By Bjorn Fehrm
August 14, 2026, ©. Leeham News: Last week, we looked at manual dry-fiber infusion layup of composites in our series on aircraft structures. Now we start looking at methods that use more automation and equipment to increase production rates and reduce labor content in the composite cost.
We start with the most common method for large-scale composite structures: thermoset prepreg tape layup, which is subsequently cured in an autoclave. It’s the method used for the primary structural parts for the Boeing 787 and the Airbus A350. The earlier 777 and A330/340 generations also employed the same composite material system but limited it to the horizontal and vertical tails and, for the 777, the floor beams.
Figure 1. The Automated Tape Layer (ATL) laying up the Airbus A350 wingbox cover. Source: M Torres.
The importance of the matrix type for the production of composites
What we have learned so far is that the decisive part of an aeronautical composite is the matrix material. The production methods center on the resin used, the layup method it supports, and the matrix curing method.
The matrix is most often a hardened epoxy resin, with varying polymerization temperatures and times. This type of matrix is called a thermoset, i.e., it will polymerize with temperature. Another group of matrices is the thermoplastic matrices. Here we have a material that acts like the plastic we use in our daily lives. It’s hard at room temperature but melts at higher temperatures. Therefore, it can be reformed and welded using heat. For aeronautical matrices, special plastics such as PEEK or PEKK are used, which melt above 300°.
The most common composite matrix today is thermoset epoxy, so we start by describing the automated production with thermoset epoxy matrices.
The fibers in a composite are often called the “reinforcement” by the industry, indicating that they play a secondary role in describing a composite. The reinforcement can be glass-, carbon-, or aramid (Kevlar) fiber-based. Common forms of reinforcement are fabric, which is used in manual layups, and unidirectional tape, which dominates automated layups.
Automated thermoset layup
We have described how sensitive a composite is to the direction of the fibers. If a fabric is used, the direction of the fibers is set by the fabric producer; the most common type has fibers woven at a 90° angle.
If a unidirectional tape is used, the fibers are parallel, most often enclosed in a tape format. The dominant form today is the prepreg tape, in which the fibers are enclosed in a hardener-activated epoxy tape. The epoxy in the tape has hardened to a sticky consistency when the tape rolls are placed in a freezer and delivered frozen to the production site, where they are stored again in a freezer.
When it is time to tape out the layup, the rolls are loaded onto the tape layer. The tape layer traverses the mould and tacks the sticky tape onto the previous plies using a heat source (Figures 1 and 2).
Figure 2. The tacking of the tape to previous plies. Source: ScienceDirect.
The advantage for the composite designer of the unidirectional prepreg tape is the freedom to choose the directions of the different plies in the layup. It can then be optimized to align with the structure’s major directional forces.
The designer has many plies to optimize. The A350 wingbox cover being laid up in Figure 1 has more than 250 plies at the wing root and more than 40 at the tip of the wingbox. The gantry-style tape layer in Figure 1 is of the large type used for large flat composite laminates. There are many types of tape layers for different part types; we will look at other variants in the following Corners.
The layup time for a tape layer is divided into several types of activities, with the laying down of tape often accounting for less than 30% of the time (Figure 3).
Figure 3. A typical time slice chart of a prepreg tape layer. Source: Leeham Co.
This is with the classical tape layer, where the inspection of the placed ply is done by a person after the ply is complete. A lot of research has gone into automating this with computer imaging methods.
During the layup, the plies need to be compacted by placing a temporary vacuum bag over the layup and using a vacuum to drive out air pockets that form as the tape is laid down.
After the layup is complete, a one-sided female mould, as shown in Figure 1, is bagged and transported to the autoclave for curing.
Thermoset Composite Cure
One of the most critical manufacturing steps to achieve a high-quality aeronautical composite is the curing of the resin matrix. This is when the polymerization takes place, the component structure is consolidated, and the structural materials obtain their mechanical strength and stiffness. Polymerization is irreversible, so curing will include any qualitative deficiencies caused by inappropriate processing in previous steps (material preparation, mould design and preparation, lay-up, and vacuum set-up).
Curing of parts such as the wingbox cover in Figure 1 is performed in an autoclave at temperatures of 180°C and pressures of 7 bar. The process of cycling an autoclave to these values is lengthy; for the wingbox cover in Figure 1, it takes 13 hours to cure the cover, after which it is extracted, fitted with stringers, and these are co-bonded to the cover in the autoclave for a further 8 hours.
Figure 4. The wingbox cover layup on its mould entering the Airbus Stade autoclave. The first wingbox cover is already in the autoclave. Source: Airbus.
The Airbus Stade autoclave used for the top covers (Figure 4) can take two molds per cycle. Such autoclaves cost around $50 million apiece and require a nitrogen atmosphere and substantial energy; the process of prepreg tape laydown with autoclave cure is expensive and demands a high investment in production equipment.
At the same time, it’s a slow process. The present process is adapted to the widebody production pace of 10-15 aircraft structures per month.
We will discuss what is currently being done to identify alternative methods to achieve a composite process capable of supporting a new generation of narrowbodies, in which up to 100 structures per month must be produced.
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