Temperature-Controlled Filament Stretching During Micro-Extrusion of Highly Filled Polymer Suspensions
For successful metallization of silicon solar cells, a metal grid with fine lines below widths of 30 mm is printed onto the solar cell surface. Parallel dispensing is an upcoming technology which allows the extrusion of a highly filled polymer-suspension through up to 150 parallel micro-nozzles. In order to further increase process speeds under stable conditions, a careful adaptation of the suspension rheology is crucial. Besides the optimization of the suspension formulation, the rheology can be adjusted by changing the suspension temperature during printing. Therefore, this study presents an experimental evaluation of the maximal attainable process speed by adjustment of process temperature. It is found that an increase in temperature promotes dynamic filament stretching between the nozzle exit and the contact point on the substrate. This effect allows to adjust the relative velocity between print head and paste extrusion, resulting in an increased production throughput as well as in a shrinkage of printed lines due to the dynamic stretching of the filament. This shrinkage of printed structures does not require a further reduction of nozzle openings and therefore bypasses the challenge of increased clogging whenever nozzle diameters are further reduced in extrusion processes of highly filled suspensions. The rate of stretching is investigated for different polymer-solvent combinations, temperatures, process speeds and nozzle diameters. It was possible to achieve an increase in process speed of up to 56% to 430 mm/s compared to the reference process at room temperature, resulting in a reduced, printed line width of 25 mm. The presented data highlights how a systematic control of the process temperature in industrial micro-extrusion processes of polymer suspensions can not only improve the process stability and throughput-rates but also further reduce the dimension of printed structures beyond the conventional limit without filament stretching.