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Coating and treatments for enhanced moulded enclosure performance

28 February 2012

Paul Hoath describes how carefully selected coating materials and techniques can significantly enhance a moulded plastic enclosure in terms of its EMC, ESD and anti-microbial performance

Plastic enclosures have many benefits: a large number of suppliers serve the market with a great number of standard products available in different styles and sizes at economical prices, reducing time to market for new projects. Products are readily available from the manufacturers themselves and from both broad based and specialised distributors. The units are lightweight, complex shapes and features are easily incorporated into the design and they are easily modified to accept displays, switches, connectors and other hardware.

However, moulded enclosures have one specific shortcoming: plastic offers no inherent attenuation to the passage of electric or magnetic fields. In many applications, this deficiency is of no consequence, but if radiation emitted by the housed electronics or their susceptibility to external fields is a potential problem, the lack of any intrinsic screening will be an issue. Enclosure manufacturers have addressed this issue, typically by adding conductive coatings to the inside surfaces of the enclosures.

The majority of plastic enclosures are constructed from a top and base moulding; in some designs there may be battery compartments and removable end panels as well. One of the key requirements for effective screening is that the internal surface of the enclosure should be as continuous as possible to ensure electrical conductivity between all its parts. In particular, long slots should be avoided. To prevent slots between the mating halves of a typical enclosure, a tongue and groove structure forms an effective complex path, improving the attenuation performance.

Obviously, the higher the frequency, the lower the wavelength, so even very small gaps can have a detrimental effect on EMC performance. If there are removable end panels, they need to be secured into an interference fit slot or, if they are assembled using fixings, a conductive gasket should be fitted to the mating surface. The rear face of plastic panels will require a conductive coating or, if aluminium panels are used, the front surface will normally be anodised and the rear left with a natural finish or iridised, a RoHS-compliant conductive finish.

The best methods of providing suitable continuity in enclosures with dedicated battery compartments will depend on the design. If the battery box is constructed with solid partitions between it and the main internal space of the enclosure, the only precautions that need to be taken are to ensure that the hole for the wires into the enclosure is as small as possible. If the battery is just clipped into mouldings in the enclosure without a partition, then the lid of the battery box will form part of the overall screening and will be a potential weak spot in the screening as there will typically only be a flat surface interface with the body of the enclosure itself.

Ni, Cu and Ag coatings
Meeting general commercial level requirements, a nickel based colloid offers acceptable attenuation of 50dB at 1GHz at a competitive cost with a 50-micron thick film. For more severe requirements, a copper colloid, formulated with silver-coated copper particles and conductive resins provides a conductive layer. It provides an effective shield against RFI and EMI and can act as a ground plane to protect against electrostatic discharge (ESD).

The material is COSHH, RoHS and REACH compliant and provides attenuation greater than 70dB at 1GHz at 50 Microns when tested to ASTM ES7-83, a standard test method for measuring the electromagnetic shielding effectiveness of planar materials. Temperature ageing of seven days at 29.4oC at 95% RH causes no degradation of properties after environmental testing.

As can be seen, high frequency performance is much better than that of nickel; inevitably, given the relative costs. The material has been tested up to 10GHz to Military Standard MIL STD 285, typically providing 78dB at 10GHz.

The highest attenuation is achieved using a silver colloid, formed of silver flakes and conductive resins. It provides an effective shield against RFI and EMI and can act as a ground plane to protect against ESD.

The material is COSHH, RoHS and REACH compliant and provides attenuation levels in excess of 80dB at 1GHz. Temperature ageing for days at 85oC and 85% RH, and 10 cycles of temperature cycling of 75oC for one hour, ambient for one hour and –30oC for one hour, and 56 days of high humidity testing at 35°C and 95% RH for 56 days, results in no reduction in attenuation.

Many applications require a display to be incorporated into the enclosure, normally viewed through a transparent window. Two main alternatives are available to preserve the integrity of the internal conductive coating when a window is required. A wire mesh will provide continuity at the expense of clarity; the better option, originally developed for use on military helicopters, is a clear conductive coating that provides the required electrical conductivity without obscuring the display.

Electrostatic discharge (ESD) can be minimised by the application of graphite or carbon-based high conductivity coatings. ESD events can occur without a visible or audible spark at low voltages of around 10V - sufficient to damage sensitive electronic components, causing outright failure or reduced long-term reliability and performance reductions. Copper coating gives good ESD protection in addition to its EMC benefits.

External coatings
Internal coatings protect the electronics, while external ones can protect the user and the enclosure itself. Hospital acquired infections from species such as MRSA, E.Coli, Listeria, Salmonella, Campylobacter and others have become a serious problem, and while improved housekeeping and new guidelines for staff have reduced the problem to a certain extent, the underlying problem of bacterial growth on surfaces remains.

To help address the issue, plastic enclosures and panels can be doped with a silver-based antimicrobial additive in the material, which provides an effective means of reducing bacteria by up to 99.99% for the active life of the product.

The key benefit is that the silver ion material is added to the raw ABS before the moulding process in the case of plastic enclosures. As the additive becomes an integral part of the material, the efficacy of the protection does not diminish over time, giving far superior protection to external coatings that will inevitably degrade through normal wear and tear, routine cleaning and accidental damage.

For front panels and extruded aluminium enclosures trials, where the antimicrobial material is added to the anodising process, are currently being conducted; preliminary results show similar reductions in bacterial growth rates to those found in plastic enclosures.

For OEMs involved in the medical equipment and instrumentation markets, specifying antimicrobial versions of products that can be machined to suit the application without reducing the effectiveness of the antimicrobial protection will reduce time to market and offer a demonstrable benefit in terms of enhanced performance.

In addition to internal coatings to give EMC attenuation and doped materials to provide antimicrobial properties, external coatings can be applied to plastic enclosures to give high gloss finishes and wear-resistant attributes, enabling them to be used in a wide variety of circumstances.

Paul Hoath is with Vero Technologies

Protected moulded enclosures from Vero Technologies
Vero Technologies’ moulded enclosures are available in sizes from hand-held units to desktop instrument housings, providing style and functionality at low cost. Easily modified according to requirements, the company’s well-known brands include Veronex, IDAS, Apollo, General Purpose Box and Patina.

All types can be supplied as standard with one of three alternative EMC coatings (as described in the main article); for highly specialised uses, other coatings are also available.
Striking the optimum balance between cost and performance, the copper colloid is Vero Technologies’ default coating material, although higher or lower performance coatings can be specified as required. Clear coatings can also be applied to any window.

Antimicrobial systems offered by Vero have been tested on over 50 different species of bacteria, and independent testing to ISO 22196:2011, using MRSA and E.Coli, has confirmed the effectiveness of the additive. In essence, the test procedure consists of inoculating test samples with a nutrient mix containing a known amount of bacteria. The samples are maintained at 37°C for 24 hours with the amount of viable bacteria colonies being measured.
 
All sizes and colours of enclosure can be supplied with the antimicrobial additive in the plastic, so the external appearance is identical to standard untreated versions.
 


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