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35 The AM industry is maturing, and developing certification processes to allow 3D-printed components to be used in aircraft. This will also drive the use of AM into high-reliability UAV systems and driverless cars. Types of AM Three main types of AM are becoming common for building unmanned systems. They use materials ranging from polyimide plastics and metals such as aluminium and titanium, to cured plastics and even ceramic materials. Composite materials that combine plastics with carbon fibres are also being used with a variety of manufacturing techniques. Fused deposition modelling Fused deposition modelling (FDM) is a common material extrusion process using a heated polymer. This is drawn through a nozzle, where it is heated and deposited layer by layer. The nozzle travels horizontally while the build bed moves up or down vertically after each layer has been deposited. The design of the nozzle, the temperature and the material used are all tightly coupled to produce the optimum flow of material for depositing a layer. Adding layers to build up the structure can cause problems with the binding between them, but FDM printers use a heated build bed – essentially an oven – to keep the polymer flexible. The heated bed prevents ‘curl’ owing to contraction stresses and helps the layers to bond, but strength across the layers is still a significant issue for FDM parts as they are highly anisotropic. To address these issues, new materials and approaches are being developed. One of these is a semi- crystalline polyimide called poly ether ketone ketone (PEKK), which can operate in higher temperatures and is stronger than traditional thermoplastic materials; it can be used with fuels and hydraulic fluids as well. The performance of Nylon materials used in FDM can also be improved by adding stiffening agents such as carbon fibres to provide more strength. One advantage with FDM over other AM processes is that it is not limited to short carbon fibres, as it can handle longer ones to provide more strength in the direction of the fibres. That then converges with traditional carbon fibre manufacturing techniques used for UAVs, where layers of carbon tape are laid down to build structures Some FDM materials have a soluble support, where the material can be wrapped in carbon fibre to leave a composite surround as a sacrificial element that is dissolved. This approach is used more for prototypes and tooling. There is substantial intellectual property regarding the optimisation of materials for the manufacturing system, the design of the nozzles and deposition head, to enable high repeatability and long life. Additive manufacturing | Focus Unmanned Systems Technology | June/July 2018 Additive manufacturing is being used to produce production parts with high-performance graphite-based materials (Courtesy of Graphite AM) A prototype of an imaging system for the European Space Agency was built using FDM (Courtesy of the ESA)