Honeycomb Air Ventilation Panels – The Polarity Principle

Composite materials have been replacing metal structures throughout the aircraft industry primarily to save weight, improve fuel economy and reduce costs.  But the lack of electrical conductivity in these materials is a disadvantage when compared to the conventional metal airframes of the past. The conventional, metal airframe allowed designers to take advantage of the natural Faraday cage it formed to protect equipment against interference. There were many opportunities to ground items of equipment reliably by connecting directly to a convenient surface ground.

Today, a typical airframe consists of around 50% composites. Major structures include the fuselage and wing fairing, as well as large sections of the wings, fin and horizontal stabilizers. 

Inside the aircraft there are numerous electrical systems capable of generating EMI which can potentially disturb the operation of critical systems. These include fluorescent lights, light switches, dimming circuits, AC-powered window heaters, motors and generators, data and power cables, and transmitters such as radio and radar. 

External storms are also a major source of potentially disruptive electrical interference and can cause physical damage to the aircraft through lightning strike impact.  

Replacing metal structures with composites means compromising the EMI shielding and lightning strike protection of the aircraft, as the composites themselves are not electrically conductive. To overcome this issue, woven or non-woven copper-aluminium mesh, or an expanded foil, can be embedded in composite structures to restore lost shielding and grounding properties. The embedded metal provides an optimal combination of electrical conductivity, weight, and corrosion resistance. Solid metal strips can be used in the radome area to handle very high concentrations of lightning energy. 

Embedded conductors, however, do not solve all the technical challenges that come with the increasing use of composites. It is very difficult to ensure reliable electrical continuity between individual composite panels after the airframe is assembled and still promote conduction of lightning energy. 

Electrical components are typically bonded or grounded directly to the airframe. These connections to the mesh can often fail to meet the very low impedance requirements because of environmental stresses such as vibration and temperature variation. The exposed mesh in the locations where grounded or bonded modules are made (fig 1, at left), can be vulnerable to environmental exposure (temperature, humidity, oxidation) that increases electrical impedance.

To overcome this challenge, a lightweight coating such as Parker Chomerics CHO-SHIELD technology can be applied to optimize conductivity in this area. CHO-SHIELD® 4994 is a highly conductive, silver-filled polyurethane coating designed for aerospace applications and has superior EMI shielding properties. The coating provides excellent adhesion and wear resistance and is resilient to most operation and environmental fluids. The coating is compatible with many primers and top coat systems.

In areas where high corrosion protection is needed, a copper-based urethane coasting such as Parker Chomerics CHO-SHIELD® 2002 can be used. When used on a composite, CHO-SHIELD 2002 provides the conductivity necessary to achieve excellent shielding effectiveness while maintaining its electrical and mechanical stability in hostile environments. CHO-SHIELD 2002 is designed to be used with Chomerics CHO-SHIELD® 1091 primer to ensure correct adhesion. 

The aircraft antennas will also need to be shielded and grounded against lightning strike. This can be achieved by using an expanded woven MetalasticTM EXP-URE gasket material. Electrically conductive grease can be applied at ground connections, to support reliable electrical connectivity under temperature and vibration. Attention must be paid to viscosity and surface-wetting properties when formulating greases for aerospace applications. Parker Chomerics CHO-LUBE® 4220 has a resistivity better than 100mΩ-cm and is an example of an aerospace-grade grease. It is formulated to support electrical interconnections, improve metal to metal contact and provide long-term oxidation protection for exposed mesh or electrical terminals.

Conductive sealants such as Parker Chomerics CHO-BOND® 2165 or CHO-BOND® 1019 can be applied at locations requiring electrical continuity and environmental protection. Typical airframe areas treated are screw holes, fasteners, antenna connection points and exposed conductors on external areas. Where conductive gaskets are used to promote electrical continuity between composite components, a conductive sealant can be applied to provide improvement in continuity. These areas are generally around the wheel wells, engine mounts, wings and the tail section, where high vibration occurs (figure 2, above). 

In addition to these methods which will improve EMI performance throughout the airframe, a lightweight conductive heat shrinkable tube such as Parker Chomerics CHO-SHRINK® 1061 can be used to shield the aircraft’s cabling against the effects of EMI and can provide a weight saving of up to 60% compared to traditional methods.

This blog post contributed by Mel French, marketing communications manager, Chomerics Division Europe

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Form-in-place EMI gaskets, also known as FIP EMI gaskets, is a robotically dispensed electromagnetic interference (EMI) shielding solution that is ideal for modern densely populated electronics packaging.

The most important distinction of form-in-place EMI gaskets is that they were developed for applications where inter-compartmental isolation is required to separate signal processing and/or signal generating functions.

Simply put, form-in-place gaskets are meant to reduce “noise” between cavities on a printed circuit board (PCB) or in an electronics enclosure. 

