Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

87 Composites | Focus Additive manufacturing As mentioned, when a composite part is designed with geometries that are too complicated to be formed in a mould – as is increasingly common – conventional manufacturers might split the part into subsections to be joined together afterwards. Increasingly however, additively manufactured parts are being used as serial production components, whereas just a few years ago, 3D printers were largely relegated to being used solely for quick prototyping. Although some companies insist that additively printed composites cannot be made to the quality of traditional isotropic materials, as they still lack strength in the z axis, numerous organisations in the aerospace and automotive space have not let that stop their uptake of additive manufacturing (AM). Most now agree that eventually, all composite manufacturing will be performed using some kind of sintering or extrusion process under the AM umbrella. By and large, high-end suppliers of additively manufactured composites use selective laser sintering (SLS) to shape and cure plastic powders amid chopped fibre reinforcers. UAV and motorsport manufacturers increasingly use SLS- produced carbon and fibreglass parts with polyamide matrices, for their low weight and cost. Rollers (essentially rolling pins actuated across each new layer of additives to ensure even distribution before laser sintering continues) in modern SLS machines help greatly to orient the chopped fibres to achieve the directional performance desired, and skilled AM engineers can produce composite parts with superior qualities to those using traditional isotropic plies. In the unmanned world, demand for 3D-printed parts is higher among small- to-medium UAVs than other vehicle types, mainly because of the limited dimensions of available SLS machinery. At the moment, the largest commercial additive printers for composite materials have internal spaces of around 500 x 500 x 500 mm. Naturally, it would be possible to make larger pieces than this by bonding two or more parts together, but as mentioned that can create operational or cost issues. AM printers are gradually getting larger though. For example, we recently covered Dive Technologies’ use of Cincinnati Big Area Additive Manufacturing (BAAM) machines ( UST 37, April/May 2021) to additively form custom sections of flooded hulls for large UUVs. However, such machinery depends on using fused deposition modelling (FDM), which produces rougher surfaces than SLS. That would mean having to apply some kind of coating for hydro- or aerodynamic performance, adding weight and cost. Moreover, FDM is sometimes prone to creating air voids in the material, which would be unacceptable to aerospace composites customers. In lieu of composite AM machine developers enabling the production of bigger parts, composite AM part suppliers have made efforts in recent years to maximise their materials’ ultimate tensile strength, elastic modulus and other factors that are critical to aerospace applications. This can enable small unmanned vehicles to have, for example, complex motor arms to help vibration damping or elliptical wings for efficient lift, with all the necessary ribs and spars for structural loading without needing to rely on adhesives to install them after manufacturing. Flat and relatively simple parts in need of high stiffness, such as propeller blades and tubes, are likely to continue to be produced to high quality and cost-effectiveness using traditional manufacturing approaches for many years to come. However, main body sections and electronics housings across the sub-25 kg UAV world are converging towards additively printed composites, and this user base might expand over the next few years as much larger composite 3D printers capable of manufacturing metre-long parts are expected to come on the market in the near future. One company is able to achieve this with its fused deposition technology through a combination of nozzles and jigs on rotating joints, mounted on robotic arms. With multiple arms for printing and holding components, a part can be swivelled and tilted as necessary for depositing additives (including thermoplastic matrix and extruded fibre) in any dimension. Very large composite structures can be additively manufactured by mounting Unmanned Systems Technology | December/January 2022 Additive manufacturing is gaining traction for serial production of small-to-medium UAVs (Courtesy of CRP Group)