Utilizing laser as a method to join plastic components is growing in
popularity. The ability to create clean, strong, and consistent hermetic seals
is very attractive to manufacturers. However, clamping the components to create
good physical contact between the parts (one of the necessary process
requirements) created challenges for assemblies that were beyond simple
Consequently, a new laser-joining method has been designed to assist in
welding three-dimensional components or large two dimensional assemblies. The
core concept is to utilize the optic element to both precisely deliver the laser
energy and to apply the necessary contact force, thus eliminating the need for a
vertical, mechanical clamping device. To accomplish this, a glass ball on a
frictionless air-bearing is put into position and pressure is applied via a
self-contained pneumatic slide.
Lasers have become an indispensable method for metal processing and have been
very popular in plastics, mainly in the areas of marking, cutting, and drilling.
Plastic assembly has long been dominated by ultrasonic welding, vibration
welding, and hot plate welding. Over the last decade, laser welding has gained
popularity as a complementary joining process for plastics and has been
successfully introduced in many industrial application areas.
Laser transmission welding offers an attractive alternative where
conventional plastic joining technologies reach their limits. The most common
concepts currently pursued are contour welding, simultaneous welding,
quasi-simultaneous welding, and the patented mask welding method. Despite all
these new process developments, the laser welding technique and other
conventional joining processes have not managed to overcome the ultimate
technical barrier, three-dimensional welding. Vibration, ultrasonic, and laser
transmission techniques, with all their established process concepts, have been
limited to welding components with two-dimensional seam geometries or
applications with slight curvature or contour. There remain many industrial
applications that require exterior tube sealing or elaborate three-dimensional
Laser transmission welding and its technical limitations
With respect to the nature of beam delivery, there is little difference between
the techniques used for metal and plastic welding. The essential difference in
the plastic welding approach is the through transmission IR concept.
Figure 1: The laser transmission welding
The necessity for physical contact in the plastic welding process arises from
the basic principle of laser transmission welding as shown in Figure 1. Once the
parts to be joined have been brought into contact, the laser beam penetrates
through the top transparent layer/component. The beam energy is transformed into
heat by the absorbent joining part, plasticizing the material at this point. The
transparent part is melted by thermal conduction as a result of the physical
contact with the absorbing layer. An impermeable weld is produced between the
two joining partners in the weld area.
External contact pressure is applied to achieve an uninterrupted contact
between the plastic components in the weld area. Good welding quality therefore
depends on the regulation of laser energy, the interaction between the laser
beam and the plastic material, and good physical contact. The standard method
used to create a physical contact between the two components is to incorporate a
clamping system, typically utilizing pneumatic cylinders to push the parts up
against a metal frame or glass plate. Good clamping conditions easily can be
achieved for smaller two-dimensional weld contours; however, larger
two-dimensional and three-dimensional welding contours make it difficult to
maintain static contact between the joining areas along the entire welding
Welding concept for three-dimensional joining applications
A new concept was recently introduced to eliminate the technical limitations
encountered in the use of clamping systems and to facilitate the use of laser
for three-dimensional joining. With this new process, the contact pressure
required for the joining process is constantly regulated to act dynamically,
selectively, perpendicularly, and precisely at the desired joining area.
The welding concept essentially works on the contour welding principle,
whereby the laser spot follows a contour and the component is sequentially
welded. A laser spot is focused on the joining plane by means of an air bearing,
frictionless, rotating glass sphere as shown in Figure 2.
|Figure 2: Schematic diagram of the welding
The glass sphere lens serves as a mechanical pressing tool applied
perpendicularly at each point on the joining plane. This ensures that the laser
beam is only incident at the site where the contact pressure is applied. This
process concept offers the possibility of applying the necessary contact
pressure concurrent with the laser beam being continuously moved along a welding
contour. The air bearing glass sphere lens is fitted in a robust and compact
processing head together with the optical fiber connector and other optical
systems and process monitoring sensors.
