Injection moulding of graphite composite bipolar plates

SGL Technologies GmbH has been developing different moulding technologies for graphite composite bipolar plates (BPP) for PEM fuel cells since 1997. Injection moulding of polypropylene (PP) and phenolic (PF) bonded graphite compounds have been identified as most promising and cost-effective production processes.SGL Technologies has invested in injection moulding machines for moulding both thermoplastic and thermoset materials. In addition, materials and processes have been developed in order to mould highly filled compounds with a graphite filler content above 80 wt.% needed for the BPP application.The PP bonded compound ®Sigracet PPG86 can be mould with complicated flow field structures and manifold through holes in a single process step. The material and process can be considered production ready for the low temperature (T ≤ 80 °C) PEMFC application.The PF bonded compound ®Sigracet BBP4 shows improved electrical conductivity and temperature stability over PPG86. It has been so far available as compression moulded plates only. However, our recent developments show that BBP4 can be injection moulded without major change of the attractive properties in comparison to the compression moulded plates. Anyhow, the injection moulded BBP4 will remain more expensive than PPG86 due to more complicated processing and a longer cycle time.The pros and cons of the two materials and processes will be discussed in detail.