The widespread deployment of fuel cell technology requires the development of new manufacturing technologies to turn it economically viable. Stencil printing is known as one of the highest throughput techniques for applying adhesives, where a moving squeegee forces the adhesive through pre-defined apertures in a stencil onto a substrate. Thus, stencil printing is investigated as an innovative method to reduce production costs and manufacturing cycle times of fuel cell sealings. Moreover, in order to allow a greater design freedom, adhesive layer thicknesses up to 500 µm should be printable under reproducible conditions and within cycle times <3 s, which have not been realized or implemented until today. With the aim of printing closed-loop structures (sealings), a mesh located on the upper region of the aperture needs to be integrated. However, this mesh can produce additional air bubbles and surface irregularities, which might affect the sealing performance and diminish the process stability. This paper describes the experimentally identified formation mechanisms of bubbles during the separation step. The quantity and size of these bubbles were measured for five separation speeds, two mesh opening sizes and three adhesive systems (UV-curable acrylic). Primary focus was placed on relating the print results with the adhesive rheological properties, which are decisive to successfully implement the sealing application process. It was found that a previously derived empirical relationship to predict the adhesive tendency to stretch filaments can be employed as a model to quantitatively assess the characteristics of bubbles that emerge during the separation step and thus specify rheological properties to enhance print quality.
Cold-joined connectors (CJC) are widely used, owing to the simplicity of the assembly process, the low initial investment, and the possibility to join several contacts in parallel. Due to the fastgrowing number of high power applications in the automotive industry, increasing efforts are made in order to make these advantages of the cold joining technology (CJT) suitable for high currents. New finite element method (FEM) models are still developed for supporting the new design cycle. The preferred technology to produce the assembly parts for high current CJC is die-cutting. Two punched edges will come in contact during the joining process to form a high power connector (HPC). Therefore, accurately describing the material properties at punched edges is crucial for a correct understanding and finite element (FE) modeling of the cold-joining process (CJP). Diecutting causes a strong strain hardening in the contact zone and changes the material properties significantly. Thus, the material properties of the bulk material cannot be assumed at punched edges for the FE-simulation. This paper presents a new method for an improved material description of punched edges. The hardening mechanisms caused by die-cutting are studied by means of electron backscatter diffraction (EBSD) and quantified with nanoindentation experiments. The hardness distributions at punched edges are correlated to flow curves, building a new inhomogeneous material law for the cold-joining simulation. The new material description is validated by means of a small-punch test on a real assembly part and shows a significant improvement of the component's material characterization.
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