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.
Figure 1. The Resin Transfer Molding (RTM) method. Photo credit: Hexcel.
RTM
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:
- The preform and mould can have final geometry. It then requires only minimal post-processing to remove any resin leakage into form gaps and resin inlets/exits.
- With a careful preform and mould geometry, the resin-to-fiber ratio can be closely defined and controlled.
- With careful flow control, the void content can be below 1%.
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.
VARTM
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).
Figure 2. The VARTM or Liquid Resin Infusion method. Photo credit: Hexcel.
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:
- The single-sided mould is much simpler and therefore lower cost to produce than the high-precision matched molds of an RTM process.
- The lower cost makes it suitable for producing large parts like wingbox covers.
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).
Figure 3. The CRTM process. Photo credit: ScienceDirect.
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.
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