As with a standard diode laser, the light is emitted in a conical shape. The
focal distance between the two internal lenses can be adjusted to create the
desired weld width regardless of the thickness of the top component. The contour
motion of this processing head is typically controlled with the aid of a
six-axis robot (see Figure 3a). A pneumatic cylinder is integrated into the
laser head to accurately and consistently control the force applied between the
glass ball and the component. A second air supply is applied internally behind
the glass ball and the pressure is equalized.
Figure 3: (a) The contour motion with a
six-axis robotic system. (b) The two-step manufacturing process for
a rigid component using a robotic assembly system.
Since the relative motion between the processing head and work piece is
always subject to mechanical contact, the air bearing of the glass sphere lens
not only serves to protect the glass surface against mechanical damage, but the
moving glass sphere effectively avoids the risk of lateral shift of the
component. If the components cannot be nested sufficiently into a fixture that
would maintain precise alignment, a robot can be utilized to maintain relative
positioning during the weld process (see Figure 3b). Other techniques, such as
snap-fits or automated timed clamping elements, can be used to maintain
positioning and eliminate a spot welding step.
This new welding concept also is used for welding large, flat assemblies or
flexible sheets/fabrics utilizing, for example, an XY gantry system. Continuous
roll-to-roll applications also are possible.
Process implementation and process monitoring
The positioning of the processing head perpendicular to the joining plane is a
core functional requirement. Good welding quality only can be achieved if the
welding process takes place entirely under contact pressure.
Figure 4: (a) Process head mounted on pneumatic
slide. (b) Frictionless rolling glass sphere lens pressed on the
assembly. (c) Camera image through the glass sphere lens with the
optical object on the welding plane.
The laser head is mounted on a pneumatic slide. This design allows for easier
robot programming, using the stroke of the slide to act as a buffer and
compensating for slight part deviations (see Figure 4a). The new process offers
all the established options for process monitoring. The surface temperature on
the joining plane can be monitored using an infrared sensor integrated in the
processing head. Laser power is regulated online on the basis of the measured IR
to create a constant joining temperature. Some robots can receive a feedback
signal that allows for synchronized laser power adjustment. This is important
when welding assemblies that have weld patterns with a combination of high-speed
straight portions (requiring high laser power) and sharp or tight corners where
the robot has to slow down (requiring lower laser power). Adjusting the laser
power allows for a consistent weld pattern regardless of speed.
Lastly, the joining process can be visualized online directly beneath the
glass sphere by means of a camera integrated in the optical system (see Figure
4c). The information also can be used to assess the welding seam quality.
Laser welding is emerging as an important welding technique in plastics
processing. The diverse fields of application always call for new techniques and
innovative problem solving approaches. Though various laser transmission welding
techniques have been introduced that complement conventional joining methods,
the innovative potential of laser transmission welding has yet to be fully
By virtue of its precise and controllable application of the necessary
joining force, this process produces an optically perfect welding seam, which is
of crucial importance in the manufacture of decorative components. A typical
example is the manufacture of automobile headlights or tail lights, which
require a three-dimensional welding seam (see Figure 5).
Figure 5: Application example - Welding of
automobile tail lights. Patented.
Another trend is the use of highly transparent plastic materials. The welding
seams remain visible as decoration. The decorative effect has thus become a
decisive criterion for such products.
The method presented in this paper integrates the clamping mechanism into the
laser head, making it possible to extend conventional laser plastic welding from
small- to medium-sized two-dimensional applications to three-dimensional or
large two-dimensional assemblies. This technical advancement eliminates the
constraints associated with standard clamping methods and allows for enhanced
welding quality via a localized application of contact force.
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Leister Technologies, LLC is the North American sales and service center
for NOVOLAS™ laser plastic welding systems. Its facility in Itasca, Ill. houses
a laser laboratory to perform initial laser testing, parameter optimization, and
training. For additional information, visit
www.leisterlaser.com or call (630)