Thermoplastic composites, in particular, offer companies kitting out aeroplanes 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, 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: Material and installation costs
For safety reasons, all metal mountings for electrical plug-in connections in commercial aviation must be earthed. 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 earthing. This earthing 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 earthing 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 earth connection.
Metal substitute removes the need for earthing
The work and costs involved in an earth 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.
For Bucher, 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.
Thermoplastic composite components
The breakthrough was achieved during their first cooperative venture with Ensinger GmbH. On the basis of Bucher’s requirement specifications, the Otelfingen (Switzerland)-based Composites division developed a solution on the basis of thermoplastic fibre-reinforced composite. A pre-preg approved for the aviation industry with glass fibre and a PEI matrix (polyether amide) 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 pre-pregs. Unlike comparable components, these blanks are not cut to size from a thick laminate and thermoformed but, using individual pre-preg 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’s subsidiary Trig Engineering in Bridgwater, Somerset (UK). Trig Engineering, which specialises 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 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.
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.
Thanks to Ensinger’s preform technology, differences in wall thicknesses are also possible when producing angles. So in a similar way to with a forked branch, the corner section under high strain can be reinforced, while in the sections which are not under as high strain (e.g. on the arms), material can be conserved. 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.