In the aerospace and defense industries, enclosure sealing and insulation needs to meet challenging and complex requirements. For example, the EMI gaskets that are used in military touchscreens must shield sensitive electronics from electromagnetic interference (EMI) while providing electrical conductivity and ensuring environmental sealing. These enclosure gaskets must also cushion the unit from mechanical shock and avoid interfering with the display's touch function.

Custom enclosure gaskets can meet these and other requirements, but designers need to specify the right materials and manufacturing methods. Unlike commodity rubber, specialty elastomers come with higher prices and minimum order quantities (MOQs). If part features such as holes, notches, or chamfers are required, manual gasket cutting may not be able to achieve the required degree of precision. Poor-quality cuts don't just suggest poor workmanship; they can result in sealing or insulation failure.

Designers also need to comply with the aerospace standards or military specifications listed on part drawings. Manufacturing at an AS9100D certified facility may be required, too. Meeting all of these demands can add considerable costs, which is why outsourced fabrication is now used with many projects. Examples of these engineered solutions include finished gaskets for EMI shielding or high flex-fatigue resistance, and custom insulation for heat and noise control.

EMI Shielding and Flex-Fatigue Resistance

With the proliferation of sensors and electronics, many enclosure gaskets need to provide a combination of sealing, shielding, and shock absorption. Thanks to innovations in material compounding, gasket materials can combine the advantages of elastomers with the electrical properties of metals. Silicones, a family of synthetic elastomers, offer thermal stability over a wide temperature range and resistance to ozone, water, and sunlight. When packed with metal or metal-coated particles, silicone sheets and rolls can provide EMI shielding and electrical conductivity.

Water jet cutting can create precise holes, notches, and chamfers to accommodate fastener heads and enclosure features.

Older particle-filled elastomers could be too hard or too brittle, but newer EMI silicones come in durometers as soft as 30 Shore A. During gasket cutting, these materials won't stretch or become deformed. Connector holes align properly, and the material's structural properties support greater tear resistance – a key consideration for gaskets with thinner walls. For shielding applications where Z-axis conductivity is required, EMI enclosure gaskets support the use of electrically conductive adhesives. Designers who choose an engineered solution can also specify an adhesive backing for ease-of-installation.

EMI enclosure gaskets support the use of electrically conductive adhesives. Enclosure designers have a choice of bonding methods.

Importantly, enclosure designers now have a choice of fill materials. Historically, many EMI gaskets were filled with silver or silver-aluminum particles. Today, nickel-graphite particles can provide silver-like connectivity. These gasket materials cost less than silver-filled compounds and can meet the lettered requirements of MIL-DTL-83528, a U.S. military specification.

Acoustic and thermal insulation can absorb sounds, dampens vibrations, reflects radiant heat, or resist the spread of fire.

For cost-effective fabrication, water jet cutting can be used. Unlike manual cutting, water jet equipment limits mis-cuts and material waste. Fluorosilicones have physical and mechanical properties that are similar to silicones, but provide improved resistance to fuels, oils, and solvents. In some aerospace and defense applications, fluorosilicone enclosure gaskets are required because of the splash of jet fuel, deicing fluids, or cleaning agents. During compound selection, designers can choose compounds that meet SAE AMS standards, or that meet the EMI shielding requirements of the MIL-DTL-83528 specification.

Silicone compounds have many desirable properties, but some have inadequate flex-fatigue resistance – a measure of a material's ability to withstand repeated flexing or bending without cracking. That's a problem in aerospace and defense, where materials with high flex-fatigue resistance are needed for door seals, window seals, and the vibration-isolating mounts for alternators, compressors and assembly bolts. Aerospace and military enclosures need gaskets with high flex-fatigue resistance, too.

During material selection, designers can specify materials that meet the full requirements of the A-A-59588 3B specification – with no exceptions for flexural testing. This standard from the U.S. General Services Administration (GSA) references the DeMattia Flex Resistance Test, which measures crack growth in inches over thousands of flexural cycles. Some A-A-59588 3B materials aren't fully compliant, however, so it's important to partner with a fabricator that can source certified 3B materials.

