By Bjorn Fehrm • Aerospace Analyst
September 18, 2026
Last week we looked at dry fiber layup methods that use a binder to tack fabrics or the tape in an AFP/ATL system to the previous ply, creating a porous but rigid fiber preform. The binder is a heat-sensitive plastic that allows the generated preform to be handled and even reshaped if reheated.
Once the preform is made, epoxy resin infusion can be done in three ways, grouped under the label Liquid Composite Molding. The first, called Resin Transfer Molding (RTM), is shown in Figure 1.
The first method, Resin Transfer Molding (RTM), requires a closed mold. The preform that will fit in the closed mold can either be made in the mold, which would mostly be a manual layup of dry fabric, or be produced external to the mold, like for the CFM LEAP fan blade preform we showed in Part 17.
A closed mold is necessary because heated resin (to reduce viscosity) is injected under pressure, forcing it through the dry preform to fully wet out the fibers. The mold is then heated to the resin’s curing temperature.
A closed mold is expensive but brings several advantages:
Precise control of process geometry and fiber and resin content makes this method a candidate for Out of Autoclave (OoA) production of limited-size parts, like the spars for Airbus’ Wing of Tomorrow project.
The second method is vacuum-assisted RTM, or VARTM, also called Liquid Resin Infusion (LRI). It uses a one-sided female mold, where the preform is surrounded by a vacuum bag (Figure 2).
The preform is normally laid up on the female mold before applying resin flow channels, porous media, and the vacuum bag.
Then a vacuum is applied to consolidate the preform, after which a heated resin pot is attached, and the resin infuses the preform. The temperature is then increased to the point where the epoxy begins to polymerize, achieving a fully cured part in, for instance, a hot-in, hot-out oven.
The advantages of the VARTM process are:
CRTM
The third method, Compression Resin Transfer Molding (CRTM), is a variant of the RTM process, optimized for rapid high-volume manufacturing of composite parts.
A dry fiber preform is placed inside a dual-sided tool, but the mold is not completely closed during the initial resin injection. A calculated volume of resin is quickly fed into the gaping cavity, after which a hydraulic press forces the mold completely shut (Figure 3).
By forcing the mold closed after injection, mechanical compression drives the resin rapidly through the dry fibers, yielding the fastest cycle times among LCM processes. Its limitations are that it demands a highly complex mold design and expensive automated press systems to carefully orchestrate the injection and compression.
The CRTM process is newer than the other two, only becoming mainstream over the last decade. Because CRTM can produce composites faster and with interesting properties but relies on a hyper-dynamic, split-second balancing act between mechanical pressing and fluid dynamics, it’s a focus area in modern composites research.
CRTM, or a very close allied process, has been fully implemented for decades at Boeing for the manufacture of Interior sidewalls. BAC5441 started the process chain, its last revision date is in 1994. BAC5441 covered the compression molding of parts between 2-sided tools resembling giant waffle irons making cabin sidewalls. Decorative Tedlar finished surface material was loaded into the cavity face down, followed by fiberglass fabric that formed the structure. Resin was poured into the tool which closed and cooked off the part. The clamshell opened when the catalyzation process was complete. Cycle time was initially in the low minutes range. Subsequent developments over time have been incorporated in BAC5090 last revised in 2020, BAC 5326 rounds out the chain of specs for compression molding as it contains the changes needed to do TPC materials. Thousands of parts a week are made to support the needs of the assy lines and a tremendous variability one part to the next occurs to create the correct decorative surface for each panel as the airlines are free to choose their interior config.
Another application of a CRTM derivative was in process in Sedro Wooley Wa where Freightliner R&D was compression molding 1 piece Semi Truck hoods in a heated tool where the reinforcements are loaded into a 2 sided nickel plated tool and the tools are mounted in a manual hydraulic ram. In this application, the resin is pigmented to the finished part color eliminating painting and increasing part durability as road wear from rock chips is mostly hidden by the color of the resin being consistent through the entire part thickness. Cycle time back then on a fully manual press was just under 15 minutes. I watched this happen around 2005/6…..
This is a very common process in place across industry.