Conductive elastomer O-rings for EMI shielding

Where conductive O-rings are used

The round section suits any joint that already has an O-ring groove: a connector or bulkhead flange, a round cover on an enclosure, a waveguide flange, a lens or window frame. MIL-DTL-83528 defines connector flange mount gaskets and waveguide gaskets as their own sheets, which shows how often the round or framed shape carries the shielding job [S3]. Manufacturers list molded O-rings as a standard form for every filler they offer, so the material choice is the same one you make for a flat gasket [S10][S13]. The conductive elastomers page walks through the fillers and the flange questions; the MIL-DTL-83528 guide explains the type letters.

Sizes: AS568 dash numbers and metric

Conductive O-rings are called out the same way as any other O-ring: an AS568 dash number, or an inside diameter and cross-section for a metric or non-standard size. The AS568 size chart lists every dash number with its dimensions and tolerances, and each size has its own page, for example AS568-214; the metric sizes guide covers ID and cross-section callouts. On a military drawing the size usually appears as a dash number on the /2 sheet, and a size that is not on that sheet is quoted against the /5 non-standard sheet [S3]. Send the dash number or the ID and cross-section and the filler your drawing names, and the quote names that compound.

Fillers available

Silver-plated aluminum silicone is the grade manufacturers recommend on aluminum flanges and in salt fog; silver-plated copper carries the highest published shielding figures for protected joints; silver-plated glass and nickel-graphite are the lower-density and lower-cost commercial choices; fluorosilicone bases take the same fillers where fuel or oil is present [S9][S10][S11][S12]. Published resistivity runs from 0.004 ohm-cm for silver-copper to 0.1 ohm-cm for nickel-graphite, and shielding from 90 to 120 dB at 10 GHz for the metal-filled grades [S10][S14]. The full filler table, with the galvanic notes, is on the conductive elastomers page.

Availability by form and filler

Every filler in the manufacturer form-factor lists is offered as a molded O-ring, and the size decides whether the ring is molded or spliced from circular strip [S15][S13][S9].

The AS568 dash number fixes the inside diameter and cross-section and the manufacturer's size list fixes whether that size is molded or spliced; the quote names which [S9]. The link between the /2 sheet's own dash numbers and AS568 sizes was not verified from a primary page, so a military drawing's /2 dash number is quoted as written and the AS568 number is confirmed against it.

Groove notes from the manufacturers

A conductive O-ring takes less squeeze than a plain sealing O-ring. One handbook limits solid conductive O-rings to 25 percent deflection, sets the groove depth at 0.75 times the free cross-section and the groove width at 1.1 times it, and allows a maximum gasket fill of 95 percent of the groove [S9]. Another manufacturer's table gives 20 to 25 percent for a solid O section, and one manufacturer catalog states a gland fill of 85 to 95 percent, 95 percent where the ring must also seal, and warns against stretching the ring more than 5 percent on installation [S10]. Where the joint holds pressure, the groove bottom needs a 32 to 64 microinch RMS finish, and machining marks that run around the groove should be avoided [S9].

Both manufacturers state that the elastomer behaves as an incompressible material, so the groove must leave void for the displaced volume or the flange will not close [S9][S10]. Closing force is a function of the profile and the flange, not of hardness alone; a hollow section or a softer grade reduces it, and the handbook advises against choosing a material on hardness for that reason [S9][S10]. The groove design guide covers the plain-O-ring gland; use the figures above where they differ.

The handbook's detailed solid-O procedure, from its November 2000 edition, designs around a nominal 18 percent deflection, so the nominal groove depth is 0.82 times the nominal ring cross-section, with 10 percent minimum and 25 percent maximum deflection checked at worst-case tolerances, cover bowing and non-conformity [S9]. Its fill limit is disputed within the same edition, 95 percent in the flange-design section and 100 percent at worst case in the solid-O procedure and on the molded-ring page, and the molded-ring page itself asks for 10 to 20 percent deflection [S9]. A spliced ring needs a groove corner radius at least 2.5 times the strip width, and the handbook asks for generous groove corner radii on any ring spliced rather than molded [S9]. The EMI gasket design guide carries the whole procedure with both figures and their sources.

What to send for a quote

Five items let us quote a conductive O-ring in one pass.

1. The size: the AS568 dash number, or the inside diameter and cross-section with tolerances.
2. The filler and base elastomer, or the MIL-DTL-83528 type letter from the drawing [S14].
3. The hardness, if the drawing names one [S10].
4. The quantity and the paperwork the order needs.
5. The environment: flange metal and finish, salt fog, fuels or oils, and the temperature range [S9][S10][S14].

Send those through the request a quote form and a Conquest representative replies with a quote in the compound you specify. The galvanic compatibility guide covers the flange, and the failure modes guide what to look at when a fitted ring stops shielding.