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Silicone, PORON, or ePTFE Vent: Choosing Avionics Enclosure Sealing Materials
An enclosure seal is specified backward more often than any other part on this estate: an engineer picks a gasket, then asks what rating it gives. The right order names the ingress severity first, then sizes the seal and decides whether the box should be sealed or sealed-and-vented. The families on the H-O avionics enclosure page sort by ingress, altitude, closure force, and RF duty, and this guide walks each one by the decision it wins, with methods named by designation.
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Name the water, then size the seal to it
An enclosure seal is specified backward more often than any other part on this estate: an engineer picks a gasket, then asks what rating it gives. The right order is to name the ingress severity first, the wash-down, the condensation, the immersion the box can actually see, and then size the seal and, critically, decide whether the box should be sealed at all or sealed-and-vented.
So the families on the H-O avionics enclosure page sort by ingress severity, altitude behavior, closure force, and any RF or outgassing duty. This guide walks them by the decision each one wins, with methods named by designation: RTCA DO-160 for the airborne environmental and ingress framing, IEC 60529 for the IP code, ASTM D1056 for the compression-force-deflection classes, ASTM D395 for compression set, ASTM E595 for outgassing, UL 94 for flammability, and MIL-DTL-83528 where the seal must also shield.
Per-grade values are the maker-TDS figures cited on the parent page.
The perimeter seal: silicone sponge and foam
The lid and flange perimeter is the primary seal, and it runs on flame-retardant silicone sponge graded to the closure force: the BISCO RS flame-retardant sponge (the RS-870 soft through firmer grades) is the lid and flange default with compression-force-deflection per ASTM D1056, the kSil V-0 super-soft sponge covers the UL 94 V-0 rated soft closeouts, and the BISCO BF foam and HT-800 medium silicone foam handle the softest, lowest-force lids.
Where a thin lid bows between fasteners and leaks midspan, the answer is a softer, higher-deflection grade that the available closure force can fully seat across the unsupported span.
Thin sections and grooves: PORON and solid silicone
Thin-section seals, connector patterns, and tightly toleranced grooves want different families. PORON microcellular urethane (the supported PORON 4701-40V0 and slow-rebound 4790-92 grades) excels at thin sections and intricate connector gasket patterns where a sponge would be too coarse, with compression set behavior per ASTM D395 on the TDS.
Grooves and RF frames that deliver high even force step to BISCO HT-6xxx solid silicone (the tight-tolerance HT-6135 and transparent HT-6240 grades), and where fuel or hydraulic fluid reaches the bay the seal moves to fluorosilicone sponge.
Venting and pressure equalization: ePTFE membrane
A sealed avionics box that climbs and descends needs to breathe without letting water in, which is the job of a vent. Gore ePTFE venting membrane equalizes pressure across the altitude cycle while blocking liquid water, so the box does not pump in its own condensation or stress its primary seal on every climb. Where a chemical-resistant gasket sheet is wanted, Gore GR ePTFE gasket sheet and the ePTFE sheet stock carry the joints elastomers cannot. The vent is part of the seal system, not an accessory, and it goes where wash fluid does not sit.
RF apertures and filtered cooling inlets
Two specialty seals round out the page. Where the enclosure seal must also be conductive, a solid silicone base carries an EMI-conductive grade and the seal is designed per the MIL-DTL-83528 framing so it shields without attenuating the antenna it surrounds. Where cooling air must pass without shedding particles, reticulated filter foam (the 10-PPI PVC-coated, 20-PPI, and 30-PPI grades, UL 94 HF-1 rated where listed) filters the inlet air.
An open-cell filter foam is not a seal, and specifying one where a closed-cell perimeter gasket belongs is a classic ingress failure.
Enclosure Sealing and Venting Families by Job
| Family | Job | Cell / form | Closure force | Where it lives |
|---|---|---|---|---|
| BISCO RS / kSil V-0 | Perimeter seal | Closed-cell sponge | Soft to firm | Lid and flange perimeters |
| PORON urethane | Thin-section seal | Microcellular | Low, fine pattern | Connector patterns, thin sections |
| BISCO HT-6xxx solid | Groove / RF frame | Solid silicone | High, even | Toleranced grooves, RF frames |
| Gore ePTFE membrane | Vent / equalize | Microporous membrane | n/a | Pressure equalization, breathing |
| Reticulated filter foam | Air filter | Open-cell, not a seal | n/a | Cooling inlets, airflow paths |
Illustrative and relative only; confirm each grade on its current technical data sheet. CFD and set per ASTM D1056 / D395; ingress framing per RTCA DO-160 and IEC 60529; DO-160 qualification belongs to the box builder.
