High-precision bipolar plates by injection compression molding with dynamic mold temperature control

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In order to utilize the urgently needed cost and lifetime advantages of polymers compared to metallic- materials for the application of bipolar plates, high filler contents are necessary to achieve sufficiently high electrical conductivity. However, thermoplastic melts modified in this way show deteriorated replication and mold filling behavior due to significantly increased viscosity and the negative effect of an increased melt thermal conductivity. This results in limitations in the possible thickness dimensions and the molding of fine channel structures of current bipolar plate geometries in the injection molding process. Since the output voltage per cell is only 1.23 V max., several cells connected in series are required for higher voltages, which also results in considerably larger installation space volumes and costs for thicker bipolar plates made of polymer compound. The desired reduction in plate thickness can only be achieved to a limited extent by higher injection speeds and mold temperatures in the injection molding process, since the reduced flow cross-section results in excessive pressure requirements during filling and inhomogeneous pressure and temperature fields along the flow path, irrespective of the type of mold temperature control. At component level, this is reflected in a deterioration of the flow path-dependent dimensional accuracy and molding precision of the channel structures. In addition to replication problems, highly filled polymer components also exhibit a pronounced direction-dependent mechanical and electrical component behavior, which is largely determined by the temperature-pressure-shear conditions in the process. The aim of the project is therefore to transfer the knowledge of the analytical and experimental design of the required time-temperature-pressure-shear conditions in injection compression molding with dynamic mold temperature control to manufacture thin-walled and highly filled bipolar plates with the highest possible dimensional stability and molding accuracy of the channel structures with simultaneously optimized electrical and mechanical properties. In this context, the flowability of the polymer melt is to be maintained during the molding process by means of dynamic mold temperature control. The combination of this modern process strategy with an embossing process allows a two-dimensional holding pressure effect and the realization of high plate aspect ratios while at the same time achieving the highest possible dimensional accuracy and replication quality of the channel structures over the flow path. By controlling temperature and pressure and indirectly influencing shear in the process, the existing knowledge of how to influence electrical and mechanical component properties can also be translated into the manufacturing process of injection compression molding with dynamic mold temperature control to maximize the material potential.