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.

Introduction
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 weld areas.

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.

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 contour.

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 technique.

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.

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.

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.

Conclusion
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 exploited.

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).

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.

References
[1] Korte, J.: Laserschweißen von Thermoplasten, Dissertation an der Universität – GH Paderborn (1998)

[2] Haberstroh, E.: Untersuchung prozessübergreifender Qualitätssicherungskonzepte beim Ultraschallschweißen unter Anwendung statischer Methoden; Abschlussbericht zum AiF-Forschungsvorhaben 11.372 N; Aachen (2000)

[3] Watson, M. N.: Welding Plastics: A Primer; Automotive Engineering, Heft 94 (1986) 12, S.55-61

[4] Pecha, E.: Trennung auf längere Sicht ausgeschlossen, Plastverarbeiter 35 (1984) 3, S.106-109

[5] Pütz, H., Hänsch, D., Treusch, H.G. und Pflueger, S.: Laser welding offers array of assembly advantages, In Modern Plastics, S.121-124 (1997)

[6] Chen, J.-W.: Laserfügeverfahren und Vorrichtungen zum Verbinden von verschiedenen Werkstücken aus Kunststoff oder Kunststoff mit anderen Materialien, Europäisches Patent, EP 0 997 261,1 Anmeldetag: 28.01.1999

[7] Chen, J.-W. und Zybko, J.: Laser assembly technology for planar microfluidic devices, Annual Technical Conference, May 2002, San Francisco, CA

[8] Chen, J.-W.: “Globo-Welding” - Verfahren zum dreidimensionalen Kunststoffschweissen, Wettbewerbsantrag für «Swiss Technology Award 2004»

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) 760-1000.