Technical Info

Design Guide

Compression Sets

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

Compression set

The ability of a seal to recover after being compressed into its original shape - is one of the things you must know before of the production of your seal or gasket takes place. This test is outlined in ASTM D-395 and D-1056 guidelines. The set test is used to determine the quality of rubber compounds and their applicability to certain types of usage.

Compression deflection

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?

For other seal design considerations such as the environment to which it will be exposed: oil, grease or fuels, UV, weathering, water and more, visit our Design Criteria for Quoting page on this website. Or, if you prefer, pick up the phone and call one of our technical staff at 800.683.0676 for help.

Quote help - If you don't know the answers to your design questions and need some help - visit our resource center for a complete downloadable version of our design guide - or just pick up the phone and give us a call at 800-683-0676 or send an e-mail.

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. Click here to access our quick reference chart to compare differences in application on open and closed cellular materials.

Compression Set Comparative Table

Open Cells	Closed Cells
Air is free to pass through the open cells. There is no effect of the 158° F (70° C) test temperature on the air pressure in the cells.	Air is not free to pass through the closed cells. The 158° F (70° C) test temperature causes an increase in air pressure in the closed cells.
All of the compressing pressure is on the rubber during the test.	Part of the compressing pressure is on the rubber, but part of it is on the air in the cells during the test.
There is no air diffusion effect through the cell wall structure.	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 free to recover immediately after the test. Air can go back into the open cells immediately.	The rubber is not free to recover after the test. Air cannot go back into closed cells immediately.
The sample retains the compression set after the test.	The sample continues to recover after the test is over.
The compression set test result indicates the state of cure of the rubber sample. An under cured sample shows a high compression set.	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 otherwise equivalent, the test results are not affected greatly by the thickness of the sample.	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 in not directly affected by the hardness of the open cell sponge.	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

The Compression Deflection (C/D) test, as outlined in ASTM D-1056, provides these important pieces of information:

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