Quasi-simultaneous Welding Research Articles

Modelling and thermal simulation of absorber-free quasi-simultaneous laser welding of transparent plastics

The growing demands on the quality of plastic components and the trend towards miniaturisation are posing a great challenge on plastics processing technology. As many complex components can no longer be manufactured in a single step, joining processes such as laser transmission welding are gaining in importance. In classic laser transmission welding, the joining partners have different optical properties. The upper joining partner is transparent in the laser wavelength range, whilst the lower partner is absorbent due to the addition of absorber materials. In medical and biotechnological applications, the addition of absorber materials is often undesirable due to strict biocompatibility requirements. If, on the other hand, radiation sources are used which emit radiation in the area of the natural absorption of the plastic (λ = 1600–2000 nm), untreated transparent plastics can also be welded. In this work, a theoretical model will be presented to calculate the temperature distribution and progression during quasi-simultaneous welding using a thulium fibre laser (λ = 1940 nm). A sensitivity analysis is carried out to investigate the influence of different parameters on the heat affected zone (HAZ). The simulated HAZ is then compared with the HAZ from the experimental work.

Improvement of Energy Deposition in Absorber-free Laser Welding through Quasi-simultaneous Irradiation

Laser transmission welding is usually known to put little thermal stress on the joining partners, indicated by a small heat affected zone (HAZ). However, this only applies when the joining partners have adapted optical properties. When it comes to welding of optically equal thermoplastics without absorbers, the main issue is the HAZ extending far from the interface. To enable welding without absorbers, lasers emitting within the polymer's intrinsic absorption bands are used. So far, different beam shaping approaches have already been investigated to achieve a selective energy deposition at the interface but, with little success to date. The approach presented in this paper is irradiating the welding path quasi-simultaneously to exploit the poor heat conductivity of polymers. Therefore, the influence of the irradiation regime on the seam formation is considered in detail. Another aspect investigated is the length of the irradiated contour which is a crucial factor in quasi-simultaneous welding. The results show that the energy deposition can be significantly improved when the welding contour length does not exceed a critical length determined by the capability of the welding system. However, by welding in segments the approach can also be applied to longer contours without any noticeable loss in welding time. The ideal irradiation regime obtained in the trials corresponds to an effective welding speed of 37mm/s and reduces the vertical extent of the HAZ by 30%.

Advanced laser welding of high-performance thermoplastic composites

Thermoplastic composite structures based on continuous carbon fiber reinforcements are gaining importance in many industrial applications. These comprise basic technical functions as well as high-performance applications in the aerospace sector. Welding techniques are applicable for composites based on thermoplastic matrix materials in contrast to thermoset systems. In this context, welding steps are not limited to the joining of carbon fiber reinforced plastic (CFRP) parts among themselves, but extend to the connection of various components, consisting of unreinforced and glass fiber reinforced thermoplastics to CFRP components. In this work, a laser transmission welding process is evaluated with respect to the influence of the carbon fiber reinforcement within the laser absorbing part as well as the glass fiber reinforcement within the laser transparent part on the weld seam formation. Thermoplastic base material nylon (PA 6.6) and polyphenylene sulfide are used. By applying two different strategies, contour and quasisimultaneous welding, the influence of continuous fiber reinforced composites on the welding process is studied. Significant differences to the process characteristics known from the joining of unreinforced thermoplastics emerge from the fiber reinforcement inducing high thermal conductivity and fluctuating absorption properties for the laser wavelength, resulting in an essentially altered plastification performance which is directly mirrored in the inhomogeneous formation of the weld seam structure.

Evaluation of a Pyrometric-based Temperature Measuring Process for the Laser Transmission Welding

Abstract An on-axis pyrometric-based temperature measurement method suitable for different laser transmission welding techniques is presented. A temperature detection during contour and quasi-simultaneous welding is demonstrated to fit the requirements of laser transmission welding with part adapted temperature fields. For the experiments a pyrometer is connected to a scanner optic with a non-color-corrected lens. The experiments are performed with a high-speed pyrometer detecting in spectral range of 1.65 - 2.1 μm. Potential errors caused by the measurement method are evaluated and its limits depend on the welding technique performed are examined by creating different defects in the weld seam.

Experiments Regarding the Influence of Pressure Profiling on Laser-Transmission Welding

Laser transmission welding of thermoplastics is being increasingly used to join complex, assembly-oriented parts by virtue of the small heat-affected zone and the very wide and flexible range of application. High joining pressures on the one hand allow short cycle times, on the other hand, they lead to small remaining melt layers in the joining and thus to lower strengths of the weld. In this work, an experimental setup to realize a time-dependent joining pressure profiling is prototyped for quasi-simultaneous welding processes and experiments to show the influence of pressure profiling on weld strength are carried out with different materials. It is shown, that a pressure profiling can lead to higher weld strengths for some materials. Especially, pressure profiling for welding polypropylene semi finished parts can improve the welding quality.

Fibre laser welding for packaging of disposable polymeric microfluidic-biochips

An essential step in the development of microfluidic-biochips is represented by the assembly process. Among the thermal bonding processes used for the assembly of such devices the laser transmission welding of polymers offers several advantages, especially when it comes about local deposition of energy and minimum thermal distortion in the joining components. The research presented in this paper proposes a new approach for the laser transmission welding developed for the packaging of disposable polymeric microfluidic-biochips. The new approach based on the use of a fibre laser and a tailored method for the laser energy deposition was tested on the sealing of polymeric biochips made from plexiglas and polypropylene with a covering foil. This method combines the characteristics of the polymer contour welding and quasi-simultaneous welding and allows the achievement of contamination-free, high quality weld seams as narrow as 100 μm with a high dynamic making it suitable for the high volume production also.