• Industry   Thermoplastic panels help aircraft construction “slim down”

Thermoplastic panels help aircraft construction “slim down”

Climate protection goals and ever-rising fuel costs are forcing the aviation industry to develop and build more efficient aircraft. Alongside the optimization of engines and aerodynamics, the use of thermoplastic fiber-reinforced composites (FRC) is also showing high potential. The main problems arising to date are low automation levels, long process times and high costs. But developments are ongoing.
The arguments are weighty and persuasive: weight reductions of up to 40% coupled with improved mechanical properties are a clear endorsement for using lightweight structures in aircraft construction. No surprise therefore that Airbus has meanwhile become a leading player in this materials campaign, alongside Boeing. The group-wide Center of Excellence at Airbus’s Stade facility now ranks as Europe’s largest production plant for lightweight structures. Add to this the adjacent CFK-Valley Stade and we have a competence network of CFK lightweight technology that is fairly unique.

Professionals who would like to find out more about the current state of affairs should speak to Christian Peters of Faserinstitut Bremen e.V. (FIBRE). He is an acknowledged expert on

thermoplastic fiber composites in aircraft construction. As Peters states: “Until now, most FRC components consisted of duroplastic matrix systems.” However, handling these materials is far from easy in the case of smaller components. The matrices and impregnated fiber pre-forms have to be kept in cold storage, the processing of pre-impregnated fiber is performed in autoclaves at high costs, and the long hardening time at temperatures of up to 180°C also slows down series production.

Thermoplastic glass fiber or carbon fiber reinforced fiber composite components are alternatives that present themselves. Christian Peters: “These materials have been employed in aircraft construction since 1990 and Boeing and Airbus are now using them increasingly in many interior and exterior applications.” Fiber reinforced thermoplastic pre-forms, so-called thermoplastic panels represent the starting point for their manufacture.

The German term “Organobleche” for thermoplastic panels is an invented name, since glass, carbon or aramide fibers with a thermoplastic, organic matrix – usually polyphenyl sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) for aviation applications and mostly polyamide (PA), polypropylene (PP) or polyurethane (PUR) for other non-aviation applications – are pressed by static or in some cases also continuous double belt presses to form plates of almost any size.

These can then simply be stored and processed in the same way as sheet metal plates. The pre-form has good mechanical properties: high impact strength, good impact behavior and minimized crack propagation. Thermoplastic panels are weldable, cuttable and recyclable. Only filaments are used as fiber or non-crimp fabric for high performance fiber composites, with the fiber length corresponding to the subsequent component size.

Bearing analogy to metal plates

The components are then produced by way of a thermoforming process, already automated where possible. The plates are gripped by a robot, heated using infrared radiation, inserted in a tempered thermoforming tool, formed under pressure and then cooled in the tool.  And depending on component size, the total process time lasts 60 to 120 seconds. This procedure is now already very similar to the processing method used for metal plates and is therefore increasingly finding use in other sectors besides the aviation industry.

According to Christian Peters: “Thermoforming delivers components with excellent laminate quality in short process times, has become established as state of the art in terms of its processes and is laying the groundwork for economical mass production in the aircraft construction industry.” Fully automated production cells for high grade thermoplastic fiber composite components are now in place.

But who is producing thermoplastic panels for the aircraft industry? Here again CFK-Valley provides the answers. Andreas Neugebauer, Sales Manager of TEN CATE Advanced Composites from Nijverdal in the Netherlands, discusses CETEX. His company has been producing this pre-form since the early 1990s, is now the European market leader and a certified system supplier for Airbus, Boeing and Embraer. CETEX complies with the Airbus Material Specification.

The material is tested for flammability, smoke emission and toxicity and has to demonstrate tensile, compressive, flectional and peel strength, in addition to which the manufacturer, production facility and production plant are also scrutinized. Accordingly, the product can only be manufactured at the certified facility. A change of plant must be agreed with the owner of the specification. CETEX is manufactured based on different carbon fiber and glass fiber materials using the polyetherimide (PEI) or polyphenylene sulfide (PPS) matrix systems.

PEI is a high performance, very strong thermoplast that is resistant to UV and gamma rays. The processing temperature is between 320 and 400 °C and the tool temperature between 120 and 180 °C. PPS is a semi crystalline high performance plastic, which retains its good mechanical properties at temperatures of well over 200 °C and is chemically resistant to nearly all solvents, many acids and alkalis, aggressive hydraulic fluid and atmospheric oxygen even at high temperatures while exhibiting good mechanical properties. The melting point of PPS is 280°C. It is formed at around 300°- 320° C and its continuous operating temperature is 220° C. The individual reinforcing fibers are impregnated with these polymers, constructed according to the client’s requirements and then pressed into plates.

Purely unidirectional structures and combinations of several such layers can be produced. During forming a phase transition takes place, for which the thermoplast matrix is heated until it softens. The individual fiber layers are shifted alongside each other and formed into the desired shape by draping. This sounds good and Andreas Neugebauer is certainly convinced: “Thermoplastic fiber composites will become increasingly important as demand for automated processing methods continues to rise. Short cycle times and the related opportunities to cut process costs are another factor in their favor.”

