Elevated system pressure can sometimes forcibly extend, or extrude, part of an O-ring into the gland’s diametral clearance gap. The extruded portion of the seal is susceptible to being chewed away to the point of failure. Even if permanent extrusion is avoided, small bits may still be “nibbled” away from the low-pressure side of the seal.

This nibbling is the result of pressure fluctuations within the system. Increasing pressure expands metal components, often enlarging the clearance gap. The larger the gap, the easier it is for the O-ring to flow into it. When pressure later returns to normal, the O-ring’s memory allows it to regain its original shape, but it does not evacuate the retracting gap before a small chunk is torn away. Repeated instances of this nibbling can lead to seal failure. Though extrusion and nibbling are most often seen in dynamic rod or piston seals, static seals facing high pressure pulsations may also suffer. Figure 132 is an example of O-ring failure due to extrusion and nibbling.

No matter what the application, excessive system pressure will obviously increase the likelihood of seal extrusion, especially if no back-up rings or other anti-extrusion devices are employed. Even if they don’t increase under pressure, clearance gaps that are inherently too large or irregularly shaped are dangerous. O-rings that are too soft or too large for the gland (either initially or after swelling in system fluid) are to be avoided. Temperature increases can also soften O-rings and make them more susceptible to extrusion. Sharp edges within the gland will be problematic and should be radiused to .001" to .002". Use of polyurethane O-rings and/or back-up rings is suggested to avoid both extrusion and nibbling.