In addition, form-in-place gaskets provide excellent electrical contact to mating conductive surfaces, including printed circuit board traces for cavity-to-cavity isolation. Parker Chomerics form-in-place gasket materials are known as CHOFORM. 

1. Small form factor - form-in-place gaskets can be dispensed in smaller bead sizes than most traditional EMI shielding gasket solutions, 0.018” tall by 0.022” wide. 
    
2. Excellent adhesion - 4-12 N/cm adhesion on prepared surfaces such as machined metals, cast housings, and electrically conductive plastics.
    
3. High shielding effectiveness - Parker Chomerics CHOFORM materials can provide more than 100 dB shielding effectiveness in the 200 MHz to 12 GHz frequency range.
    
4. Quick programming - Because form-in-place EMI gaskets are robotically dispensed, a standard CAD file can be used to program the dispensing system and quickly map out the dispensing pattern.
    
5. Complex geometries - The positional tolerance of the gasket can be held to within 0.001” and is able to follow very complex geometries including sharp turns, corners, and serpentine patterns. Other gaskets such as die cut sheets or o-rings manufacture and/or fabricate into such shapes and patterns. 
    
6. “T” joints - Traditional extruded gaskets are difficult to mate at intersections or “T” joints. The robot dispensing systems produce reliable junctions between bead paths to provide continuous EMI/EMC shielding and environmental sealing.
    
7. Integrated solutions - CHOFORM technology combined with a Parker Chomerics supplied metal or conductive plastic housing provides an integrated solution ready for the customers’ highest level of assembly. This approach requires no additional assembly or process steps for the installation of gaskets and/or board-level auxiliary components. 

1. Large form factor enclosure sealing that can accommodate a groove. For larger areas such as machined covers that can accommodate a gasket groove, other EMI shielding solutions are better suited. In most applications, conductive elastomers such as the CHO-SEAL product line by Parker Chomerics will provide better shielding and sealing. Form in place gaskets can be dispensed in bead sizes only as large as about 0.062” tall x 0.075” wide.
    
2. Enclosures requiring submersion or durable weather sealing. Because of the small form factor, FIP gaskets will not meet stringent environmental sealing requirements such as IP 67 or higher. While silicone-based, the material is better at preventing dust and environmental moisture from entering an enclosure. FIP gaskets can be paired with additional sealing gaskets for enhanced weatherproofing. 
    

This blog post was contributed by Ben Nudelman, market development engineer, Chomerics Division.

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Material selection for military applications requires careful consideration, as there are strict requirements to ensure maximum durability, security and of course performance. In munitions, or missiles and missile launch systems, materials that provide electromagnetic interference (EMI) shielding and environmental sealing are critical for the functionality and field life of the application.

Let's look at three areas of a munitions application -- specifically nose cones, cable shielding, and connectors, as each of these areas exemplify why EMI and environmental shielding are a necessity.

Nose cones are what goes over the top of missiles, planes and other airborne technologies to assist with aerodynamics and to protect the electronic components inside. In missiles, all the electronics are stored within the nose cone and the fuel is held inside the canister. If these two parts are not properly shielded from each other, contamination can become a catastrophic event.

Therefore, shielding the nose cone from EMI and other outside environmental dangers and shielding the components of the missile from each other is of utmost importance.

Another threat to missile electronic malfunction is external tampering from malicious forces. Unintended or intentional EMI can result in misfires, false trajectory, and other problems. Often, anti-jammers are installed to help prevent this problem in combination with EMI shielding materials.

Cable shielding is a woven fabric that goes over cables to prevent electromagnetic cross-talk between the cables and the components. Typically, a metal mesh is wrapped around the cables that will prevent any EMI from interacting with the cables or emitting from the cables.

Different amounts of layers can be added to increase EMI shielding effectiveness, however adding more layers will also add more weight. Cable shielding that is lighter, typically non-metal based, is ideal for applications where weight is of concern like in munitions.

Connectors are where wires are plugged into to keep electric circuits intact. In munitions, connectors can be a failure point because environmental agents can more easily enter which is why they require more attention to be shielded properly.

The complexity of military electronics has increased significantly on air, sea and land-based applications. The environments in which systems are required to operate are often extreme. Design engineers need to consider wide variations in ambient temperature, shock and vibration, and electromagnetic interference (EMI).

With a wide choice of shielding materials and a range of advanced shielded optical windows, Parker Chomerics helps ensure the protection of complex electronics from damage and compromised reliability caused by EMI.

Sensitive electronic components can be kept within their operating temperature range limits by using heat management materials that include highly conformable, thermally efficient gap fillers and gels.

Parker Chomerics offers the products, technical know-how, close customer support and supply chain capabilities to meet these challenges and deliver superior, reliable and cost-effective solutions.

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