Conductive elastomers for EMI shielding gaskets and O-rings

Fillers: what each one is chosen for

The filler sets the resistivity, the shielding figure, the density and how the gasket behaves against the flange metal; the base elastomer sets the temperature and fluid limits [S10][S11]. The figures below are the ones manufacturers publish for their own compounds, so treat them as the shape of each family rather than a guarantee for a specific grade [S10][S14].

Silver-plated copper and silver-plated aluminum are the two fillers most drawings name, and the choice between them is usually the flange: silver-copper for the shielding figure on a protected joint, silver-aluminum wherever the flange is aluminum or the enclosure sees weather [S9][S12]. Nickel-graphite is the usual fit when the drawing is commercial and cost matters more than the last 10 dB [S11]. Silver-plated aluminum and silver-plated copper differ in the filler core only; the silver skin carries the current in both, and the aluminum core is what changes the galvanic behavior and the density [S9][S10]. Nickel-plated aluminum is the newest common filler and carries no type letter; the galvanic compatibility guide sets the maker's salt fog figure beside the 2000 handbook's coupon data [S15][S9].

Shielding effectiveness and how it is measured

Shielding effectiveness is the ratio, in decibels, of the field measured with and without the barrier in place, so 100 dB means the barrier cut the field by a factor of one hundred thousand. It changes with frequency and with the kind of field, and the manufacturer tables run from a 200 kHz magnetic field through 2 GHz and 10 GHz plane-wave columns [S9][S10]. Most published single figures are the 10 GHz value from the specification's own fixture test, and manufacturers state that a fixture result may not transfer to a real flange, because the flange's stiffness, fastener spacing and tolerances can raise or lower it [S9][S10].

Three test methods are named on drawings and datasheets. IEEE 299-2006 measures the shielding effectiveness of enclosures with every dimension of 2.0 m or more, from 9 kHz to 18 GHz, and IEEE marked it inactive-reserved on 30 March 2023 [S6]. MIL-STD-285, the older enclosure method dated 25 June 1956, was cancelled on 24 October 1997 with IEEE 299 named as the replacement [S7]. ASTM D4935-18 measures planar materials under far-field plane-wave conditions from about 30 MHz to 1.5 GHz and does not apply to cables or connectors [S8]. Transfer impedance is a fourth method some manufacturers report for gasket materials [S10].

Published values for silver-filled and nickel-graphite grades sit between 90 and 120 dB at 10 GHz; carbon grades sit near 30 dB [S10]. The value on a datasheet answers what the material can do in a fixture; your flange design answers what the joint will do.

Shielding effectiveness by frequency, per filler

Only values with a source are in the rows below, each with the method and the edition the source states; where a source gives one frequency point, that is the only point shown. The 10 GHz values are the manufacturers' minimums in the MIL-DTL-83528 fixture as their sheets label them [S15][S11][S14].

The five-point table shape on the manufacturer sheets is 200 kHz magnetic field, 100 MHz and 500 MHz electric field, and 2 GHz and 10 GHz plane wave, with 40 GHz on newer sheets; the rows above carry the points the sources publish per grade, not the whole sheet [S15][S11]. Nine manufacturers held QPL-83528 qualification on the DLA Qualified Products Database on 2026-09-14, and the MIL-type grades quoted here come from QPL-listed makers; the commercial grades are named as their makers publish them [QPD-1]. What each fixture measures, and why the fixture value never equals the flange value, is on the testing guide; the groove that keeps the pressure on is on the design guide.

Compression, deflection and closure force

Deflection is the change in gasket height under the closing load, and manufacturers publish it per profile rather than per material. Read together, the Parker Chomerics handbook and the Laird catalog put solid sections at about 15 to 30 percent, with solid conductive O-rings held to 25 percent, hollow sections up to 50 percent and flat gaskets about 10 percent, and the Laird catalog states that actual deflection should never fall below 10 percent [S9][S10]. Size the free height so that the flange gap, including the unevenness of the mating faces, lands inside that range [S10].

For a grooved gasket, one manufacturer catalog states a gland fill of 85 to 95 percent, 95 percent where the gasket must seal as well as shield, and warns against stretching the part more than 5 percent on installation [S10]. Hardness in the tables runs from 45 to 85 Shore A, but the handbook states that hardness is not a reliable guide to closing force for a filled elastomer; profile shape is, and hollow sections with softer grades cut the force needed [S9][S10]. Compression set is listed at 25 to 50 percent maximum by ASTM D395, and electrical stability is shown as resistivity limits after heat aging, during and after vibration and after EMP exposure, which is how a datasheet shows that conductivity holds after the gasket has taken a set [S10]. The full groove procedure, the disputed solid-O lower bound and the fastener spacing rules are on the design guide.

