GlossaryKind of term: Manufacturing termSealing face
What is a sealing face?
A sealing face is the surface a gasket, O-ring or mating face seals against, with its own requirements for roughness, flatness and sometimes lay.
The seal can be a gasket, an O-ring in a groove, or in a metal-to-metal seal a second metal face. On the drawing you recognise it by requirements of its own: a roughness, a flatness and sometimes the direction of the machining marks or a requirement that the face be free of scratches. Those requirements apply to that face; the rest of the part may be coarser.
What does a sealing face depend on?
What a sealing face needs depends on the seal, the medium, the pressure and whether the seal is static or dynamic. A soft gasket fills small irregularities and may grip better on a slightly rougher face; a metal-to-metal seal needs fine roughness and tight flatness. For O-rings, ISO 3601-2 gives guidance values for groove and contact surfaces. The direction of the marks matters: marks running across the seal, from inside to outside, can form a leak path, whereas circular marks run along the seal. Facing on a lathe leaves such marks, although strictly they form a spiral; for a critical seal, for example on gas, the drawing may exclude a spiral or call for another finish. The drawing calls for the circular direction with the lay symbol C in the surface texture symbol.
How does a sealing face work in practice?
Whether the face meets its requirements in the last pass on the machine depends on material, tooling, machine and setup; a fine finishing pass in a setup you already have can make grinding or lapping unnecessary. Tight flatness needs a stable setup with little clamping distortion and a check, for example on the CMM or a surface plate. A scratch or dent across the face can make the part unusable, so handling and packing per piece belong in the route.
What is a sealing face often confused with?
A sealing face is not just a locating face: a locating face has to be on size and in position, a sealing face also has to be smooth and flat enough not to leak. And the lowest roughness is not automatically the best: some gaskets seal better on a slightly rougher face.
Frequently asked questions
Does a sealing face have to be ground or lapped?
Not necessarily. A good finishing pass on the machine often reaches the roughness and flatness already. Grinding, lapping or polishing comes in for very low roughness, very tight flatness or a material that is hard to machine to a fine finish.
Why does a sealing face carry a direction with its roughness?
Because marks running across the seal can form a leak path. Circular marks, as from facing on a lathe, run along the seal and make that less likely. The lay symbol C in the surface texture symbol asks for such a circular texture.
How do you protect a sealing face until delivery?
With careful handling, a protective cap or film and packing per piece. A dent or scratch across the face can make the part unusable, so allow for that care in time and packaging.