EMI gasket forms compared: molded, extruded, cut sheet, form-in-place, mesh, fingerstock and foam

The one question, and the answer by need

Laird®'s telecom case study shows the sequence for a cast-aluminum outdoor radio: binder from the temperature and fluid table, filler from the galvanic table against the flange metal, a hybrid conductive plus non-conductive design for weather, then the cross-section for closure force and gap, with groove fill above 90 percent and compression of 25 percent or more for a poorly coplanar chassis [S11]. The form falls out of the last two steps: how much the joint can deflect, how long the seam is, and whether the gasket must also keep water out [S9][S11].

Seven forms side by side

Shielding is the band the sources publish for the family, deflection the window the sources publish for the form, and the seal column says whether the form can hold pressure or weather on its own. Where no source read states a relative cost, the cell says so rather than guessing.

Molded conductive elastomer

Molded parts cover sheet stock, O-rings and D-rings, flat washers, waveguide and connector-flange gaskets, jam-nut seals, grommets, and over-molded or reinforced parts [S15][S9]. The handbook's limits: any solid cross-section down to 0.040 in diameter, outer dimensions to 34 in in any direction (larger by splicing), flash up to 0.008 in wide by 0.005 in thick [S9]. Molded D- and O-rings are interchangeable with non-conductive seals of the same size, are meant for a groove giving 10 to 20 percent deflection, and rings over 2 in inside diameter may be spliced from extrusion rather than molded when the groove corner radii are generous [S9]. A die-cut sheet gasket with expanded-metal reinforcement controls creep into a waveguide opening, and raised lips around the cut-out improve power handling and pressure sealing [S9]. Molded O-rings by dash number are on the conductive O-ring page.

Extruded conductive elastomer

Extrusions run from a 0.028 in solid O to a 2.00 in wide flat ribbon in solid O, hollow O, solid D, hollow D, mushroom D, solid and hollow rectangle, channel, hollow P and open V profiles, with hollow walls down to 0.020 in and no limit on length, supplied on reels [S9]. The MIL-DTL-83528 sheet list mirrors that catalog: circular strip 0.040 to 0.250 in (/1), solid D (/3), rectangular D (/6), hollow D (/7), hollow P (/8), solid rectangle (/9), channel (/10) and hollow O (/11) [S3]. Cross-section tolerances are plus or minus 0.005 in under 0.101 in, 0.008 in to 0.200 in, 0.010 in to 0.300 in, 0.015 in to 0.500 in and 0.020 in above on Laird's guide, and 0.005 in under 0.200 in, 0.008 in to 0.349 in, 0.010 in to 0.500 in and 3 percent above on the handbook's [S11][S9]. Fabrication options include cut-to-length, spliced rings with a minimum 2 in inside diameter and a groove corner radius at least 2.5 times the strip width, frame assemblies, bonded compression stops and clip-on or friction-fit mounting profiles [S9]. The solid O is the most-specified profile because it has the widest deflection range among solid sections, needs little space, installs easily and has no corners to damage [S9].

Cut sheet: die-cut and waterjet-cut flat gaskets

Molded sheet comes in 0.020, 0.032, 0.045, 0.062, 0.093, 0.100 and 0.125 in thicknesses and in 10 by 10, 10 by 15, 15 by 20 and 18 by 18 in sheets from one manufacturer, with thickness tolerances from plus or minus 0.004 in at 0.020 in to 0.010 in at 0.093 to 0.125 in [S11][S9]. A second manufacturer lists 0.020 to 0.125 in in 10 by 10, 10 by 15 and 10 by 20 in sheets, and one fabricator adds continuous rolls 12 to 36 in wide with flash, waterjet and CNC cutting as the conversion methods [S14][S31]. Handbook limits for cut parts: width never less than thickness, holes no closer to an edge than the sheet thickness, webs no narrower than the thickness, and flat gaskets deflected only about 10 percent [S9]. Flat gaskets suit sheet-metal and machined flanges where the flange is bent outward so bolts stay outside the shielded volume; if bolts must pass through, the holes must fit the threads closely, from machined dies rather than rule dies, or an EMI seal must sit under each bolt head [S9]. This is the one form cut in our shop: the conductive silicone sheet page has the fillers, thicknesses and what to send.

Form-in-place beads

A robot dispenses a bead of conductive silicone along a programmed path on a metal or plated-plastic housing and it cures in place, so the bead adheres and no groove or mechanical retention is needed [S9][S37]. The handbook's process facts from 2000: minimum bead 0.020 in high by 0.026 in wide with a recommended 0.034 by 0.039 in bead, placement accuracy 0.001 in, height tolerance 0.004 in, flanges as narrow as 0.020 in, surfaces sloped up to 80 degrees, part-to-part elevation within 0.012 in or fixturing, deflection between 10 and 50 percent with 30 percent nominal and a compression stop, and shielding above 75 dB from 200 MHz to 10 GHz for a 0.85 by 1.0 mm bead [S9]. A wave-pattern bead cuts closure force 20 to 40 percent, from 4 N/cm to 2.6 N/cm in the example [S9]. TE lists nickel-graphite, silver-aluminum, silver-copper and silver-nickel silicones dispensed by a pressurized system on a CNC XYZ table [S37]. Nolato's magnetically shaped bead aligns the particles after dispensing into a tall triangular section: 48 percent less material than a D bead, a case study cutting screws from 23 to 13 and cover weight from 135 g to 117 g, and 3.51 N/cm against 4.47 N/cm for three parallel rows at 30 percent compression [S16].

