Very often, the design of the equipment enclosure will itself dramatically limit the choices of gasket materials. Some enclosures are light-weight aluminium or even plastic with very little potential for withstanding distortion during compression of the shielding gasket.
EMC shielding
Other considerations for deciding the correct type of gasket
Normally, the more robust the enclosure is, the wider the range of materials which can be employed for gasketing. Similarly, wide, and well reinforced flanged permit higher compressive forces particularly where limit stops are fitted.
Possibly the most difficult configuration for conventional shielding exists where a ‘knife-edge’ return on a door or cover is to mate with a conductive gasket in addition to providing an environmental seal to IP55 or higher. If the enclosure can be modified to feature an additional return edge or flat where the knife-edge originally finished, the problem is easily solved – but this is not always possible and is usually costly.
Also remember;
- Mating surfaces must be clean, flat, smooth, and conductive (no pain, preservative, oil or grease should be visible)
- Ideal fixings are outside the shielded envelope and particular attention should be paid to screws/bolts passing through the enclosure to secure cover plates in position.
- Avoid gasket materials which are too hard for the type of enclosure.
- Avoid knife-edges wherever possible.
- Consider the environment and select gasket materials which are suitable and will not degrade, e.g., Fluorosilicone rubber instead of plain silicone where hydrocarbon contamination is likely.
- Consider the operating temperature range.
- Do not over-engineer the design. The most expensive materials are not always the right ones for the job in hand.
- Do not under-engineer to save money, this does not work in the long-term.
- Do not over-specify. 100dB may be comfortable but will 60dB solve the problem?
- Use standard or ‘catalogue’ products wherever possible.
- Consider the possibility of Galvanic Corrosion.
The two most common adages in the shielding industry are :
“Design the enclosure with shielding in mind, not as an expensive after-through – retrofitted shielding hardly ever works as planned!”
“one day engineers will design-in standard materials from the catalogue and enjoy considerable cost savings – but they rarely do!”
- Nolato PPT
- EMC products
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