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Robot-based Plastic Hybrid Welding

Stäubli Robotics

Metal and glass automobile headlight and taillight assemblies are yesterday’s technology. Plastic is the modern material allowing the new trends to be set in car design. Now another design enabling technology opens up even more freedom for the design and size of automobile lights; the turnkey hybrid welding machine. It has the ability to join large 3-Dimensional assemblies with the integrated Stäubli TX90L industrial robot, which handles the energy inputs and provides a flexible clamping tool for the innovative welding process.
 
What would modern automobiles be like today without the ever more cleverly designed plastic lighting assemblies? Unknown to the end users, the designers were limited until now in their choice of size and shape by the production processes that were available. Stress-free welding with the conventional vibration or hot plate welding methods was only possible with workpieces up to a certain size and with relatively flat geometries. As if this was not restrictive enough, irregular welding seams had to be expensively hidden from view by the black 2-K edges of the transparent PMMA lens.
 
Laser contour welding brushes aside the restrictions of traditional plastic welding techniques, and is therefore ideal for welding today’s standard light housings made, for instance from ABS or an PC/ABS blend, with transparent PMMA lens. Laser beams produce high quality 3D welding seams with the additional benefit of only exposing the workpieces to very low mechanical stresses. The secret of laser welding is the very precise application of the energy – a technique that LPKF and its partners have mastered. LPKF Laser & Electronics AG in Erlangen in cooperation with the Bavarian Laser Center developed the robot-based laser hybrid welding method which has been successfully tested since 2005 in several prototypes. This patented method has also proved its credentials in volume production; the very large tail lights of the Hyundai Equus have been welded using the laser hybrid welding method for some time now.
 
The latest development of this technology with the integrated Stäubli TX60 six-axis robot was showcased at Fakuma 2009. In its latest series solution for light production, LPKF now uses the elegantly housed six-axis TX90L and a rotary table. Thanks to these innovative features, the fully-automatic LPKF TwinWeld3D welding cell boasts the potential to revolutionize plastic welding. Short processing times, slashed material and tool costs, no time-consuming annealing procedure, perfect looks and new design freedoms all highlight the potential of this innovative technology.
 
Laser welding in a thermal field
 
Unlike pure laser welding, the heat energy in hybrid welding is not only applied in the underlying, laser-absorbing component, but also in the transparent upper plastic component. This is done by pairing the monochromatic laser beam, which cannot heat the transparent joining partner directly, with long wave polychromatic halogen light. This enables selection during the welding process itself of the optimum temperature for the material-specific melting points of the two parts being welded together. The marriage of these energy sources enhances the process speed, and guarantees extremely good looking and stress-free welding seams. The more uniform heating up and cooling down of the two parts being
joined means that the annealing essential to release stress in other plastic welding methods can be completely dispensed with here.
 
All these features yield crucial advantages in assembling lighting systems for automobile subcontractors in particular. These, and the new ability to weld even very large sized plastic lights with bold free-form surfaces, were probably the reasons behind the orders for the first three turnkey LPKF TwinWeld3D systems.
 
Laser, halogen light and mechanical joining force all-in-one
 
The Stäubli TX90L robot controls operations inside the air-conditioned hybrid welding cell with a rotary table. Controlled by its precisely guided hand, the laser beam and the halogen light meet on the workpiece. While the robot tracks the contours of the workpiece, the integrated pneumatically-sprung tensioning roller applies forces of up to 300N to ensure a precise positively locking fit between the two plastic parts being joined. The result is a stress-free, precise and slim welding seam with no fraying at the edges, so good in fact that it is welcomed as a quasi-decorative line in the visible part of the component!
 
“Realizing hybrid welding is only possible with the flexibility provided by a six-axis robot with extremely high path accuracy,” explains LPKF Product Manager Manuel Sieben. “There are many reasons why we chose the Stäubli from the many robots on the market: at ±0.035 millimeters repeatability, the TX90L boasts much better path repetition accuracy than other robots, and is also very compact with a reach of 1200 millimeters even in the “L model” despite the specified payload capacity. The open VAL3 control system also satisfies our stipulations for integration in the cell. This enables us to supply our clients with an all-round highly user-friendly system.”

