PROTECTING NON-METALLIC SEALING RINGS Protection of non-metallic sealing rings from extruding Allows the use of PTFE as a sealing ring at high pressures Materials available for all fields of applications back-up rings are used to prevent non-metallic sealing rings from extruding into the space between the piston rod and packing cup. Back-up rings, which are typically made of cast iron or bronze material, seal only the end face of the packing case cup. The bore hole has a small clearance from the piston rod. They are suitable for all fields of application in the process, natural gas, or food industry.