Thin sheets for the hydrogen sector: production technologies for series manufacturing

To produce complex geometries and flow field structures on bipolar plates with high precision, toolmakers must manufacture forming tools using correspondingly precise machining processes. The dimensional accuracy of these tools is in the single-digit micrometer range and, thus, reaches the limits of manufacturing technology in toolmaking. At Fraunhofer IPT, we use high-precision micro-milling machines to produce mold inserts and structural components for precision forming tools.

In order to be able to use the expensive tools for as long as possible, we simulate and measure the tool wear and thereby improve the tool technology for forming tools for the production of bipolar plates. Our aim is also to integrate additional functions into the tool, for example heating technologies to improve forming quality or the integration of inline quality monitoring.

Handling of bipolar plates in a discrete process

For many processing steps in sheet metal working, for example cutting or joining, suitable intelligent fixture technology is essential to achieve optimum process results. This technology ensures that processes remain reliable and can be scaled to production needs. Fraunhofer IPT develops fixing jigs for various processing steps in thin sheet metal processing. These can clamp, position and handle the components. We take an integrated view of all components in order to implement even demanding machining processes with complex components and processes. For us, fixture technology is a subsystem of an interlinked production line in which we take tolerances and requirements of upstream and downstream process steps into account.

Handling bipolar plates in a continuous process

In the continuous production process, we convey the feed material with high precision in order to carry out the respective production steps in the desired quality. The most important control parameters are the web tension, the speed, and the positioning of the web. 

While we carry out web positioning in our roll-to-roll systems using standard systems, we regulate the web force and speed using our own control system. This has enabled us to achieve web tensions of up to 3000 N with high accuracy and consistency at a web speed of 5 m/min. The optimal web parameters are highly dependent on the individual manufacturing processes and are currently part of the research work being carried out at the Fraunhofer IPT.

Processing with the laser: Cutting and welding bipolar plates

We use lasers to cut and weld bipolar plates from thin sheet metal. We have mastered laser process control in both discrete and continuous production processes. While individual sheets can be clamped and processed one after the other in discrete production, a metal foil runs over rollers in continuous sheet metal processing. This allows the bipolar plates to be cut and welded directly one after the other. Both production processes are scalable and suitable for series production.

From titanium and stainless steel to aluminum and nickel – we can handle materials as thin as 0.02 mm and process them with the laser. Precise cuts and weld seams can be produced with a laser spot diameter of just a few micrometers.

Simulatively testing the tool design and predicting wear

Tool wear is an important cost factor in industrial series production. We have, therefore, developed a test bench which we use to examine the forming process and generate data to predict tool wear. In the simulations, the contact pressure and the sliding distance in the contact area are calculated. In this way, we can gain insight into the wear behavior during sheet metal forming and predict tool wear in detail. On this basis, process reliability can be increased and cost-efficient series production ensured.

Thanks to a modular tool design and variable process speeds, we can emulate various industrial processes and boundary conditions.

Cleaning bipolar plates

The metallic bipolar plates must be cleaned to ensure electrolyzers and fuel cells function perfectly and maintain high quality. At Fraunhofer IPT, we use ultrasonic cleaning to remove particles and films gently and effectively. We take the material properties of the bipolar plates into account and adjust the temperature, cleaning time and composition of the medium.

Our aim is to achieve a reproducible surface quality without damaging the sensitive structures of the bipolar plates. After cleaning, we dry the bipolar plates with hot air, leaving no corrosion or residue. Tailored packaging solution ensures that the bipolar plates can be stored temporarily and processed further without contamination. By measuring the water contact angle (WCA) or using relative fluorescence units (RFU), we can detect residues and assess the cleanliness of the bipolar plates.

This coordinated process chain, consisting of cleaning, drying and packaging, provides the technological basis for processing the workpieces reliably in subsequent processes such as functional coating or gasket application.

Apply sealing contour

The seals are applied directly to the bipolar plates. Compared to conventional insert seals, this direct application has the advantage that the seal adheres firmly to the bipolar plate and cannot slip during handling and assembly. Seals can be applied to metal bipolar plates using various methods. We select the most suitable process based on component size, gasket geometry and application rate. Frequently used application processes are screen printing and dispensing. In various process routes, we investigate if these processes are suitable for the materials, the process speeds and the accuracy of the applied gasket. Using screen printing, we have already applied 150 µm high silicone seals with low surface roughness to bipolar plates. Further investigations have shown that higher gaskets of over 400 µm can also be achieved with the screen printing process.

Leaking bipolar plates, which are used in electrolyzers and fuel cells, reduce performance and pose a safety risk. To avoid this, manufacturers carry out leak tests. At Fraunhofer IPT, we use a testing device that can determine leakage rates using a relative pressure method. We also use suitable devices to reliably test bipolar plates without their own seal: A frame tool provides the necessary contact pressure, while a sealing tool ensures that the test specimen is sealed off from the environment.