Laser transmission welding is a well-known joining technology for thermoplastics. Because of the needs of lightweight, cost effective and green production nowadays injection molded parts usually have to be welded. These parts are made out of semi-crystalline thermoplastics which are filled to a high amount with glass fibers. This leads to higher absorption and more scattering within the upper joining partner and hasa negative influence onto the welding process. Here a ray tracing model capable of considering every single glass fiber is introduced. Hence spatially not equally distributed glass fibers can be taken into account. Therefore the model is able to calculate in detail the welding laser intensity distribution after transmission through the upper joining partner. Data gained by numerical simulation is compared to data obtained by laser radiation scattering experiments. Thus observed deviation is quantified and discussed.

Laser transmission welding is an established single-stage plastic joining process, which enables hermetically sealed joints under the workpiece surface. The process requires joining partners with proper degrees of transmission and absorption to the processing wavelength. For reaching a stable process an in-process quality assurance is very valuable. Current monitoring systems have a limited usage, as no quantitative information of the weld itself is obtained without its destruction. In medical and pharmaceutical applications a weld with leakage is e.g. unacceptable. The main objective of this paper is the presentation of the optical coherence tomography as a tool for the quality assurance in laser transmission welding. This approach enables the measurement of any residual gap, weld geometry, internal pores and leaks. The presented results show that this technique allows even the characterization of welds using joining partners with thicknesses of 2 mm or with glass fiber reinforcement levels of 30% per weight.(C) 2014 The Authors. Published by Elsevier B.V.

In near future the use of plastics in the automotive industry will continue to grow strongly because of the increasing use of lightweight construction. The range of applications of plastics already extends from smallest electronics to large structure components. The rising use of plastic parts causes a high demand for efficient technologies to join thermoplastics as well as hybrid structures made out of dissimilar lightweight materials. Therefore laser-based joining technologies are becoming increasingly important. In this paper, two approaches for the laser-based joining of large thermoplastic parts with three-dimensional weld contours and for the production of thermoplastic aluminum hybrids are presented. Both technologies are considered suitable for flexible and easy automation. However a big issue is still their small gap bridging capability. A promising approach to improve the gap bridging capability is a highly dynamic laser beam oscillation leading to quasi-simultaneous intensity distributions. In this paper a system technology designed for laser-based joining of three-dimensional parts performing such laser beam oscillation is introduced. Further experimental results of polyamide welding and polyamide aluminum hybrids are discussed.

Thermoplastics are present in a wide variety of industrial applications. Different welding techniques like vibration, resistance and laser welding are used to join these materials. Laser transmission welding is known for high flexibility and extraordinary possibilities of process automation. Today, there is rising interest in joining large thermoplastic parts with three-dimensional formed weld seams. To successfully obtain these seams, the parts have to be clamped in such a way that is nearly gap-free. Gaps could result in processing issues such as welding failures, poor achievable process speed and low weld seam strengths. To overcome these problems, a new laser transmission welding technique was developed to achieve part adapted temperature fields based on a combination of contour and quasi-simultaneous welding. In addition, the authors are investigating the effectiveness of this strategy with a thermal finite element model. This model focuses on the heat affected zone geometries, temperature gradients and simulated melt pool geometries depending on the parameters creating part adapted temperature fields dynamically. The results from the model were verified by experiments with a diode laser. Furthermore, process control strategies were developed, which can later be used to ensure a constant weld seam quality. The process control is carried out by an on-axis pyrometer, which allows online temperature detection within the weld seam.