Transparent plastics can be joined via laser beam at a wavelength of around 1,700 nanometers. Image: Fraunhofer |
Laser
welding is on the advance, but it also has its limits: it has been
impossible to fuse two transparent plastic components together – up
until now. Researchers have now succeeded in circumventing this hurdle –
by choosing the right wavelength. The new welding process is
revolutionizing bioanalytics.
It’s
a quick process, generates almost no waste and is extremely precise:
within a few seconds, a laser beam has welded the casing and speedometer
cover together – without any screws, clamps or glues whatsoever. The
result is a perfect weld seam scarcely visible to the naked eye. There
are no sparks or particles flying through the air during welding. What’s
more: the resulting heat is confined to a minimal area. This protects
the material. Many industries have now turned to welding plastics with a
laser.
Still,
the technology has its limits; when it comes to fusing two plastic
components together, for instance, there is little freedom of choice. Up
until now, the upper joining part had to be transparent to permit the
laser to shine through unimpeded while the lower joining part absorbed
the radiation. This usually meant soot particles had to be blended into
the plastic. These particles absorb the energy of the laser beam and
transmit the fusion heat generated to the upper joining part.
“Up
until now, you usually had to choose a single plastic combination:
transparent and black. There are lots of applications – in medical
technology, for instance – where what’s needed is a combination of two
transparent plastics,” explains Dr.-Ing. Alexander Olowinsky, project
manager at the Fraunhofer Institute for Laser Technology ILT in Aachen,
Germany. The researcher and his team have now managed to erase the
previous boundaries of laser welding.
“The
industry now also makes infrared absorbers that are nearly transparent,
but these are not only very expensive but also have a green, yellowish
tint to them,” Olowinsky elaborates. “So our goal was to find a way to
get the job done completely free of absorber materials.”
To
accomplish this, researchers studied the absorption spectra of a range
of transparent polymers in search of wavelength ranges within which
plastic absorbs laser radiation. Then the scientists tested and
perfected the laser systems to match: systems that emit light of the
right wavelengths.
“Before,
you didn’t have the right light source,” Olowinsky adds. “It was only
during the past few years that laser sources have been developed that
emit light in these wavelength ranges.”
To
deliver the light energy to the joining level – to the seam along the
border between the two transparent plastics – the experts at ILT came up
with special lens systems. These systems focus the beam so that the
highest energy density occurs at the beam waist – where the beam
diameter is the smallest – so that the highest temperature is delivered
precisely to the joining level. The researchers’ most promising results
were achieved at a wavelength of around 1700 nanometers.
“This
is the peak welding-efficiency range,” Olowinsky summarizes.
Nevertheless, the researchers are also continuing work on the EU
Commission-sponsored “PolyBright” project (www.polybright.eu)
in search of the combination of the right absorption bands with the
matching light sources. “The result has to be the most cost-effective
laser system possible that can execute high-precision welding tasks at
the highest possible speed.”
Medical
technology and bioanalytics in particular are among the main
beneficiaries of the new welding process: The magic word is “lab on a
chip.” This refers to automatic, miniature-sized laboratory analysis on
the surface of a chip. Whether fluids, protein or DNA analyses – the
spectrum of applications is a broad one.
