Compression Tests

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

Compression set

Compression set is the ability of a material to recover and resist permanent deformation after being compressed. It is one of the key attributes required of a thermoset material. This characteristic is defined as Compression Set and is outlined in the ASTM D-395 and D-1056 testing guidelines. This data can be critical in the selection of a material that will meet performance requirements.

Seal and Gasket Design Guide

Compression deflection

Compression deflection is the method of compressing the rubber piece at a rate of 12.5 to 50 mm/min (0.5 to 2 in./min) gently without impact as outlined in ASTMD-1056. It measures the force it takes to compress a standardized test specimen to a deflection of 25%.

Seal Considerations

There are many elements you will need to know when providing information for a quotation. Those related to compression include:

  • Is the application dynamic or static
  • Does it need an attachment system
  • If so, should that be a pressure sensitive adhesive or a mechanical fastening system and more?

Understanding Compression Set Results

If the material has good compression set resistance, it will recover sufficiently when the load is released to affect a repeated seal according to ASTM D-395 and D-1056 standards. 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. See our quick reference chart below to compare differences in application on open and closed cellular materials.


Compression Set Comparative Table

Open Cells
  • All of the compressing pressure is on the rubber during the test.
  • There is no air diffusion effect through the cell wall structure.
  • The rubber is free to recover immediately after the test. Air can go back into the open cells immediately.
  • The sample retains the compression set after the test.
  • The compression set test result indicates the state of cure of the rubber sample. An under cured sample shows a high compression set.
  • On the samples that are otherwise equivalent, the test results are not affected greatly by the thickness of the sample.
  • The compression set test result in not directly affected by the hardness of the open cell sponge.
Closed Cells
  • Part of the compressing pressure is on the rubber, but part of it is on the air in the cells during the test.
  • During the time that the closed cell structure is under pressure in the test, there is some air diffusion through the cell walls. (This is the same diffusion effect that occurs when air pressure decreases in an automobile tire over a period of time, even though there is no specific leak in the tube. This effect is a basic characteristic of the rubber or synthetic polymer. It cannot be changed significantly by the cellular rubber product manufacturer.
  • The rubber is not free to recover after the test. Air cannot go back into closed cells immediately.
  • The sample continues to recover after the test is over.
  • The compression set test result does not necessarily indicate the state of cure of the sample. It is more an indication of the amount of air that has diffused from the closed cells and has not yet diffused back.
  • On the samples that are equivalent in other respects, the test results are greatly affected by the thickness of the sample tested. This is because of the diffusion effect as noted above.
  • The compression set test result is affected by the hardness of the sample, harder materials showing lower percentages of set. This is because in the harder material, the rubber portion supports a relatively higher amount of total pressure in comparison with the air cells.
Understanding Compression Deflection Test
  • 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.

Important Distinction of 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

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