The approach of constructing structural components in a sandwich design consisting of a lightweight core and outer layers of fiber-reinforced composite material is not new in itself. However, in the past, the comparatively higher costs of this method often led to the choice of aluminium solutions which were heavier but more cost-efficient. Frimo (Lotte, Germany) took on the challenge of developing clever and economical lightweight construction solutions in this multi-sandwich design, which also include those in conjunction with bionic surfaces. For instance, this includes the cooperative Street Shark project, where a shark skin structure was attached to complex sandwich components at various stages of development.
The latest version, Street Shark 4.0, is a rear wing for a Corvette (Picture 1). On the automotive side, the partner in this collaboration is Callaway Competition, a company specializing in car body manufacture, vehicle repairs and modifications for racing. The rear wing for this Corvette features an innovative three-dimensional sandwich construction fitted with a bionic surface structure based on shark skin in certain areas (Picture 2). Another important cooperating partner in this project is Eschmann Textures (Gummersbach, Germany).
Functionally-optimized multi-sandwich in 3D format
As part of further development, a comparatively complex three-dimensional component had to be selected as a demonstration part in contrast to previous parts that were flatter. A Corvette rear wing was chosen. Such a component, through its special cross-section and the resulting different air velocities on the upper and underside, leads to increased downforce for the vehicle on the road. A weight-optimized sandwich construction is used for the new Street Shark 4.0. In doing so, the manufacturing process for a three-dimensional foam core on a serial production scale is the focal point here.
Previously, a variety of core materials were used for the different sandwich structures, such as PET, PP or PUR foams as well as balsa wood or bamboo in the form of plates or sheets. Flat sandwich plates produced in this manner were necessary at the beginning of development and helpful in documenting results on real components as well as in interesting potential customers about the sandwich principle and manufacturing process, and to convince them of its advantages (Picture 3).
A sandwich lightweight construction
Over the course of continued development by Frimo, more complex three-dimensional sandwich structures can now immediately be produced from polyurethane using molded foams. For this purpose, a test tool was designed in which the three-dimensional core was first produced using hard PU foam. In addition to the tool and mold carriers, the Frimo TechCenter also features a PURe Mix metering unit (Picture 4) specially designed for matrix materials. Even while manufacturing the lightweight core, it is possible to integrate additional functions, such as for example taking fixation points or electronic elements into account.
Once the three-dimensional core is produced, the external skins are produced from a fiber structure by use of a special PU matrix system with a Frimo high-pressure RTM process. The first step is to insert the lower layer of the preform made from reinforced fibers (carbon, glass, natural) into an RTM tool. The molded foam core and then finally the upper fiber layer is inserted. After that, the RTM tool is closed and a special-low viscosity PU matrix system is introduced.
The low viscosity of the PU system enables rapid entry into the tool, and due to the snap cure, the start and duration of the hardening process can be adjusted fairly accurately in terms of time. The process releases a relatively low amount of heat with a maximum reaction temperature of well below 100°C. Nevertheless, the material is thoroughly suitable for surface coating in KTL systems with a glass transition temperature (TG) of more than 200 °C. The concept is significantly more efficient for certain applications in large-scale productions and at the same time sturdier than classic matrix materials. Due to low tool temperatures of less than 100°C and the low internal tool pressures, the system requires a relatively small investment, making it practically predestined for additional application in top layers on sandwich components, since it does not damage the foam core. As only low clamping forces are required for the tool, ceramic tools can be used.
Differently than for most processes typical on the market, more complex, large-volume three-dimensional structural components can also be directly produced in the desired geometry.
Decorative and functional surfaces
As part of the Street Shark project, Frimo managed, with the help of its cooperation partners, to produce a bionic vehicle surface that reduces drag. As the name suggests, the skin of a shark served as inspiration to improve the aerodynamics of a vehicle, an area in which car manufacturers in particular are constantly looking for improvements. The shark is a fast and skilful swimmer. The tooth-shaped scales on its skin significantly reduce drag. We were also able to prove this in this project using wind tunnel tests. While the first shark skin textures were attached to substrates using special films, it is now possible to apply delicate surfaces directly onto lightweight construction component surfaces by use of polyurethane matrix materials with the corresponding tool and systems technologies. In doing so, the surface structure was optimized in each new version by scaling, and tested and approved in wind tunnel tests. Ever since its initial presentation, numerous new trials and enhancements have been implemented with interested parties from a wide range of industries, for instance the wind power, sport and leisure industries, who are currently at the prototype and pre-production stage.