Blind disks are formally known as Flange covers, sometimes also referred to as Blind flanges or pipe plugs. They are flanges without holes in the center, used for sealing pipe openings. They serve the same function as caps and pipe ends, but the blind disk seal is a removable sealing device, whereas the seal of the cap is not intended to be opened again. There are various types of sealing surfaces, including flat, convex, concave, tenon and mortise, and annular joint surfaces. Materials include carbon steel, stainless steel, alloy steel, copper, aluminum, PVC, and PPR.