FEATURE RACKS, CABINETS & ENCLOSURES THE PERFORMANCE TO WEIGHT RATIO


Gerard Young AMIMechE MIEEE, applications engineering team leader at Parker Chomerics investigates new opportunities to save weight, by replacing metal shielding with conductive plastic


T


hermoplastic compounds, when formulated so as to be conductive,


are an alternative option to sheet aluminium material for fabricating EMI shielding. Such compounds offer various advantages: injection moulding enables the creation of parts to fit complex geometries, and conductive polymer materials are less dense than aluminium. They can also offer superior corrosion resistance while offering moderate levels of shielding compared to the rather higher levels offered by aluminium. In practice, many designers of automotive, aerospace and other weight- sensitive applications have had a conservative attitude towards the replacement of metal shielding with conductive plastics. This is because of the higher inherent level of shielding offered by aluminium housings, and by concerns over the reliability and the consistency of performance of the conductive thermoplastic compounds. Recent developments have enabled


manufacturers of conductive polymer resins to manufacture a new generation of materials which eliminates the drawbacks of earlier products. At the same time, examination of many applications of metal aluminium shielding shows that the actual level of attenuation achieved is far lower than the theoretical maximum provided by an unbroken piece of sheet metal. This means that the latest conductive polymer products may provide sufficient attenuation to meet many applications’ requirements while enabling the designer to save weight compared to aluminium housings.


HOW NEW CONDUCTIVE POLYMERS ARE FORMULATED


The traditional approach to the formulation of conductive polymers was to add conductive particles and fibres to the base resin, and then process the material in such a way as to ensure the particles and fibres are evenly dispersed in the final component so as to provide a homogeneous network of particles and fibres. Failure to achieve this even and homogeneous dispersion results in poor and inconsistent shielding performance. Many of the early conductive plastics suffered from poor dispersion, but this has been solved by the Chomerics Premier range (Figure 2).


22 APRIL 2018 | ELECTRONICS Fig. 2:


Even dispersion of conductive fibres in a finished housing formed from Chomerics Premier conductive polymer


applications, because the actual level of attenuation required is often far below the >130dB that thin continuous metal shields provide. If the requirement is, in fact, at or below the 40-80dB range, conductive thermoplastic compounds such as PBT-225 might be a valid choice, and with no requirement for secondary plating or painting. It should be noted that the additives


Figure 1:


EMI shielding moulded plastic from Parker Chomerics


In some of these conductive polymers,


two differently formulated pellets are supplied, blended in the correct proportions by the manufacturer. These blends have formed the mainstay of most conductive plastic suppliers’ portfolios in recent years, especially if the requirement is for a high level of shielding.


Parker Chomerics’ most recent offering


eliminates the need for blending. The Premier PBT-225 product is a single- pellet, polybutylene terephthalate (PBT) based, electrically conductive plastic which provides approximately 65dB of attenuation over the 6-40GHz frequency range. While consistent, the attenuation


provided by products such as PBT-225 is markedly less than that of sheet metal, at >130dB, and is typically similar to that of vapour phase deposition (VPD) plating or conductively painted components. The difference is stark on paper. But how important is the difference in practice? In most electronic shielding


applications, a housing which provides shielding has various apertures for fasteners and for cables, and seams where parts are joined. These holes and joints in the metal enclosure substantially reduce the actual level of attenuation below the theoretical maximum provided by a continuous metal housing. These structures are nevertheless successfully used in many


which provide the plastic’s desirable conductive and mechanical properties tend to affect the surface finish of the component to the extent that it might not be acceptable for applications in which the cosmetic appearance is important, especially when high levels of shielding (≥60dB) are required. The first task when considering conductive polymer as an alternative to a non-conductive polymer extrusion is to estimate the level of shielding required. If the value required exceeds the published shielding performance of the conductive polymer for the frequency range of interest, then it is unlikely to offer a viable solution. It might then be necessary to consider plating, painting the plastic with a conductive coating, or introducing a conductive laminate on the inner surface. In many cases, however, the shielding performance of an existing enclosure is limited by the holes and seams it contains: the actual level of shielding provided by an existing solution might be as low as 40dB, in which case a conductive plastic might well be suitable. The mechanical, chemical and thermal properties of the conductive polymer solution must also be considered. If these are also suitable, the material should next be tested. Suppliers such as Parker Chomerics will give users a reasonable quantity of pellets – 3-5kg – for injection moulding tests free of charge, and provide appropriate processing parameters.


In fact, electrically conductive plastics can offer considerable weight savings: the density of conductive plastics is normally at least 30% lower than that of aluminium. Figure 2 summarises this and the other attributes of the various options described in this article.


Parker Chomerics www.parker.com T: 01494 455400


/ ELECTRONICS


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