Lasers reinforce lightweight structures

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

At the JEC Composites Show in Paris, researchers from Germany’s Fraunhofer institutes showed how lasers can make the manufacturing of fiber-reinforced thermoplastic structures more efficient, reliable and automated.

Fiber-reinforced plastics are used in applications including sporting equipment and in the aerospace and automotive industries. Parts using these materials combine impressive stability and breaking strength and are 50 to 70 percent lighter than steel and 15 to 20 percent lighter than aluminum. Nevertheless, there is room for improvement, fueled in part by the trend of using lighter parts to save energy.

When it comes to improving the way these parts are made, “it makes a big difference which kind of polymer material you are using,” said Michael Emonts of the Fraunhofer Institute for Production Technology. There are two general classes of polymers that can be used, he said, and each behaves very differently when exposed to heat.

Thermoplastic polymers are normally produced in one step and made into products in a subsequent process. They become soft and formable when heated and, when cooled below their softening point, turn rigid and become usable as formed parts. On the other hand, thermosetting polymers usually are produced and formed in the same step. They also soften somewhat when heated but cannot be shaped or formed to any great extent and will not flow because of their almost crystalline structure.

Most of today’s fiber-reinforced materials use thermosetting polymers because they can be processed at room temperature. Making parts involves lining a mold with glass or carbon fiber mats. For high-performance applications, the air is removed before fluid resin is injected so that the matting can become fully saturated, and no air bubbles that would impede stability are generated on the fibers.

Finally, an oven that can accommodate the part is needed to harden the material, which can be gigantic if designed for aircraft, for example. The result is a part that is fully cross-linked by the fiber structure and the almost-crystalline plastic surrounding it. Although this sounds great, it also has a big disadvantage: If damaged, cracks can propagate through the entire part, causing damage or failure far away from the impact – even inside the structure.

Thermoplastic materials avoid this problem as they remain more elastic: Because of their noncrystalline structure, cracks remain local. But the downside is that they cannot be processed at room temperature, which is not compatible with classical production methods.

A recently developed alternative uses “tapes” consisting of carbon fibers integrated into kilometer-long strips of meltable thermoplastic resin. To assemble sturdy components from these tapes, multiple layers are stacked on top of each other, partly overlapping, and they are heated locally just before being laid down and pressed together. In this way, highly customized structures can be made and adapted to the application without requiring huge furnaces.

“The big issue with this approach to date, however, has been the availability of suitable sources producing localized heat,” Emonts said. Gas flames have been tried, but their capabilities controlling the heat are very limited. Infrared radiation and hot air generators are not very energy efficient. This is where the laser comes in. It heats the material in a localized and efficient manner, enabling the tape strips to fuse with each other and to cool off quickly.

Lasers not only open the path to making single fiber-reinforced parts, but also help make difficult-to-form, bulky components of fiber-reinforced plastic by joining them together.

Lightweight components are manufactured using a new method: combining fiber-reinforced tapes with laser-induced heating. Courtesy of Fraunhofer Institute for Production Technology.

A new technique presented by researchers from Fraunhofer Institute for Laser Technology ILT offers sturdy connections that satisfy the standards of automotive and aerospace industries, said ILT’s Dr. Wolfgang Knapp. “All we need for this is a laser that emits infrared light. The infrared laser melts the surface of the plastic components. If you compress them when they are still fluid and then let them harden, then the result is an extraordinarily stable bond.”

Directly applied high-power diode lasers are the first choice for these applications, predominantly for economic reasons. Gaussian beam quality is not really needed here; in fact, the dispersive beam converts in a homogeneous profile at the workpiece – exactly what is needed. In principle, fiber lasers could be used, especially if they were cheaper or if their better beam quality were needed; e.g., for scanning applications with a long focal distance.

The main challenge when using lasers is the fiber reinforcing the typically transparent polymer material. It induces scattering and (multipath) reflections because of the difference in refractive index between the thermoplastic matrix and the glass, reducing the absorbed power. The lack of absorption in the fiber itself can be the other challenge. Carbon fibers – as widely used in high-cost, high-performance parts – are black and absorb a wide spectrum. Glass fibers, however, do not.

Solving this problem is the next thing the researchers are working on – to address mass uses for glass-reinforced plastic, also known as fiberglass.