Fluffy Sandwich with Strong Features
Thermoplastic Sandwich Structures Automatically Made
There is a high demand for concepts of weight saving in aviation and automotive industry. Their large potential for lightweight construction therefore makes sandwich structures a very attractive option for these branches. Both sides of a lightweight core structure are covered with a face sheet. This way, weight is reduced, while excellent mechanical properties are achieved at the same time, e.g. flexural strength and energy absorption. What is more, sandwich structures offer good properties of thermal and acoustic isolation.
Talking about the production of sandwich structures, a characteristic feature, until today, has been time-consuming and expensive manual work. Automated processes can rarely be found, yielding only very simple components with restricted functionalities at present. That is where the MAI Sandwich research project comes in. Its aim was to develop a completely automated process that integrates forming and joining of coatings and core as well as component functionalization.
The project even went so far as to focus on recyclability of the materials, at this early state. Therefore, the engineers deliberately used sorted materials for the sandwich structures in order to make recycling of the components as simple and efficient as possible.
One Mold Concept to Fit Many Materials
To make sure components have a high compressive strength, the researchers used fiber-reinforced plastics from automotive (PP-GF, PP-CF, PA-CF) and aviation (PESU-CF) applications, respectively. While processing conditions are different for the materials, finding a mold concept to fit all of these materials represented a particular challenge. For example, PESU-CF must be heated up to approx. 400°C (while PP requires only just above 200°C) and coatings are only one millimeter thick, thus cooling down very fast. This is why a very fast handling system had to be designed for the sandwich layers previously heated up in peripheral units, to transfer them to the mold without major loss in temperature.
An entirely new kind of process technology was also necessary so that sandwich structures could be functionalized, and to prevent deconsolidation of the coatings during joining. This is because it is impossible to mold materials directly upon a sandwich structure due to the high pressures applied (approx. 1500 bars are required with the aviation material PESU-CF), which would make the foam core collapse. A novel type of slide-table mold with a dummy core of steel solves this challenge. To put it simply, the thermoplastic coatings are pressed against a steel version of the foam core, to be formed. The functional elements can now be molded on without any problems, because the steel dummy core can resist the high pressure. The actual foam core is inserted into the mold in a following step, when joined to the coatings.
High Degree of Automation with Favorable Cycle Times
The partners are satisfied with the overall outcome of the project. Operating on a plant with only one mold, the partners to MAI Sandwich were successful in producing thermoplastic sandwich structures including their functional elements in a process almost completely automated. The process can even handle several types of plastics with the most diverse properties. The novel process reaches a high degree of automation and outstanding cycle times - in an all-automatic mode, processing times of under 2.5 min for automotive materials, and under 5 min. for aviation materials are possible. Another outstanding feature is the application of sorted materials, making the components extremely well suited for recycling.
The MAI Sandwich research project was funded by the Federal Ministry of Education and Research. The partners to the project were:
- Airbus Defence and Space GmbH Airbus Group Innovations
- BASF SE
- BMW AG
- Foldcore GmbH
- Hofmann – Ihr Impulsgeber (Werkzeugbau Siegfried Hofmann GmbH)
- Neenah Gessner GmbH
- Neue Materialien Bayreuth GmbH
- SGL Carbon GmbH
- TU München – Chair of Carbon Composites