We began focusing on the production of thermoplastic composites last week. PEEK variants have a melting point as high as 380°C to 400°C, while LMPEAK (low-melting-point PAEK) has a melting point exceeding 305°C. They have a high viscosity when melted (similar to honey), which renders the dry fiber impregnation method unfeasible. Instead, a heated closed mold transfers heat to the matrix to force the composite to take shape, usually through hydraulic pressure applied to the mold surface.
Since thermoplastic composites can be joined by welding, they are attractive for manufacturing composite fuselages, where the main challenge lies in connecting thousands of parts. However, the need for closed heated molds, often accompanied by pressure, makes it difficult to produce skins 30 to 50 feet long using this method.
When Airbus fabricated the Multi-Functional Fuselage Demonstrator (MFFD) by welding thermoplastic composites together (see Figure 1), the in-situ consolidated AFP skins had to be post-consolidated in an autoclave before stringers, clips and frames were welded to the section.

Figure 1.Eight-meter-long single-aisle MFFD fuselage section.
Manufacturing large thermoplastic composite parts
Last week, we introduced several thermoplastic manufacturing methods that utilize heat, pressure and closed molds. These methods are used to produce the internal components of the fuselage section in Figure 1, which can be assembled by welding.
To manufacture the fuselage skins, the MFFD project adopted a quarter-circumference concave mold, in which the AFP system applies thermoplastic tapes and fuses them in place via in-situ consolidation (that is, the nip-point laser heating laydown technology adopted by the Royal Netherlands Aerospace Centre NLR) (Figure 2).

Figure 2: An AFP robot applies thermoplastic tapes to the MFFD skin layup
Subsequently, these skins are joined by welding in large tooling to form the lower half fuselage skin for the next-generation narrow-body aircraft (Figure 3).

Figure 3. Two quarter-circumference skins are welded together in large tooling to become the lower half fuselage skin.
The spliced skins were then shipped to the German Aerospace Center (DLR) in Stade, Germany, for post-consolidation treatment in an autoclave at 7 bar and 400°C (Figure 4).

Figure 4.MFFD lower fuselage skin undergoing post-consolidation treatment in the autoclave in Stade, Germany.
The reason for carrying out post-consolidation treatment is that the AFP (Automated Fiber Placement) process for thermoplastic tapes has problems such as tape burn-through and insufficient fusion between plies and tapes during the movement of the head. The speed of this process is only one-fifth that of thermoset AFP; even so, the strength without post-curing is still less than 50% of that of a fully consolidated composite. This means that post-curing treatment is mandatory, usually carried out in an autoclave or in an oven after bagging and applying vacuum.
The low-speed AFP process and the need for post-consolidation are inconsistent with the goals of the MFFD (Multi-Functional Fuselage Demonstrator) and the American HICAM (NASA and Industry High-Speed Composite Aircraft Manufacturing Program) to develop high-speed, low-cost composite structure technologies for next-generation narrow-body aircraft.
Alternatives to in-situ AFP consolidation
Another approach is to use closed heated molds to manufacture skins using the techniques described last week. This will produce closed molds of unprecedented size. If the preform is made in the form of a flat-laid AFP ply layup, the mold also needs to act as a pressure mold to achieve a circular shape with the help of a large press. It will be quite challenging to set up door and window cutouts on the flat preform, because it is difficult to achieve the correct final position in the final skin shape after compression molding.
Fuselage manufacturing must cover a length of 40 to 50 meters, which requires this skin mold to be divided into several sections. Figure 2 and Figure 3 illustrate the size concept of the closed mold. Once the mold is manufactured and the process is verified, thermoplastic skins can be produced at an extremely high rate. Given the high efficiency of heating and pressurization, large presses can be shared by multiple segmented molds to produce each segmented skin in batches.
Before providing challenging thermoplastic fuselage skins for the new generation of composite narrow-body airliners, the economic feasibility of this giant closed mold component technology must be studied, refined and demonstrated, as a large number of expensive molds are required.