Galvanic pairing and corrosion

Galvanic corrosion needs two dissimilar conductors and an electrolyte, so a silver-filled gasket on an aluminum flange corrodes the aluminum when salt water or humidity is present and does little in a dry office environment [S10]. The DoD reference for the pairing is MIL-STD-889D, Galvanic Compatibility of Electrically Conductive Materials, active on DLA ASSIST; Revision D judges compatibility by the corrosion rate of the anodic member of the couple rather than by the potential difference alone [S4][S5]. Manufacturers publish their own couple charts from salt spray tests, and one manufacturer catalog states a 500 hour exposure that records the weight loss of the metal coupon and the resistivity change of the elastomer for each pairing [S10].

On aluminum the guidance is consistent across manufacturers: silver-plated aluminum is the compatible filler, nickel-graphite is an acceptable lower-cost alternative, and silver-plated copper, silver-plated nickel and pure silver corrode the flange faster in salt fog [S9][S11][S12]. A chromate conversion coating on the aluminum counts as a conductive finish; the handbook cites the MIL-C-5541 Class 3 limit of 200 milliohms after 168 hours of 5 percent salt spray at 200 psi contact pressure [S9]. Where you plate the flange, choose a finish close to the filler in the galvanic series and protect the outside edge of the joint so the electrolyte cannot reach the gasket [S9][S10].

Environment: temperature, fluids and weather

The base elastomer sets the temperature capability: for its filled conductive compounds, one manufacturer lists silicone at -45 C to 200 C, fluorosilicone at -55 C to 175 C and EPDM at -50 C to 125 C [S10]; the unfilled family ranges are on the silicone, fluorosilicone and EPDM pages. The filled compounds carry tighter published limits, a -55 C minimum by ASTM D1329 and a 125 C or 160 C maximum operating temperature by type, so use the compound datasheet rather than the base-elastomer figure [S10][S14]. Fluorosilicone grades survive the specification's fluid immersion test and silicone grades do not, which is why fuel and oil exposure moves a drawing from Type A or B to Type C or D [S10][S14].

For outdoor or salt fog service the filler decides more than the base: silver-plated aluminum silicone is the grade one manufacturer recommends for dissimilar mating surfaces, salt fog and uncontrolled environments, and it lists the grade as ozone resistant [S12]. Shelf life is stated per compound on the manufacturer datasheet; ask for it with the quote. One manufacturer catalog states that every part in it is lead-free and RoHS compliant, so ask for the declaration for the exact compound when the order needs it [S10].

Forms: molded, extruded, cut sheet and form-in-place

Conductive elastomers come as flat gaskets cut from sheet, molded shapes including O-rings, extruded and co-extruded profiles, gaskets bonded to a metal carrier, co-molded parts and form-in-place beads [S13]. Flat sheet is molded in standard thicknesses of 0.020, 0.032, 0.062, 0.093 and 0.125 inch, and a flat gasket should not deflect more than about 10 percent [S9]. Form-in-place is a dispensed bead cured on the housing; the handbook gives it a 10 to 50 percent deflection window, a 30 percent nominal and a mechanical compression stop [S9]. That process is run by the gasket manufacturer or the housing maker, not stocked as a part.

Conquest Seal supplies the molded and extruded forms, made to MIL-DTL-83528 by manufacturers on its qualified products list where the drawing calls for it, and conductive silicone sheet is waterjet-cut to your drawing for flat gaskets in our shop in Orange, California. Send a DXF, DWG or PDF with the sheet thickness and the filler, or see the conductive silicone sheet page for the stock thicknesses, the forms guide for the seven forms side by side, and the conductive O-ring page for round sections.

How to specify a conductive elastomer

Six items describe the part completely.

1. The MIL-DTL-83528 type letter, or the filler and base elastomer for a commercial part [S14]; the MIL-DTL-83528 guide explains the letters.
2. The form: cut sheet with its thickness, an extruded profile with its free height and width, a molded shape, or an O-ring by dash number or ID and cross-section [S13].
3. The hardness, if the drawing names one, from the 45 to 85 Shore A range in the tables [S10].
4. The flange metal and finish, with the filler your drawing names; the galvanic compatibility guide shows the pairings [S9][S10].
5. The environment: temperature extremes, fluids, salt fog or weather [S10][S14].
6. The quantity and the paperwork the order needs.

Send those with the drawing through the request a quote form and a Conquest representative replies.

Figures on this page are the manufacturers' published values for their own compounds, cited by source; the compound datasheet and your flange design govern the final choice.