Fuel cells are claimed to be one of the technologies for the cars of the future. For us manufacturers getting ready for the type of machining it will require may well be essential for survival.
New drive systems
Drive systems that can be operated with electricity produced from renewable sources are becoming more and more important as an alternative to the internal combustion (IC) engines that are currently being impacted by international legislation and fossil fuel availability. In addition to the battery technology that is currently the focus of attention, fuel cell technology represents another possibility for the drive technology of e-vehicles. In view of the specific advantages such as sufficient supplies of the inputs hydrogen and oxygen, low-emission outputs, greater efficiency than IC engines and the possibility of fast refuelling, fuel cells are a serious option for vehicle drives.
What does this technology imply? New breakthroughs come in the face of new challenges and manufacturers are wondering what production resources they will need to meet the market’s demands. New car models such as the Mercedes GLC F-Cell are being launched and now coexist with more established cars such as the Toyota Mirai.
The basics
First, let's go back to the basics. What is a fuel cell car? This type of car uses an electrical motor connected to a battery. The electricity needed to run the car comes from a battery charged by a fuel cell.
In hydrogen fuel cells the electricity is generated when molecules of hydrogen and oxygen react, resulting in water (H2O), electrical and thermal energy. Bipolar plates constantly guide the reaction gases so they are evenly distributed in a specific structure over the complete active face of the catalyst layer. As a single cell only generates approx. 0.7 V a stack of 250-1,000 bipolar plates is necessary to power just one car.
There are several types of fuel cell. The Proton Exchange Membrane Fuel Cell (PEMFC) is mostly used for car mobility. The different types of fuel cell can be also used for many other applications such as power supply, combined heat and power plants, and aerospace and submarine applications. Not only hydrogen but also methanol and methane can be used in some types of fuel cell.
Production requirements
Regardless of the fuel cell in question, the bipolar plate thickness is as little as 50 µm. Currently, each manufacturer makes its own design of fuel cell as no standard design has yet been established.
Given the high number of plates and the application in question, these plates have to meet strict requirements in terms of parallelism, flatness, contour accuracy and surface quality. And of course, low volume production costs are necessary for so many plates.
There are several ways to meet these production requirements:
Progressive stamping
Hydroforming
High-velocity forming
All of them are only as good as the tool used. And this is where Makino comes in.
Forming die requirements
The tool used has to fulfil really demanding accuracy requirements, especially on a lengthy processing part.
Here is one example of the typical forming die requirements (which may vary slightly from the real ones):
Tolerances:
Contour accuracy: +/- 5 µm
Flatness: 10 µm
Parallelism: 10 µm
Roughness Ra: <0.2 µm
Materials:
Approx. 60 HRC
Machining times:
Approx. 150 - 200 hours
Smallest tool diameter:
0.3 - 0.4 mm
Why the Makino iQ500?
The active cooling system in the iQ500 enables excellent thermal stability in the z-axis over long machining times (>150 hours).
The machine is equipped to achieve even tighter contour accuracies so the shapes and radii of the dies will not be a problem, even when they become even smaller.
Thanks to its linear motors and high-precision path control the iQ delivers the fastest feed rate while maintaining accuracy, even in small movements.
A large number of tools are necessary. All of them have to be measured precisely so there are no differences between them in the z-axis. This can only be achieved through a precise tool setting on the spindle and precise measurement, which in our case is ensured by the Precision Tool Image Measurement Device (PTIM).
Tool wear can be kept to a low level thanks to smooth spindle running and excellent axis control. The surface quality is also superior to other machines, even with standard tools.
Finally, and as important a factor as the machine, our application engineers are experienced in milling the plates and can support end-users in technology terms and the development of a cutting strategy.
