Extrusion occurs when fluid pressure forces the seal material into the clearance gap between mating hardware. Dynamic motion further promotes extrusion, as surfaces in motion tend to pull material into the extrusion gap, generating additional frictional forces and heat. This can cause premature failure via several modes. Extruded seal material can break away and get caught underneath sealing lips, creating leak paths. As material continues to break away, seal geometry erodes, causing instability and eventual leakage. Additionally, heat generated from added
friction will cause the seals to take a compression set, dramatically shortening their life.

Careful design considerations should be evaluated to prevent extrusion. For example, minimizing
clearance gaps and selecting a proper material based on system temperature, pressure and fluid are both helpful in reducing the risk of extrusion. As clearance gaps increase, less pressure is required in order for extrusion to occur. Higher temperatures can also play a role in this effect by causing seal materials to soften, encouraging extrusion at lower pressures. If the seal material chosen is not suitable to be used in the system fluid, softening due to chemical attack can also decrease its ability to resist extrusion.

The following Table 2-3 lists possible causes of extrusion and troubleshooting tips for preventative or corrective measures.

By definition, the radial gap is one-half of the diametrical gap. The actual extrusion gap is often
mistaken as the radial gap. This is too optimistic in most cases because side loading of the rod and piston will shift the diametrical clearance to one side. Often, gravity alone is sufficient for this to occur. Good practice is to design around worst case conditions so that extrusion and seal damage do not occur. Table 2-4 provides maximum radial extrusion gaps for various seal compounds.

As a general rule of thumb, the pressure rating of dynamic seals will be approximately one-half
that of static seals.

As noted in Table 2-4, pressure ratings decrease when wear rings are used due to the larger
extrusion gaps required to eliminate metal-to-metal contact. If wear rings are used, be sure to consult the Wear Guide Ring Section of our web-site for appropriate hardware dimensions.
Wear ring hardware dimensions for the piston and rod throat diameters always supersede those dimensions called out for the seals themselves.