What Are Vacuum Flanges and How Do You Choose the Right Type?

A vacuum flange is the standardized mechanical joint that connects two vacuum components—tubes, chambers, pumps, valves, or feedthroughs—while maintaining a sealed environment. Choosing the right type means matching three things at once: the flange standard (KF, ISO-K/ISO-F, or CF), the seal material (elastomer vs. metal), and the size and bolt pattern so mating parts physically fit. Get all three right and you have a demountable, leak-tight connection you can open and reseal repeatedly.

What a Vacuum Flange Actually Does

Every vacuum system is assembled from separate parts, and flanges are where those parts meet. A flange must:

• Seal against atmosphere so gas doesn't leak in (or out, for over-pressure systems).
• Locate components precisely so bores align and flow paths stay continuous.
• Allow disassembly so you can clean, replace, or reconfigure parts without cutting and re-welding.

Kurt J. Lesker Company (KJLC) lists "Vacuum Flanges & Components" as a core product category alongside pumps, valves, and pressure measurement—flanges are the connective tissue that ties those categories into a working system.

The Three Main Flange Standards

Most vacuum work uses one of three families. They differ mainly in how they seal and how much vacuum (and heat) they tolerate.

KF flanges

Small, quick to assemble with a single clamp or chain clamp. Best for forelines, roughing lines, and anywhere you need to break the connection often. The elastomer O-ring sets the practical temperature and vacuum ceiling.

ISO-K and ISO-F flanges

Used for larger-diameter connections. ISO-K uses claw clamps around the outside; ISO-F uses bolts through a fixed bolt pattern. Same sealing principle (centering ring with O-ring), so choose based on whether you want fast clamping (K) or a more rigid bolted joint (F).

CF flanges

The knife edge on each flange bites into a soft copper gasket, creating an all-metal seal. This is what you use for UHV and for systems that get baked to drive off water and other adsorbed gases. The trade-off: copper gaskets are typically single-use, and assembly needs even, sequential bolt tightening.

How to Match Flanges Between Components

Mating parts must agree on all of these, or they won't seal:

1. Standard — a KF part will not bolt to a CF part. Adapters exist, but each adapter adds a joint and a potential leak path.
2. Nominal size — e.g., KF-25, ISO-63, CF-35 (the number refers to the nominal bore, not the outer diameter).
3. Bolt/clamp pattern — ISO-F and CF have fixed hole counts and spacing; ISO-K and KF use clamps sized to the flange.
4. Seal interface — the centering ring or gasket must match the flange face geometry.

A practical rule: stay within one standard for a given section of the system, and use a single adapter only where you must transition (for example, CF at the chamber, KF at the roughing pump).

Metal vs. Elastomer Seals: When Each Is Appropriate

• Elastomer (Viton®, Buna-N, silicone): convenient, reusable, lower cost, good for rough and high vacuum. Choose when you don't need UHV or high-temperature bakeout.
• Metal (copper, sometimes aluminum): required for UHV and bakeout. Choose when residual gas levels matter—deposition, surface science, or any process sensitive to water and hydrocarbon background.

If your process involves PVD, ALD, or CVD (all listed in KJLC's application areas), the chamber side is often CF precisely because those processes demand clean, bakeable, low-outgassing joints.

Common Causes of Flange Leaks and How to Check Them

Most flange leaks trace back to a few fixable mistakes:

• Dirty or scratched sealing surfaces — a fingerprint or a nick across a knife edge defeats the seal.
• Damaged or reused gasket — copper CF gaskets are generally single-use; a deformed O-ring won't reseat.
• Uneven bolt torque — tighten in a star/cross pattern in stages, not one bolt fully at a time.
• Wrong or missing centering ring on KF/ISO joints.
• Mismatched standard or size forced together.

To check: isolate the suspect joint and use a helium leak detector, or perform a simple pressure-rise (rate-of-rise) test on the isolated section. Spraying a volatile liquid at a joint while watching a gauge or RGA can localize a leak, but helium detection is the more definitive method for small leaks.

Making the Choice

• Roughing/foreline, frequent assembly: KF with elastomer seals.
• Larger high-vacuum connections: ISO-K (fast clamping) or ISO-F (rigid bolted).
• UHV or bakeout, clean processes: CF with copper gaskets.
• Transitioning between standards: use one purpose-built adapter, and keep the rest of the run consistent.

KJLC's product portfolio covers flanges and components across these standards, and the site offers a quotation path if you need help specifying a custom chamber or matching a full assembly.