Manufacturing of integral frames by means of co-consolidation in thermoforming process

Efficient production of carbon fiber reinforced plastics is still an issue in transport industry. In series production, the processing of thermoset prepregs in an autoclave is common, which leads to a high component quality in comparatively small quantities. The repeatability of temperature and pressure control ensures robust process conditions. One disadvantage of processing in an autoclave is the low eco-efficiency due to the high cost of consumables, long cycle times and the high energy input for generating the vacuum as well as for temperature control of the autoclave. Research and development in the field of so-called "out-of-autoclave (OoA)" processes is dedicated to this disadvantage. In the field of thermosets, liquid impregnation processes, such as resin transfer moulding (RTM) or wet compression molding, are well-known representatives already used for series production. Regarding thermoplastics, the use of organo sheets and their processing in isothermal molding processes is a typical application in series production.

In OSFIT (One-Shot Fully Integrated Thermoplastic Frame) aerospace research project, Leibniz-Institut für Verbundwerkstoffe (IVW) is working on the development of an efficient process chain for processing fiber-reinforced thermoplastics into structural components for the aerospace industry. Premium Aerotec GmbH acts as consortium leader and potential user of the developed technology. In addition to IVW, the project partners Fraunhofer IGCV from Augsburg, Automotive Center Südwestfalen and Fraunhofer IFAM from Stade are developing the technologies of laser-assisted automated tape laying, molding and injection-molding as well as automated tolerance management in a one-shot process. The combination of tape laying of two-dimensional Tape Preforms, subsequent thermoforming in an isothermal molding process and the integration of a new type of fusion joining process for stiffeners should take advantage of the special properties of thermoplastics and thus contribute to efficient component production. Fibers are laid down in accordance with the load path. Placing them on the plane allows higher travel speeds and inexpensive tools. Fusion joining instead of rivets as a connection technology saves weight and costs. The concept developed to integrate the fusion joining process into the forming process was applied to the production of integral frames and validated with the production of demonstrator components.

Figure 1 shows the production of the demonstrator components. The basic principle of processing fiber-reinforced thermoplastics is heating the polymer above its melting temperature range and cooling down under pressure. Molecular chains can slide off one another or diffuse above the melting temperature, which means that forming and joining processes can be carried out.

The cooling from the melting range under pressure leads to solidification. This process, called consolidation, is decisive for the subsequent component quality. Incorrect process parameters  during consolidation lead to air entrapment and ultimately to less mechanical performance. The used tools and heating equipment allow simultaneous melting of the Tape-Preforms and the local stiffeners so that molding and joining can take place during one press stroke. Less than 10 minutes elapse from melting to component removal after the consolidation process is complete. The process management requires both the control of the heating behavior of the Tape Preforms and the local stiffeners, as well as the draping behavior during shaping.

Figure 2 shows an integral frame with fabric-locked stiffeners after the molding and joining process. One of the research topics  IVW is dedicated intensively is the determination of the influence of Tape-Preform consolidation after tape laying but before molding on the component quality after the molding process. It has been shown that heating in the radiation field can lead to deconsolidation of the Tape Preform. With a suitable process control during the subsequent forming and consolidation, however, components can still be produced without air inclusions that meet all quality requirements

Project “OSFIT – One-Shot Fully Integrated Thermoplastic Frame” is funded by the Federal Ministry of Economic Affairs and Energy (BMWi) on the basis of a decision by German Bundestag (funding reference 20W1706C).

Further information:

M.Sc. Julian Weber
Scientific Staff Roving & Tape Processing
Leibniz-Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Straße 58
Telefon: +49 631 2017-437
E-Mail: julian.weber@ivw.uni-kl.de