By Bjorn Fehrm
July 31, 2026, ©. Leeham News: In our series on aircraft structures, we look at the production of composite parts and laminates. The composite’s quality and, thus, strength depend on the fiber, matrix, and production method. We start with the most common method: manual, or hand, layup of fiber fabric impregnated with epoxy resin on a female mould.
In Figure 1, we see a composite production operator laying epoxy-impregnated carbon fiber fabric in a fuselage-half female mould at Diamond Aircraft in Austria, cutting the composite ply to size while laying it down on top of previous layers.
The hand layup of composites
The hand or manual layup of a composite is the original and most common method. It requires few specialized machines and tools, but the person doing the layup must have special training. The hand layup system for aeronautical use is for smaller and medium-size aircraft or parts, up to fuselage sides or wing covers for gliders and general aviation aircraft.
For hand layup, thermoset resins are used, as thermoplastic resins require laydown at temperatures where the resin melts (over 100°C up to 400°C depending on the resin), making hand handling impossible.
For general aviation and gliders, production rates and demands on part strength do not motivate a faster, higher-quality production method. For the eVTOLs, it’s a way to get underway with a composite material system which can be easily certified by using a certified material system and production method, like the NCAMP ones we described.
The fabric is placed in a female mould (Figure 1), often made in the same composite material system, as it then has the same thermal expansion ratio, to avoid any layup-to-mold expansion stress when there is a temperature change due to using oven curing of the layup in the mould.
To increase production rate at an acceptable cost, a more automated production method is necessary where the balance of equipment investment versus manual labor is more skewed towards a higher equipment investment. We discuss these methods in coming Corners.
Matrix and Fibers
The common matrix is epoxy, and the fiber is most often in a fabric format, pre-impregnated with the epoxy. The supplier can pre-impregnate the fiber,which is then called a prepreg. It keeps the resin content closely controlled for the intended use. The drawback is that the epoxy with its hardener is now active, delivered in a sticky state that has active polymerization at room temperature. This is why delivery and storage are in a cooled state.
The impregnation before fabric laydown gives easier handling and a wet fabric, as in Figure 1. This results in a more variable epoxy-to-fiber content ratio, potentially reducing composite quality. The quality of a composite part is dependent on the resin-to-fiber ratio, but also on the porosity of the finished part. Air inclusions in the composite function as stress concentrations, and as we learned last week, composites are sensitive to stress concentrations around holes or notches. The porosity of a composite is therefore a closely observed parameter, which for a high-quality composite shall be below 1%.
Layup and curing of composites
There are a multitude of resin systems that can be used, with different polymerization temperatures and cycles. In general, the longer and hotter the curing process, the stronger the epoxy matrix. The fibers are called reinforcement and can be different types of carbon, fiberglass, or Kevlar in different forms such as multidirectional fabrics, non-crimp fabrics (where the fibers are straight, mixed in directions using stitching to pack the layers), or for smaller parts, chopped fibers wetted with epoxy. In summary, the variation of resins and fiber types in a hand layup is almost endless.
The use of composite to produce laminate parts does not have to be because of superior strength. A flexible fabric laid down on a mould allows very complex surfaces to be made. To, for instance, make a general aviation or eVTOL fuselage skin, a wooden pattern of the shape of the fuselage is made by NC machining wood parts to the appropriate shape.
The pattern is then surface-treated to have an absolutely smooth shape, upon which a layer of release agent is sprayed, and the layers of the composite mould are draped over the pattern. It’s then bagged, with sticky tape closing off the bag at the perimeter of the mould so that a vacuum pump can compact the composite. The compacting is important to drive out any trapped air.
Then the curing starts. The curing is mostly done in an oven for manual layup parts. It can be done in a slow-ramp oven cure of up to 170°C for a high-quality part, where the oven cycle can last up to 24 hours, or at room temperature with a resin and hardener adapted to this curing process. The room-temperature-cured part is movable after 24 hours, but final curing can last up to a week. A low-temperature cure gives a lower-strength part.
If the layup is done with industrially pre-impregnated cloth (in contrast to the fabric spray impregnators that Diamond used in the video we referenced in Part 9), it means the ratio between resin and fibers is precisely controlled to be close to the ideal 40% to 60% to create a higher quality part. The part quality can be further improved by curing under pressure and temperature in an autoclave to achieve a composite with low porosity.
If we look at the autoclave curing cycle for an NCAMP-certified material system using the Hexcel 8552 resin, we have a five- to six-hour cycle in the autoclave at 7 bar pressure Figure 2.
Figure 2. The curing cycle in an autoclave when using Hexcel’s 8552 resin system. Source: Hexcel.
Autoclaves give a high-quality composite but use a lot of energy and nitrogen. The flow through an autoclave is often a bottleneck as there is a long ramp-up and down cycle. When curing in an oven or hot room, the part enters and leaves without any lowering of the temperature (hot in, hot out).
Hand layup, not only for small aircraft
I have described the hand layup as being used for smaller aircraft/VTOLs. For larger aircraft, it will be used too, as there are many types of composite parts in an airliner and these will probably use all of the production methods we will discuss in the series.
Related
