Long Fiber Injection (LFI) is a new polyurethane molding process developed successfully in recent years. The process has the advantages of high automation, short molding cycle, light weight and low manufacturing cost.
In the automotive industry, the LFI process was first used to manufacture structural and semi-structural panels, such as roof components. According to reports, a sports car's LFI polyurethane roof is 20% lighter than a steel roof and more than twice as rigid as an aluminum roof or other fiberglass roof. In addition, LFI polyurethane composite materials are also used in the agricultural and commercial vehicle industry, such as tractor covers, heavy truck panels, bulldozer body panels, bus luggage racks, etc.
In recent years, composites experts from private industry, the military, and research institutions have been collaborating to explore whether the LFI process can also be used to produce carbon fiber-reinforced composite (CFRP) parts needed for high-performance materials markets such as aerospace, enabling mass production at a lower cost. The study has been funded by the U.S. Air Force Research Laboratory (AFRL) since 2022, and participants include AFRL's Manufacturing, Industrial Technology, and Energy divisions, prime contractor Lockheed Martin, equipment manufacturer KraussMaffei, and the University of Dayton Research Institute (UDRI).
The manufacturing process for carbon fiber composite components currently used in aerospace is lengthy and labor-intensive, and typically features carbon fiber prepreg being manually placed on a single-sided tool, bagged, and then rolled into an autoclave for a full day of curing. In contrast, the LFI process is fast, efficient and automated. The LFI process first cuts the glass fiber to the desired length, the chopped fiber is mixed with a two-component liquid resin, then sprayed into a preheated open mold, and finally cured under low heat and pressure. The entire process is one of the lowest-cost, fewest waste methods for making composite materials and can take anywhere from a few minutes to a few hours, depending on the complexity of the part.
The first important problem the team faced was the degumming of the carbon fiber, which must be dispersed enough to be uniformly dispersed within the resin during mixing, and they found that Zoltek's large tow carbon fiber provided the best mechanical properties and degumming properties. The researchers first broke up large 50K bundles of carbon fiber into smaller 2K to 3K bundles, and then rewound these bundles back into 50K bundles. When a cut occurs in the LFI head, they are easy to unravel.
Selecting the right resin is also a focus of research. The research team used the Covestro polyurethane resin system, which is specifically designed to reduce the density of the final cured panel, by adjusting the mixing method to minimize or eliminate potential void content.
In terms of processing equipment, the researchers are analyzing several variables including cutting speed, fiber length, fiber/polymer ratio, air pressure, pressing time, mold design and mold temperature, one of the important issues is the processing capacity of carbon fiber, especially the blade and drum of the cutting machine, which can obtain better cutting results after the cutting blade upgrade.
The research team plans to conduct two aerospace related manufacturing demonstrations using LFI/ carbon fiber technology by the end of 2025. Although a high-performance polyurethane resin system is currently used, the team prefers to use epoxy resin because it offers better performance. But this would require major modifications to the LFI hardware system, and there is currently insufficient funding to cover these costs.
This collaborative research project can promote the development of the entire composite materials industry. Leveraging these companies' innovations in low-cost composite processes could accelerate aerospace original equipment manufacturers (Oems), such as Lockheed Martin, to find manufacturing solutions for next-generation aircraft.