BMW cooperated with the Munich University of Technology to combine automatic fiber placement with 3D printing using continuous fiber reinforced materials for the production of automotive demonstration parts, and significantly reduced mold costs.
Middle chassis roof frame made without high cost molds: the core layer made by 3D printing technology based on extrusion, combined with the upper and lower skin layers laid by AFP technology, is used to make the middle chassis roof frame. Its strength and stiffness can be comparable to that of hollow steel parts, but it reduces the mass by 40%, reduces the cross section by 50%, and also eliminates the injection mold cost required by the previous MAI Skelet design
In 2019, a development project called MAI Skelet led by BMW of Germany showed a composite windshield frame, which was designed to replace the previous thermosetting composite material technology with unidirectional carbon fiber reinforced thermoplastic (CFRTP) pultruded parts.
In a 75s two-step process, the hot formed pultruded parts are first formed, and then coated with injection molding. The performance of the structural roof components produced thereby exceeds all the requirements of the previous components.
In 2020, SGL Carbon and Koller Kunststofftechnik won the order of BMW for many years, and began to mass produce carbon fiber reinforced thermoplastic for the frame of front windshield and rear window.
The thermoplastic composite windshield frame displayed by MAI Skelett project is made by coating and injection of carbon fiber pultrusion part (lower), which is used on BMW iX SUV
In 2019, BMW engineers began to cooperate with the Munich University of Technology (TUM) to study how to reduce the injection cost of such parts by using 3D printing.
For years, TUM has been carrying out various project studies to explore how to combine traditional composite production processes such as automatic fiber placement (AFP) with 3D printing using continuous fiber reinforced materials. The technical development engineer of BMW Lightweight Structure and Technology Center explained: "The mold for injection molding is very expensive. Our goal is to obtain a solution that is completely comparable to MAI Skelet technology, but we hope to use 3D printing to significantly reduce costs."
In order to carry out the next generation innovation of Skelett roof frame, the researchers made two different demonstration parts using two different 3D printing methods and continuous carbon fiber reinforced thermoplastic. They used selective laser sintering (SLS) and injection or AFP technology to improve the front roof frame demonstrated by the MAI Skelet project.
The components described in this article combine 3D printing based on extrusion with AFP to produce a middle roof frame at the connection of B-pillars between chassis frames. Both frames are slightly bent, forming a closed box structure with the chassis frame, providing the necessary stiffness and torsion resistance. However, the front roof frame also needs to be matched with the windshield and a number of accessories for interior trim components.
SLS and Extrusion
The BMW team led by Maidl identified SLS and extrusion as the most suitable 3D printing process for the roof frame.
SLS uses laser to sinter powder materials into solids defined by 3D CAD models of components. For example, CRP Technology of Italy uses its Windform materials of multiple levels added with carbon fiber crushing materials and SLS process to manufacture composite components.
In order to combine the SLS process with CFRTP materials, the BMW team used the SLS process to print out very complex structures to form cavities that can inject or lay thermoplastic CFRP materials.
Relevant personnel said: "We first print out this part, then put CFRP reinforcement, and then put metal inserts." Metal inserts can be used to connect the roof frame to the surrounding structure.
The second process option is based on the extrusion of the robot, that is, after the plastic particles melt, the 3D print head installed on the robot arm deposits the plastic melt. "This is more attractive to us because it is cheaper than SLS." Relevant personnel said.
"The use of extrusion technology can also make use of recycled materials." TUM's 3D printing research assistant and expert said, "We used an injection grade material, which is 10 times cheaper than SLS material."
Relevant personnel said that the cost of less than 5 euros per kilogram is impossible for SLS. The problem with SLS process is that although the powder collected from previous printing can be reused, the material will lose some properties and processability because it has been heated once. Therefore, new powder with a volume ratio of at least 50% must be added.
In order to ensure the correct proportion of participation, it is also necessary to monitor the cost. By extrusion, the previous materials can be crushed and put into the melt hopper for reuse. Extruder for this project is provided by Hans Weber Maschinenfabrik Company in Germany.
"Extrusion also allows us to use mass produced material grades, so this is a very cost-effective process." Relevant personnel said, "On the other hand, the advantage is the high deposition rate of materials.
In large-scale processing, the maximum material deposition speed can reach 2~35 km/h, while the processing speed of other processes is mostly limited to 1 km/h. "
In order to produce the demonstration parts of the project, the AM Flexbot printer of TUM (provided by CEAD of the Netherlands) and a standard polyamide (PA) mixture reinforced by chopped carbon fiber (CF) (provided by AKRO-PLASTIC of Germany) are used to print the basic shape, and then the unidirectional carbon fiber PA tape is laid with AFP technology to strengthen the local. These strips are supplied by SGL Carbon.
Because the materials for printing and local reinforcement use the same type of fiber and matrix materials, it is easier to recycle. "Our design is to build a sandwich structure, that is, to lay AFP skin layers above and below the core layer of 3D printing." The relevant personnel explained, "This allows us to eliminate the mold cost and help us create an economically attractive business case."