Prismatic cell Cases

Prismatic cell Cases, this image is copyright to Schuler.

Prismatic battery cells are one of three different formats for Li-Ion Battery cells, next to pouch and cylindrical cells. All formats have their share in the market and to continue the growth path of the EV market as anticipated all formats will be around for the next years and keep growing.

Prismatic cell housings are characterized by its geometry (length x width x height, wall and bottom thickness, etc.) and the raw material. Currently the majority of such cell housings are made of aluminum or aluminum alloy, only exotic variants are known using stainless steel or plastic. In the next years the aluminum prismatic cell case will be required in massive numbers. To choose the ideal manufacturing method one has to take into account the physical limits of metal forming, the available production machines and the market requirements, here especially the target production costs.

Some basic know-how may be taken from the manufacture of metal packaging which is a mature technology. However, since the prismatic cell housings have different sizes to beverage or aerosol cans, we may not transfer the lessons learned from the manufacture of such cans to the new application one-to-one.

In principle, two different forming methods are applicable for prismatic cell cases made of aluminum: deep draw or impact extrusion. Both methods are combined with wall ironing to come to the final geometry and reach the thin walls as specified and in tolerance.

The process route of deep draw consists of the following steps in the manufacturing line:

  • Blanking of a coil material
  • Deep draw
  • Redraw
  • Ironing (1, 2 or 3 rings)
  • Trimming

The blank 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 the

length of the housing is increased. The decrease in wall thickness or increase in length is given by the ironing ratio.

 

Looking at the impact extrusion route we see a process which is completely different:

  • Use of a slug material
  • Reverse impact extrusion
  • Ironing (1, 2 or 3 rings)
  • Trimming
Impact Extrusion process

Impact Extrusion process, this image is copyright to Schuler.

 

As with deep draw, these procedures are carried out at room temperature. The process limits are given by the minimum wall thickness, which is determining the pressure tension of the punch. Next to the resilience of the tooling the length of the container is limited as buckling of the punch needs to be taken into account. Wall ironing is the same follow-up method as applied with deep draw. One difference worth mentioning is that with deep draw the bottom thickness of the housing is the starting gauge of the coil material, whereas with Impact extrusion the bottom thickness is adjustable and by adding forward extrusion one may produce special bottom shapes for cooling, pressure relief, etc..

The machines for the two process routes are completely different. Deep draw requires transfer presses with multiple dies. The cases are transported by transfer systems from die to die. The size of the case is influencing the speed of this automation and the table length of the press. Those speed are between 20-30 spm for cases with a length of 200mm.  Table length of the press may be required up to 5m. Not to forget is that the working energy is an important machine specification. The forming loads multiplied with the forming travel will determine the required working energy of the press.

On the other hand, impact extrusion is a one-step process on dedicated machines, which can reach speeds of 60-100spm depending on the machine size. The rectangular area of the housings, the wall thickness and the flow curve of the material will determine the machines size. Typical sizes are between 4.000kN-10.000kN.

The required output will determine which production method is more cost efficient. Material utilization is an important cost driver. For deep draw the coil material is used only partly and 20% or more of the material is scrapped. On the other hand, the slug material is used in its entirety. This makes a difference in the cost calculation considering different raw material prices for coil and slug material as well. Investment cost for impact extrusion is higher than with deep-draw so a high machine utilization (e.g. three-shifts) is key when comparing ROIs.

In short, when producing prismatic battery cell housings, one has the choice of two different forming methods. Both processes are proven and commonly available, but to do the choice method planning is required followed by the choice of suitable machinery and two calculations of production cost to get to the full picture.

Acknowledgments
Insights of forming methods are taken from the Schuler Metalforming Handbook, ‎ Springer; 1., Edition (12. Juni 1998).