With the challenge of decarbonising mobility being set forth as a green growth strategy in conjunction with attaining carbon neutrality by 2050, there are particularly high expectations for fuel cell (FC) vehicles. Fuel cell stacks, which act as the power supply unit, are being developed with higher power densities, made possible by dramatic technological innovations such as the high-precision flow channels of the thin metal sheets called bipolar plates, several hundred of which are mounted in the FC stack. Higher performance of FC stack elements, such as the bipolar plate and membrane electrode assembly (MEA), is a factor in improved power output density. However, the bipolar plate must have three main features, namely:
- complete separation of hydrogen and air flowing on the front and back surfaces;
- fine and complex channel shapes to increase the contact area with hydrogen and air; and
- thin cells to increase the overall unit density.
However, in conventional press moulding, the thickness of the plate is not uniform due to insufficient machining accuracy of the mould, resulting in extremely thin parts. This can lead to holes in the plate and the mixing of hydrogen and air. Therefore, the bipolar plate requires a high-precision mould that can press the fine channel shape and plate thickness uniformly.
This article looks at achieving a high-precision bipolar plate mould shape using the UVM high-precision machining centre from Shibaura Machine Co. (Shibaura), shown in figure 2.
Figure 1
Machining technology for higher accuracy
Two main evaluation criteria are used for accuracy in high-precision machining, these being form accuracy and surface quality (roughness). Using 3D CAD/CAM software to ensure high calculation accuracy and a high-speed rotating spindle to minimise chip formation is not sufficient for achieving the required bipolar plate mould quality; so Shibaura Machine developed a machining technology that allows for tool path compensation, namely utilising vector information on the cut location to fix a tool shape error.
VectriComp tool path vector compensation software
The high-precision machining process generally consists of four tool checking steps, namely:
- model creation in CAD;
- machining path generation in CAM;
- actual machining; and
- measurement analysis.
These tool checking steps are key for improving machining quality. Many kinds of tool measuring devices are installed in conventional machining centres to check the position and diameter of rotating tools, but they only check if an ideal shape can be defined for tools. This means the error of tool contour is directly transferred to the machined workpiece as a form error component. To solve this problem, Shibaura has developed software called VectriComp that affords an error compensation function. The software can create a vector information-added tool path by integrating CAD data to identify the tool location on the workpiece. This process, known as vector compensation, automatically corrects the commanded tool position coordinate values on the machine as shown in figure 3, thus minimising the effort needed to achieve good shape quality. In contrast, on conventional machining centres, a change of tool position coordinate values necessitates re-calculation of the 3D model, meaning a lot of time is lost. VectriComp enables significant reductions in loss of production time from non-machining processes.
Figure 2
FormEye tool shape detector
Although vector compensation affords significant benefits, highly accurate tool contour information is still missing. Shibaura therefore investigated the tool shape detecting method as a more accurate and efficient means of measuring than conventional methods such as line sensing and standalone types of image capture. As a result, the company developed the FormEye tool shape detecting system, which involved brushing-up the image processing software and integrating synchronised capture function into the aerostatic spindle manufactured in-house. This system can identify which direction a milling tool is facing during high-speed rotation, making it possible to complete the measurement using a minimum number of images. Furthermore, minimising the number of images enables faster processing and gives detailed tool shape information such as radius, roundness, tool run-out, worn value, etc. The measurement results for a tool contour are shown in figure 4. They show that even a brand-new tool has an error of about 4 μm from the ideal round shape, which directly reflected on a machining error.
Figure 3
Machining verification
Shibaura undertook machining verification of the form of a bipolar plate using VectriComp and FormEye, with the aim of improving accuracy. The 3D model and cross-sectional view of the bipolar plate form for machining verification are shown in figure 5. The workpiece material was stainless steel for moulds (HRC52) and the tool was a cubic boron nitride (CBN) ball end mill with a radius of 0.5 mm. The contour accuracy of the ball end mill and the tool compensation amount obtained by FormEye are shown in figure 6.
Figures 4 and 5
When machining without vector compensation, the form accuracy is 2.5 µm + α (effect of wear or disturbance) because of the detected tool contour. In contrast, machining with vector compensation, resulted in improvement of the overcutting phenomenon seen without vector compensation and therefore a form with almost no error relative to the design form. A comparison of the results is shown in figure 7. The form accuracy was -0.6 to +0.7 µm (PV: 1.3 µm), which is less than ±1 µm, this being what is required for bipolar plate moulds.
Conclusion
This paper describes a single machining technology development for high-precision bipolar plates. The performances of many other industrial products are improving at a remarkable pace, and their parts are becoming increasingly miniaturised and precise. Consequently, more and more markets will be requiring nanofabrication technology, and there will be more areas where conventional manufacturing processes cannot be adapted, so the production process itself needs to be examined beyond the existing framework. Shibaura will continue to identify needs from new perspectives and strive to expand the application scope of nanofabrication technology for manufacturing.
Shibaura Machine Co.