With a time of 7 minutes and 4.957 seconds, the Xiaomi SU7 Ultra production version made its debut at the Nürburgring, becoming the fastest production electric vehicle in Nürburgring history! From prototype to production version, the application of carbon fiber is not just a material selection, but a systematic engineering project integrating materials science, craftsmanship aesthetics, and safety verification.
This seemingly simple "black gold" carbon fiber material has undergone a complete transformation in Xiaomi Auto's technology system, from fiber filaments to high-performance vehicle exterior components. Every process reflects Xiaomi Auto's exploration of the boundaries of performance.
In an era that pursues ultimate performance and lightweight design, carbon fiber, with its superior properties, has become an indispensable key material in many high-end fields, earning it the reputation of "black gold." A seemingly ordinary carbon fiber hatchback requires nearly 11 complex processes from raw material to finished product. Its formation process can be summarized by three key elements: carefully selected materials, intricate processes, and rigorous strength testing.
The complexity of the carbon fiber market is beyond imagination, with the primary challenge being the difficulty in distinguishing genuine from counterfeit products. Commonly available carbon fiber stickers and water-transfer printed carbon fiber products may appear indistinguishable from real carbon fiber, but their essence is vastly different. Genuine carbon fiber is the darling of the aerospace industry, favored in high-end equipment such as aircraft and rockets. Its outstanding durability, wear resistance, corrosion resistance, and stable performance even in extreme environments are the core reasons for its selection.
Therefore, in practical applications, precise material selection and clear process specifications are crucial. In the selection of carbon fiber products, full-carbon versus non-full-carbon are key indicators. Some so-called carbon fiber products on the market are actually "carbon cloth patches," with only a surface layer of carbon fiber and the interior filled with resin, fiberglass, or other materials, not a full-carbon structure. While these "carbon cloth patch" products bear the name of carbon fiber, their performance is significantly inferior to full-carbon products, making them unsuitable for applications with high performance requirements.
The curing process of carbon fiber is also very important, mainly divided into dry carbon and wet carbon processes. Wet carbon fiber processing is relatively simple, but it has many drawbacks, such as difficulty in controlling resin content, use of unsaturated resin, strong odor, and non-compliance with production standards, resulting in heavy, low-strength, and easily damaged products. Dry carbon fiber processing uses pre-impregnation technology to precisely control the resin content before high-pressure, high-temperature curing, resulting in lighter and stronger products. Different curing processes directly affect the final performance and quality of carbon fiber products, so careful selection is necessary.
The "fineness" of carbon fiber is classified by the number of monofilaments, with common specifications including 3K, 6K, and 12K. 3K carbon fiber contains 3000 monofilaments per strand, resembling an "embroidery-like" process, possessing characteristics of fineness, lightness, high strength, and high toughness. In contrast, 12K carbon fiber contains 12000 monofilaments per strand; the larger number of monofilaments results in a relatively coarser texture, typically suitable for manufacturing large components.
Due to differences in the number of monofilaments, carbon fiber varies significantly in appearance, performance, and applicable scenarios, making it a crucial factor to consider when selecting materials. Brand selection is also paramount. We chose Toray Carbon Fiber, an industry-leading brand also used by luxury brands like Ferrari. On the carbon fiber hood of the Xiaomi SU7 Ultra, both the outer and inner panels utilize three layers of carbon fiber, covering an area of 1.73 square meters, resulting in a more sophisticated texture and a more precise structural layout.
In the automotive industry, carbon fiber has demonstrated immense value. The Xiaomi SU7 Ultra production model and prototypes extensively utilize carbon fiber materials in various interior and exterior components.
This high-carbon-content design brings significant improvements to vehicles in multiple aspects. It reduces vehicle weight, making the vehicle more agile and greatly enhancing handling; it also reduces energy consumption; furthermore, it enhances the body's strength and rigidity, providing solid support for excellent performance on the track. Throughout the research and development process, we have maintained a meticulous approach, rigorously controlling every detail of the carbon fiber material to strive for perfection in every aspect.
This is the essence of carbon fiber materials: from distinguishing genuine from counterfeit, selecting wet and dry processing methods, considering full-carbon versus non-full-carbon, meticulously differentiating the number of monofilaments, and finally, rigorous brand selection—behind every carbon sheet lies a comprehensive and meticulous approach to materials. It is this unwavering pursuit of every detail that forges the superior quality of our products.
