ASTM D395 covers three test methods for determining the compression set of rubber compounds. Test Method A uses constant force, Test Method B uses constant deflection, and Test Method C accounts for material hardness. These are applicable to rubber used in machinery mountings, vibration dampers, and seals.
Standard test specimens are cylinders 29mm in diameter and 12.5mm thick. Specimens are conditioned at 23 ± 2°C for at least 3 hours before testing. The rubber is compressed between plates, held at the specified temperature for the specified duration, then released and the recovery measured.
Compression set is expressed as a percentage of the original deflection (Method A) or original thickness (Methods B and C). The test measures how much permanent deformation remains after the compressive load is removed, which is critical for sealing and vibration damping applications.