Knitted wire mesh

A single wire is knitted into interlocking loops in round, rectangular, round-with-fin and double-round-with-fin sections, all metal or over a solid or sponge elastomer core [S34][S9]. Monel is the wire most often used for aging, tensile strength and spring quality; tin-plated copper-clad steel gives the best magnetic-field performance; aluminum suits aluminum flanges, and other metals and alloys are available [S9][S34]. The handbook's summary for the family is 60 to 100 dB between 20 MHz and 10 GHz, suited to military work that does not need more than 80 to 90 dB above 1 GHz or exceptional sealing; all-metal mesh cannot give a pressure or weather seal, is not recommended for joined gaskets under 2 by 2 in, and takes a compression set, so TE advises against frequent panel opening [S9][S34]. Combination strips place mesh in parallel with an elastomer weather strip, both halves touching both flanges, and frame gaskets crimp mesh into an extruded aluminum frame that doubles as a compression stop [S9]. Temperature is set by the core material rather than the wire, silicone cores carrying the wider range on the core maker's rating, and mesh cross-section tolerances are one-sided, plus 0.015 to 0.062 in and minus zero by size [S17][S9].

Beryllium-copper fingerstock

Stamped or etched beryllium-copper CA 172 strip is formed and heat treated for spring memory, with eight finish options: clean and bright, gold, silver, tin-lead, bright tin, nickel, zinc with clear chromate, and electroless nickel; stainless versions exist for D-connectors and card cages [S17][S35]. Tech-Etch publishes effective compression of 20 to 70 percent of free height for most families, 20 to 50 percent for spherical and reverse-bend contacts and 20 to 80 percent for twisted contacts, with free heights from 0.03 to 0.44 in covering gaps from 0.01 to 0.35 in [S17]. Mounting is stick-on, clip-on for flanges of 0.03 to 0.125 in, hook-on, snap-on into slots or onto tracks, riveting, spot welding or soldering [S17]. TE's design note is the one to remember: over-compression damages fingers and loses effectiveness [S35]. Brull's 40 GHz work found a fingerstock gasket's attenuation falling continuously above 12 GHz because of the slot between the fingers [S22].

Conductive foam and fabric-over-foam

Open-cell polyurethane or PET foam is plated copper then nickel so it conducts in all three axes, often faced with nickel-copper polyester fabric, and supplied die-cut or in sheet or strip [S17][S36]. Tech-Etch rates conductive foam at -40 to +70 C, 2 to 3 psi at 50 percent compression, UL 94 V rated, a dust shield with generally no environmental seal; TE lists 1.5, 2.3 and 3.4 mm thicknesses, UL 94 V-1, sheet to 560 mm wide, with or without conductive adhesive, for I/O panels, connectors, backplanes, displays and indoor access panels [S17][S36]. Fabric-over-foam wraps metallized nylon or polyester fabric around an open-cell urethane core in rectangular, square, D, C-fold, knife-edge, flat and domed sections, -40 to +70 C, profile tolerance 0.020 in, surface mounted on adhesive [S17]. The handbook's data sheet for the same construction states a minimum 10 percent deflection to reach the plotted shielding, compression set under 20 percent at 25 percent deflection, and resistance rising from 0.006 to 0.032 ohm over 10,000 cycles at 50 percent deflection [S9]. Brull found I/O connector gaskets performed better with a conductive foam core than a non-conductive one, and a fabric-over-foam gasket held a steady response from 1 to 40 GHz [S22].

Outside the seven: oriented wire, combination seals and vents

Oriented wire stands thousands of Monel or aluminum wires perpendicular to a silicone or fluorosilicone sheet, crimped into a slight zigzag so they act as springs: TE quotes up to 140 wires per square centimeter, 0.8 mm minimum thickness and both environmental and EMI sealing in one gasket; Tech-Etch gives 600 to 900 contact points per square inch, 40 Shore A silicone, a temperature range set by the silicone or fluorosilicone carrier, and a fluid and pressure seal that requires relatively high closure pressure [S38][S17]. The solid elastomer flows rather than compresses, so the groove must leave room for the material to move, and the handbook places the family in the same 60 to 100 dB band as knitted mesh [S9]. Co-extruded strips put a conductive elastomer in parallel with a non-conductive one, the non-conductive half outboard for corrosion protection and adhesive; Nolato's co-extruded and multilayer constructions claim shielding above 90 dB after 600 temperature cycles, compression set improved by more than 30 percent and compression force reduced by more than 20 percent against a single-material part [S9][S20]. Honeycomb vents make each aluminum cell a waveguide below cutoff: MAJR's rule is a 4 to 1 cell-depth-to-opening ratio, 0.060 in cells above 18 GHz and 0.125 in below, with an example of 70 dB at 10 GHz plane wave for tin-plated aluminum; Metal Textiles reports more than 80 dB at 10 MHz for single-layer chromated honeycomb and 105 dB for a cross-cell double layer [S39][S40].

How each form is specified

What a drawing or a purchase order has to carry for each form, from the manufacturers' ordering guides.

Conquest Seal supplies the molded and extruded forms and the O-rings, made by manufacturers on the relevant qualified products list where the document has one, and cuts conductive sheet on its own waterjet; mesh, fingerstock, foam and form-in-place are described here so the comparison is complete, and the material behind the elastomer forms is on the conductive elastomers page. The deflection windows and groove rules that decide between a solid and a hollow section are on the design guide, and the type letters on the MIL-DTL-83528 guide. Send the drawing through the request a quote form with the form and the filler named.

Figures on this page are the manufacturers' published values for their own products, cited per row; cost appears only as the words the sources use.