Vacuum systems demand seals that stay tight and do not pollute the chamber.
What goes wrong
- Volatiles (water, solvent, plasticiser, uncured chemicals) leave the rubber and raise the pressure.
- Permeation from the atmosphere side limits ultimate pressure.
- Seals stick to flanges and tear on opening.
- Outgassing products deposit on optics and wafers.
Material choices
| Material | Notes |
|---|---|
| FKM (post-cured) | The standard in high vacuum, typically to 10-7 mbar after bake |
| FFKM | For clean, chemically harsh process vacuum; very low outgassing when specified |
| VMQ | Good low-temperature elasticity but high permeability |
| EPDM (peroxide) | Used for vacuum with plasma processes, with specific grades |
| PTFE and encapsulated rings | Clean, but stiffer and prone to leakage on rough flanges |
| Metal seals (copper, indium) | For ultra-high vacuum |
Processing for low outgassing
- Post-cure for a long time (FKM: 24 hours or more at 200 to 230 degrees C).
- Vacuum bake after cleaning.
- Clean in compatible solvent; avoid silicone-containing fluids.
- Packaging: clean polyethylene bags, no talc.
- Handling: gloves and lint-free wipes.
Standards and tests
- ASTM E595 (total mass loss and collected volatile condensable material) is the screening test used in space and vacuum work; a typical limit is TML below 1 percent and CVCM below 0.1 percent.
- ASTM E1559 for outgassing kinetics.
Design hints
- Use smaller cross-sections and higher squeeze to reduce permeation.
- Choose dovetail grooves to hold rings on vertical flanges.
- Lightly grease only with approved vacuum grease; grease can also outgas.
- Avoid scratches on flanges. Finish about Ra 0.4 micrometres.
- Replace elastomer seals after bake cycles, which harden them.
Cleanliness
The quality system should cover raw material controls, dedicated moulds and cleaned tooling for vacuum grades. Ask your supplier whether a dedicated low-outgassing compound with documented TML/CVCM is available.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
