From passenger to cargo aircraft
24.04.2024 | Petra Rehmet

There is even a separate word for this: preighter. Created during the coronavirus crisis, when hardly any people were traveling, it refers to a passenger jet used (temporarily) as a cargo plane. And the permanent solution is also common: aged airships that no longer offer the latest comforts still cut a good figure as cargo aircraft. Demand is high, as the purchase price is only a fraction of that of new aircraft. 

Amongst many other tasks, passenger to cargo conversions require replacing acrylic window plugs with metallic alternatives for ease of maintenance. This costly and time-consuming metallic solution can be further optimized by modern fiber-reinforced thermoplastic composites that offer the same stability at lower costs, less weight, and in a fraction of the time it would take to machine the metallic solution.

The National Institute for Aviation (NIAR) at Wichita State University (Kansas, USA) and its Advanced Technologies Lab for Aerospace Systems (ATLAS) are investigating issues such as these, and at the same time, training the specialists of tomorrow. A total of around 1,600 employees are active at six NIAR locations, with the total annual volume of R&D projects amounting to more than 300 million dollars. With the KraussMaffei partnership, NIAR has gained active support with industry expertise on plastic molding.

With FiberForm, a fully consolidated fiber-reinforced thermoplastic sheet known as organosheet is inserted into the tool, formed and overmolded with a thermoplastic polymer. In the case of the window plugs, this is done on a GXW 450-2000/1400 with a swivel platen. In an extensive project sponsored by the US Air Force for developing affordable and sustainable manufacturing technologies for aerospace applications, Dr. Waruna Seneviratne’s team is working with Eugen Schubert, Sales and Application Manager KraussMaffei.

Together they are developing the manufacturing process supported by process simulations to ensure the part performance meet the technical requirements. Numerous simulations conducted at NIAR were instrumental in part and tool design. The process simulations carried out were verified through inspections at various stages of the process so that these tools can be used for future developments.

The window plug was designed with the oval-shaped geometry of the original airplane window with appropriate design modifications, ex., adding ribs to stiffen it to withstand pressure loads. In order to achieve the desired mechanical properties, the team used high-performance plastics LM-PAEK reinforced with AS4 carbon fibers for the 16-ply organosheet and 30% chopped-fiber-filled PEEK for overmolding the ribs.

Due to the higher melting temperature of the PEEK, the two materials fused together optimally when the flow front meets the organosheet.

Different variants were tested for fully automated production, and the following sequence emerged: First, the flat organosheet is heated in an infrared (IR) oven, inserted into the first station of the mold, formed and pressed, the mold opens, the turning unit rotates and transports the blank to the second cavity for overmolding, while the next organosheet is inserted at the front, getting it ready for thermoforming.

The cycle time is so short that 40 window closures can be produced within an hour. This is one of the major advantages of fast injection molding compared to metal machining and also compared to reaction processes.

The thermoplastics used for FiberForm offer the possibility of welding components, have a high impact strength, are resistant to high temperatures, chemical and environmental influences and are also flame-retardant.

Compared to its metal counterpart (590 grams), the composite overmolded version of the window plug weighs 20 percent less. The team is already working on further optimizing the structure to make it about 40 percent lighter than the metallic counterpart.

In safety-relevant applications such as aviation, particular attention is paid to the traceability of production processes. With the dataXplorer, KraussMaffei offers a perfect solution here. Over 500 signals can be recorded and stored every 20 milliseconds.

The next step for the NIAR program is to subject the window plugs to further functional tests required for certification – for example with regard to durability – so that it is ready for series production. It is a win-win scenario for everyone involved: NIAR can successfully implement automotive manufacturing technologies into the aviation industry, and KraussMaffei strengthens its commitment in support the needs of the aviation sector.

But first, Nolan Strall (President of KraussMaffei Corporation North America) is looking forward to a joint appearance at the trade show (May 6-10, 24, Orlando/Florida): "The NPE is the showcase for the plastics industry in North America and we will be welcoming customers to our booth with our partners from NIAR. The combined expertise will certainly be attractive to many trade fair visitors." More projects are therefore likely to follow the window plug project. 

The project has already received its first award. The accompanying technical paper "Ultra-High-Rate Manufacturing of Thermoplastic Window Plug Using Hybrid Overmolding" by Dr. Waruna Seneviratne and Eugen Schubert won first place in the SAMPE 2024 Technical Paper Awards. The winners will be honored at the SAMPE Awards Breakfast on May 23, 2024.