Sixth axis for pneumatically-sprung tensioning roller
 
The high rated payload of almost 15 kilograms specified for the machine only seems amazing on paper.  One glance at the robot itself in the welding cell reveals all, in addition to the high pressing force applied by the tensioning roller, the robot also has to shoulder the entire welding system. The laser energy source with its complex high-performance cooling set up (based on the Peltier effect) also has to be carried to dispense with long fiber-optic cables and all the associated disadvantages.
 
“This kind of machine concept would have been impossible only two years ago because back then the cooling alone of a comparable laser was still the size of a 19” rack,” said Manuel Sieben. “A customized wrist created by the Stäubli development engineers in Bayreuth made it possible to mount a complex welding head with a circular halogen source onto the fifth axis instead of a co-rotating housing. This and the laser beam which shines through the center are imaged at infinity via a complex, gold-coated reflector. The two radiation sources are then focused onto one point by a shared lens. These are the new and very sophisticated features of this system.”
 
The tensioning roller needed for the laser welding technique is also located in the hand of the Stäubli robot on its sixth axis and generates the vertically acting clamping force on the components. This is made up of a toothed wheel with planet gears that drive a pneumatically-sprung gear rim with an arm for the cooled clamping roller. The roller is decoupled from the rotary movements of the welding head. The integrated pneumatic-springing guarantees the gentle surface-protecting guidance of the roller – even where there are large component tolerances.
 
Integrated quality assurance
 
The innovative process monitoring system measures the height of the components during the welding process, even as it welds at speeds of 80–150 mm/s. This measurement allows tolerances in each of the components to be compensated by influencing the control parameters. The quality assurance system is based on the reflection properties of the welding seam, and is also integrated within the welding head.
 
Manuel Sieben stated, “A large number of mathematical calculations are carried out during the welding process to control the power output of the laser and the halogen light dependent on the welding speed. A PLC with interfaces to the PC control acts as the master that actuates all the movements. This communicates with the Stäubli CS8C robot control which is VAL3-based, and provides significant programming benefits. The results are stable processes with very high levels of user-friendliness via the system touch panel. This gives a top view of the product enabling operators to very quickly re-teach the robot path points – as can happen fairly frequently depending on the amount of dimensional deviation of each batch of plastic components. The Stäubli manual controller can also be used if necessary. We are highly satisfied, and will market this innovative cell exclusively with Stäubli robots.”
 
Robot flexibility slashes processing times and costs
 
Unlike other plastic welding methods, this system needs no upper die because the whole welding head, including tensioning roller, is integrated with the robot. The only component-specific tool costs are associated with the lower die which is constructed by LPKF as part of the engineering and design support activities. The speed and flexibility of the six-axis robot is therefore passed on to the entire welding cell. Together they set new benchmarks, with only around 30 seconds processing time to weld an average tail light, and completely saving the 30 minute annealing procedure.
 
The system delivers the wide range of automobile tail light versions per car model with usually two-part designs for the trunk and trunk lid on the left and right hand sides.  This is because the tracking of different geometries is a natural part of intrinsic robotic flexibility. The rest is dealt with by a fully automatic tool changer which swaps the tools within two minutes, using a quick-action system to change the two lower dies on the rotary table. Further automation by linking up the turnkey welding system via a conveyer belt system is already incorporated conceptually.
 
The laser specialists from Erlangen are charting a very promising course with the hybrid welding method and the innovative plant concept which integrates the complete state-of-the-art welding technology package within the robot. The almost completely unrestricted design of automobile tail lights made possible in this way, which can even be fitted with LEDs before welding, may well influence the design of the whole vehicle in future.
 
For more information please contact:
Stäubli Robotics (Germany)
Sonja Koban
Head of Marketing
+49 (0)921 88 32 12
 
About Stäubli: Textile Machinery, Connectors and Robotics
Stäubli is a mechatronics solution provider with three dedicated divisions: textile machinery, connectors and robotics. With a workforce of 4000, Stäubli has a presence in 25 countries and agents in 50 countries around the world.
 

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