is defined as the decrease in thickness of a rubber which has been deformed under specific conditions of load, time and temperature. Normally shown as a percentage, it is typically tested to ASTM
D395-18 Method B (Compression Set
Under Constant Deflection in Air). Generally, a lower percentage of Compression Set
indicates a better quality of material.
A good quality Solid Rubber would have a Compression Set
below 20%; most Sheet Rubber will have a Compression Set
of around 30%, whereas poor quality, under cured or badly mixed Rubber Sheeting will exhibit a Compression set of 45% or more.
Another definition that is commonly used is Material Recovery
or even springiness!
So why does Compression Set matter?
The answer is simply that if your material has a low level of Compression Set
it will perform its function as a seal more effectively for longer. By taking on a Compression Set
, the material is no longer effectively resisting compression and ‘pushing back’ against the load. Once this has happened the internal pressure will have a greater influence and start to create leak paths, requiring the gasket to be further tightened until there is sufficient load to effectively break the mechanical bonds within the material causing catastrophic failure.
Don’t be fooled if the data sheet for the rubber sheeting material that you are viewing says Compression Set
is greater than 50% or, even worse, that it is Not Applicable, as the likelihood of failure in situ has increased dramatically and the effective working life of these high Compression Set
materials is considerably shorter than good quality, properly mixed Rubber sheeting.