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Thinking outside the box: lightweight battery housing

The lightweight structure of electric vehicles (EV) is a key factor to help extend the cruising range, which is a necessary condition for accelerating the popularization of electric vehicles. A specific weight reduction object is the battery box used to place the battery.

So far, the requirement to support the weight of the battery while protecting the battery has often made these housings themselves heavy. However, now, some research projects are seeking to alleviate this situation by using new lightweight material applications.

At the Fraunhofer Structural Durability and System Reliability Research Institute in Darmstadt, Germany, a newly completed project developed a lightweight battery case made of continuous fiber reinforced thermoplastic (CFRTP) using innovative foam injection molding Craft made. The structure uses two layers of CFRTP, with a foam layer in the middle, which is said to be able to reduce the weight by 40% compared with the aluminum structure.

Sandwich structure

The project leader, Dr. Felix Weidmann, confirmed these details. "This structure is based on a sandwich structure and aims to achieve very high mechanical properties at low weight. This is a classic method that follows lightweight design methods in components and can be found in many applications," he said. "So far, this has been very expensive, but our new materials and processes are more competitive in cost-sensitive applications."

In the manufacturing process, the outer layer is first formed by a cross unidirectional (UD) tape laying procedure. The necessary consolidation process is done using a dual-belt press, which Dr. Weidmann describes as "the most cost-effective method of making thermoplastic composite laminates." Then in the process of laser cutting and local thermoforming, "3D pre-forming" is performed, and only the parts of the structure that need to be bent are heated and cooled to achieve the desired shape. Dr. Weidmann explained: "The cuts and their specific contours can be folded into 3D prefabricated parts, similar to packing boxes/cartons."

In addition, Dr. Weidmann said, this "hot bending" technique avoids more extensive changes in the crystalline properties of the material, which might otherwise cause problems in the actual battery case manufacturing process. "We have introduced localized thermal cycling where we need to bend the profile to achieve 3D preforms," he points out.

During the injection molding process, the outer layer is combined with the foam layer to create load-bearing capacity. "This is basically a melt bonding process at the polymer interface," said Dr. Weidmann. "During the processing, the temperature of the mold and the material is determined in a way that enables a firm bond between the foam core material and the composite panel," he said. The injection pressure required is very low, and the injection of the foam core requires approximately 5 seconds. Currently, the cooling time required to remove the workpiece from the mold is close to 120 seconds, but Dr. Weidmann believes that a shorter time can be achieved.

Although the sandwich material formula is already a good way to achieve the combination of light weight and high strength, Dr. Weidmann said that this project still has many innovative features. "In the field of thermoplastic composite injection molding technology, this in-situ CFRTP sandwich molding process is a very new process," he said, "but we are the only company in the world to push it to such a large scale and extremely high technical requirements. 3D structure (such as battery housing)."

Fraunhofer process:

A particular problem that must be solved involves the prediction of shrinkage and warpage of the sandwich structure, and the development of key bonds between the foam core and the composite panel. As Dr. Weidmann pointed out, "There is currently no commercially available simulation solution." However, the project used a new method developed by Dr. Weidmann himself to successfully verify proper simulation of shrinkage and warpage, as well as the latter case. Under the bonding behavior.

Therefore, Dr. Weidmann believes that the final combination of materials and processes provides an attractive hybrid function. "For structural reasons, it does not require any metal," he said. "This is a battery structure made entirely of polymer and composite materials that can accommodate batteries of any shape." He added that in addition to light weight, low cost, and non-conductivity, the development of this technology has promoted freedom of design. , Including the choice of geometry and materials to support "almost any application requirement, including very strong flame retardancy."

The project involves the development of battery shells for plug-in electric vehicles and all-electric buses involving Fiat and Iveco, respectively, but Dr. Weidmann said that there has been encouraging feedback from the wider industry. In addition, he said, this technology is not limited to battery structure, but also applicable to any components that require cost-effective and lightweight design.

Solutions developed by the alliance

Elsewhere in Germany, a consortium of companies has developed a battery case in the past few years. The case uses glass fiber-reinforced SMC (sheet molding compound) mounted on an aluminum substrate. Compared with the commonly used existing material combination, the 10% weight will not affect the mechanical properties. The companies involved include: fiberglass product manufacturer Lorenz Kunststofftechnik; material chemistry expert Evonik; materials and production process developer Forward Engineering; battery supplier LION Smart; composite material expert Vestaro and battery case supplier Minth.

