Researchers from the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal (Germany) have developed a new method for the production of thermoplastic fiber composite materials designed for large-scale use in vehicle construction.
Once these materials have reached end of life, they can be shredded, melted down and reused to produce high-quality parts. They also perform significantly better in crash tests: thermoplastic components reinforced with textile structures absorb the forces generated in a collision through viscoelastic deformation of the matrix material without splintering.
Modern cars are now built from a mixture of steels, aluminum and fiber-reinforced plastics. Highly stressed load-bearing structures and crash components that are designed to buckle on impact help to reinforce the body in order to protect the vehicle’s occupants in the event of a collision. Automakers have previously constructed these parts from composites using a thermoset (i.e. infusible) matrix. However, notes Fraunhofer, this approach has a number of disadvantages: as well as being difficult to implement efficiently in a mass production environment, it can also be potentially hazardous since this material tends to delaminate into sharp-edged splinters in a collision. A further problem is the fact that thermosets cannot be recycled.
Researchers had previously failed to come up with a suitable manufacturing technique for thermoplastic composite structures made from high-performance fibers. The ICT engineers have now developed a process suitable for mass production which makes it possible to manufacture up to 100,000 parts a year.
Our method offers comparatively short production times. The cycle time to produce thermoplastic components is only around five minutes. Comparable thermoset components frequently require more than 20 minutes.
—Dieter Gittel, a project manager at ICT
The Fraunhofer researchers have named their technique thermoplastic RTM (T-RTM). It is derived from the conventional RTM (Resin Transfer Molding) technique for thermoset fiber composites. The composite is formed in a single step.
We insert the pre-heated textile structure into a temperature-controlled molding tool so that the fiber structures are placed in alignment with the anticipated stress. That enables us to produce very lightweight components.
The preferred types of reinforcement are carbon or glass fibers, and the researchers have also developed highly specialized structures. The next step involves injecting the activated monomer melt into the molding chamber. This contains a catalyst and activator system required for polymerization. The researchers can select the system and the processing temperature in a way that enables them to set the minimum required processing time.
As a demonstration, ICT engineers crafted a trunk liner for the Porsche Carrera 4 that weighs up to 50% less than the original aluminum part. To improve the crash behavior of the vehicle’s overall structure, the ICT engineers also calculated the optimum fiber placement.
The cost of the thermoplastic matrix material and the cost of its processing in T-RTM are up to 50% lower than the equivalent costs for thermoset structures.