Extrusion (of O-rings). When pressurized, O-rings behave as incompressible liquids: the O-ring in the installed state is mechanically deformed; with rising pressure it is subjected first to further elastic deformation and then to plastic deformation, filling the cavities existing on the side facing away from the pressure. With a further increase of pressure the deformation becomes so great that the O-ring finally penetrates even into narrow gaps and is destroyed. This extrusion (gap penetration) does not only destroy the O-ring, however, but also reduces the mobility of the seal face to such an extent that it is no longer able to follow the shaft movements to the degree necessary. The result is an increase of leakage. All other forms of secondary seals for mechanical seals made of elastomers and non-elastomers (e.g. PTFE) tend to behave in the same way as an O-ring. Only the pressures needed for the extrusion differ. It is also known that the pressure necessary for the extrusion is primarily a function of gap size and material hardness. Further parameters are the material itself, the temperature, the number of pressure cycles and the possible additional chemical load on the material. If the operating conditions are known, extrusion damage can be prevented by selecting suitable materials of commensurate hardness and, if necessary, by using a support ring with lock.





O-ring destroyed by excessive extrusion