Why Does a Used O-Ring Take the Shape of Its Groove Instead of Remaining Round?

Rubber materials used to manufacture O-rings may have the same name, but their manufacturing processes and compound formulations can differ. These differences affect their properties, including recovery after compression.

An O-ring with good elastic recovery can regain much of its original shape after use and removal from its groove, so its cross-section remains relatively round. A lower-quality rubber with poor recovery may instead retain the shape of the groove in which it was installed.