Stencil printing of adhesive-based fuel cell sealings: The influence of rheology on bubble formation during the separation step

Abstract

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.