02 Craftsmanship and refinement: meticulous carving and craftsmanship to create exceptional quality.
The application of carbon fiber materials is merely the starting point for manufacturing lightweight components. Transforming the base material into a carbon fiber front hood requires a series of precise and rigorous processes.
Carbon fiber initially exists in the form of threads. To transform it into a practical carbon fiber front hatch, the first step is to weave individual carbon fibers into carbon cloth. Then, the carbon cloth is immersed in resin and cryopreserved at -18 degrees Celsius to ensure resin stability. After thawing, it is removed and cut according to design requirements. At this point, the carbon cloth has partially solidified, resembling sheets of ready-to-use fabric, preparing it for subsequent molding processes.
This process, while seemingly simple, actually demands extremely high precision in manufacturing and environmental control. Even subtle differences in each step can affect the quality of the final product.
Once the fabric is ready, it needs to be laid onto the mold. This step demands extremely high standards from both the mold and the workers.
Taking the carbon fiber hood of the Xiaomi SU7 Ultra as an example, the inner and outer panels require two specially customized molds, all made of high-strength P20 steel. P20 steel not only possesses high hardness and wear resistance, but more importantly, it can withstand the high temperature and pressure environment inside an autoclave. Because after the carbon fiber fabric is laid, the entire mold needs to be placed in an autoclave for shaping, even the slightest deformation of the mold under high temperature and pressure will render the carbon fiber hood unusable.
For the Xiaomi SU7 Ultra model, more than 100 sets of molds have been developed. Each mold is equipped with two skilled workers for manual laying, a process that takes approximately six hours and must be completed in a clean environment. Even more stringent is the requirement that the carbon fiber be laid precisely layer by layer, perfectly fitting at every corner, and even the carbon fiber grain must be strictly aligned. Even a deviation exceeding 0.5 millimeters in the bonding process can severely impact the appearance quality and may even render the product unusable. Such precise process requirements mean that a single mold can only complete a maximum of two bonding operations per day, truly embodying the spirit of meticulous craftsmanship.
After bonding, the carbon fiber hood requires further curing, a process inseparable from autoclave technology. Unlike room temperature curing, Xiaomi Auto's autoclave process uses heating and pressurization to tightly bond the layers of carbon fiber. Inside the autoclave, the temperature must reach 145 degrees Celsius, and the pressure 0.7 MPa, under which the carbon fiber must withstand 6 hours of rigorous testing. Only under these conditions can the dimensional stability and appearance performance of the carbon fiber hood reach their ideal state. Carbon fiber cured in an autoclave not only withstands more complex working conditions but also possesses significant advantages such as light weight and high strength.
After the product is removed from the autoclave and demolded, it undergoes CNC machining to remove any burrs and machine the necessary holes for subsequent processing. Next, sandblasting and polishing are performed, with each step aimed at achieving the smoothest and most perfect surface finish. Furthermore, since the carbon fiber hood consists of an inner and outer panel, the connection between them requires a high-precision bonding process. We use robots for the bonding operation to ensure a tight bond between the inner and outer panels.
Even after completing the above steps, only a "rough shell" is formed; the crucial painting process is still required. Painting just one carbon fiber hood takes three days. There are two painting options: clear varnish and colored paint. Regardless of the choice, the process is extremely complex.
Firstly, paint adhesion must be considered. Due to the unique properties of carbon fiber, special carbon fiber paint must be specially formulated for each product. Xiaomi specially formulated a carbon fiber-specific paint for the SU7 Ultra to ensure strong adhesion to the carbon fiber surface.
Secondly, uniformity of the spray must be guaranteed. Carbon fiber surfaces differ from metal and plastic parts, making painting more difficult. The Xiaomi SU7 Ultra uses a two-color spraying method. For example, in colors like Space Silver and Lightning Yellow, the central carbon fiber texture and the colored areas on both sides are painted separately. Even under magnification, no jagged edges appear at the seams, achieving completely uniform spraying without any color difference.
Thirdly, after painting, the durability of the paint surface must be tested. The paint adhesion, durability, and quality standards of the carbon fiber hood of the Xiaomi SU7 Ultra are the same as those of sheet metal parts, which can effectively avoid problems such as weak paint and paint being washed off by high-pressure water guns during car washes.