Gasket fabricators differ in terms of manufacturing capabilities, too. In addition to the cutting method, designers need to consider the best way to bond rubber gaskets. Hot splicing requires clean, straight cuts but support higher production volumes. By contrast, vulcanization is more forgiving since the cuts don't have to be smooth and precise. Cold bonding is a manual process that's performed with a brush or an adhesive or glue. It's ideal for low-volume quantities, but cold bonded gaskets won't last as long as hot spliced ones. Molding is the only bonding technique that can create rounded corners.

Thermal Protection and Noise Control

Ground support equipment (GSE) contains several enclosures, including the cabin, that require sealing and insulation.

In addition to rubber gaskets, aerospace and military enclosures need insulation that can absorb sounds, dampen vibrations, reflect radiant heat, or resist the spread of fire. Depending on the application, designers can specify thermal, acoustic, or thermal-acoustic products. These engineered solutions feature multi-layered structures and can meet FAR 25.856, which defines requirements for materials that are installed in an aircraft's sides or underneath the flooring. Thermal, acoustic, and thermal-acoustic insulation is also used in ground support equipment (GSE).

Acoustic insulation absorbs, transmits, or redirects sound waves – vibrations in the air that pass-through objects and result in audible noise. There are four main types of acoustical materials: absorbers, barriers, dampers, and facings. Sound absorbers are made of open cell acoustical foams, typically polyester, polyurethane, urethane, or melamine. Sound barriers are also made of foams but are denser. Vibration dampers come in extruded vinyl, asphalt-impregnated paperboard, metal foil, or fiberglass. Facings include materials that resist dirt, mildew, abrasion and chemicals.

Thermal, acoustic, and thermal-acoustic insulation are all used in GSE. GSE gaskets need to withstand wind, water, a range of service temperatures, and possible contact with jet fuel and de-icing fluids.

Thermal insulation also comes in a variety of materials with specific properties. Examples include Mylar films and aluminum foils that reflect radiant heat, melamine foams that resist fire, and vinyl rubber that meets UL 94 V0 flam-mability requirements. Like acoustic insulation, these thermally insulating materials are laminated together in a sandwich-like structure. They can also be combined with sound absorbers, dampers, barriers or facing materials for noise control. To support temporary or permanent fastening, custom insulation can be taped or used with a pressure-sensitive adhesive (PSA).

The engine bay insulation that's used with GSE provides several examples of engineered solutions. To keep heat and noise in the engine bay from reaching the cabin's interior, thermal and acoustical materials are layered together, laminated, and then water jet cut into precise geometries with part features such as notches and bolt holes. Some thermal-acoustic insulation consists of an aluminum facing or metallized Mylar that's laminated to a sound-absorbing foam. The facing reflects radiant heat and resists engine oil, but also withstands the soap and water used in engine washdowns.

Engine bay insulation can also use lightweight, melamine fire-resistant foams that are laminated to facing materials and supplied with PSA liners. Open-cell melamine foams combine high-temperature resistance with strong sound-absorbing properties. Another type of engine bay insulation sandwiches a layer of silicone foam between a reinforced fabric facing and a removable PSA liner. Along with chemical and oil resistance, this type of insulation resists mildew.

The GSE cab or cabin where an operator sits is another type of enclosure with custom insulation. If the vehicle uses a headliner, the insulation's facing material may feature small holes for enhanced noise control. Sound absorbing foams that resist moisture, dirt, and petroleum products are heat laminated to these vinyl facings, which come in custom colors. Rubber flooring isn't generally associated with enclosure sealing and insulation, but floor mats that are water jet cut conform to contours and help to absorb sound.

Enclosure designers need to meet multiple requirements for aerospace and defense applications. Material selection, manufacturing capabilities, AS9100D certifications, and standards like MIL-DTL-83528, A-A-59588 3B, and FAR 25.856 are just some of the factors to consider. By choosing a value-added manufacturer instead of a component supplier, designers can find engineered solutions for superior sealing and insulation. For mission-critical applications, the requirements are for nothing less.

This article was written by Roberto Naccarato, Sales Manager, Elasto Proxy (Boisbriand, Quebec, Canada). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the December, 2019 issue of Aerospace & Defense Technology Magazine.

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