In simple terms, compression set is a measure of how much a material does NOT rebound. This is expressed in a percentage, for gasketing, the lower % the better (to ensure “apples and apples” confirm test temperature).
Why do we care about compression set?
Compression set, sometimes called C-set, is an indicator of how a material behaves under certain conditions. The compression set test for foams and sponges include a 50% compression at various temperatures. Room temperature 23°C, intermediate 70°C, and high (100°C for silicone). Since heat is an “accelerator” of compression set, an engineer would roughly relate these temperatures to their application. A gasket material that takes a C-set at elevated temperature will quite often stop functioning as a gasket over time. This would be metered against the expected life of the product, whether there is re-compression, level of sealing (e.g. NEMA 4X vs NEMA 3R), preventive maintenance, cost, and most important the cost of a leak/equipment failure due to a leak.
Below is a graph produced by Rogers Corporation, this shows compression set values of different gasket materials at different temperatures, silicone is tested at a higher temperature (100°C) since it is considered a high temp gasket material and resist the effects of heat.
A couple years ago Stockwell had a customer who had remote monitoring equipment in the desert; the only way to access the enclosures for this equipment was by helicopter. The gasket needed to seal under desert conditions; temperature cycling, rain sealing, dust sealing, and not be affected by ice. The cost to access and repair the equipment would have been enormous. Stockwell Elastomerics’ Applications Engineers recommended a closed cell silicone foam material that could withstand high temperatures without taking a compression set. Had the application been for a piece of lab equipment, a very different material may have been selected.
Rubber Compression Set in summary:
Compression set is just one data point that follows an ASTM test procedure in order to help engineers distinguish the performance properties of various materials on an even playing field. There are many other factors and key properties to sealing successfully such as stress relaxation and cell structure that need to be considered for the overall best gasket material choice. Selecting a commercially available material and how to use it can be daunting, Stockwell Elastomerics’ Applications Engineers are available to discuss your project.