Christian Peters is of the same opinion. He believes that “the percentage of thermoplastic fiber reinforced CFK components in the Airbus A350 XWB and future aircraft generations will increase hugely, particularly for structure-bearing components in the aircraft fuselage. Around 1.5 million thermoplastic fiber composite components per year are currently planned for the upcoming A350 runup phase.” Any number of parts then, but they will all have to be machined prior to installation. Drilled holes and grooves will be inserted, measurements corrected and the required quality for connecting areas produced.

Dr.-Ing. Martin Garbrecht, Head of Department at the Institute for Materials Technology (IWT) in Bremen has worked intensively on these issues. “Components made from thermoplastic panels are not easy to work with, but our systematic investigations have shown them to be excellent,” he says.

Any delamination must be avoided

Among the challenges to be overcome, Garbrecht cites the high abrasive wear of the tools. In addition, drills and milling cutters are subject to high cyclic stress levels on account of the non-homogeneous material structure. During the cutting process, delamination and material damage must not occur under any circumstances when the drill is removed. Attention must also be paid during processing to the anisotropy and the speed and temperature influence of the matrix.

Overall, this can give rise to conflicting requirements in terms of tool geometry and cutting materials. Ralph Hufschmied, general manager of Hufschmied Zerspanungssysteme in Bobingen, Bavaria, is familiar with all these problems. He is also a member of CFK-Valley Stade and his company has specialized for 20 years in tool production for processing new materials. Hufschmied explains the difficulties: “The main problem with thermoplastic panels is combining the fibers with a thermoplastic matrix. A suitable tool needs a relatively blunt cutting edge on the one hand, to provide sufficient resistance to the abrasive fibers, while on the other hand the cutting edge needs to be as sharp as possible, to prevent heat building up in the thermoplast and causing massive smearing.”

Martin Garbrecht’s investigations have shown that coated hard metals produce good results. Ralph Hufschmied confirms these findings. “We have developed a special hard metal both for drilling and mill cutting operations, which has high resistance to the fibers. And we combine this with a sharp cutting edge. A relatively thin, nanocrystalline diamond film then rounds off the perfect pairing for cutting thermoplastic panels.“ The Hufschmied NC cutter geometry also prevents delamination of the fibers both during drilling and mill cutting.

This separation of layers in the material composite would not be tolerated at Premium AEROTEC (PAG) in Bremen either. The Thermoplastics Parts Production Division has been manufacturing fiber reinforced, thermoplastic formed parts from thermoplastic panels for the Airbus fuselage structure since 2006. The plates formed are made from carbon fiber material and polyphenylene sulfide (PPS) or polyetheretherketone (PEEK). PEEK is a high performance plastic with excellent mechanical properties, high temperature resistance up to 250 °C and good chemical and radiation resistance.

PAG manufactures clips from both PPS and PEEK thermoplastic panels as connecting elements between the aircraft fuselage skin and frame. Since May 2010 Teilefertigung Bremen has manufactured all the clips for the entire front fuselage section S13/14 and for the entire rear fuselage section S16/18 of the A350XSWB.  At more than 4,000 clips per aircraft in total, Teilefertigung Bremen therefore produces the majority of the thermoplastic components of the A350XWB. An automated thermoform cell went onstream at the turn of 2009/2010 to heat, form, press and cool in one process step. The parts produced are then milled to their final shape in the factory and subjected to dimensional, visual and ultrasound testing.

Preparing series production

Till Fesser, Development Engineer for CFK technology, gives his verdict on the material advantages of thermoplastic panels over conventional fiber composite materials: “Very short process cycles are possible, the pre-form can be stored almost indefinitely, the formed workpieces exhibit high impact resistance and can be repeatedly melted and also welded.“ And what can we expect in the near future?

Gaby Soehner, Head of the Future Technologies Division: “We are currently working on a waste optimizing solution for the slabstock and the existing processes are being further developed.” This sounds promising. Achievable cycle times of 60 seconds for automatic forming of thermoplastic panels makes series production of large quantities of these highly stable and lightweight parts an extremely interesting proposition.

Robert Wouters

Spotlight Organobleche

Thermoplastic panels are fiber reinforced thermoplastic pre-forms. The German term “Organobleche” for thermoplastic panels is an invented name, since glass, carbon or aramide fibers with a thermoplastic, organic matrix – usually polyphenyl sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) for aviation applications and mostly polyamide (PA), polypropylene (PP) or polyurethane (PUR) for other non-aviation applications – are pressed by static or in some cases also continuous double belt presses to form plates of almost any size.

German  Summary

Klimaschutzziele und immer weiter steigende Treibstoffkosten zwingen die Luftfahrtindustrie zu Entwicklung und Bau effizienterer Flugzeuge. Neben der Optimierung von Triebwerken und Aerodynamik weist der Einsatz von thermoplastischen Faserverbundkunststoffen (FVK) dafür ebenfalls ein hohes Potenzial auf. Hauptproblem dabei bisher: ein geringer Automatisierungsgrad, lange Prozesszeiten und hohe Kosten. Aber die Entwicklung geht weiter. Denn: Organobleche (thermoplastic panels) helfen „Abspecken.“ Der deutschsprachige Beitrag ist nachzulesen auf www.aerotec-online.com/

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