Peter Ooms, Chief Operating Officer of Lorenz Kunststofftechnik, said that the project uses Evonik's epoxy curing agent to develop a new SMC with a density of 1.5-1.7gm/cm. "It has excellent properties, such as bending strength greater than 350 MPa, bending elastic modulus greater than 18,500 MPa, and impact resistance greater than 150kJ/m2," he reports. "In addition, by using epoxy resin instead of ordinary polyester resin, other problems that usually occur when glass fiber reinforced SMC materials are used can be eliminated."

Ooms said the thermal performance is also impressive. "The material can withstand 10 minutes at 800°C without burning through, and its insulating properties can protect surrounding components and materials from temperatures exceeding 300°C. In addition, by using epoxy resin instead of the usual polyester Resin can eliminate other problems that usually occur when glass fiber reinforced SMC materials are used."

But Ooms also pointed out three special innovations in the new development. He said: "First, the raw materials can be stored for several months at a normal temperature of 20°C." "The second is to add a release agent to the material so that the material will not stick to the mold. Third, any traditional or High-tech SMC customers can use this material without changing the process."

According to Ooms, further work is aimed at the development of a fully composite structure, but he confirmed that the new product has aroused interest in the automotive industry. "Several car manufacturers have already made promises," he said.

Some companies have jointly developed a battery case that uses glass fiber-reinforced SMC to be mounted on an aluminum substrate, which is said to be 10% lighter than commonly used material combinations.

Lightweight laminate

At the same time, in the United Kingdom, Stalcom Automotive Technology in Pershore has launched a new lightweight laminated electric vehicle battery substrate technology, temporarily called lightweight laminate, which combines aluminum and polypropylene composite materials. , To create a so-called "heterogeneous mixed material using the best performance of two materials". The product uses the CFRTP material layer produced by Huesker Synthetic Company in Germany, and uses the adhesive-free powder bonding process of the British company Powdertech Surface Science, which is said to provide three times the bond strength of similar adhesive products.

According to Bob Mustard, Managing Director of Stalcom, a key element of the product is the use of this connection technology, which was specially developed by Powdertech to solve problems related to bonding thermoplastic materials to metal. He said: "This is a breakthrough process to connect metal and polypropylene, simple, fast and clean."

Mustard said that, if needed, the manufacturing process begins with the pre-cutting and forming of composite materials and aluminum layers, which have already been coated with an adhesive coating, and mechanical fasteners are inserted into the aluminum. Then the aluminum plate is heat-melted to the polypropylene composite material in the short-cycle compression molding process, and the finished component can be used directly from the mold. He added that the processing technology and cycle time can be changed to accommodate low, medium or high yields with only minor process changes. He said: "Once cooled and removed from the tool, the resulting component requires little or no post-processing before use."

Due to the different thermal expansion coefficients of composite materials and aluminum, Mustard's design, tools, and processing need to adapt to this to avoid subsequent problems. However, he emphasized that the basic manufacturing process is completely simple. "These materials are assembled and molded in a traditional compression molding process," he confirmed.

"For safety-related reasons, the top and bottom layers are usually insulating composites, and the middle aluminum layer provides a ground plane for EMC shielding and grounding safety," Mustard said. "So far, three-layer and five-layer designs have been built and tested. The five-layer design has two independent conductive layers, providing interconnection possibilities similar to multilayer PCBs."

Mustard can list a series of performance advantages of this material in EV battery casings and substrates compared with monolithic composite materials or homogeneous metals. These include: fail-safe internal connections between layers; very high puncture resistance; improved thermal insulation performance compared with all-aluminum substrates, and an internal peak temperature lowered by 60°C; and easy removal of the final laminate to facilitate material removal Reuse and recycle.

In fact, the British small car manufacturer Watt Electric Vehicle Company (Watt Electric Vehicle Company) has designated this material technology for its upcoming Coupe sports car platform, and the customer plans to deliver it in the first quarter of next year. Mustard said: "It constitutes the bottom plate of the battery and is fully integrated into the chassis with a ‘cell to chassis’ design," Mustard said. "This will be one of the first vehicles to showcase the concept of a chassis or battery box."

Source: China Battery Network

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