Plugs, Stoppers and Inserts

Plugs, stoppers and inserts fit into holes or cavities and are designed for the ends of shafts, tubes, pipes, fittings, connectors, screws, bars, or other shapes. Plugs are available in many different shapes and materials.  Plugs come in a variety of different cross sections.  These include round, button plug, square, rectangular, and screw cap or plug.  Common features include threaded, tapered, and expanded tube plug.  Threaded plugs, stoppers, and inserts have a female thread.  The thread can be straight or tapered (NPT).  Tapered sides provide a snug fit.  Some can be used as either a cap or a plug.  Tapered caps and plugs are versatile and can be used for straight tubes and bars, male and female NPT threads, and male straight threads.  Expanding tube plugs are used in moderate pressure applications, such as condenser and heat exchanger tubes.

Important round plug specifications when searching for plugs, stoppers, and inserts include the NPT size, BSP size, diameter, and round plug features.  NPT threads for round plugs, stoppers, and inserts range from 1/8” to 6”.  BSP sizes include ¼ BSP, 3/8 BSP, and ½ BSP.  The diameter applies to round caps and plugs only.  For straight threads the major thread diameter is the important diameter.   (For example: .25 for a ¼ straight thread).  This does not apply where cap/plug or object being capped/plugged has a tapered (NPT or BSP) thread.  Features of round plugs include flange, knurled or faceted head, slotted head, and recessed socket.  Important dimensions to consider when searching for square or rectangular plugs include width and length.  The width is the smaller cross sectional dimension for rectangular caps or plugs.  If the section is a square, the width and length are the same dimension.  The length is the larger cross sectional dimension for rectangular caps or plugs.  If the section is a square, the width and length are the same dimension.

Common materials of construction for plugs, stoppers, and inserts include aluminum, EVA, fiber, cork, or wood, nylon or polyamide, polyetheretherkeytone (PEEK), polyethylene (PE), polypropylene (PP), silicone rubber, styrene butadiene rubber (SBR), stainless steel, and vinyl.  Additional features to consider for plugs, stoppers, and inserts include lift or pull tab or ring, ribbed, and vented.  Lift or pull tabs allow for easy removal.  Pull plugs are designed for "through-hole" applications.  They use a smaller diameter "pull section" to pull the plug until it is snug in the hole.  Ribs are found on plugs only.  The ribs are on the mating part of the plug and help to provide a secure fit inside a tube.  Ribs are available on round, square, and rectangular plugs.  Vented plugs are commonly used in the medial industry.  The vent allows air flow during gas sterilization and prevents the cap from blowing off during extreme heat conditions.

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