Continental Structural Plastics Focuses on EV Lightweighting

Bulky batteries are the heart and soul of electric vehicles. But, their weight creates numerous challenges that demand lightweight materials.

Continental Structural Plastics Inc. (CSP), a Teijin Group company, is at the leading edge when it comes to creating innovative composite and multimaterial technologies. The company’s engineers are focusing their attention on developing materials that address the unique environmental and safety needs of EV manufacturers.

Composites are ideal for reducing weight, improving strength and stiffness, and improving vehicle safety when made from chemistries that are flame retardant and help to contain thermal runaway.

CSP recently introduced several new advanced composite formulations that meet stringent EV battery enclosure performance standards for flammability, thermal runaway and volatile organic compound (VOC) emissions, while offering the design flexibility of sheet molding compound (SMC). These new composites include a low-VOC formulation, an alumina trihydrate-filled system, an intumescent system and a phenolic system.

Hugh Foran is executive director of CSP’s Advanced Technologies Center, a 47,500 square-foot R&D facility in Auburn Hills, MI, dedicated to developing a wide variety of next-generation materials and processes.

“We are developing technologies and processes here that leverage [our] expertise in thermoplastic and thermoset composites, carbon fiber and manufacturing to provide our customers with new options for existing and future vehicle programs,” says Foran. “We can reduce weight while improving durability and occupant safety—all key features needed for autonomous, connected and electric vehicles.”

Foran’s team recently developed a full-sized, multimaterial battery enclosure featuring a one-piece composite cover and a one-piece composite tray with aluminum and steel reinforcements.

In addition, CSP engineers are focusing on developing a new honeycomb manufacturing process that produces ultra-lightweight Class A panels. Considered a “sandwich” composite, these panels use a lightweight, honeycomb core clad with natural fiber, glass fiber or carbon fiber skins that are wetted with polyurethane resin. This process enables the molding of complex shapes and sharp edges, and results in panels that offer very high stiffness at a very low weight.

Autonomous & Electric Mobility recently asked Foran to discuss those lightweighting efforts, and some of the challenges that lie ahead for OEMs and suppliers.

AEM: How can the weight of electric vehicle structures be reduced?

Foran: There is tremendous opportunity to address lightweighting. And, we can go far beyond aluminum. For instance, there is a lot of opportunity for engineers to use composites for structural member applications, such as A-arms, engine cradles and frames. It’s just a matter of getting engineers to understand that. Fuel cells will be another big opportunity for lightweighting efforts. For instance, fuel tanks that hold hydrogen and fuel cell plates can be made out of lightweight materials, such as composites.

AEM: Why does so much EV lightweighting activity today involve battery enclosures?

Foran: It has the potential to reduce a lot of weight and there are many features we can put in. Even beyond thermal features, we can add materials to provide EMI and RFI shielding in certain areas, if required. While we design most of our battery enclosures to withstand 3 psi, some go as high as 10 psi, which almost becomes a pressure vessel.

AEM: Are all battery enclosures basically the same or is there much difference between them?

Foran: They’re all the same basic size and shape, but they are different. However, the geometries and requirements are different for every customer. For instance, because of electronics and seal requirements, battery boxes can be much different from vehicle to vehicle. Depending on where the battery pack is within the vehicle, some enclosures may even become structural load floors.

AEM: What is the latest trend in battery enclosures? What makes your new design unique?

Foran: Our battery enclosure consists of a one-piece composite cover, a one-piece composite tray with aluminum and steel reinforcements, and a mounting frame that uses structural foam for energy absorption. This enables a reduced frame thickness and weight, while improving crash performance.

By molding the cover and the tray each as one piece, we created a system that is easier to seal and can be certified prior to shipment. The full-size battery enclosure we built measures 1.5 by 2 meters. To produce it, we made a P20 set of molds consisting of a tray and a cover.

We currently produce more than 30 different battery box covers for different OEMs. However, our new lightweight battery enclosure is generic. Our goal is to get into complete systems, because customers now want us to perform leak tests. They also have various types of thermal runaway requirements. Composites can be used to reduce weight, improve strength and stiffness, and improve vehicle safety when made from chemistries that are flame retardant and help to contain thermal runaway.

AEM: What type of weight savings have you been able to achieve?

Foran: Our multimaterial battery enclosure is 15 percent lighter than a steel battery box. Although it is equal in weight to an aluminum enclosure, it offers better temperature resistance than aluminum, especially if a phenolic resin system advanced composite is used. Just for the frame alone, we’re saving 30 percent. However, overall weight savings depend on the material and the amount of batteries. Typically, battery packs in electric vehicles can weigh anywhere from 1,000 to 1,100 pounds.

AEM: How have you been able to reduce complexity or eliminate parts with your new battery enclosure design?

Foran: Traditionally, a metal battery enclosure features about 40 parts. Those extrusions or stampings are welded together. The big issue that OEMs are concerned with is leakage—primarily water leakage and rusting in areas where the seams are. Metal enclosures require more than just a cover and a frame. There needs to be multiple attachment points, such as brackets that locate and hold batteries down. That requires multiple bosses or studs that have to be welded, in addition to extra reinforcements. With composites, you can mold in all of those features in one piece. It’s also possible to locally thicken material where needed. Most areas in a composite enclosure are 2 to 3 millimeters thick. But, we can increase wall thickness anywhere from 3 to 7 millimeters where necessary.

AEM: How does your new enclosure simplify assembly?

Foran: The traditional battery cover that we make requires us to drill holes. Sometimes, we encounter issues with cracking due to point loading and seal loading issues. With our composite enclosure, we developed a clip system that replaces the need for bolts. This improves the seal, reduces assembly costs and makes it easier to access batteries if service is needed.

Each steel clip is about 30 millimeters long and allows us to spread the load. It’s a standard clip that we can use with many different battery boxes.

These clips affix to the battery enclosure via molded-in features, instead of using secondary machined holes and welds to join the top and bottom sections of the box. Unlike a point load created by using a bolt, these clips spread the seal load more evenly across the upper and lower flange. We use 68 clips designed to hold 3 psi of internal pressure to replace 72 bolts that would all need to be torqued to a specified value. Each clip provides 950 newtons of clamp force.

By eliminating the need for bolts or bonding of any sort, a box made with these clips is easier to assemble. It’s also much easier to service if the internal cells need repair or replacement. By using these clips in place of multiple fasteners and different types of welds, we can reduce assembly cost, complexity and the amount of scrap involved in the production of each enclosure.

AEM: Besides battery enclosures, what other EV lightweighting applications are you exploring?

Foran: We have been working on honeycomb cells that are more than 50 percent lighter than comparable steel parts. We are making SMC body panels that are 1-millimeter thick. We’re not just doing flat load floors with honeycomb. We are molding shapes such as hoods, lift gates and roofs. The parts are lighter and stiffer. Our process also adds insulation and acoustical properties. In addition, we can mold in hardware or headliner material. In fact, we can even run the material through an electrocoat paint oven, which runs about 210 C.