KF vs CF Flange: A Complete Guide for High Vacuum and UHV Systems
When assembling vacuum pipelines, choosing between KF and CF flanges is a fundamental decision for every engineer. These two most popular vacuum flange standards serve different vacuum ranges, operating temperatures and application scenarios. Understanding their differences helps you pick the right combination with copper gaskets, O‑rings and vacuum bellows to maintain stable sealing performance.
KF flanges, also known as quick flanges, are widely used for rough and high vacuum systems. They rely on a centering ring, a Viton O‑ring and a clamp to form the seal. Installation is fast and straightforward without torque calculation; you only need to tighten the clamp to lock the two flange faces together. This design makes disassembly and maintenance highly convenient. KF flanges cover common sizes such as KF16, KF25, KF40 and KF50. They work well for vacuum coating machines, general laboratory lines and low‑cost vacuum setups. However, the elastomer O‑ring limits their baking capacity. They cannot endure high bake‑out temperatures and are not suitable for true ultra‑high vacuum requirements.
CF flanges, named for their conflat knife‑edge design, are the standard for ultra‑high vacuum. A sharp metal knife edge presses into an oxygen‑free copper gasket under evenly torqued bolts, creating a permanent metal‑to‑metal hermetic seal. This structure supports baking temperatures up to 450°C and achieves extremely low helium leak rates. Since the copper gasket plastically deforms during compression, it must be replaced once the joint is taken apart. CF assemblies demand careful torque control and clean flange surfaces to avoid scratches on the knife edge. Common sizes include CF35, CF63 and CF100, frequently found in semiconductor process chambers, surface analysis instruments and particle accelerators.
From the cost perspective, KF components are cheaper and easier to source. CF hardware, including precision machined flanges and copper gaskets, carries a higher price tag because of stricter machining tolerance and material standards. If your system needs frequent modification and only requires high vacuum without baking, KF flanges are economical. If your target pressure falls into the UHV range or thermal baking is necessary, CF flanges are the only viable option.
Both flange types can be welded to vacuum bellows, viewports and vacuum valves. Many complex vacuum stations mix KF sections for auxiliary lines and CF sections for core UHV chambers. We supply a full range of vacuum hardware: KF and CF flanges, copper gaskets, Viton O‑rings, vacuum bellows and custom welded assemblies. Our technical team can assist you in designing your vacuum layout and selecting matching components. Reach out for datasheets, CAD drawings and quotations.