Application of Carbon Fiber Components
Recently, BMW disclosed the application technology of a continuous carbon fiber reinforced thermoplastic (CFRTP) structure in the body-in-white of automobiles. Starting from the conceptual design, a new lightweight design was formed by combining UD thermoforming and injection molding technology. , and finally achieved mass production on the BMW iX car.
This series of processes from design to processing and series production is carried out at the BMW Group's plant in Landshut. The resulting structure is both cost-competitive and sustainable. This official account will briefly introduce some technical details of this progress in two articles.
In 2013, an important milestone in the start of production of the BMW i3 series, the carbon fiber reinforced plastic (CFRP) body structure was mass-produced according to the automotive production process. Although the cost of structural parts of CFRP for automobiles has been greatly reduced compared to the aerospace field, CFRP body structures still cannot compete with common sheet metal structures.
For structural parts that can be mass-produced, cycle times well below three minutes must be achieved, and material costs should be significantly reduced. In addition to reducing the manufacturing cost of carbon fiber, it is also important to avoid process-related waste. Based on the need for sustainability and recyclability, thermoplastics are increasingly becoming the matrix material for carbon fiber reinforced plastic parts.
To increase market penetration, components need to be integrated into well-known automotive production processes, including painting processes. Since it needs to be passed through a furnace after cathodic immersion plating (CDP), it is required to have a temperature resistance of 180°C or higher without any adverse effect on the performance of the CFRP part.
01. Structural Design Concept
Considering various requirements and possible lightweight construction methods (Fig. 1), a CFRTP processing method was designed and developed, the core of which is to convert multifunctional injection molded parts into integrated structural components. At the same time, the use of continuous carbon fibers must be limited to the main load paths. As shown by the black bars/rods in the middle of Figure 1, these are unidirectional carbon fibers contained in rectangular profiles and impregnated with a thermoplastic matrix.
Construction Concept of Lightweight Structural Parts
The rod itself is injection-molded into a thermoplastic polymer, which is also reinforced with carbon fibers. The rib-like structure that can occur in injection molding leads to a truss structure, through which the individual bars are reinforced to form the overall profile. For cost and thermal deformation reasons, the thermoplastic polymer chose a polyamide 6 (PA6) matrix, a common material in body construction.
The innovation in the production of components is not in injection moulding, but more in the production of 3D shaped rods, their integration and fixing in injection moulding tools and the technologically reliable connection between materials. At the same time, injection molding opens up additional lightweight construction potential. Functional integration greatly simplifies the subsequent assembly process and reduces the number of parts.
The relatively low cost of injection molds and the simple adaptability of materials, such as by changing the fiber content in the injection molding compound, offer additional weight and cost reduction potential.
02. Conceptual Research
The above ideas were tested within the framework of the MAI Skelett joint project, which is part of the MAI Carbon frontier cluster. The main frame structure was derived from the BMW i3's front roof bow (Fig. 2), and was intended to clarify many fundamental issues, including rod forming techniques and boundary conditions for the bond between the rod and the injection-molded material. Another focus is the use of pre-formed in-process trimmed fibers produced by the BMW i3 as reinforcement for injection-molded composites.
MAI Skelett Concept Structural Part (a), Bending Test (b), Test Results (c)
The first component test was decisive, showing good agreement between the predictions calculated by the simulation and the flexural shock test (Figure 2). Therefore, the MAI Skelett project also technically confirmed the product geometry and design calculation method developed for the frame.
03. Preliminary Development
As the core of the skeleton structure, continuous carbon fiber-reinforced PA6-based composites must withstand the most diverse load distributions, which are largely influenced by production considerations.
For reasons of lightweight construction, a high fiber volume content and stable, good monofilament impregnation are required. Among the possibilities, a PA6 matrix and a CFRTP rod with a fiber volume fraction close to 50% were selected, and by optimizing the fiber impregnation and profile formation, a constant rod mass was obtained compared to the concept stage.
After the main requirements have been met at the concept stage, a 360° inspection and confirmation must be carried out from the product and its function in the vehicle. The main challenge is how to meet the requirements for its overall integration with the vehicle without requiring new body structures and painting processes. Side sheet metal sheets are used for integration into the entire vehicle (Fig. 3b). These labels provide clear access to skeletal components to ensure force transfer. They can be used in standard spot welding processes during body shell construction. With these sheet metal labels on the front roof bows, full vehicle testing is now possible.
Components of the front roof bow of the BMW iX series