The use of fiber composite materials and in particular the sandwich construction method offer particularly great potential for lightweight construction applications in commercial vehicles. Such sandwich structures consist of highly rigid and strong cover layers and a lightweight core. In addition to high material efficiency, this combination also enables very high weight-specific bending and buckling stiffness. The sandwich architecture is therefore also suitable for heavy-duty structural components, for example in aircraft and vehicle applications. It enables potential weight savings of up to 70 percent compared to monolithic structures, with a load-bearing capacity of the components that corresponds to or even exceeds that of (significantly heavier) metallic components.

One challenge to date has been that these components can hardly be manufactured in large-scale production processes. The use of thermoplastic fiber composite sandwich semi-finished products can be the solution here. These can be produced efficiently in continuous processes and turned into components in automated manufacturing processes. The participants in the joint "HyWaSand" project, which has now been successfully completed, have shown what is possible with this.

A flap for a storage compartment in a truck cabin was produced as a demonstrator. "We wanted to prove that the sandwich construction method with continuously produced thermoplastic sandwich semi-finished products is possible for such applications. Together with our partners, we succeeded," says Dr. Ralf Schlimper, who led the project at the Fraunhofer IMWS.

The thermoplastic sandwich molding technology developed at the institute proved to be particularly valuable in achieving this goal. This is based on a new type of thermoforming process and was developed specifically for the fully automated production of continuous fiber-reinforced sandwich components with a thermoplastic matrix in large series. Thermoplastic sandwich semi-finished products, consisting of a thermoplastic honeycomb core and cover layers of fibre-reinforced thermoplastic UD tape laminates, are heated in an infrared oven and, after automated transfer to the molding tool, 3D thermoformed and functionalized by means of injection molding.

"The TS molding technology proved to be very efficient in our project. We were able to show that it enables the efficient production of 3D-molded and functionalized lightweight components in sandwich construction in the injection molding cycle, i.e. with cycle times of around one minute. This shows the potential for further fields of application, such as the car body sector," says Schlimper.

In the project, ThermHex Waben GmbH focused primarily on the development of hybrid sandwich semi-finished products with a small cell width and functional polymer and metal foils to improve the surface quality. In addition, the new development of continuously manufactured semi-finished sandwich products with even lower material usage was driven forward. In conjunction with the necessary adaptation of the manufacturing process to make it suitable for series production, it was possible to achieve a further reduction in costs and weight compared to the previous standard configurations and non-recyclable material alternatives from reaction injection molding (RIM) processes. At the semi-finished product level, 30 to 45 percent of the weight can be saved with the same material thickness or up to 22 percent of the weight with comparable mechanical performance and greater material thickness. This improved resource efficiency in the use of materials also has a proportionally positive effect on material costs and CO₂ emissions.

In test structures, the project was able to demonstrate that components with Class A surfaces, which are particularly important for the automotive industry, can also be manufactured from thermoplastic semi-finished products in sandwich construction. Progress was also made for metal-plastic hybrid sandwich materials as part of the "HyWaSand" project, for example the lamination of test structures with metallic cover layers within the TS molding process.

"There are still open research questions here that we want to continue working on. This makes it all the more valuable that we have made great progress in the project in providing virtual engineering methods for process and component development and have also developed feasible test tools for various common component design features. The new possibilities for inline inspection and for suitable non-destructive testing methods to ensure component quality also enable us to rapidly bring the technology even closer to application in various cost- and weight-sensitive areas," says Schlimper.

"We are currently investigating the transferability of the results to customer development projects in order to facilitate entry into new market segments," says Matthias Biegerl, who led the project at ElringKlinger. "The potential is huge. By manufacturing the components in large series, cost-efficient lightweight structures for interior and body components can be produced."

The demonstrator will be on display at the JEC World plastics trade fair in Paris from March 5-7, 2024,  at the booths of ElringKlinger AG (Hall 6, K105), ENGEL Austria GmbH (Hall 5, L106) and ThermHex Waben GmbH (Hall 6, P108). The project partners will also present the component and the technology at the Composite Sandwich Conference on April 24-25, 2024 in Halle (Saale).