Two rules that sort the page
First, decide whether the box should be sealed or sealed-and-vented before choosing the gasket: a fully sealed avionics box that cycles in altitude pumps in its own condensation and stresses its seal on every climb, which is why a vent membrane is part of the seal system, not an afterthought, and the companion article on this estate walks that ingress-and-breathing logic in detail.
Second, match the cell structure to the job: a closed-cell sponge seals, an open-cell filter foam filters, and specifying an open-cell pad where a closed-cell perimeter seal belongs lets water straight in.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take flame-retardant silicone sponge, microcellular urethane, solid silicone, fluorosilicone, ePTFE membrane and sheet, and reticulated filter foam stock from the material manufacturers and die-cut, kiss-cut on liner, laminate, and kit it to your drawing, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We recommend a family and grade direction from your ingress severity, altitude behavior, closure force and lid stiffness, RF or EMI duty, outgassing limits, and serviceability, and grade-level values are confirmed against the manufacturer's current TDS. DO-160 qualification and ingress-rating certification stay with the box builder.
Frequently asked questions
What gasket family seals an LRU lid against wash-down and condensation?
A closed-cell flame-retardant silicone sponge graded to the closure force is the lid and flange default: the BISCO RS ladder from soft to firm, kSil V-0 for V-0 rated soft closeouts, and BISCO BF or HT-800 foam for the softest, lowest-force lids, all with compression-force-deflection per ASTM D1056 on the TDS. The ingress severity is named first, per the RTCA DO-160 and IEC 60529 framing, then the grade is sized to seat across the worst-case unsupported span of the lid, not just at the bolt line.
Why does a sealed avionics box need a vent at all?
Because a fully sealed box that climbs and descends pumps air, and moisture, in and out through any leak path as the internal-to-external pressure differential cycles, and it stresses the primary seal on every climb. A Gore ePTFE venting membrane equalizes the pressure across the altitude cycle while blocking liquid water, so the box breathes without ingress and without condensation collecting inside. The vent is part of the seal system, placed where wash fluid does not sit; the companion design article on this estate walks the breathing logic in detail.
Can a die-cut gasket get my box to an IP67-class rating?
A die-cut closed-cell gasket sized correctly to the closure force and lid stiffness is the sealing element of an ingress-rated enclosure, but the rating itself is a property of the finished box, tested by the box builder per IEC 60529 or the relevant RTCA DO-160 section. H-O converts the gasket, and where appropriate a vent membrane, to the drawing so the box can achieve its target, but the ingress rating is verified on the assembled enclosure, not certified on the gasket.
The cell structure, closure force, and lid stiffness all have to be right for the rating to hold.
What seals a radome or antenna interface without hurting the signal?
Two different requirements have to be reconciled. Where the aperture seal must be RF-transparent, a non-conductive silicone or ePTFE seal closes the joint without adding loss in the antenna's band.
Where the seal must instead be conductive to maintain a shielded boundary, a conductive silicone designed per the MIL-DTL-83528 framing shields without attenuating the signal it surrounds, which is the failure to avoid: an aperture gasket that attenuates the antenna it was sealing. The drawing has to state whether the aperture is RF-transparent or a shielded boundary.
Does H-O qualify enclosures to DO-160 or certify ingress ratings?
No. H-O converts the documented sealing, venting, and filter materials to the qualified drawing, with material traceability and lot-code TDS records that support your qualification file. Qualifying the box to RTCA DO-160, certifying an IEC 60529 IP rating, and holding platform certifications belong to the box builder and its test houses. Our role is bounded: die-cut, kiss-cut, laminate, and kit the gaskets, vent membranes, and filter foams to your drawing, repeatably and traceably.
Get an enclosure sealing recommendation
Tell us the ingress severity, the altitude behavior, the closure force and lid stiffness, any RF or EMI duty, the outgassing limits, and the serviceability, and we will recommend a family and grade direction and quote die-cut seals, vents, and filters to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- RTCA DO-160 (airborne environmental and ingress framing for civil avionics, cited qualitatively; qualification belongs to the box builder).
- IEC 60529 (the IP ingress-protection code, cited qualitatively).
- ASTM D1056 (compression-force-deflection classes for cellular rubber); ASTM D395 (compression set).
- ASTM E595 (outgassing, per grade); UL 94 (flammability listings); MIL-DTL-83528 (conductive elastomer framing where the seal must shield, by designation).
- Manufacturer technical data sheets for the silicone sponge, microcellular urethane, solid silicone, fluorosilicone, ePTFE membrane and sheet, and reticulated foam grades named on the parent application page.
This article summarizes enclosure sealing and venting material-selection principles for avionics and electronics using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your closure force, altitude, and installed condition. Standards (RTCA DO-160, IEC, ASTM, UL, MIL-DTL) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not qualify enclosures to DO-160, certify ingress ratings, or hold platform certifications; those belong to the box builder and its test houses.