Dr. Jörg Schwartz, email@example.com
PARIS and AACHEN, Germany – 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
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
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.