Application - welding
Plastic welding using a laser beam in three-dimensional joiningspace
Brief introduction
Laser transmission welding is a relatively new joining process for plastics. Due to the special process requirements, namely the good physical contact between parts during the welding process, this technology was, until now, limited to a two-dimensional joint seams. The new joining method presented here works according to the principle of contour welding and is designed to weld complex components with a three-dimensional joint seam. The concept – to selectively, dynamically and precisely apply the necessary contact force at the desired joining site using a glass sphere while welding with a laser beam – offers completely new process possibilities. The expansion of the application spectrum through this new technology is illustrated by potential applications, both in the automobile industry and the technical textile industry.
Theultimate technical hurdle
Following a long period of dominance of ultrasonic, vibration and hot plate welding in joining technology, laser welding has, in recent years, grown in importance as a complementary joining process well-suited for plastics, and has been successfully introduced in many industrial application areas. Laser transmission welding offers an attractive alternative where conventional technologies reach their limits, by virtue of its cost-effectiveness and particularly its well controllable local transfer of energy and low mechanical stress exerted on the joining partners.
The plastics industry is very clearly characterized by its extreme diversity and fields of application extending across a wide range of sectors. As distinct from metal processing, where beam delivery and energy dosage are of paramount importance, the laser welding technique for plastics is strongly dependent on product design, process requirements and the materials used, as well as the appropriate beam delivery for the particular task. The innovative potential of this new welding technology is already evident from the constant stream of process concepts developed and implemented in practical applications. The concepts currently pursued are contour welding, simultaneous welding, quasi-simultaneous welding, as well as mask welding established three years ago.
Despite all these new process developments, neither the laser welding technique nor the other conventional joining processes have managed to overcome the ultimate technical barrier, that of process independence. The vibration, ultrasonic and laser transmission techniques with all their established process concepts have, until now, been limited to welding components with two-dimensional seam geometries as a consequence of their different technical requirements and functional principles. Industrial applications, however, usually feature a three-dimensional joining seam. A three-dimensional laser-based joining technique is therefore very much sought after in many industrialsectors.
Problemsencountered in laser transmission welding
The process requirements for physical contact arise from the basic principle of laser transmission welding. Once the parts to be joined have been brought into contact, the laser beam penetrates the transparent joining component without heating it significantly. The beam energy is transformed into heat by the absorbent joining part, plasticizing the material at this point. The transparent joining part is also melted by thermal conduction as a result of the physical contact. An impermeable weld is produced between the two joining partners in the joining zone by the pressure applied.
The external contact pressure applied is firstly required to compensate for the expansion in volume when heating plastics and to avoid material deformation. Secondly, this contact pressure serves to achieve the most uninterrupted contact possible between the plastic components in the joining zone and to avoid the formation of cavities caused by plastic volume shrinkage on cooling of the welded components.
Good welding quality therefore not only depends on the interaction between the laser beam and the plastic material or the regulation of heat energy required for plastification, but also on the physical contact, which satisfies the basic requirement of the transmission welding principle. This physical contact is created statically by use of a clamping system. Good clamping conditions can be achieved relatively easily by mechanical means for two-dimensional welding contours. However, as soon as there is a three-dimensional welding contour, static contact between the two joining planes along the entire welding contour is technically almost unachievable.
This is the reason why laser transmission welding for plastics was, until now, always limited to two-dimensional joining planes. The maximum area to be processed using this technique is also limited by the need for suitable clamping systems. The design of conventional clamping systems for the large-area components found in the technical textile industry is stretched to its technical limits. Consequently, laser transmission welding remains applicable for small joining componentsonly.
Newsphere welding concept for three-dimensional applications
A completely new contact pressure concept was introduced to eliminate the technical limitations encountered in the use of clamping systems and to facilitate the transformation of laser welding technology for plastics in real three-dimensional joining space.
Figure 1: The principle of the sphere welding technique
(Source: LEISTER Process Technologies,CH-6060 Sarnen)
The sphere welding concept essentially works on the contour welding principle, whereby the laser spot follows a contour and the component is sequentially welded. In the new concept, a laser spot is focused on the joining plane by means of an air bearing, frictionless, rotating glass sphere as shown in Figure 1. The glass sphere lens serves as a mechanical pressing tool applied perpendicular at each point on the joining plane. This ensures that the laser beam is only incident where the contact pressure is applied.
