Manufacturing CFK parts

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Parts manufactured from fiber reinforced plastics are used since decades in aerospace industry and sports. The superior ratio of strength or stiffness to part weight is a major advantage of these materials and corresponding constructions. Therefore significant weight savings compared to conventional metal-based designs can be achieved. However, manufacturing processes used in aerospace exhibit only little automation and hence lead to long production cycles. This has been a major reason for high part costs impeding the use of these innovative materials in mass production, e.g. within the automotive industry.

With the RTM process (resin transfer molding) significantly reduced cycle times, reproducible product quality and a high degree of automation of the manufacturing processes become possible. With RTM processes fiber material is positioned within a closed mold, resin is injected at increased pressure, components are cured and subsequently removed from the mold. Especially the injection stage offers potential for reduced production cycles. However, this requires an accurate prediction of the resin flow through the fiber material within the mold. Employing numerical simulations this aspect of the process can be investigated efficiently and in great detail. Also causes of defects, e.g. non-wetted areas due to non-optimal placement of the inlet ports, and local fiber re-orientations due to excessive injection pressure can be detected and hence avoided during design of the manufacturing process.

Prior to the analysis of components, the fluid mechanical properties of the employed fiber material need to be characterized. This is performed by detailed  fluid-flow simulations through small material specimens, whose fluid mechanical properties can be extrapolated to an entire fiber mat. The result of these simulations are directional permeabilities, which depend on the specific fabric employed and on its degree of straining or draping.

Figure 1 shows results of such a simulation for a generic fiber fabric. The resin flow front, areas of higher flow velocities at constrictions between the rovings of elliptic cross-section, and the saturation process of rovings including the temporal entrapment of air bubbles within rovings are clearly visible. When analyzing components, individual rovings can not be resolved. Rather the fiber fabric is modeled by a porous medium having corresponding directional permeabilities, which have been determined previously. In Figure 2 resin flow fronts of an injection simulation of a flat plate is shown. The plate's dimensions are chosen similarly to bonnets or roof structures as used in automotive industry. The fiber layup consists of three layers of unidirectional (UD) carbon fibers, oriented in angles of +30°, +150° and +270°. The thickness of each of these UD-layers is resolved by three mesh cells. The injection simulation clearly reveals the preferred flow directions along the fiber axes within each layer. Also computing times of a few hours only when running on HPC-systems allow for multiple optimization loops of the manufacturing process also for larger parts.