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38 bed. Using aluminium powder and then changing to titanium for example can lead to faults and fractures, particularly in engine nozzles in space systems, which are under a lot of thermal and mechanical stress. SLS is a common technique for building metal components, but it is also suitable for carbon fibre composite. Originally developed for the motorsport industry, the composite in question is now being used to build a fully modular UAV system. The body is made from a reinforced composite polyamide carbon-filled material. This has greater resistance to shock, vibrations and deformation compared to other materials from the same supplier. The material also shows increased impact strength and elongation at break, as well as excellent thermal properties and resistance to high temperatures. The four arms supporting the body frame were 3D-printed using a less stringent version of the material. Using 3D printing gave the engineers more flexibility to make design improvements and avoid the limitations of traditional tooling, which struggles to make thin struts. That has helped reduce the cost of the UAV system substantially. The critical problem was scaling up from a prototype to a full-size version. There is a 0.15 and 0.20 mm tolerance that can compromise the assembly of the UAV, but being able to produce different versions quickly on the 3D printer avoided that problem. The arms consist of motor supports plus the removable arms and interlock base, which allows a user to easily tighten the four arms with the support of a patented ‘tension ring’. That allows the user to connect and disconnect the interchangeable arms as required. The same graphite thermoplastic material used to make the UAV is also being used on a range of space systems, including the hatches for the unmanned Orion system and small cylindrical satellites. The satellites will be released from the International Space Station and need a dedicated launch tube, which has been 3D-printed using the graphite material. These graphite materials are used for clips and brackets but they are also becoming popular to make impellers in pumps that operate at up to 175 C. The impellers have the same strength as a metal part but are lighter and can be sealed and shielded. The graphite material is 10-20% lighter than metal and can be used for components with a honeycomb structure that reduces the weight even further. Entire UAVs are now being printed using AM, with the base printed in a graphite SLS to give them impact resistance. Also, thermoplastic materials can be used for underwater vehicles as the components do not expand after sealing. These components can be made in 1.6 m sections and with a diameter of 300 mm. Stereolithography The stereolithographic apparatus (SLA) process first appeared in the early 1980s, with commercial machines appearing in 1984. SLA components are created by successively printing thin layers of a liquid photopolymer, or resin, that is cured using UV light. The technology is used more for prototypes and tooling such as moulds for carbon fibre or glass fibre structures. So-called ‘right-side-up’ SLA machines are built around a large tank containing the liquid resin, and the build platform. A UV laser is focused onto the surface of the resin, tracing the pattern of a layer of the 3D model. The build platform then descends by a distance equal to the thickness of a single layer, and a resin-filled blade sweeps across the vat to re-coat it with fresh material. This approach appears mostly in large- scale industrial systems, providing some of the largest build volumes, low forces on the parts during the print process, high detail and accuracy. The drawback is that the entire build area must be filled with resin, which can easily mean 10 to 100-plus litres of raw material. That makes SLA a time- June/July 2018 | Unmanned Systems Technology A hatch for the Orion space module was built using the Windform XT graphite- based 3D printing material from CRP (Courtesy of Stratasys)