The Application of Engineering Plastics in the Automotive Industry as Seen Through “Automotive Lightweighting”

　　There is no doubt that lightweight design is one of the key directions for the automotive industry. Theoretically, for every 100 kilograms reduction in vehicle weight, fuel consumption per 100 kilometers can be reduced by about 0.4 liters. Provided that performance remains unchanged, lightweight vehicles not only save on fuel costs but also protect the environment, bringing significant benefits to both consumers and the environment.

　　Generally, automotive lightweighting involves focusing on materials and manufacturing processes. With advancements in technology, the combination of new materials, novel structures, and innovative processes has given rise to a special type of lightweight body structure: the monocoque body.

　　1. Integrated molding can reduce weight by 60%.

　　The body of a typical passenger car generally consists of a series of components, including the door outer panels, roof panel, front and rear fenders, side sill panels, and floor pan. The complete vehicle is assembled through four major processes: stamping steel sheets, welding sheet metal parts, painting the bare body, and final assembly. As a load-bearing structure, the car body accounts for the majority of the vehicle’s weight and plays a crucial role in protecting the safety of occupants inside the cabin.

　　But have you ever heard of another method for manufacturing car bodies—one that doesn’t require welding or adhesives? That’s the monocoque body construction, which looks like the one shown in the image below.

　　In fact, monolithic body construction goes by an even more surprising name—“all-plastic body.” As the name suggests, the main body of the car is made from a lightweight rotational-molding plastic material. This body structure differs from traditional manufacturing methods in that it replaces steel with polymer materials and employs a rotational-molding process to create the entire body in one piece. Since the raw materials can be color-adjusted, the body no longer requires painting—a significant simplification that eliminates processes such as stamping and spraying. That’s precisely what we mean by “one-step molding.”

　　Plastics are widely used in automobiles, but would you be surprised to learn that entire car bodies can now be made entirely of plastic? This innovative process and material combination can significantly reduce the vehicle's weight.

　　Thanks to their lightweight and simple structural design, this type of body structure is primarily used in electric sedans, aligning perfectly with the development trend of new-energy vehicles. Take, for example, the Danish energy-efficient electric vehicle ECOmove QBEAK: its body dimensions are 3,000×1,750×1,630 mm, and its curb weight is only 425 kg. By contrast, a conventional sedan of similar size typically weighs over 1,000 kg. Even the smaller Smart, with body dimensions of 2,695×1,663×1,555 mm, has a curb weight ranging from 920 to 963 kg.

　　It’s no exaggeration to say that a monocoque body with a simple structure and lightweight plastic can reduce weight by more than 60% compared to a metal body of similar specifications.

　　II. Rotational Molding Integrated Molding Process: Faster New Vehicle Development

　　As we’ve discussed earlier, we’ve learned about the benefits of this forming process. So, what exactly is the rotational molding integral forming process? Simply put, it involves adding plastic raw material into a specially designed mold, then continuously rotating and heating the mold around two perpendicular axes. Under the combined effects of gravity and thermal energy, the plastic inside the mold evenly coats and melts, adhering uniformly to the entire surface of the mold cavity, thereby taking on the desired shape. After cooling and solidifying, and undergoing subsequent steps such as demolding, an integrally molded product is obtained. The figure below shows a simplified schematic diagram of the process principle.

　　A major feature of the rotational molding integrated forming process is that it allows for the one-time production of large or extra-large hollow plastic products with complex curved surfaces. This precisely meets the requirements of passenger car bodies, which are characterized by their large volume and sleek, smoothly curved exterior lines.

　　Some people might confuse rotational molding—a holistic forming process—with one-piece stamping—a process aimed at simplifying welding procedures, enhancing structural strength, and improving aesthetic appeal. The latter is commonly used in car door stamping, but it still falls within the realm of traditional body manufacturing methods. In contrast, the former represents a disruptive approach that completes the entire body manufacturing process in a single step.

　　Although this technology is not yet fully mature and is still in its early stages of development, it nonetheless boasts many advantages. For example:

　　▎Traditional vehicle development typically costs around 100 million RMB, which to a large extent constrains the advancement of the automotive industry. In contrast, this new process simplifies the body structure, reduces the complexity and cost of component manufacturing, and shortens the product development cycle.

　　Compared to traditional body structures, all-plastic bodies weigh more than half as much, which greatly facilitates vehicle weight reduction and helps lower fuel consumption.

　　▎Generally, the one-step molding technology features a variety of modular kits, offering great design flexibility and enabling customized production, thereby enhancing the degree of body personalization.

　　▎Since the vehicle body is made of eco-friendly plastic, its production process does not pollute the environment, and the body itself is resistant to corrosion during daily use, boasting high durability.

　　▎A-grade surface body panels can be directly obtained by adjusting the color of raw materials. Compared with traditional painting processes, this approach significantly reduces the substantial investment required for phosphating and electrophoretic coating, making the production process more environmentally friendly and lowering energy consumption.

　　3. Plastic bodies can also be safe.

　　We know that vehicle bodies have extremely high safety requirements. Can a monocoque body—formed in one piece—really meet these strength requirements? Can it truly protect our safety? What are its advantages, and what are its disadvantages?

　　Take a certain domestically produced electric vehicle as an example: its body uses one-piece molding technology. After the doors are removed, the body weighs about 71 kilograms, and the entire vehicle weighs only 500 kilograms. However, the vehicle’s safe speed is limited to no more than 60 kilometers per hour. Does this mean that a one-piece plastic body simply can’t guarantee sufficient strength? No—there are several reinforcement solutions available.

　　Due to the inherent strength limitations of plastics and their tendency to shrink and deform, a purely plastic structure is insufficient to meet strength requirements. To address this issue, many monocoque body designs incorporate built-in steel mesh structures or add reinforcing materials such as glass fibers to enhance the structural integrity of the body.

　　Take the built-in steel structure as an example: Specifically, a steel mesh is embedded inside the mold. During the rotational molding and heating process, the raw material envelops the steel mesh—much like reinforced concrete. The steel mesh counteracts the shrinkage and deformation of the plastic, thereby significantly enhancing the body’s strength. Furthermore, to further boost body strength, some manufacturers also add an aluminum frame on the inner side of the body. Although this increases the overall weight somewhat, it effectively ensures the safety of the powertrain components mounted on the frame.

　　Of course, since one-step molding of an all-plastic body places higher demands on the machining accuracy of molds, the speed of mold opening and closing, and the uniformity of the finished products, the process is quite challenging. If fiber reinforcement is used solely—whether through pre-mixing or post-mixing—the fibers cannot be uniformly dispersed throughout the raw material, directly resulting in less stable mechanical properties of the body parts.

　　Summary:

　　One-step molding significantly reduces the vehicle body weight from both material and structural perspectives. Although, at this stage, such body designs still have numerous drawbacks and remain in the early stages of development, there are already proposed solutions to enhance their strength.

　　Currently, this technology is still limited to the low-speed electric vehicle market, but in the future, its application scope is expected to expand. Enhancing safety will be the key to its widespread adoption.

　　In the future, if you happen to see an electric vehicle on the street, someone might just say, “Look, that car’s made of plastic.” But you can confidently and with style reply, “Honey, that’s a one-piece body!”

　　Automotive Lightweighting Online