drivEcomp II – Development of a high-performance SMC for structural applications in electric drive systems

Press & Joining Technologies8News25

In the project "drivEcomp II", new materials and manufacturing processes are being developed for producing components of electric drive machines for use in bus and rail applications. Weight reduction at component level with fiber composite technology increases the power density of the drives, thereby saving resources and reducing energy consumption over the service life.

Compared to metallic materials, fiber-reinforced plastic composites (FRPC) often involve higher raw material and manufacturing costs as production usually consists of only a small proportion of automated processes. In the 1960s, therefore, Sheet Molding Compounds (SMC) were developed which for the first time enabled the fully automated production of FRPC. These compounds are based on polyester or epoxy resins which are produced as flat semi-finished products from a resin paste and chopped fibers in a fully automated process. For component manufacture, the semi-finished product is cut to size, stacked until component weight is achieved and placed in a hot tool. Due to the heat effect in the pressing tool, the viscosity of the SMC decreases, it starts to flow and fills the tool. The flow of the material allows great design flexibility due to the feasibility of implementing complex geometries such as ribs and webs. SMC made it possible for the first time to manufacture FRPC components suitable for large-scale production at competitive prices compared to metallic components. Today, glass-fiber-reinforced SMC is primarily used in structurally low-stress components such as outer skin components in the automotive industry. In order to economically exploit the weight-saving potential of FRP in other areas, recent developments are aiming at using SMC in structurally highly stressed components. For this purpose, carbon fibers and higher fiber volume fractions are used to improve the mechanical properties of the SMC.

In the project "driveEcomp II", housing components of an electric traction motor, which are usually made of metallic materials, are developed as carbon fiber-reinforced SMC components. The research activity of Leibniz-Institut für Verbundwerkstoffe GmbH consists of the development of the material and process technology. The component is subject to high dynamic and thermomechanical stresses during its operation. For the application under consideration, it is essential that the component stiffness remains as constant as possible over the entire operating temperature range (- 20 °C to 120 °C). A change in stiffness leads to a shift in the natural frequency of the overall system. At certain excitation frequencies, this can lead to undesirable resonance effects which in the worst case can result in component failure. Commercially available carbon fiber-reinforced SMC materials show drops in dynamic flexural modulus of up to 16 % at the maximum operating temperature of 120 °C. SMC material based on an epoxy resin system developed and investigated as part of the project shows a drop of 6 % at the same temperature which is on a par with that of aluminum.

Another research focus is on optimizing the process technology. During the production of semi-finished products, complete and uniform impregnation of the reinforcing fibers must be ensured in a manner that the occurring loads can be transferred into the reinforcing fibers via the matrix. For this purpose, the processing viscosity of the resin system is adjusted according to the applied resin film thicknesses, the fiber type and the fiber quantity. In part production that is using heated sealing edge tools on parallel-controlled hydraulic presses, the process parameters (mold temperature, cavity pressure, holding time of the pressure, closing speed) and mold occupancy are adapted to the part geometry and material properties. This ensures complete filling of the mold and curing of the resin system.

The goal of the project is to produce functional prototypes of the components which will be tested in further functional tests under realistic conditions.

Project partners:
Siemens AG
CirComp GmbH
Gustav Gerster GmbH & Co KG

Project ‘drivEcomp II - Advanced composite solutions for electric drives to increase the power density in ground-based mobility applications’ is funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) on the basis of a decision by German Bundestag (funding reference 19I20017D).

Figure 1: Schematic depicting the production stages of a C-SMC component

Figure 2: Schmidt & Heinzmann SMC Line 600

Figure 3: Results of dynamic mechanical thermal analysis (DMTA) - comparison between commercially available reference material and project developments

Figure 4: Microsection of C-SMC with 50 % fiber volume content


↰ News