Threaded Plug Selection: Performance Comparison Between External‑Thread and Internal‑Thread Plugs

When selecting threaded sealing plugs, working pressure, installation space and disassembly frequency shall be comprehensively considered. External‑thread plugs and internal‑thread plugs differ greatly in sealing principle and pressure‑bearing capacity. Many equipment‑circuit leakage failures result from improper plug selection. Clarifying performance differences helps reduce water leakage, pressure relief and thread‑damage risks and improves system stability.

1. Differences in Sealing Structure and Working Principle

External‑thread plugs screw into internal‑thread holes of work‑pieces. Taper‑thread versions realize self‑sealing by thread‑profile extrusion deformation. Cylindrical external‑thread plugs mostly rely on O‑rings or gaskets for end‑face sealing; threads only undertake locking force instead of direct sealing. Internal‑thread plugs, also known as cap‑style plugs, sleeve over external‑thread joints of work‑pieces. Sealing is completed by compressed end‑face gaskets or built‑in O‑rings. Threads merely provide locking function. Thread‑meshing zones keep away from internal fluid medium and do not participate in sealing.

2. Comparison of Pressure‑Bearing Capacity and Failure Modes

External‑thread plugs deliver higher pressure‑resistance. Threads are fully embedded inside work‑piece holes and supported by base material for uniform stress distribution. They adapt to long‑term high‑pressure water circuits and hydraulic oil circuits in injection molds. Common failures include base‑thread tooth‑crushing caused by over‑torque and leakage from auxiliary‑seal aging. Stable high‑pressure sealing can be achieved under proper tightening torque. Internal‑thread plugs own limited pressure‑bearing capacity. Locking force acts on external threads of work‑pieces, generating large tensile stress at thread roots. Under high‑pressure impact, thread slipping and plug loosening are likely to occur. Fluid pressure produces outward thrust counteracting locking force. Vibration and temperature cycling raise leakage probability, so they are not recommended for permanent high‑pressure sealing.

3. Installation‑Space and Thread‑Protection Characteristics

After installation, most bodies of external‑thread plugs sink into holes with only wrench‑operation positions exposed, occupying little external space and avoiding component interference. Nevertheless, work‑pieces need pre‑tapped internal threads. Tap quality determines sealing reliability, and damaged base threads are difficult to repair. Iron chips must be fully cleared before assembly. Internal‑thread plugs are mounted outside joints and consume more external installation space, which may cause interference for compact component layout. Their advantage is full coverage and physical protection for work‑piece external threads against collision and dust contamination during storage and transportation. Disassembly‑replacement operations are convenient without deep‑hole work.

4. Temperature‑Resistance, Medium‑Resistance and Repeated‑Disassembly Performance

Metal external‑thread plugs suit high‑temperature conditions. Stainless‑steel and brass variants adapt to high‑temperature cooling water and hydraulic oil for molds. Threads inside base holes suffer less oxidation‑corrosion from external environment. Sealing performance stays stable under multiple disassembly cycles if threads remain intact. Repeated disassembly will wear base internal threads; thread‑sealant or PTFE tape can compensate enlarged gaps. Metal internal‑thread plugs feature decent rust‑proof performance, while plastic versions only apply to low‑temperature low‑pressure temporary protection. Built‑in sealing rings age rapidly under sustained high‑temperature and oil‑liquid immersion and lose sealing effectiveness. Disassembly will not damage base threads, so they fit temporary blocking and protection scenarios requiring frequent maintenance.

5. Practical Selection and Application Key Points

Choose external‑thread plugs for mold cooling circuits, hydraulic oil circuits and high‑pressure process holes for permanent sealing. Match thread specifications completely with work‑piece holes, control tightening torque strictly, and select proper auxiliary sealing accessories. Internal‑thread plugs are for equipment‑transportation protection, test‑phase temporary blocking and low‑pressure port dust‑proof purposes rather than high‑pressure permanent sealing. Confirm joint external‑thread specifications and inspect aging status of built‑in sealing rings regularly. Avoid using them under heavy pressure fluctuation.

Conclusion

External‑thread plugs feature high pressure‑resistance and small external‑space occupation for long‑term high‑pressure fluid sealing, at the cost of high base‑thread‑machining requirement and thread wear during disassembly. Internal‑thread plugs excel in thread protection and easy disassembly for low‑pressure protection and temporary blocking with limited pressure‑bearing ceiling. Beyond thread‑specification matching, working pressure, temperature, disassembly frequency and space constraints shall be weighed in plug selection to prevent leakage and pressure‑release failures caused by wrong component adoption.