Simplified Guide to Flangeless Fittings

What are flangeless fittings? Flangeless fittings are mechanical connectors used to join tubing (typically 1/16" or 1/8" OD) to flat-bottom or coned receiving ports without the need to "flange" or flare the end of the tube.

These connectors are abundant in microfluidics applications and chromatography due to their broad compatibility. While stainless steel flangeless fittings are the industry standard for UHPLC, plastic flangeless fittings are the go-to choice for microfluidic instruments, automated liquid handlers, and benchtop research configurations. 

Components and How They Work

The flangeless connection relies on mechanical compression to create a leak-proof seal using a threaded nut, ferrule, and optional stainless-steel ring.

• The Nut: Acts as the driver, applying axial force. While it should not contact the fluid, it must be chemically resistant to protect against leaks or leaching. 
• The Ferrule: The sealing element. The nut pushes the ferrule into the port’s internal seat, compressing it until it grips the outer wall of the tubing. The ferrule has minimal or no contact with the fluid.
• Optional SS Ring: Sometimes known as a "lock ring," this component minimizes tubing rotation (twisting) during installation and prevents "creep" (material relaxation). This ensures the seal remains tight during long-term operation or temperature fluctuations. Like the nut, the SS ring should not have direct contact with the fluid in the tubing.

Choosing Nut Threads and Material

Various nut threads and nut materials are available to ensure leak proof connection.

Thread size follows a two-part naming convention sepearated by a dash (-) where part one indicates the hole size and part two indicates the threads per inch, with the exception of metric-based threads like M6-1 (often short hand as M6). Here are some common notes about thread size selection:

• 1/4-28 vs M6: Both are used for low-pressure requirements with 1/4-28 being most popular. Some use M6 in place to 1/4-28. M6 deceivingly fits in 1/4-28 ports but will result in leaks. Commonly used valves, reagent bottles, sensors, etc. 
• 10-32: For manufacturers that want a more compact connector than 1/4-28 and used for sensors and valves. 
• 6-40 & 6-32: Smallest and most compact fittings for semi-permanent installation. Most popular for high-density liquid connection designs such as fluidic manifolds and rotary valves with 20+ ports.
• 5/16-24 & 1/2-20: Largest ports used for tubing with 3/16" outer diameter or 1/4" outer diameter, respectively.

Nut materials can be made from polyether ether ketone (PEEK), polyphenylene sulfide (PPS), polyoxymethylene (POM, or trademarked name Delrin), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkane (PFA), and polypropylene (PP). While the nut should not contact the liquid, one should choose a material that is resistant to the fluid/chemicals used because if leaks were to occur, the threads of the ports would be damaged. Some general points for nut material are provided below:

• PEEK: Most popular and commonly used for high pressure requirements with excellent chemical compatibility. Higher cost.
• PPS: Excellent rigidity and popular due to broad chemical resistance but not good for prolonged exposure to organic solvents (THF, methylene chloride, etc). Commonly used for applications above room temperature due to creep resistance and lower cost than PEEK.
• POM: Low-cost alternative with good stiffness, limited chemical compatibility, and low friction which minimizes tube twisting during installation. Good for budget applications that do not require temperature or chemical resistance such as waste bottles.
• PTFE/PFA: High chemical inertness but susceptible to creep and loosening over time. Good for low pressure such as waste bottles.
• PP: Lowest cost, least chemical resistant. Excellent for waste bottles or simple applications.

Choosing Ferrule Material

The ferrule provides the seal to the system and experiences minimal to no contact with the liquid when installed correctly. Most popular ferrules are made ETFE and PEEK but are also available from other materials such as PTFE/PFA and PP. Some pointers on choosing material:

• ETFE: The most common flangeless ferrule material. It is more rigid than PTFE, providing a higher-pressure rating (up to 1,000 psi) and better grip on the tubing. Not recommended with PEEK tubing.
• PEEK: Used when extreme chemical resistance or higher pressures are required. PEEK ferrules are permanent once compressed onto the tube. Recommended material for PEEK tubing if SS ferrules are not an option.
• PTFE/PFA: Soft and rarely used for permanent applications because it tends to loosen over time but can be used for highly corrosive chemicals at very low pressures when other materials are not resistant.
• PP: Extremely soft, lowest cost, least chemical resistant. Excellent for waste bottles or simple applications.

The Optional Stainless-Steel Ring

For critical applications, choosing a ferrule with an integrated Stainless-Steel "Lock Ring" is highly recommended.

1. Anti-Twist: It allows the nut to spin freely without "grabbing" the tubing, preventing stress on tubing.
2. Anti-Creep: It provides a mechanical backbone to the polymer ferrule, preventing the plastic from relaxing under pressure.
3. Ease of Use: Many designs use the ring to "capture" the ferrule on the tubing, ensuring it doesn't fall off repeated assembly and disassembly.