Tim Kearvell, product manager, Parker Chomerics Europe outlines why urethane-filled expanded aluminium gaskets can now offer significant advantages
What are the premium requirements of gaskets for flange-to-flange aerospace assemblies such as antennas, access panels, radars and lighting? Electrical conductivity? EMI (electromagnetic interference) shielding? Fluid and pressure sealing? Galvanic corrosion mitigation? In short, the answer is all of these things and more.
For this reason, designers choosing fully-cured, urethane-filled expanded aluminium gaskets will not only find a solution that meets all of these criteria, but one that offers numerous performance improvements when compared with alternative sealing solutions. In fact, users can expect a 33 per cent lower total cost of ownership (TCO) resulting from the avoidance of assembly rework and material replacement.
The secret, of course, is in the gasket material. Impressive levels of electrical through-resistance, shielding effectiveness, corrosion resistance and fluid resistance, as well as an extensive temperature operating range are all attributes of the latest urethane-filled expanded aluminium gaskets.
For any design engineer, the foremost consideration of a gasket is sealing performance. Here, the high conformability of low-modulus urethane binder systems adapts to surface irregularities to ensure leak-path elimination and efﬁcient sealing characteristics.
Gasket materials must also perform well in terms of both atmospheric pressure and fluid sealing, and again urethane-filled expanded aluminium gaskets perform well. The high surface afﬁnity and conformance associated with urethane binder systems creates an efﬁcient sealing solution in variable applications such as those found between externally mounted aircraft devices and the fuselage. When subjected to speciﬁed torque values for such devices, pressure sealing of circa 2.8 bar should result, while also providing sealing capabilities and chemical resistance against common aviation ﬂuids.
Among other major gasket functions is electrical conductivity, as lightning strike survivability and EMI shielding (to avoid antenna signal interruption) are paramount. Expanded aluminium improves upon electrical performance compared to particle-ﬁlled or woven mesh alternatives by eliminating the electrical contact resistance associated with point-to-point conductance requirements. The homogenous nature of expanded metal results in a highly efﬁcient electrical system which optimises attenuation and electrical grounding performance.
So what of airframe pitting? Clearly, it is vital to avoid the surface pitting of airframes and any associated stress fatigue and cracking. Here, 3000 series aluminium alloys offer a notable advantage. When these materials act as the gasket’s conductive medium, they become the sacriﬁcial entity of the assembly when interfaced with harder aluminium components. Expanded aluminium also promotes an even distribution of interfacing surface load, resulting in the elimination of concentrated stress points that can increase pitting occurrence. Microscopic inspection of substrates after gasket deﬂection shows no negative interaction with expanded metal-based gaskets. Conversely, woven-wire based solutions can create stress points at wire-overlap locations that cause surface pitting.
With regard to minimising aircraft TCO, the mitigation of corrosion is a significant contributor. Using urethane-filled expanded aluminium gaskets, moisture ingress, and the resulting electrolytic environment, can be minimised through increased sealing performance. Additionally, as aluminium is the typical base material for substrate design, deploying 3003 expanded aluminium as the gasket’s conductive medium galvanically matches the sealing solution with the mating assembly. In these two ways, aircraft owners can expect signiﬁcant reductions in rework and component replacement requirements commonly associated with corrosion.
New generations address old issues
There are of course some historical concerns with conventional urethane-based antenna gasket technologies, such as the need for multiple material trimming operations, loss of fastener torque retention and silicone contamination. These challenges have been successfully addressed by Parker Chomerics with its latest Metalastic EXP-URE gasket solutions.
Multiple material trimming operations due to cold ﬂow has been a common issue with urethane gel gaskets. However, it is now possible to provide application-speciﬁc analysis of gasket footprint size to limit material evacuation from the installed device, as well as the associated trimming steps. As a point of note, it is worth performing compression stop integration in parallel with gasket size evaluation.
This leads nicely to addressing the next concern: loss of fastener torque retention. The latest urethane-filled expanded aluminium gaskets feature optional compression stop integration in the mechanical design to ensure that any stress relaxation of the urethane binder has no ill effects on fastener performance, thus eliminating the need for multiple torque sequences during installation.
Silicone contamination is a well-documented contributor to poor paint adhesion. As aircraft go through several repaints during their lifespan, developing a silicone-free gasket solution is crucial. Not only do urethane-based binder systems facilitate easy removal without mechanical aids, but clean-up operations prior to painting are straightforward using commonly available chemicals such as isopropyl alcohol.
Urethane-filled expanded aluminium also lends itself to assembly integration in line with predefined designs. The material accommodates a wide range of existing application parameters while maintaining a high level of electrical and sealing performance. Urethane gel can be deﬂected easily and accommodate various mechanical designs to suit fastener spacing, fastener torque requirements, ﬂange thickness and so on. Indeed, electrical conductivity occurs at gasket deﬂection as low as 15 per cent. This guarantees a higher probability of surface-to-surface electrical contact between the gasket and the mating surfaces. In turn, more surface contact ensures higher electrical performance (in terms of lightning energy transfer and EMI shielding) in highly variable or contoured assemblies.
Finally, installation is straightforward thanks to a symmetrical gasket cross section. This allows for gasket installation without consideration of Z-axis orientation, thus allowing process yields to improve.