Back-up rings, also known as anti-extrusion rings, are plastic or rubber backing rings used in conjunction with o-rings in both dynamic and static sealing applications. Anti-extrusion rings have no sealing function and are generally used in high pressure hydraulic applications to support the sealing element and prevent elastomer gap extrusion (when the o-ring flows into the clearance gap between two mating parts causing its failure). Depending on the system pressure, the O-ring can be combined with one or two back-up rings, in double acting applications it is necessary to use one anti-extrusion ring on each side of the O-ring. Anti-extrusion rings are commonly supplied in NBR 90Sh, virgin PTFE, Polyester (TPE), Polyurethane (PU) and POM. Back-up rings can come in the form of a solid flat ring, the NBR version comes as a solid ring with a contoured concave cross-section, while the PTFE version comes in three different types: solid, split and spiral.