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Polyurethane-coated flat cable for automotive systems |
Polyurethane and other foams
Polyurethane-coated flat cable for automotive systems
Presented by Alfons Bertels, Head of Business Unit Extrusion, Thermoplastic Polyurethanes, Elastogran GmbH, Lemförde
Trade Press Conference K 2004, June 22, 2004, Ludwigshafen, Germany
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As motor vehicles are packed with more and more electrical and eltronic devices, the wiring systems that connect them are increasing in complexity, size and weight. There is scarcely room left for conventional round wire conductors. Moreover, traditional wiring harnesses are expensive to produce, often requiring much manual labour and offering little scope for automation. Conventional wiring technology in modern cars has clearly reached its limits and will not be able to meet the requirements of future electrical systems (figure 1).
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Flat cable – the low-profile solution
One solution is to use flexible flat cable coated with Elastollan® thermoplastic polyurethane (PU). Such cables are not only lighter and require less space, they also provide better performance and safety. The cable consists of a number of parallel tape-like copper conductors - sometimes of differing width - which are insulated by extrusion coating with a polymer to form a ribbon-like structure. Extrusion coating eliminates the need for an adhesive and so saves costs (figure 2).
Elastollan is well-proven as a coating for automotive cables and plug connectors. Its optimal set of properties - which include flexibility, abrasion and chemical resistance, and flame retardance - guarantee reliable operation. Unlike most other insulation materials, Elastollan can be used at temperatures of up to 125°C, making the coated cable also suitable for hot environments such as the engine compartment (figure 3).
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Space saving, flexible and easy to assemble
Flat conductors are able to carry more current than round ones because their greater surface-to-volume ratio allows them to dissipate heat faster. Another advantage is that external stresses are distributed evenly over a greater area. (It is often the case that the minimum thickness of a single round conductor is dictated by strength requirements). As a result, flat conductors can be made thinner, lighter and more compact - compactness being of particular importance when it comes to installing wiring in confined spaces such as in the headliner, doors and instrument panel.
Further advantages derive from the design freedom provided by the cable’s flat form, as well as its ability to resist creasing. This not only allows the cable to be hidden away easily - in the headliner for example - but to be routed continuously to hinged features such as the tailgate or door mirrors without the need for intermediate connectors. Flexible flat cable is suited to automated assembly because it is easy to handle and position mechanically. The fixed position of the conductors relative to each other means that finishing operations - attachment of plugs and terminals etc. - can also be mechanized; it is also easier to detect and suppress any electromagnetic interference emanating from the cable (figure 4).
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Ready for the future
Extruded flexible flat cable has reached commercial product status and meets the demands of the automotive industry. Application-specific design and customized features are possible thanks to the use of multiple, variable-width conductors, the ability to vary insulator thickness, and easy colour coding of the jacket. Although flat cable is currently more expensive to produce than round wire, it turns out to be the more cost effective solution when potential savings from cable termination and assembly operations are taken into account (figure 5).
In its first-ever automotive application, Elastollan flat cable is now being fitted inside the headliner of the Mercedes M-Class off-roader. Flat cable is due to appear in the instrument panel, door linings and seating of various other models. Consideration is already being given to the role of flat cable in the new 42-volt electrical systems due to appear in new cars in a few years' time. (42-volt systems are necessary to meet the increased power demand as more vehicle systems are electrified). Space saving power distribution will therefore become even more important, with PU-coated flat cable replacing bundles of wire in many places on the vehicle.
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