A relatively large cut-out in the sheet metal of the bulkhead between the engine and passenger compartment was the starting point for the new design of a plastic component used to route cables and ventilation ducts in the Audi Q7. In the right-hand drive version of the vehicle, the main wiring harness and the air intake duct have to be routed on the left-hand side because the engine alignment remains the same even if the steering position is changed. The resulting space conditions proved to be extremely challenging.

Two feed-throughs in one part

Making both openings in a confined space and creating an economically viable, stable and permanently watertight construction proved to be more complicated than it seems at first glance. This is all the more true as the sheet metal cut-out provided by Audi is so close to the nearest contours that stable fixing points in the form of welded-on threaded inserts were not feasible.

For this reason, the frame must have high rigidity in places: Deformations from the forces generated by the injection-molded TPS sealing lip must remain within very narrow tolerance limits. In addition, there were the typical automotive requirements such as mastering the operating temperature range of -40 to 80 °C as well as resistance to oil mist and low water absorption. "In similar designs, assemblies consisting of several parts to be mounted on the vehicle have been used in the past - and are still used today," explains Armin Leipold, product developer and project manager at supplier ETG. "This is exactly what Audi no longer wanted at this point. The idea of realizing the two bushings in one easy-to-install part was new for Audi during development."

The requirement for "guaranteed tightness" between the interior and engine compartment for the air duct and cable feed-through already proved to be a major hurdle for the design in the initial analysis. Tests were carried out on the duct in its installed state in a water spray simulator at a vehicle inclination of 30° and statically under a 200 mm high water column for 24 hours at various temperatures. The front wall, air intake and cable passage interfaces were also subjected to various static and dynamic loads from driving,

With PP to the 2-component part

"Most designers in this environment and with such mechanical requirements probably tend to use a highly reinforced polyamide grade almost as a reflex," says Armin Leipold. "Nevertheless, development manager Matthias Dietz from ETG looked into the possibilities of a high-performance polypropylene (PP) alternative. On the one hand, PP is more cost-effective, but above all the combination with a

thermoplastic elastomer (TPE) in the form of a 2-component part is much easier to realize due to the good adhesion to PP and the available TPE selection is considerably larger.

In order to meet the requirements formulated in the specifications, only a high-performance compound could be considered - if at all. The company was brought on board during the concept phase to check whether a grade of the PP product series Altech NXT developed by material supplier Albis Plastic could be considered. The polypropylene-based compound solution is an interesting alternative to polyamide-based solutions, particularly in terms of performance, price, availability and weight. Altech NXT PP is versatile and is also used in automotive applications, not least due to its good thermo-mechanical properties at comparatively low cost.

After an initial assessment of the available installation space, the possible wall thicknesses and other requirements of the specifications, such as temperature and oil resistance, made the use of polypropylene appear possible, detailed investigations and simulations began. The

Albis' experience regarding the optimal processing parameters of PP compounds from a large number of customer projects and ETG's design ideas led to a material and design proposal, which Albis optimized both in structural calculations and in the injection moulding simulation. The software tools used included Ansys for strength and structural evaluations of the component and Moldex3D for 2K injection molding simulation. The resulting tool concept was implemented by a mold maker. Only two optimization loops were run until the part was ready for series production, which made use of the re-setting options provided in critical mould areas.