This process concept offers the possibility of applying the necessary contact pressure concomitant with the laser beam continuously along a welding contour where welding takes place and where the contact pressure is actually required. 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. The focal plane of the laser beam and the welding seam width can be set and adjusted according to the components processed. The contour motion of this processing head is typically controlled with the aid of a 6-axis robot (see Figure 2). The process head positioning and motion contour are matched and programmed for each component according to the delivery of the laser beam and the contact pressure.
Since the relative motion between the processing head and workpiece to be welded 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 thecomponent.
Figure 2: The conour motion with a 6-axis robot system
(Source: LEISTER Process Technologies,CH-6060 Sarnen)
Processmonitoring
This sphere welding concept can be realized in practice with a variety of engineering processes. The positioning of the processing head perpendicular to the joining plane is another core functional principle of this sphere-welding concept. The reason is that the contact point must always be on the optical system axis to ensure that it coincides with the laser radiation zone. Good welding quality can only be achieved if the welding process takes place entirely under contact pressure.
The sphere welding concept offers all the established options for process monitoring available in the conventional laser welding technique. The surface temperature on the joining plane can be monitored directly using an infrared sensor integrated in the processing head as part of the active welding process monitoring. Laser power is regulated online based on the temperature measured. The constant joining temperature can, however, also be achieved through passive control methods. Speed can be measured and the laser power required can be continuously adjusted by the control system to ensure constant energy dosage.
Through motorized displacement of optical components, online modification of the welding line width can be achieved. The required radiant energy density can also be maintained with high precision through adjustment of the laser power. The welding contour can therefore be completely programmed for each component. The joining process can also be visualized online directly beneath the glass sphere by means of a camera integrated in the optical system. The information can also be used to assess the welding seam quality should this berequired.
Sampleapplications
The sphere welding method presents a new process variant for three-dimensional laser plastic welding. By virtue of its precise and controllable energy delivery, this process also produces an optically perfect welding seam, which is of crucial importance in the manufacture of decorative components. A typical example for the application of this method is the welding process in the manufacture of automobile head or rear lights, which require a three-dimensional welding seam geometry and are mass-produced in the automobile industry. The trend towards aerodynamic design is here to stay. This often means that the parts are designed with profile curvature in two dimensions. Another trend is the use of highly transparent plastic materials. The welding seam remains visible as decoration. The decorative effect has thus become a decisive criterion for such products.
A joining method is also required in the technical textile industry or paper industry, where products with gigantic dimensions of 10x50m² are manufactured, which not only fulfils the demands imposed by the processing dimensions presented, but also ensures the option of high production throughput. As these processing dimensions do not permit a static clamping system, the sphere welding method reveals its enormous potential through its flexibility of motion and effective contact pressureconcept.
Figure 3: Application example - Welding automobile rear lights
(Source: LEISTER Process Technologies,CH-6060 Sarnen)
Figure 4: Application example- Gas analysis
chamber without use of a complex clamping device
(Source: LEISTER Process Technologies,CH-6060 Sarnen)
The laser welding method is developing into an important joining technique in plastics processing. The diverse fields of application continue to call for new techniques and innovative problem solving approaches. Following the introduction of various laser transmission welding methods as complementary process options alongside the conventional joining methods used in industrial series production, such as ultrasound or vibration welding, the innovative potential of laser transmission welding has yet to be fullyexploited.
In the sphere welding concept, the clamping mechanism is elegantly integrated into the processing head. This has made it possible to extend conventional laser plastic welding from a two-dimensional to a real three-dimensional sealing technology. This technical advancement eliminates the difficulties previously encountered in this process and results in a major process benefit, namely significantly enhanced welding quality through the localized application of contact pressure. A technical solution is now available for three-dimensional joining structures, as well as large-area joining components. Above all, this technical accomplishment offers significantly greater freedom and flexibility of the welding system.
The Globo Welding concept is geared towards specific problems and is astounding in its simplicity. The laser joining methods already established will not be replaced, but they will be complemented by a new and interestingoption.
(Copyright © 2004 by LEISTERProcess Technologies, CH-6060 Sarnen
Author: Jie-Wei Chen, Leister Process Technologies)