In-tool polymerization realizes continuous fiber-reinforced composites

Continuous fiber-reinforced composites with thermoplastic matrix resins are very well suited for use in automotive manufacturing. However, to manufacture them is complicated. A new approach involving in-mold polymerization of the matrix resin developed by the Fraunhofer Institute for Chemical Technology ICT (Pfinztal, Germany) and Engel Austria GmbH (Schwertberg) now reportedly makes it possible to use the injection molding process.

In-mold resin matrix polymerization and continuous fiber reinforcement in injection process realize high-strength parts in rapid cycle times.

To date, it has been very laborious to manufacture fiber-reinforced composites with a thermoplastic matrix in large quantities according to the development partners. On the one hand, textile-like dense continuous fiber-reinforced structures are difficult to shape; on the other, joining the continuous fibers with a highly viscous thermoplastic matrix material is a highly complex process. Until now, there is no economically profitable production technology for large-volume component series according to the ICT.

Together with Engel, the ICT has, for the first time, brought a technology to production readiness that reportedly allows the series production of such continuous fiber-reinforced thermoplastic composites with an injection molding process. So far, it has only been possible to use the injection molding process for fiber-reinforced composites made of short fibers or long fibers.

"Continuous fiber-reinforced composite structures with a thermoplastic matrix are becoming increasingly popular, and will be used increasingly in the automotive industry," states Lars Fredrik Berg, scientist and project manager at the ICT. "With the injection molding process, components that have high fiber contents by volume and therefore outstanding mechanical characteristics can be produced efficiently in high volume series."

Together with Engel, the ICT has developed a prototype machine for the injection molding of continuous fiber-reinforced composites. The Engel e-victory 120 can handle all the necessary working steps in a single machine. The reactive components are prepared and mixed, and the formulation is injected into the injection tool after the textile reinforcement structures have been introduced. In-situ polymerization then takes place in the tool.

"The ICT and Engel have developed a robust, compact and fully automated technological system to series readiness that is flexible and rapid at the same time. The say it is exactly this technology that the automotive industry has been lacking for continuous fiber-reinforced thermoplastic composite structures. The process, which to date had been distributed across several machines, can now be carried out on a single one", says Peter Egger, Head of the Technology Center for Lightweight Composites at Engel. e-victory has already passed its first crucial test: Engel produced, as an example, a brake pedal insert made of fiber glass-reinforced polyamide for the automotive supplier ZF Friedrichshafen.

In contrast to the injection molding processes for fiber composite materials to date, where only short or long fibers could be processed, continuous fiber-reinforced composite structures can be fed into the e-victory and be impregnated with a very low viscosity plastics matrix. "We have developed a process in which the in-situ polymerization of thermoplastic matrix materials works," says Berg. "We allow monomers, which are highly reactive molecules, to polymerize directly in the machine. The monomers have a shorter molecule chain than polymers, and therefore a lower viscosity." He adds, "When being processed, the viscosity of the reactive plastics matrix is similar to that of water. This means that the fiber structures can be wetted down in an ideal manner, without displacing the structures in the form.

In October 2012, the Reinforced Plastics Industrial Association AVK awarded the ICT and Engel an AVK Innovation Prize in the "Processes" category for this new technology.

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