Reducing the weight of EV battery protection

Weight reduction in electric vehicles (EVs) is an important goal for automakers because it can help to extend vehicle range, a key consideration for consumers. Rocker panels featuring thermoplastic honeycomb-based structures can cut weight by up to 40% vs. incumbent extruded aluminum solutions while delivering excellent energy absorption and potential cost reductions of up to 20% via part consolidation. 

Today, most EVs use structural components that are made of steel and/or aluminum to meet stringent crashworthiness requirements for batteries. Structures such as rocker panels are intended to absorb energy during a crash to avoid shock and intrusion into the battery that can lead to thermal runaway and fire. Therefore, rocker panels would benefit from additional reinforcement. To help reduce the weight of EV battery protection without adversely affecting the dynamics, durability or crashworthiness of the vehicle structure, SABIC developed two concepts for energy absorption in the rocker region using thermoplastic honeycomb structures based on NORYL GTX resins.

Developing these concepts required SABIC’s Specialties business to address specific considerations:

• Packaging space constraints: The solutions had to fit within the depth and height limitations of the rocker panel space/sill area in the body-in-white (BIW) while providing the required combination of bend and crush resistance.

   

• Variations in bending stiffness:  In EVs, side impact protection is required all along the length of the vehicle floor, where the batteries are located. But the bending stiffness and dynamic response of the vehicle are not uniform all along the vehicle length. This variability presents challenges in balancing weight with crush performance in different vehicle locations and highlights the need for multi-material-hybrid/locally stiffened structures that provide similar energy absorption and bending stiffness along the length of the vehicle.

Incorporating Thermoplastics

High strength rocker panel reinforcement structures are typically made from steel and extruded aluminum. However, metal reinforcements are heavy and require secondary operations. In contrast, reinforcements based on thermoplastics are both lighter and easier to manufacture. Traditionally, however, substituting lighter materials for metals has meant a trade-off in stiffness. To this end, SABIC has developed two thermoplastic-based rocker panel reinforcements with high stiffness that meet test requirements for side pole impact. The following proposed designs provide energy absorption during side impact plus added value such as light weighting and the potential for ease of manufacturing.

• All-thermoplastic solution 

  The all-thermoplastic rocker panel reinforcement design consists of a front honeycomb structure and a rear double-wall structure. The rear double-wall structure provides the additional bending stiffness required for the front honeycomb structures to absorb energy during an impact.

  The honeycomb structures and the double-wall structure can be injection molded together from a ductile thermoplastic such as unfilled NORYL GTX™ resin. If higher bending strength is required, the double-walled structure can be formed from a stiffer polymer (such as a filled grade of NORYL GTX resin), which is used as an insert. The honeycomb structures are then overmolded on the front. While this solution would allow manufacturers to avoid the E-coat process, it would come with additional design complexities and higher costs vs. extruded aluminum.

• Multi-material solution

To maintain bending stiffness, self-supportive structures are needed within the rocker panels to enhance crash performance. This design consists of a rear composite or metal component, and a thermoplastic component comprised of an overmolded or an injection-moldable honeycomb/tubular structure. These solutions can be made as continuous parts or in modules assembled with metal support brackets, allowing for additional design freedom. The composite or metal components added as an insert can provide the required bending stiffness and support for progressive crushing of the honeycombs during a side pole impact. Furthermore, limiting the length of the thermoplastic component could simplify molding and manufacturing, reduce the risk of warpage and potentially ease the assembly process.

SABIC’s concept designs offer several potential benefits versus the traditional extruded aluminum design, as shown in Table 1. In particular, SABIC’s multi-material solutions offer enhanced design flexibility and excellent potential for weight savings and reduced system costs. 

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Features and Benefits of multi-material solutions:

• Stiff backbone: The rear metal component, made of steel/composite, acts as the backbone to support and enable progressive crushing of the thermoplastic component in a crash.

   

• Excellent energy absorption: The front thermoplastic component, made of a ductile, impact-modified polymer like NORYL GTX resin, consists of injection-molded crush structures such as honeycombs, which are designed to absorb impact energy.

   

• Local tuning for optimized weight: Even though the bending stiffness and dynamic response of the vehicle are not uniform, the crush structures can be designed with varying cross-sections to deliver crush performance at every point along the vehicle’s length.

Simulations using finite element analysis revealed that sill intrusion performance for the multi-material solution was better than that of the traditional extruded aluminum design. The hybrid solution was also as much as 40% lower in weight. This result is due to the bending stiffness provided by the rear metal component, which supported the energy-absorbing structures in the front as they crushed and absorbed the impact energy. In addition, with lower raw material use and part integration, this solution can achieve cost savings of up to 20 percent compared to aluminum solutions.

Conclusion

SABIC’s conceptual rocker panel reinforcement designs that utilize the company’s specialty thermoplastics can be considered to replace heavy structural reinforcements made of steel/aluminum to enable lighter-weight electric vehicles. These lower-cost, weight-saving reinforcements can contribute to improved electric driving range, while their excellent impact performance could help reduce the risk of fire following impact to the battery during vehicle crashes.

Below is a list of grades from our extensive portfolio that may provide your EV battery design with the needed material solutions:

NORYL™ GTX964W resin (crash protection): High impact resistance, and high flow

NORYL™ GTX830 resin (crash protection):  30% GF, high stiffness and high heat resistance

NORYL™ NHP6012 resin (cell retainers / battery modules / housings): High flow, V0 at 1.5mm

NORYL™ GTX4610 resin (cell retainers / battery modules / housings): 5VA at 2.0mm

LNP™ KONDUIT PX11311U compound (thermally conductive applications): Wi-fi signal transparent, V0 at 1.0mm