Mountings for electrical plug-in connections

Lightweight construction in aeroplane interiors: composite-based series-produced components

Lightweight and high-strength materials have become indispensable for high-end applications in the aviation industry. In passenger aircraft every kilogram counts, which is why metals are increasingly being replaced with plastics-based solutions, also in interior design. Thermoplastic composites, in particular, offer companies kitting out aeroplanes significant weight reduction without requiring them to make compromises in terms of mechanical properties. A project by companies Bucher Leichtbau and Ensinger shows that making the switch to fibre-reinforced composites can provide additional technical advantages.

Bucher Leichtbau AG, headquartered in Fällanden (Switzerland), is a specialist for aircraft interiors and automotive equipment. Among other things, the aviation division of the corporate group develops and produces galleys and cabinets for passenger aircraft. A full module of this type can accommodate a load corresponding to eight times its own weight and has to be able to withstand accelerations that are at least nine times that of gravitational acceleration. Here, a distinction is drawn between flight loads (normal flight conditions such as take-off, landing, turbulence) and emergency landing loads (controlled emergency landing or aborted take-off). The required minimum operating life of the modules produced, is for example 25 years for Airbus and Boeing.


Initial situation

For safety reasons, all metal mountings for electrical plug-in connections in commercial aviation must be grounded. With electrically conductive supporting structures in the Bucher aluminium galleys, an existing fastening screw equipped with the necessary corrosion protection measures is often sufficient. If, however, the metal connector mountings are positioned on non-conductive structures in the galleys, an additional cable is required for the electrical grounding. This grounding cable must be secured at regular intervals which drives a requirement for additional fastening threads, spacer bolts, cable clamps and screws.

In addition to the material input for the cable routing, high costs arise for planning and installation: The design work includes making an entry in the schematic circuit diagram; what is more, a resistance measurement must be defined in the test plan for every grounding cable. This must also be incorporated into the relevant maintenance guidelines and operating instructions, as well as into the authorisation documents. During production, the contact resistance must be recorded and a corrosion protection coating be applied with every ground connection.


Thermoplastic composite substitute removes the need for Grounding

The work and costs involved in a ground connection can be completely eliminated if the connector mountings previously made from metal are replaced by a non-conductive variant. Because of the operating temperature and fire safety requirements, however, it is not possible to use a low-cost, standard plastic.

The sought-after technical alternative needed to be capable of replacing a large number of existing sheet aluminium parts one-to-one so as to minimise the work and costs involved in switching when they were introduced. For this demand to be met, a rigid, high-strength material is required.

Owing to the relatively small quantities of the different connector mountings, a solution with high tool costs was not an option. Initial trials with locally remodelled parts made from fibre-reinforced, thermoplastic sheet material did not show satisfactory results in the remodelled zones.

Working with Ensinger’s Composite division, a suitable solution on the basis of thermoplastic fibre-reinforced composite was quickly found. A prepreg approved for the aviation industry with glass fibre and a PEI matrix (polyetherimide) was chosen for the material. After a brief optimisation phase, it was possible to manufacture the first series-produced parts. Because of the specialist processing techniques, two of the Ensinger’s sites are involved in the production of the connector mountings:

At the Otelfingen site, Ensinger's composite experts produce semi-finished parts in an angular shape from the PEI prepregs. Unlike comparable components, these blanks are not cut to size from a thick laminate and thermoformed but, using individual prepreg layers, pressed in a tool into the defined form. Among other things, this process offers the advantage of reducing the springback effect. The CNC processing of the fibre composite parts takes place at Ensinger GmbH in Cham in Bavaria.

The Cham machining department, which specializes in the production of precision parts from thermoplastic high-performance plastics and composite materials, operates in accordance to the aerospace AS9100 revision D standard and is a member of Ensinger’s international machining group. From the angular blanks produced in Otelfingen, Trig Engineering Ltd precision machine the connector mountings to final product specification.

For most fastening brackets there are several versions with different dimensions. Thanks to the flexible process, it is also possible to produce high-quality, special designs with contours or drilled holes and with short lead times.


Simpler, more cost-effective, and lighter

The manufacturing costs for the new plastic mountings are significantly higher than the previous version in aluminium. However, the savings in terms of engineering and production of the cabinets means that the new solution is still considerably more straightforward and importantly more cost-effective in total. As the galleys are lighter thanks to the use of composite components, the airlines benefit in the long term, because every kilogram of additional weight which does not have to be taken into the air saves money.

BEFORE CNC PROCESSING

A blank for a bracket for a plug-in connection is constructed from a fibre composite pre-preg, approved for the aviation industry with glass fibre which is embedded in a PEI matrix.

The connector mountings come in versions with different dimensions. Thanks to the continuous, straightened fibres, the thermoplastic composite material has significantly higher strength and rigidity than short fibre-reinforced plastics. 

PREFORM TECHNOLOGY

Thanks to Ensinger's preform technology, differences in wall thicknesses are also possible when producing angles. So similar to a forked branch, the corner section under high stresses can be reinforced, while in the sections which are not under high stresses (e.g. on the arms), material can be saved. The manufacturing technique is referred to as the ‘near net shape’ process.

Depending on the component geometry, significant material and weight savings are possible with this technology – whilst maintaining comparable strength and rigidity.