Self-sealing fasteners have an elastomeric insert or captive o-ring that provides simultaneous locking and sealing. They offer positive sealing under vacuum pressures and are often reusable, providing high-integrity seals even after many applications. There are two basic types of products: threaded fasteners and rivets. Both types of devices are made of aluminum, alloy steel, brass, bronze, carbon steel, molybdenum, stainless steel, plastic, or polytetrafluoroethylene (PTFE). Products made from proprietary materials such as Hastelloy® and Inconel®, registered trademarks of Haynes International, Inc., are also available. Many self-sealing fasteners are treated with coatings or finishes that enhance corrosion resistance or provide other desirable characteristics. For example, aluminum fasteners are often anodized or finished with chrome. Copper fasteners are plated with silver or zinc. Black oxide, bright nickel, dull nickel, cadmium plating, chromium plating, and zinc plating are other common surface treatments. Passivation, the process of removing iron compounds from the surface of stainless steel, is used to form a thin, protective oxide layer that resists corrosion.
Types of Self-sealing Fasteners
There are three types of threaded, self-sealing fasteners: screws, nuts, and bolts. Products differ in terms of head style, drive type, English size, and metric size. Self-sealing fasteners with hexagonal heads have six flat, vertical sides. Fillister heads are similar to machine screw pan heads, but do not have rounded top corners. Flat heads lay flush with the surface of the fastened material. Products with socket cap, oversized, and tamper-proof heads are also available. There are four basic drive styles for threaded, self-sealing fasteners: slotted, Phillips, Allen, and six-lobe. Screws with English sizes are numbered or measured in fractions of an inch. Numbered screw sizes include 2, 3, 4, 5, 6, 8, 10, and 12. Fractional screw sizes include 1/4 in, 5/16 in, 3/8 in, 7/16 in, 1/2 in, 9/16 in, 5/8 in, 3/4 in, and 7/8 in. Metric sizes for nuts and screws include M2, M2.5, M3, M3.5, M4, M5, M6, M8, M10, M12, M14, M16, M18, M20, M22, M24, M27, and M30.
Rivets are self-sealing fasteners that consist of a smooth cylindrical shaft with heads on both ends. Typically, one head is factory-formed. The other head is formed by clenching metal after the rivet has been inserted. Some rivets have round heads. Others have flat heads with angled sides. Hole-diameter is an important specification to consider when selecting rivets. Most suppliers provide products that are designed to fit holes with a diameter of 3/32 in, 1/8 in, 5/32 in, 3/16 in, 1/4 in, 5/16 in, or 3/8 in. Metric products are also available.
Self-sealing fasteners use a variety of sealing materials. Nitrile and Buna-N provide good resistance to petroleum hydrocarbons and fuels. Butyl has low gas permeation and is capable of providing high-energy absorption. Ethylene propylene (EPDM) and neoprene offer good resistance to sunlight, weathering and ozone. Products that use fluorosilicone, polyphenylene sulfide (PPS), polyurethane, and silicone rubber are also available. Proprietary sealing materials include Kalrez® and Viton® (DuPont Dow Elastomers), and AFLAS® (Asahi Glass Co., Ltd).