Compression set is one of the things you must know before of the production of your seal or gasket. You need to know if it is dynamic or static - does it need a pressure sensitive adhesive or a mechanical fastening system and more?
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The term compression set test is the amount, measured in percentage, by which a standard rubber test piece fails to return to its original thickness after being subjected to a standard compressive load or deflection for a fixed period of time. The set test is used to determine the quality of rubber compounds and their applicability to certain types of usage.
If the material has good compression set resistance, it will recover sufficiently when the load is released to effect 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.
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 differences in application and interpretation of the compression set test on open and closed cellular materials are shown in the comparative tabulation listed below.
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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. |
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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. |
The Compression Deflection (C/D) test, as outlined in ASTM D-1056, measures the force it takes to compress a standardized test specimen 25% of its height. The reported result is 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 as expressed by the Compression Deflection test. Digits 0 through 5, as seen in the Classification of Flexible Cellular Materials, denote these grades.
It is important to understand why C/D is a more useful tool than a durometer reading. A durometer gauge, which uses a very small pointer, only provides single point readout on the sample where no significant deflection of the sample actually occurs. Depending on the cell structure of the sponge compound, the durometer readings could have a very wide span. C/D on the other hand, is geared to provide engineers with some standardization of load force for any given compound. They can then determine which grade of sponge will work to give the closure force necessary for a given application.
