Understanding Compression Tests

The optimum performance of your seal depends on many variables. Compression Set and Compression Deflection Tests are two important factors.

While this page outlines basic testing procedures, our experts can work with you to determine the right polymer compound and profile design for your application.

Compression Set 

This refers to the ability of a material to recover and resist permanent deformation after being compressed for a specified time and temperature. It is one of the key attributes required of a thermoset material.

Compression Deflection

This is the method of compressing the rubber piece at a rate of 12.5 to 50mm/min (0.5 to 2 in./min) gently without impact. It measures the force it takes to compress a one square inch test specimen to a deflection of 25%.



If the material has good compression set resistance, it will recover sufficiently when the load is released to affect a repeated seal.

It is not necessary for a material to have 100% recovery to produce an effective, repeatable seal. However, if the seal is under constant compression, material recovery is not as important.

Material Differences Matter in Test Results

Due to the special characteristics of the closed cell cellular structure, the compression set test has an entirely different effect on closed cell materials and requires an entirely different interpretation.



The compression deflection test can be described as follows:

  • Force required to compress a standardized test specimen 25% of its height.
  • Results expressed in kilopascals or pounds per square inch.
  • Sponge compounds, whether open or closed cell, are then classified by grades. Each grade is based on a specific range of firmness of the sponge, commonly called ‘durometer’, as expressed by the Compression Deflection test. Digits 0 through 5, as seen in the Classification of Flexible Cellular Materials, denote these grades.


Compression Deflection vs. Durometer Gauge

It is important to note that compression deflection is a more useful tool than a durometer reading for the following reasons:

  • A durometer gauge only provides a single point readout on the sample.
  • Where no significant deflection of the sample actually occurs the durometer readings could have a very wide span.
  • Compression deflection is geared to provide some standardization of load force for any given compound. Closure force necessary for a given application can then be determined.