A team of experts from the professorial chair for structural lightweight design and polymer processes at Chemnitz University of Technology (TU Chemnitz), the Steinbeis Research Center for Automation in Lightweight Construction Processes (ALP), and Cetex gGmbH have been looking into ways to create fiber-reinforced thermoplastic structural components that are suitable for mass production. The aim is to develop a production technique that would improve productivity compared to conventional methods, reduce manufacturing costs, and thus open the door to new applications. To examine mass production suitability, the teams have developed a pilot plant.
The project is part of a cluster of excellence called MERGE at TU Chemnitz, which is headed up by Prof. Dr.-Ing. Lothar Kroll and backed by the German research alliance DFG. The team involved in the project developed a sequence of processes that would make it possible to combine and synchronize different steps resulting in continually functioning upstream and downstream work processes – a coiling process that would, for example, make it possible to lag and, if required, fiber-reinforce extruded sections. The process guides an “endlessly produced” strand (a coiling hub or “liner”) through a coiling unit where it is coated with a fiber-reinforced material. The machine has also been designed so that asymmetrically rotating cross sections can be coiled. It includes a pinch and compacting roller to make it possible to form different surfaces more flexibly, meaning that undercuts (concave sections) can also be layered with materials.
Orbital coiling is a combined process involving thermoplastic tape layering and coiling in one, and this is the function of a so-called coiling unit. Fiber-reinforced thermoplastic ribbons – or tapes – are used to “coat” materials. For their research project, the three partners from Chemnitz decided to completely consolidate the reinforcement fibers within these tapes and wrap them in a plastic matrix. The tapes are melted during processing by an end effector. This forms the shape and individual layers are firmly bonded to one another through molding adaptation. Assuming the coiling hub is made of the same plastic as the matrix material, it can also be joined with the material composite. Alternatively, it can be removed after production in a fixed state.
Synchronizing processes into continual upstream steps is made possible by the orbital movement of the individual coiling units. These rotate around the coiling hub which is in continual translational movement during the coating or coiling process. The pilot plant developed by the lightweight design experts for their project was based on modular principles so it can also be adapted by adding individual coiling units constructed along the same lines. By rotating individual coiling units around the coiling hub, each step works like an inverted coiling process. In essence, the kinematic elements of a coiling module consist of groups of multi-axis kinematic parts and a specially developed end effector – i.e., the orbital laying head. The groups of parts that work by interacting with one another are a mechanical drive, the tape feeder, a heating unit, a mechanism for consolidating individual layers, a cutting system, a sensor system, and a control unit for the overall machine.
The first stage of machine development and commissioning is to wind a section with a uniform diameter. Once the unit is running, the output section – in this case the consolidation roll – is guided systematically around the coiling hub at a constant speed (up to 200 mm/s). The required guiding curve is then based on kinematic criteria, and, as movements will be critical, these will have to be defined at each stage. The end effector on each coiling unit (the orbital laying head) is equipped with all features required for the technical process of processing semi-finished thermoplastic parts. The pinching force required for laying tape is applied by a special mechanism on the consolidation module. This is typically attached along standard lines to the local surface by swiveling the consolidation module into place. The machine is currently being ramped up and plans are already underway to transfer the findings of research to other applications.
Prof. Dr.-Ing. Wolfgang Nendel and Mirko Spieler are co-directors of the Steinbeis Research Center for Automation in Lightweight Construction Processes (ALP). The services offered by the Steinbeis Enterprise range from automation concepts to lightweight construction processes, the coordination of research projects, projects in the field of handling methods, special machines, and prototypes for use with automation concepts.
Prof. Dr.-Ing. Wolfgang Nendel, Mirko Spieler
Steinbeis Research Center Automation in Lightweight Construction Processes (ALP) (Chemnitz)