Conductive polymers replace metals

Enclosure front panels moulded from conductive polymer have sparked the interest of major telecomms equipment suppliers as replacements for more traditional metal counterparts.

Enclosure front panels and related components moulded from a new conductive polymer have sparked the interest of major telecomms equipment suppliers as replacements for more traditional metal counterparts in circuit board assemblies.

Currently under development by XTech (Extrusion Technology) of Randolph, Massachusetts, and injection moulded by Fielding Manufacturing of Cranston, Rhode Island, the panels and hoods are made of a new stainless steel filled compound developed by GE Advanced Materials which meets industry requirements for electromagnetic interference (EMI) shielding and electrostatic dissipation (ESD), as well as Underwriter's Laboratory V0 flame-retardancy requirements.

XTech is a leading global supplier of mechanical systems for circuit board technology and other embedded systems.

Its enclosure systems are used across a wide range of industries, including telecomms, GPS, wireless, instrumentation, security, medical, data storage, broadband, and VoIP.

According to Steve Richardson, Applications Engineer for the conductive plastics programme at XTech, the company began investigating engineered polymers for these applications as part of an overall plan to reduce costs through innovation.

"We were intrigued with the offerings that some of these EMI-filtering plastics provide", said Richardson.

"So we teamed up with GE Advanced Materials and Fielding Manufacturing to develop some applications for one of our larger OEM customers".

An optical hood and a face plate were chosen, both because of the engineering characteristics required of them and because they were discrete elements which fit well within a planned programme of design upgrades.

"Optical hoods are small, wedge-shaped pieces that act as eye shields to deflect laser light from optical connectors and other internal components".

"The face plates are roughly a foot long and three-quarters of an inch wide, and provide the interface to circuit board assemblies".

Traditionally, a faceplate family could include five to 10 or more different cutout geometries to accommodate the different circuit board assemblies to be mounted to them.

"One of the challenges was to make a single mould with different inserts that would allow the customisation of hole patterns".

But the key requirement, said Richardson, was for shielding.

The material used, GE's Faradex DS-1003 FR HI compound, is highly effective against electromagnetic radiation sources, providing EMI attenuation between 20 and 60dB, a range that satisfies most electronics and telecommunications applications.

The new grade is specially formulated to achieve a balanced dispersion of stainless steel fibres during the moulding process to help maintain the fibre length/diameter (L/D) ratio for an optimal conductive network for shielding effectiveness.

Excellent impact resistance has also been achieved by using GE Lexan EXL polycarbonate (PC) resin as the new Faradex compound's base resin.

This tough PC material maintains impact strength at both elevated and low temperatures (9KJ/m2 at -45C), offering high flow and a good balance of high-performance properties.

It has been formulated with a halogen-free flame retardant in compliance with Blue Angel and TCO'98 green requirements resulting in a UL94V0 flammability listing at thicknesses of 2.1mm compared with alternative PCs.

Optimising these properties required a moulder willing to experiment, said Richardson.

"These high-end materials can cost between $10 and $20 per pound, and many moulders are reluctant to invest the time and run the potential financial risks involved in learning how to process them effectively".

This was why XTech turned to Fielding Manufacturing, a moulder/die caster with more than forty years of experience.

"Fielding's expertise is in moulding miniature parts where process control is critical to a successful result", said Richardson.

"Also, because Steve Fielding is a die caster as well, he was in a unique position to understand the special requirements of these applications".

"So even though some were rather large parts by his standards, his immediate response was, 'We can handle that'".

"Processing the Faradex resin was not that difficult", Fielding President Steve Fielding reported, "but optimising the balance of shielding and conductivity performance with good physical properties took some tweaking".

"We were also able to add value in the tooling which allowed more economical production of customised hole patterns", he said.

Another major requirement of all concerned in bringing the project to fruition was co-operation, Richardson said.

With parts now in the final stages of functional testing, he said he is still surprised at just how much co-operation and time is required to nurture these applications from the drawing board, through all the necessary testing, to approval.

"It's a very big investment of time and resources for everybody, but the payback can be significant".

As a final, near afterthought, XTech asked GE if the material could be compounded in an off-white colour.

Other semiconductive materials are not colourable, but GE returned with an off-white material that had: "just a little veining of stainless-steel fibre in it".

But far from being displeased, Richardson said the new look has: "opened the eyes of people around here who want to maintain some industrial design latitude and take advantage of the range of colours offered in Faradex".

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