Compression Set Testing measures the ability of rubber to return to its original thickness after prolonged compressive stresses at a given temperature and deflection. As a rubber material is compressed over time, it loses its ability to return to its original thickness. This loss of resiliency (memory) may reduce the capability of an elastomeric gasket, seal or cushioning pad to perform over a long period of time. The resulting permanent set that a gasket may take over time may cause a leak; or in the case of a shock isolation pad, the ability to protect an accidentally dropped unit may be compromised.
Compression Set results for a material are expressed as a percentage maximum figure. The lower the percentage figure, the better the material resists permanent deformation under a given deflection and temperature range. Compression set results can be confusing in that they are calculated different ways under different methods of testing.
Measuring Compression Set of Solid Elastomers
Most testing follows ASTM D395, Test Method B. Solid elastomer specimens 0.25" thick x 0.52" diameter are compressed to 0.177" thick using a device as diagrammed, for 22 to 70 hour periods at elevated temperature in an air circulating oven. Compression set represents the percent of deflection that did not return. That means that if a sample neoprene disc, tested for 70 hours at 212°F, then allowed to cool for 30 minutes, measured .221" thick, the Compression Set would be 40%. Since the disc was deflected .073", yet returned .044" (back up to .221") indicating that the amount of the distance compressed that it did not return is 40%.
To run the math
Compression Set (ASTM D395 Method B) = {(orig. thickness - final thickness)/(orig. thickness - space bar thickness)}/100
40% = {(0.25" - 0.221")/(0.25" - 0.177")}/100
Measuring Compression Set of Sponge Elastomers
A similar example can be shown for a cellular material.
A closed cell neoprene sponge specimen 0.50" thick is compressed 50% for 22 hours at 158°F, the load is released and a 24 hour conditioning period is allowed. If the final thickness is .438" thick, the Compression Set would be 25%. Since the specimen was deflected .250", yet returned .188" (back up to .438") indicating that the amount of the distance compressed that it did not return is 25%. For ASTM D1056, although sponge materials are compressed 50%, the Compression Set calculation for ASTM D395 Method B is still used.
To run the math on this example:
25% = {(0.500" - 0.438")/(0.500" - 0.250")}/100
Contact Stockwell Elastomerics, Inc.
Contact Us for further assistance.
|
