Bjorn’s Corner: Aircraft Structures Part 13. Dry fiber infusion.

By Bjorn Fehrm

August 7, 2026, ©. Leeham News: In our series on aircraft structures, we have looked at hand layup with pre-impregnated fiber fabrics. We now look at the alternative manual layup method, dry fiber infusion. Dry fiber infusion, if done correctly, can deliver a higher quality composite than the wet layup we described last week.

The main difference is that the fiber fabric is laid up on the mould without pre-impregnation with epoxy resin; it’s introduced as a low-viscosity flow at a later stage. Figure 1 shows the beginning of the infusion process with the dark epoxy resin flow progressing through the dry fabric. Note the resin feed pipe and the connected flow distribution pipes inside the bagged layup.

Figure 1. The dark epoxy resin flow gradually wets the carbon fiber fabric, placed below the white infusion flow mesh. Source: Easy Composites Ltd.

The manual dry fiber layup of composites

The dry fiber layup of composites is an alternative, more complex method for fabricating composite laminates or parts. It requires few specialized machines and tools, just as the pre-impregnated layup. It can also be done more elaborately to produce larger parts. As an example, we describe how the Airbus A220 wingbox is done using a dry fiber infusion method.

Dry fiber infusion essentials

We start with the manual layup method for smaller parts, using a very good instruction video from Easy Composites Ltd. The tricky bit is how the epoxy resin is sucked into the dry fiber fabric by the compacting bag’s vacuum, and how to wet all the fibers uniformly as it propagates through the layup. The YouTube instructional video shows how to prepare the layup for the epoxy resin introduction and how to control the flow propagation through the fabric.

Here are the steps:

1. You start by making a mold with a broad border area around the composite part, where the vacuum bag is attached, and the resin introduction and exit pipes are placed.
2. The dry fiber fabric is cut and placed carefully in the mold, tacked to the mold surface by a glue sprayed onto the surface. The fiber stack is laid down onto the mould in its full number of plies using tacking glue if needed to hold down the fabric layers.
3. Several layers of special release and flow fabrics are then placed on top of the fibers. Finally, a vacuum bag is placed over the part and firmly glued onto the perimeter of the mold. It has to be absolutely tight, or there will be air inflow pits in the part’s surface.
4. The vacuum is attached to the outflow point, and the fiber fabrics are compacted by the vacuum as the operator checks the layup for leaks by sealing the in and out pipes and checking a vacuum meter while waiting for several minutes.
5. If the layup is tight, then the low-viscosity resin is attached to the in-pipe, and the flow of the resin through the layup starts by pumping away air with the vacum pump at the outflow port. The inflow of the resin is aided by special distribution pipes in the layup (Figure 1). For large parts, several inflow points are used.
6. The key is to have the resin reach the outflow pipe’s pickup as the last point in the layup. This requires extensive knowledge, and for larger parts, several tests with prototype parts to perfect the inflow, distribution, and propagation of the resin through the layup.
7. The example layup has a resin that cures at room temperature once mixed with the hardener. Infusion parts can also be oven-cured or, for demanding applications, cured with a high-temperature epoxy in an autoclave.

The Easy Composites Ltd video shows the resin infusion process well. I have witnessed a 20 times larger prototype part being infused at one of the major composite technology companies, and the procedure was the same.

The quality of a well-prepared vacuum-assisted dry fiber infusion is typically higher than for a wet pre-preg manual layup. The risk of air pockets in the layup is lower, creating a more solid composite part.

Producing the Airbus A220 wingbox

Bombardier, the developer of the CSeries, later Airbus A220, chose dry fiber resin infusion to produce the wingbox spars and covers for the CSeries. As usual for wings, only the central wingbox is in carbon fiber composite. The leading edge is aluminum (for de-icing heating with bleed air), as is the trailing-edge mechanism. The ailerons and flaps are in carbon composite but not necessarily produced with the wingbox method.

The procedure for the large A220 wingbox covers is well described in this video from the Bombardier Belfast wing production site (now an Airbus facility). The cutting of the fabric parts is automated with NC machines, but the layup of the cut fabric on the large wingbox cover mould is manual (Figure 2).

Figure 2. The layup of the dry fiber fabric onto the CSeries/A220 wingbox cover mould. Source: Bombardier video.

Figure 3 shows how the dry fibers for the stringers are placed inside special stringer molds and placed on top of the cover fabric before bagging of the cover/stringer combination.

Figure 3. The wing cover with stringer dry fabric holders in place. Source: Bombardier video.

The mould with cover and stringers is then inserted into an autoclave where the infusion is made under heat and pressure. This gives a high-quality, one-piece integrated cover with its stringers with a void inclusion of below 1%.

The technology for placing the stringer dry fabric on top of the cover layup required extensive research and tests. The result is covered in a patent registered by Bombardier at the time.

We have now covered the dominant manual layup methods for composite parts. In the following Corners, we look at different automation methods for the layup.

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