Having covered the prismatic cell housings in my previous blog I would like to spotlight here the cylindrical cell housings. Other than its prismatic cousins made of aluminium the cylindrical cell housings for electric vehicles are made of nickel-plated steel (NPS). There are some applications for light vehicles with aluminium but for EV the vast majority is steel. It´s a good question if aluminium will play a role in the future and some interesting work has be done about the advantages of aluminium – see e.g. recent whitepaper “Benefits of aluminium cell housings for cylindrical lithium-ion batteries” as published by company Speira and RWTH´s PEM institute. However,nobody can tell whether it will come true sometime, so let´s focus for now on NPS housings.
The process route for cylindrical cell housings made out of NPS is a combination of deep drawing and wall ironing similar to the application for beverage cans which is commonly known as “DWI” which stands for Draw and Wall Ironing. This process for beverage cans was already developed in the 1960s by Reynolds Metal Company and others.
To manufacture the cell housing, the process route is as follows:
- Cupping (Blanking out of a coil material and first deep draw)
- Redraw – number of times
- Ironing – number of times
In between, piercing and additional forming steps for the bottom and/or opening geometry may be required.
Recap from the previous blog: The cup size and the deep draw / redraw steps will be determined by possible deep draw ratios β of the material. Wall ironing is a reduction of the wall thickness by pressing the metal through the gap between a punch and ring. The fact that the wall thickness is reduced by ironing is directly related to the fact that thelength of the housing is increased. The decrease in wall thickness or increase in length is given by the ironing ratio.
Cupping is a method which produces multiple cups per each stroke of the double-action cupping press which is not only very productive but also advantageous in regards of material utilization. The method is simple, in principle nothing else than cutting out a round blank and apply the first draw to this blank. A cross section of the dieset shows the active elements like cut-edge, draw-pad, blank-and-draw-die and punch.
These elements are iterated multiple times in the dieset to run wide coils with multiple cups per stroke.
As a matter of fact, the material utilization improves with the number of outs as seen below:
NPS is typically available in widths of 650mm-900mm. It is necessary that one finds out the max. width available in its supply chain and try to use it best by arranging as much blanks as possible to optimize material utilization. The coil material is lubricated before processed with a smooth oil film.
Running fast (up to 1.200 parts per minute) and with good material yield is the name of the game of cupping. Full traceability is given by marking each cup with its number of die station and random checks of the cups are done to ensure quality.
The cups must be transported by mass conveying to the next press which applies process steps like re-draw, wall ironing, trimming and possibly other forming steps as necessary. The cups are moved to a mezzanine and dropped by chutes to a post processing presswhich is basically a transfer press. It runs with multiple tooling stations to perform the different tasks transferring the part from one station to the next station during the opening and closing of the press stroke. The basic concept was introduced by Louis Schuler at Expo Paris in 1900 and is still in use as of today.
Today´s descendants run multiple rows of tooling and reach up to 240 parts per minute for a battery cell housing with diameter of 46mm. For such performance, tooling is actively chilled, and parts are lubricated by oil or emulsion. Random quality checks of badges ofthe housingsright after the transfer press ensure that only ok-parts enter the next production step, which is washing.
So far, we have received a housing which is according to its drawing and within tolerance, produced with high speed and stable processes. However, another important requirement, which is the cleanliness of the can cannot be achieved by the metal forming as it is a wet process with cup lubes, drawing oils and emulsions applied.
This is the task of the washing machine which is running water-based or with solvents. For mass production and taking up two or three legs of front-endequipment, continuously working, water-based machines are used in which the housing undergoes pre-washing, washing, and rinsing zones, followed by drying. Such machines can handle 2.000 housings per minute and more.
After that, end-of-line systems with 100% camera inspection of the housing are putting the housings in trays at the end of such production line. After washing, the cans are dry and sensitive for dents, so this inspection and packing must be done with special care.
Production of cylindrical cell housings is aiming at zero defects, high speed, best usage of material and extreme cleanliness. We can manage such requirements by standing on the shoulder of giants and refining established metal forming methods to the next level.
Insights on forming methods are taken from the Schuler Metalforming Handbook, Springer; 1., Edition (12. Juni 1998).