Despite multiple rounds of testing in each preceding process, a comprehensive external inspection is conducted before the product leaves the factory. Only after passing this rigorous inspection can the product truly be labeled as qualified and shipped.
During the inspection process, even seemingly perfect substandard products can be found with careful examination, revealing potential flaws. Even a tiny particle will result in a product being deemed unqualified. Subsequent products undergo two more rounds of quality inspection before being installed in vehicles. Xiaomi consistently adheres to the principle of selecting the best of the best in its carbon fiber standards, with quality requirements bordering on the obsessive.
From carbon fiber material to the creation of the carbon fiber hood, a total of 11 processes are involved, including weaving, pre-impregnation, freezing, cutting, laying, autoclaving, cutting, sandblasting, bonding, painting, and external inspection. The standard production cycle is approximately 7 days. It is these meticulous processes that forge the superior quality of the carbon fiber hood.
03 Rigorous testing: Forging a safety "shield"
From materials to processes, carbon fiber hoods have undergone numerous tests, but the key to whether they can be put into use and ensure driving safety lies in a comprehensive and rigorous verification process.
▍Ultra-high-speed verification: Pushing the limits to ensure stable performance
As a high-performance car, the Xiaomi SU7 Ultra boasts 1548 horsepower and a top speed exceeding 350 km/h, comparable to the speed of a high-speed train. To ensure the carbon fiber hood remains stable and durable under such extreme conditions, ultra-high-speed testing is essential.
This test requires transporting the vehicle to a professional testing ground capable of reaching speeds of 350 km/h. During the test, the vehicle continuously travels at high speeds, even performing high-speed cornering maneuvers. Under these conditions, the carbon fiber components of the car body, including the carbon fiber hood, must remain unbent and undeformed.
This ultra-high-speed testing is extremely costly; only a handful of testing grounds nationwide meet the requirements, and renting the entire facility for a single day can cost hundreds of thousands of yuan. However, it is precisely this significant investment that ensures the vehicle's safety at high speeds. If the carbon fiber hood had not undergone ultra-high-speed testing, it could suddenly detach during high-speed driving, posing a significant safety hazard to the occupants.
▍Pedestrian safety test: People-oriented, safeguarding life safety
As a daily means of transportation, pedestrian safety performance is paramount for vehicles. The Xiaomi SU7 and SU7 Ultra come standard with a pedestrian protection system that plays a crucial role in low-speed collisions.
In the event of a low-speed pedestrian collision, the vehicle quickly activates the pedestrian protection device, causing the entire hood to rise rapidly. This provides sufficient buffer distance for the pedestrian's head to impact the raised hood in the event of a collision, reducing the impact force and minimizing injury, thus effectively protecting the pedestrian's safety.
During testing, we simulate real-world collision scenarios. Upon impact, the pedestrian protection device springs up instantly with considerable force, requiring the carbon fiber hood to withstand a significant impact. Xiaomi Auto has conducted extensive testing and optimization to ensure the pedestrian protection system functions effectively in critical moments, truly safeguarding pedestrian lives.
On the Xiaomi SU7 Ultra, a 2000-hour light durability test was conducted to ensure the product would not yellow, peel, or crack under prolonged exposure to sunlight. Simultaneously, 16 paint-related tests were performed to verify the paint's resistance to damage from everyday use scenarios such as small stone impacts, ensuring it wouldn't easily chip. Even the adhesives used for bonding underwent specialized mechanical testing.
Only through repeated verification tests can the quality of the carbon fiber hood be truly guaranteed, making it a "shield" for ensuring driving safety and enhancing the driving experience.
Every application of carbon fiber in the Xiaomi SU7 Ultra mass-produced version represents an ultimate exploration of performance. With its Nürburgring lap time of 7 minutes 04.957 seconds, and the 11-step carbon fiber hood protecting your journey, Xiaomi Auto demonstrates through breakthroughs in materials, processes, and verification that carbon fiber is not only "black gold" on the racetrack, but also a readily accessible technological commitment in mass-produced vehicles.
This article is reprinted from the WeChat official account of Automotive Materials Network.
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The Xiaomi SU7 Ultra's cover is from Aosheng Technology.
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