Unmanned Systems Technology 020 | Alpha 800 I Additive Manufacturing focus I USVs insight I Pegasus GE70 I GuardBot I AUVSI Xponential 2018 show report I Solar Power focus I CUAV Expo Europe 2018 show report

39 Additive manufacturing | Focus consuming process to handle, maintain, filter and swap materials. Another process, ‘inverted SLA’, uses a tank with a transparent bottom and a non-stick surface that serves as a substrate for the liquid resin to cure against, allowing for the gentle detachment of the newly formed component. A build platform is lowered into the resin tank, leaving a space equal to the layer height in between the build platform (or the last completed layer) and the bottom of the tank. The UV laser points at two mirror galvanometers, which direct the light to the correct coordinates on a series of mirrors, focusing the light upwards through the bottom of the vat and curing a layer of resin against the bottom of the tank. A combination of vertical build platform and horizontal tank movement then separates the cured layer from the bottom of the tank, and the build platform moves up to let fresh resin flow beneath. On more advanced systems, the tank is heated to provide a controlled environment, and a wiper passes across the tank in between layers to circulate resin and remove clusters of semi-cured resin. That allows the build volume to exceed the volume of the tank, as the machine only needs enough material to keep the bottom of the build vat continuously covered with liquid. This makes it generally easier to maintain and clean the machine and swap materials, and allows for a much smaller and less expensive machine size, making it possible to bring SLA to the desktop. Both types of SLA system require the use of support structures for most parts, depending on their design. In right-side-up systems, these supports hold parts at a precise location to ensure that all their details have something to attach to, and resist lateral pressure from the resin-filled blade. Inverted SLA uses supports to attach overhanging parts to the build platform, prevent deflection due to gravity, and retain newly created sections during the peel process when removing the component from the build bed. Most software tools automatically create supports during the preparation of the 3D models, but they can also be adjusted manually. Once the printing process is complete, these supports must be manually removed from the finished product. SLA is also used for printing ceramic parts. A ceramic paste is laid down and again cured by the UV light. Ceramic 3D-printed cable ties and tie bases are now being used in space systems. Up to 500 of them can form a single system, making the 30% weight saving over the metal version significant. Jetting technologies also use liquid photo-polymers, but these are deposited using piezoelectric nozzles in the same way as an inkjet printer and cured with a UV lamp attached to the print head. This is a layer-by-layer process with different materials printed from nozzles at the droplet level and using UV to cure each layer. They can be used to make coloured prototypes or moulds. Other techniques Ultrasonic fusion builds a solid metal object by welding a succession of metal tapes into a 3D shape, with periodic machining during the process to create the detailed shape. Ultrasonic vibrations at 20 kHz are applied locally to metal foils, which are held together under pressure, to create the welds. The heat and plastic deformation in the process promote diffusion and recrystallisation at the interface of the metals, resulting in a true metallurgical bond, one that is achieved at a low temperature and without any special environments. For all metals, the bonding temperature is far lower than the melting temperature. In aluminium, for example, it is always below 250 F (121 C). That can be used to create bonds between dissimilar metals without creating an undesirable brittle metallurgy, which is very different from fusion- based processes. For example, layers of molybdenum and Invar (a nickel-iron alloy) can be printed into an aluminium heatsink to provide a specific coefficient of thermal expansion of the heatsink for an electronic circuit. A low bonding temperature also allows temperature-sensitive components to be embedded in solid metal components. Microprocessors, sensors and telemetry have been successfully embedded using this approach. Another technique for AM is laser wire direct closeout (LWDC). This was developed to build high-temperature nozzles for space launcher engines. It uses a wire-based AM process to precisely close out the nozzle Unmanned Systems Technology | June/July 2018 The latest SLS metal 3D-printing system from Renishaw uses four lasers to speed up the production of components (Courtesy of Renishaw)