period. This test is particularly useful for applications in which foams would be in a constant pressure/release state. A small value indicates that the foam has a strong ability to recover when compressed for a long period of time.

A test piece is subjected to compressive strain corresponding to 25% of the thickness, for 80,000 cycles at a rate of 60 times per minute. Afterwards, the test piece is kept for 24 hours at standard conditions, and the thickness is measured again. The compression set rate C_s (%) is indicated by the following equation:

\[ C_s = \frac{t_0 – t_2}{t_0 – t_1} \times 100 \]

Where:
t₀ = prior thickness of a test piece (m)
t₁ = spacer thickness (m)
t₂ = thickness (m) of the test piece 24hrs after removal

Standard closed cell foams have Compression Set figures of <5%, making them ideal for orthotic and footwear applications.