ConFlat (CF) flanges use a copper gasket that is cold-welded between two stainless steel knife edges, creating a true metal-to-metal hermetic seal capable of achieving system pressures below 10⁻¹² torr and helium leak rates of 10⁻¹³ torr·L/s or better. KF (Klein Flange, also called NW or ISO-KF) flanges use an elastomeric O-ring with a centering ring, providing a quick, reusable seal suited to pressures down to approximately 10⁻⁸ torr. CF is the correct choice for true UHV, bakeout above 150°C, and long-term hermetic integrity. KF is appropriate for rough vacuum, high vacuum, and temporary or frequently reconfigured connections.
How CF Flanges Work
A ConFlat flange has a precisely machined knife-edge on each mating face. When bolts are tightened, the two knife edges plastically deform into a soft copper (or occasionally aluminum or gold) gasket, creating a cold-welded metal-to-metal joint with no elastomeric material in the leak path. This design eliminates gas permeation through the seal material and enables bakeout to 250°C or higher, which is essential for driving off adsorbed water vapor and other contaminants from chamber walls before reaching UHV base pressures.
CF flanges are available in standard sizes designated by outer diameter, with common sizes including CF16, CF35, CF40, CF63, CF100, CF150, and CF200. The knife-edge geometry and bolt circle are standardized, making flanges from different manufacturers interchangeable. A CF gasket is single-use – once the knife edges have deformed into the copper, the seal cannot be reassembled with the same gasket without risk of leakage.
How KF Flanges Work
KF (NW) flanges use a slip-on clamp that compresses a centering ring and elastomeric O-ring between two flat flange faces. The assembly is tool-free and can be connected and disconnected in seconds. Common sizes are designated by the nominal bore diameter: KF10, KF16, KF25, KF40, and KF50.
The elastomeric O-ring – typically Viton, Buna-N, or silicone – limits ultimate vacuum performance due to outgassing and gas permeation through the polymer. Bakeout is limited to roughly 80–150°C, depending on the elastomer. For most rough- and high-vacuum applications, these constraints are acceptable, and the convenience of quick connection and reuse outweighs the performance limitations.
Side-by-Side Comparison
- Seal type: CF uses a cold-welded copper gasket (all-metal); KF uses an elastomeric O-ring.
- Ultimate vacuum: CF reaches below 10⁻¹³ torr; KF is limited to approximately 10⁻⁸ torr.
- Leak rate: CF achieves 1×10⁻¹¹ torr·L/s or better; KF is typically 10⁻⁸ to 10⁻⁹ torr·L/s at best.
- Bakeout temperature: CF supports 250°C or higher; KF is limited to 80–150°C.
- Reusability: KF O-rings are reusable; CF copper gaskets are single-use.
- Connection speed: KF is fast with a single clamp – CF requires bolting a full bolt circle.
- Cost: KF hardware is less expensive; CF flanges and gaskets are more expensive.
- Application range: CF is required for UHV and hermetic applications; KF suits rough and high-vacuum applications.
When to Use CF
- Any system requiring pressures at or below 10⁻⁷ torr.
- Applications requiring bakeout above 150°C to reach UHV base pressure.
- Permanent or semi-permanent chamber boundaries where leak integrity must be maintained for months or years.
- Systems qualified to helium leak-rate thresholds at or below 1×10⁻⁹ atm·cc/sec.
- Semiconductor process chambers, beamline sections, surface science systems, and space simulation chambers.
When to Use KF
- Roughing lines and backing pump connections where pressure stays above 10⁻³ torr.
- Temporary diagnostic ports or instrumentation connections that are frequently reconfigured.
- Load-lock sections or transfer lines where moderate vacuum suffices and fast cycling is more important than ultimate pressure.
- Systems with bakeout temperatures below 120°C.
Can KF and CF Be Used Together?
Yes. Many UHV systems use CF for all critical chamber boundaries and long-term seals, while using KF on roughing lines, vent valves, and temporary connections. The key is to ensure that no KF joint lies in the direct leak path between the UHV volume and the atmosphere. The overall system leak rate is limited by the least hermetic joint in the vacuum boundary.
Gasket Material Choices for CF Flanges
Copper is standard and suitable for most UHV applications with bakeout to 250°C. Aluminum gaskets are softer and used where knife-edge hardness is a concern. Gold-plated or oxygen-free high-conductivity (OFHC) copper gaskets are specified for applications requiring extra corrosion resistance or in systems handling reactive gases. Always verify that the gasket material is compatible with your process chemistry before selecting.
Selecting the Right Flange for Your System
The choice between CF and KF is usually straightforward once you know your target base pressure and bakeout temperature. If your system must reach 10⁻⁸ torr or below, or if you need to bakeout above 150°C, CF is the correct interface. For everything else, KF provides a practical and cost-effective solution. MPF Products offers both CF and KF-compatible hermetic feedthroughs and viewports and can help you specify the right interface for your vacuum boundary.