Automated high-precision gap filling for thermoplastic structures

The automated gap filling process developed by Fraunhofer IFAM in Stade makes it possible to quickly and precisely close gaps of variable widths that remain when joining components. This prevents gas and moisture inclusions. A robotic end effector with extruder is guided along the gap and precisely applies the molten plastic into the gap, which has previously been geometrically measured by the end effector sensor. To ensure sufficient adhesion of the filling material, the thermoplastic contact surfaces are thermally pretreated immediately before application.

If individual fibre composite components are joined together to form an overall structure, gaps and joints (so-called "gaps") of varying widths and lengths occur during production and assembly. Such gaps allow moisture and gases to enter and escape, which can lead to corrosion and impairment of the structural strength. Particularly in aviation and aerospace, but also in maritime applications such as shipbuilding, joints must be impermeable to moisture and gases.

Sealing for thermoset FRP 

Depending on the joining process and downstream processing, gaps and joints are nowadays filled or sealed with standard sealants. These sealants are usually epoxies, polyurethanes, MS polymers, polysulphides and silicones, which are important for fibre-reinforced plastics (FRP) with a thermoset matrix. For some years now, however, products and structures with a thermoplastic matrix have increasingly been used in larger branches of industry, such as the automotive industry and, in particular, in aircraft production.

Automated sealing with high-performance plastics l thermoplastic sealants

However, the use of common sealants is usually not immediately possible with a thermoplastic matrix. Due to the low surface energy and apolarity of some technically and economically important thermoplastics, a certain pre-treatment is required to ensure sufficient bonding between the sealant and the component surface. In addition, most sealants and joint fillers are unsuitable for downstream processing - such as thermoplastic welding - as they are not sufficiently resistant to the high process and machining temperatures, especially on high-performance thermoplastics.

The use of thermoplastic sealing material is of particular importance here. To ensure material compatibility during processing, the same matrix material that the fibres of the fibre composite component are embedded in is ideally used for filling.

Fraunhofer IFAM in Stade has an extruder tool for gap-filling and sealing using thermoplastics. The extruder is equipped with a specially customized nozzle to fill gaps of different sizes. The tool is equipped with a 2D line scanner to determine the gap geometry even during the process. The intelligent communication between the measuring sensors and the extruder enables gap filling with precise volume and without excess material. The extruder allows the processing of plastics such as PA, PMMA, PPS through to high-performance plastics such as PEEK or LMPAEK. In addition to pure plastic granulate, granulate containing fibres can also be processed.

The extruder tool can be coupled to an industrial robot and guided by it. It therefore also enables precise automated gap filling for complex-shaped components and even large structures.

Different requirements for the filling and sealing of thermoplastic (fibre composite) structures often require the development of individual solutions. This is why the R&D work at Fraunhofer IFAM in Stade includes advising the user, selecting and characterizing the gap filling and sealing material as well as the appropriate selection or development of end effector and nozzle systems through to the application technology. Complete customized gap filling and sealing processes can be designed on this basis.

The research and development work on automated gap filling technology is part of Fraunhofer IFAM's Automation and Production Technology R&D department. The team from the "Joining Technologies" department has a robot-assisted system for gap filling using different end effectors. Fields of application to date are the filling of gaps in thermoset FRP structures and the sealing of gaps at the joints of shell elements of thermoplastic FRP aircraft fuselage shells.