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Smart Stacking – a Handling System for Quickly Producing Laminates

Steinbeis acts as research partner on the processing of preformed thermoplastic materials

Developing production processes for modern composite components that work well with lightweight materials is becoming more and more important in business – not just for environmental reasons, but also because of political factors. To do this, new kinds of production technology have to be tested and then continuously developed not only so they are ready for serial production, but also so that modern lightweight materials can be used economically in high volume manufacturing. One particularly promising method is to use thermoplastic- based fiber-reinforced composites (FRCs). Compared to thermoset materials, these can be processed much more quickly and they make it is less of an effort working with finished materials and semi-finished parts. To accelerate processing times, the mechanical engineering specialist S&F has been working with the Steinbeis Innovation Center for Automation in Lightweight Construction Processes (ALP) and the professorial chair for structural lightweight design and polymer processes at Chemnitz University of Technology (TU Chemnitz) on the development of a new kind of handling system. Their aim: to develop a system that makes up individual, semi-finished, thermoplastic cuts into a layered structure. This should take place directly with a mechanical gripper and the resulting laminate should be capable to bearing large loads and should be fixed.

The gripper developed by the research team; demonstration equipment

Pre-impregnated semi-finished thermoplastics with continuous filament fiber reinforcement – so-called pre-pregs – are simple to transport in rolls, which are delivered by the meter. They can be used directly in production processes and adapted to individual parts, usually coming in individual sheets for layering into laminates. The direction of fibers is matched to each part during stacking before the components are pre-heated and processed into high-performance composite parts. Thermoplastic processing involves high volume methods such as cold pressing and injection molding. The semi-finished fiber composites are laid together in tools to make complete parts. Injection molding is particularly suitable for producing complex fiber composite parts, using partial continuous filament fiber reinforcement. Overseen by the professorial chair for structural lightweight design and polymer processes, the MERGE center at TU Chemnitz is currently involved in fundamental re- search in this area. The technologies involved in these processes are key drivers of lightweight construction developments, bringing together highly automated processes and high volume manufacturing. As a result, these technologies are found in many different areas and undergo continual redevelopment.

The research being carried out by S&F and the Steinbeis Innovation Center for Automation in Lightweight Construction Processes is part of a state-funded ZIM project. The main focus of the project is the chain of events between the cutting of individual layers and the pre-heating required during final processing. The aim of both project partners was to shorten the time-consuming handling of components between the cutting of individual layers and finished laminate composition. Previously, each individual cut had to be lifted off the cutting board, turned to exactly the right angle, and temporarily lined-up before all individual layers could finally be brought together from their respective position. With the new process, there is no longer any need to put individual cuts to one side, so there are no more special movements with a complex robot gripper grabbing parts, stacking them, and holding them in posi- tion during cutting. The project partners built a gripper prototype for semi-finished parts up to 650mm long and 300mm wide. The experts decided to create a linear system that moves to one side of the gripper, picks up each individual layer with a suction element, lifts it, and feeds it back into the gripper, where the layer is ultimately placed on the stack. Once all layers are in the right position, they are joined at fixed points using an integrated 35 kHz ultrasonic welding system.

The research focused on developing suitable ultrasonic parameters and sonotrode points to ensure parts were fixed securely without damaging the fiber reinforcement in the semi-finished parts. It also looked at developing a suitable gripping system and arranging the construction of each of the functional parts in an appropriate configuration. The results of the research project speak for themselves: Depending on the number of layers required, the new technology can cut the processing time needed to go from original cuts to a prepared semi-finished stack in half. This significantly reduces process time and enhances profitability in production.