Unmanned Systems Technology 036

85 1 m size constraint limits the number of lasers that can be used to 12. That in turn limits how fast each layer can be created and so how fast a design can be built. As the technology matures, so designers and developers are focusing increasingly on the productivity of the systems, along with the qualification and certification of the components. AM system developers have therefore been looking at different techniques to get around the dual challenges of small size and slow speed. These are of course connected, as a larger component takes longer to produce in AM. One approach now being commercialised is binder jetting. This was one of the original techniques for AM but struggled with the limited range of materials available at the time, its limited accuracy and a requirement for secondary post-processing. Thanks to improvements in the technology though, binder jetting is gaining interest for high-volume production of parts above 100 units. Another technique using liquid metal comparable to plastic AM printing is also being commercialised for faster printing, especially for aluminium powder. This is a challenge for the powder-based approaches, because the aluminium powder is covered in an inert oxide layer, restricting how it can be melted together, and is also explosive in a confined space. There are also composite AM techniques that combine 3D-printed elements with layers of carbon fibre composites that can be moulded and thermoset. This speeds up the delivery of custom components with unique shapes and structures suitable for unmanned platforms of all kinds. Then there is the automation of the existing SLS systems. Automating selective laser sintering A project in Germany called IDAM, for example, is aiming to make all the steps in AM more automated, including the parts handling. The project uses a machine to automatically remove the parts from the build plate on the SLS printer as well as any extra elements such as supports. There is a bottleneck though, which is the printing itself, and which is dependent on the SLS laser system being used. There is also a size limit from the bed holding the metal component being built, which puts a constraint on the number of lasers that can be used and therefore the speed of production. Another of the challenges that developers using SLS want to overcome is combining materials. Stainless steel is a popular material for SLS but copper is also of increasing interest, because of its higher thermal conductivity. This can be used to implement component designs that cannot be produced using other techniques, for example for components with complex internal channels for cooling. Hybrid manufacturing is another approach. It combines copper with Inconel or steel, and has been found to work well. This requires a separate SLS machine for each material, to avoid contamination, and the powder bed is moved from one machine to the next for each layer. On the SLS machine that prints copper, the build chamber is not heated up, so the next layer with a steel or Inconel powder is essentially welded to the layer underneath. This allows the creation of detailed internal channels but requires more complex automation to move the powder bed between printers. Binder jetting There are serious attempts to bring metal binder jetting AM to companies that have not traditionally been able to afford it. Binder jetting is not new, but it has been argued that it evolved in the wrong direction for small companies, which need to gain access to it by reducing the complexity and cost. It uses powdered metal similar to laser-based printing but combines it with a polymer binding material in a solvent. Rather than having to generate a high temperature to melt the powder, the polymer is sprayed onto the bed of powder using an inkjet printer head. This holds the metal powder together, and a component is built up layer by layer. The technique was invented in the 1980s, and is used in the polymer industry, but it has some key challenges when used with metal powder. Since a high temperature is not required, the printing process is up to 100 times faster than SLS and other methods, but the final component has to be sintered in an oven to melt the powder together, which can take hours or days depending on the size and complexity of the design. This lends the process to batch manufacturing for production rather than quick prototyping. The sintering erases the history of the manufacturing process, as the grains Additive manufacturing | Focus Unmanned Systems Technology | February/March 2021 GKN Additive is working with BMW on automating the production of additively manufactured metal parts using selective laser sintering (Courtesy of BMW)