The first step in conduit system design is to identify the critical performance requirements of the wire enclosure. Or in other words, to determine what exactly the conduit system is supposed to do. For general duty applications these requirements might range from protecting the wires and terminations from moisture to preventing physical damage due to rough handling. In high-performance settings, additional requirements such as protection from extreme temperatures and radiation are commonly considered. In harsh environmental conditions, the ability of chosen materials to resist damage from hydraulic fluids, fuels, lubricating oils, solvents and decontaminating agents comes into play. The following properties are routinely considered in designing an appropriate wire enclosure solution:
- Field/Factory Assembly
- Environmental/NBC Protection
- Electromagnetic Protection
- Flammability
- Crush Resistance
- Temperature Tolerance
- Abrasion Resistance
- Flexibility
- Tensile Strength
- Size and Weight
- Repairability
Because the range of performance requirements is so broad, Glenair manufactures and stocks an equally broad range of conduit products: from plastic convoluted tubing to various forms of metal-core conduit. Understanding the performance properties of these products is the key to the initial selection of material.
Plastic Convoluted Tubing Material Selection
Glenair’s Annular and Helical Plastic Convoluted Tubing Systems (IAW MIL-T-81914) are available in the widest range of materials and constructions in the industry — the right weight, duty, and protection level for every application. The Series 74 (helical) convoluted tubing, for example, is available in PTFE, Tefzel (regular and irradiated), PFA, FEP, Kynar and PEEK. The tubing is fabricated in helical, close convolutions, thin wall, dual wall and reinforced versions and can be supplied in random lengths (Glenair Standard Packaging) as well as specific lengths or spools. The following summaries provide an overview of material properties for the commercial polymers used in Glenair’s line of plastic convoluted tubing:
Teflon® PFA and Teflon® PTFE - Every one of the of the plastic polymers used in the fabrication of convoluted tubing has unique properties due to its distinctive molecular make-up. But two of DuPont’s most popular materials, Telflon® PFA and Teflon® PTFE are so close in most major performance areas that selection is more often an issue of price, rather than performance.
Both materials show outstanding chemical properties for resistance to corrosive agents, nonsolubility, and nonflammability. Both are virtually identical in terms of their electrical properties for dielectric, dissipation, and surface resistivity. Even in critical melting point (service temperature) ratings, both materials have identical properties. In fact, it is only in three areas, cold flow, folding endurance and cost that these virtually identical fluoropolymer resins differ significantly. Because PFA is melt extruded, it exhibits better cold flow values than PTFE, which translates to better long-term sealing and reduced loosening of fittings. PTFE, on the other hand, exhibits better folding endurance than PFA, but at 500,000 bends before failure, PFA is well within the operational tolerances of most applications. In terms of cost, PFA convoluted tubing is typically the better performer, as the melt extruded product is less costly to fabricate and can be produced in any length, effectively eliminating the scrap associated with PTFE.
Kynar®- Kynar® is the trade name for polyvinylidene fluoride, an opaque black, thermally stable material used in the standard manufacture of annular plastic tubing. It is highly resistant to UV light and gamma radiation. Kynar® is extremely resistant to oxidizing agents and halogens and is almost completely resistant to allphatic, aromatics, alcohols, acids and chlorinated solvents. Kynar® is mechanically strong, abrasion resistant and features high dielectric strength. Kynar® is also thermally stable, resistant to low temperatures, self-extinguishing and nontoxic.
High-Temperature ETFE - Ethylene tetrafluoroethylene provides a durable, highly flexible protective enclosure for wiring systems used in harsh application environments. The material provides exceptional flexibility in cable routing and electrical system architecture and has a higher operating temperature range than standard ETFE of -55½ C to 200½ C. The tubing also withstands temperature excursions to 300½ C and is both crush resistant and cut resistant at elevated temperatures. The irradiated material provides outstanding resistance to hydraulic fluids, fuels, lubricating oils, solvents, and decontaminating agents.
ETFE - Also known by the trade name Tefzel, ethylene tetraflouroethylene provides the highest tensile strength and lubricity available. It is produced in opaque black and transparent clear. ETFE resin is a meltcut thermoplastic which combines mechanical toughness with outstanding chemical, dielectric and thermal properties. Its other main features include ease of processing, a lower density than Teflon® and improved radiation resistance. ETFE is suitable for continuous exposure at a maximum service temperature of 155½C.
FEP - Fluorinated ethylene propylene (FEP) is a lower cost material which provides relatively high thermal stability. The product provides continuous service temperatures up to 205½C . FEP has excellent dielectric properties and is unaffected by virtually all solvents and chemicals. As with PTFE, FEP also has excellent low temperature toughness and good resistance to adhesion.
PEEK - Polyetheretherketone is our only halogen free material. It is extremely light weight and crush resistant. PEEK is semi-crystalline, with both crystalline and amorphous regions. The material is very tough under an extreme range of conditions. Melting occurs at about 340½ C, continuous use is rated at 250½ C, with short-term excursions to 300½ C, or 572½ F. PEEK far outperforms other plastics in this regard, and has a V-O flammability rating down to 0.057 inches without the use of additives. The mechanical and high temperature properties of this material make processing and manufacturing of PEEK tubing more costly than other polymers. PEEK combines strength, toughness, chemical resistance, purity, wear resistance, and USP Class VI biocompatibility. The material maintains its properties under sustained pressure, elevated temperatures and most chemical environments.
As you can see from the summaries and also from Table 1, each material has certain advantages over the rest, but there is also a great deal of overlap in terms of the most basic physical properties. In fact, some of the performance measures are so close that final selection is usually based on other factors such as ease of manufacture, price, color and so on.
Metal Core Conduit
The “Cadillac” of EMI shielding, helically-wound metal conduit (IAW A-A-52440, MIL-C-13909, MIL-C-24758, AND MIL-C-28840) provides extremely high levels of EMI protection across all radiation fields and frequencies. Metal-Core Conduit is the material of choice for TEMPEST secure communications and other applications involving sensitive electronic equipment and intense levels of EMI. Available materials include brass, nickel/iron and stainless steel. The brass conduit is generally specified with bronze overbraids and rubber jacketing. Ni/Fe and stainless steel are typically braided with stainless steel braid, with or without a jacket. The choice to use Metal-Core depends on the sensitivity of the system to EMI as well as mechanical, thermal and environmental specifications.
Braided Shielding
Glenair is able to provide turnkey interconnect conduit systems—complete with braided shielding and jacketing. Alternatively, bulk tubular shielding is available as a user-installable accessory. Braided shields are effective at minimizing low frequency interference at audio and RF ranges. Braided shields also provide additional structural integrity while maintaining good flexibility and flex life. In use, the reduction of EMI is dependent upon the signal amplitude and frequency in relation to the many combinations of mesh count, wire diameter and the braid material. Generally, the higher the percentage of braid coverage, the more effective the shield against high-frequency emissions. Available materials include tin-plated copper, nickel-plated copper and tin-plated iron/copper as well as hybrid materials such as metalized Kevlar (Aracon®). Depending on the ratio of metal braid to composite plastic, Glenair's Aracon product can reduce the weight of EMI shielding up to 75%.
