The German Carbon Fiber Composites Union (Carbon Composites e.V., CCeV) is a joint organization composed of enterprises and research institutes, whose members span the entire industrial chain of high-performance fiber-reinforced composite materials. The alliance has multiple branches, and MAI Carbon is one of them. Approved by an independent jury on January 19, 2012, MAI Carbon passed the third round of selection by the German Federal Ministry of Education and Research (BMBF) for cutting-edge technology industry clusters, becoming one of the five major cutting-edge technology industry clusters, located in Munich, Augus Fort and Ingolstadt triangle, plan to form a large-scale industrial cluster of carbon fiber composite materials by 2020. In order to achieve this goal, the joint R & D projects carried out by MAI Carbon member companies focus on the entire life cycle of composite materials components, including resin fiber raw materials, component manufacturing, and material recycling throughout the entire industrial chain.
MAI Carbon is founded by Audi, BMW, Premium Aerotec, Airbus Helicopters, Voith, SGL, and IHK Swabia, German Carbon Fiber Composites Institute (LCC), Munich Technical University, etc., and currently has more than 120 member units. Since 2012, there have been 39 joint research and development projects among members of the institution, with funding ranging from hundreds of thousands of euros to millions of euros.
Today, the editor takes you through a demonstration project called "MAI Skelett" by MAI Carbon. The project is a new attempt by composites manufacturers to find ways to reduce component costs. Through continuous efforts, researchers have used a variety of materials to mix "appropriate materials for the right parts", while at the same time meeting the needs of large-scale production for automation and functional integration to the greatest extent.
Project Description
The MAI Skelett project received a grant of 1.9 million euros from the German Federal Ministry of Education and Research (BMBF) for a period of 17 months and was implemented by BMW. The partners include P + Z Engineering, SGL Automotive Carbon Fibers, CirComp and Eckerle the company. The project carries out research and development work on the windshield horizontal frame structure above the windshield and between two A-pillars, and forms a product and process demonstration. Its design is based on the existing BMW i3 model and complies with all the functional and structural requirements of the model design. The target component is not only the roof transverse frame structure, which provides good rigidity (which can effectively reduce NVH: noise, vibration and roughness), strength (help the roof component meet the impact requirements in compression experiments), and can also be used for sun visors. , Decorative parts, lighting lines and other interior parts, as well as provide support for windshields, sunroofs and roof outer panels.
This project first proposed the "skeleton" design concept, using unidirectional carbon fiber reinforced composite materials and pultrusion process, through the thermoforming-composite molding (overmolding) two-step method, producing structural parts in 75 seconds, surpassing previous stages The process requirements of the version parts have realized the large-scale application of thermoplastic composite materials in the body-in-white structure. In addition, the project increased the residual stress of body-in-white components and changed its fracture mode from brittle fracture to ductile fracture, thereby improving the crash behavior of the component. There are 4 unidirectional carbon fiber reinforced composite pultruded rods at the bend of the windshield frame designed with "skeleton", which are encapsulated in the component through the composite molding process. Of the four pultruded rods, two are near the bottom of the part and two are near the top, not in the same plane, which is convenient for providing functional accessories with torsional stiffness and complex shapes.
Material selection
The project uses relatively low-cost large tow carbon fiber as the reinforcing material. Because 50k large tow carbon fiber monofilaments are closely packed, resin wetting is very difficult. Therefore, it is necessary to optimize the fiber guidance in combination with the fiber stretching technology to achieve the desired prepreg effect, while ensuring a high fiber volume content of about 50%. SGL has mastered this technology and has included pultruded profiles in its "thermoplastic product selection box".
In addition to reinforcing fibers, the project also examined different types of PA6 resins to ensure that their viscosity and rheological properties can optimize the pultrusion rate and product quality. SGL's "Thermoplastic Product Option Box" provided the project with a variety of materials, including carbon fiber unidirectional belts, organic boards, chopped fibers of different lengths, and unidirectional carbon fiber reinforced pultruded parts. The above materials are made of SIGRAFIL 50k carbon fiber and a sizing agent suitable for thermoplastic resin substrates such as polypropylene and polyamide. There are many types of polyamide-based thermoplastic resins, including PA6, PA66, PA12, and some types of PPA as candidate materials. Some PA6s can even be obtained in situ during the molding process.
Thermoforming and compound molding processes
The material system initially selected for the MAI Skelett project was a carbon fiber reinforced PA6 composite. Later, the researchers adjusted the material composition so that the material can adapt to the shape of the part and the needs of the load on different parts. The selection of the thermoforming process mainly considers that the carbon fiber needs to be as straight as possible to show high strength and high rigidity. Therefore, the pultruded rod is stretched in the flow direction of the resin matrix, and the end is bent and stretched. .
In the second step, the thermoformed pultruded bar is placed under an infrared heater, and it is heated to a specified temperature within 50 seconds, and then transferred to an injection mold by a robotic arm. The chopped fiber resin paste is injection-molded on or around the profile through a composite molding process. The compound molding process has extremely high requirements on the precision of the mold and the process, so as to ensure that the position of the pultruded rod after hot pressing remains unchanged.
The total cycle of the two-step process of pultruded bar thermoforming and compound molding is about 75 seconds. Because the thermoplastic resin matrix can be remelted before the composite molding process, the pultruded rod after the hot pressing process can complete the final shaping of the part in a very short time and be integrated with the secondary injection molding material. This feature of thermoplastic resins even allows them to form connections with metal parts. At the same time, the thermoforming and injection molding processes of thermoplastic resin-based composite materials can also obtain consistency in product quality and controllability of the process, which is critical for large-scale production.
Ductile fracture
PPA and PA6 resin-based pultrusion profiles that are compatible with glass fiber and carbon fiber resin pastes have better toughness, and the fracture mode is tough fracture. Although the acquisition of the ductile fracture mode lost part of the load that the windshield frame could transmit, it significantly improved the structural integrity and comprehensive use performance of the body-in-white.
Although BMW did not specify its preferred material combination in the project conclusion report, the report concluded that the final simulation and test results show that the “skeleton” structure exceeds the pure carbon fiber reinforced composite components in addition to torsional stiffness All performance indicators, while torsional stiffness is not critical for windshield frames. Compared with ordinary carbon fiber composite parts, the load level and energy absorption level of the "skeleton" structural parts during collision are more excellent. At the same time, the component has a ductile fracture mode, which not only further improves the crash fracture performance of the composite structure, but also clarifies the relationship between its fracture behavior and the overall body-in-white structure.
Future applications of "skeleton" design
In the final report, BMW stated that the "skeleton" design concept could also significantly reduce production costs, raw material costs, and tooling costs when applied to six other automotive components. SGL also proposes to apply this technology to automotive and aviation seats, dashboards, robotic arms, X-ray workbenches and other fields.
The research on the "skeleton" design concept has not stopped. In the subsequent research and development project MAI Multiskelett, the design method was extended to multi-axial stress components. Large structural members with multiple load paths crossing. I won't go into details here.
The windshield horizontal frame structure designed by the “skeleton” concept uses the pultrusion process and the composite molding process, which effectively shortens the process cycle, reduces material waste, and explains the effective use of carbon fiber on unidirectional load structural components. It is a typical demonstration of the design and large-scale production of next-generation carbon fiber reinforced composite materials. At the same time, using carbon fiber scraps from other components to make resin pastes required for the composite molding process can effectively improve the functionality and performance of the components, and is an effective way to improve the sustainability of composite materials.