Plastics Weld­ing

Laser transmission welding enables a media-tight joint with low production costs and cycle times. In this process, a laser-transparent and -absorbing component are in contact with each other under pressure, while a laser beam passes through the transparent component with a low energy loss. The energy of the laser is converted into heat in the absorbing component. Due to the contact in the joining zone, the transparent component is heated by heat conduction and the weld is formed.

Scientific investigations to date have mainly targeted small weld seams. Here, the correlation between the individual process parameters of the weld temperature and the mechanical properties of the weld seam can so far be well characterised on the basis of the energy per unit length. However, these findings can only be applied to contour welding, as it is well known that other influencing variables (weld length and width) act in quasi-simultaneous and simultaneous welding. Due to the simultaneous melting process, a squeeze flow occurs in the complete joining zone. The resulting energy loss is not taken into account in the energy per unit length and thus loses its significance. The aim of this research project is to develop scale-up rules for quasi-simultaneous welding that make it possible to transfer optimal process parameters to real components by means of simple laboratory tests.

As part of the cooperation project with the Chair of Plastics at Chemnitz University of Technology, which was completed in 2021, the fundamentals of hot gas butt welding were systematically developed. The central point here was the heating behaviour and the resulting mechanical properties of the welded joints. Local measurements of the melt layer thickness enabled an analysis of the melt layer profile on the joining surface. An analysis of the weld samples was carried out both optically and mechanically. In addition, the determination of the influence of the process gas used (nitrogen and air) on the weld seam quality was a core part of the research project. The aim was to clarify what influences the type of gas has on the short-term mechanical properties and to what extent thermal damage affects the resulting weld seams.

The investigations carried out have shown that the nozzle geometry as well as its heating parameters have a decisive influence on the resulting melt profiles and thus also on the weld seam strengths. While the round nozzles introduce the heat very selectively into the joining plane, a homogeneous heating over the entire joining surface can be achieved with the slot nozzle. This means that a heating strategy adapted to the nozzle system used is necessary. With the slot nozzle, it was possible to realise a more material-friendly heating of the joined parts with low gas temperatures. This was advantageous for a non-heat-stabilised PA66, where similarly good weld strengths could be achieved with the slot nozzle using the process gas air as when welding with the round nozzle tool and nitrogen. Furthermore, it could be shown that a realistic test specimen can also be welded using slot nozzle geometry. Furthermore, the influences of the gas type and the joining direction were analysed. Here it was shown that when welding polypropylene, high weld strengths can be achieved regardless of the type of process gas. When welding polyamides, on the other hand, higher weld strengths with lower standard deviations can be achieved when using nitrogen as the process gas with the round nozzle tool.

Efficient lightweight construction is a goal that is strived for in almost all developments in mechanical engineering these days. But how durable are such lightweight solutions in active use? This question is being worked out in a cooperative project within the framework of a funded IGF project on infrared welding of thermoplastic components. Specifically, the research centres are pursuing the goal of joining glass fibre-reinforced plastics using infrared welding processes and determining their cyclic load capacity. This gives rise to two central research questions:

1.     How do the process parameters influence the morphology of the weld?
2.     How does the morphology of the weld seam influence its fatigue strength?

Within the scope of the project, a deep micromechanical understanding of the joining zone in relation to the process parameters must be established. For this purpose, the parameters are to be varied in various experimental investigations in order to examine the resulting weld seam morphologies. In addition, industrially relevant flaws will be induced in order to determine the effects of these flaws on the long-term behaviour. Various laboratory analyses are planned for the investigation of the weld morphology, which will provide different findings. For the assessment of the fatigue strength, Wöhler tests are planned. The primary objectives are as follows:

•     Description of the joining zone morphology in relation to the process parameters.
•     Improved micromechanical understanding of the joining zone
•     To establish a correlation between material condition and fatigue strength

Two materials are to be investigated and characterised in the research project. This should ultimately provide a basis for evaluating the fatigue strength in cyclically loaded thermoplastic components as a function of the process parameters.

In the automobile, metallic components are increasingly being substituted by those made of plastic in order to achieve efficient lightweight construction and reduce CO2 emissions. The need to save weight exists both for vehicles with conventional internal combustion engines and for those with electric motor drives, as the weight saving in both variants ensures significantly lower rolling, acceleration and gradient resistance. Due to increasing component complexity, the plastic components used for this purpose can rarely be manufactured in one process step and must increasingly be welded. Both vehicles with conventional combustion engines and those powered by electric motors have identical requirements for component and weld seam quality. In addition to component tightness and high weld seam strength, this includes above all the temperature resistance of the materials, as these are exposed to high thermal loads in the automobile. Consequently, high-temperature resistant thermoplastics such as polyphtalamide (PPA) or polyphenylene sulphide (PPS) are increasingly being used.

Due to the high melting points of these thermoplastics, problems often occur in current series production with multi-stage welding processes as a result of the changeover process. The changeover process causes the previously heated joining zones to cool down. Furthermore, a high-strength, long-term joint connection of the individual components is the basic prerequisite for the use of components made of high-temperature thermoplastics in the automobile. However, a statement about the long-term properties of these materials is currently not possible, or only possible through time-consuming preliminary investigations. Therefore, a service life prediction model for welded high-temperature resistant components is to be developed within the scope of the project.