Unmanned Systems Technology 036

86 Focus | Additive manufacturing grow larger than the powder size, but the polymer has to be burned out cleanly during the sintering process without leaving any carbon behind, which would contaminate the end-product. The liquid binder also has to be thin enough to run reliably through an inkjet print head, and provide a small enough resolution with the right surface tension and viscosity to penetrate into the powder bed. Different print heads can be used for different performance, for high resolution versus high speed. The resolution is determined by the shape and size of the powder – effectively the ‘pixels’ to create the ‘image’. An inkjet head has a resolution of 1200-1600 dots per inch, with a 16-20 micron droplet size for sub-millimetre features. The quality of the surface is determined by the drop size of the binder and the powder size, which can be as little as 3 microns to give a surface roughness of 3-6 microns. With this process, called Single Pass Jetting, the thickness of the powder layer is typically 50-100 microns. A 75 micron layer might have five or six layers of particles, and the system has to put enough binder down to allow the particles to bind in the layer. That means the design of the surface tension of the binder needs to go deep into the layer. Droplets of binder are fired onto each area of powder from a distance of 1 mm and at a velocity of over 5 m/s. Using multiple droplets allows for redundancy in case the nozzles clog. The speed of the process also comes from a focus on optimising the ‘up time’ of printing. This involves formulating the binder so that there is no need to wait for it to dry before the next layer is spread over the top. That means a layer can be printed every 3 seconds using a print head that is the same width as the powder bed. The print head exploits techniques used for printing large posters, where the print heads, each 3 cm long, are stitched together with an overlap. In theory this can produce a print head up to 5 m long. The print head is combined in a single production unit that moves the bed through the process without having to wait for a particular stage to finish. Currently the sweet spot for components is 25-30 cm in diameter, as beyond that the shrinkage during sintering begins to dominate. It does have the advantage though of making it easy to create alloys, as different metal powders can be mixed in the powder bed before being bound together and then sintered. For example, with a steel powder, the binder grabs nearby nickel. Binder jetting can be 100 times faster than laser AM, and can deliver 10,000 parts in a few weeks from a commercial AM printer, so although the sintering stage takes time it is not the bottleneck. Instead it is more the design limitations. The issues are similar to multi-jet plastic fusion, where there are parts in the middle of the bed so supports are not needed. The sintering side has limitations as well, as sintering the parts creates shrinkages of up to 20%, and the parts deform in different directions depending on gravity. Sinter simulation is now being developed to calculate the deformation before manufacturing. Good control of the sintering is possible, but if the part is not designed for binder jetting then most of the time it will not work. Powder quality is a major factor in the deformation, but there are other parameters such as the sintering temperature, duration, phases, sintering programs and print quality. Software to compensate for the shrinkage is key. It uses a physical model of the different materials to predict how the part will shrink through gravity and friction. A graphics-based physics engine simulates the process and adjusts the design to print a scaled, warped part that warps down to the ideal shape. This has to take into account the way different powders pack and then combine during sintering. This process, called necking, reduces the surface area and depends on the geometry of the design, so it is a complex process for the software. For a long, thin design such as a propeller, the shrinkage is driven by its cross-section. In other parts using techniques such as gyroidal infill that remove 90% of a part, this eliminates the effects of gravity and friction. However, a 10 mm-thick part can still take between 10 and 24 hours to sinter, depending on the overall design. Like the laser metal process, there are also moves to make binder jetting fully automated. It is designed to be compatible with continuous sintering furnaces, February/March 2021 | Unmanned Systems Technology Liquid aluminium can be used to build parts using a spray process (Courtesy of Xerox)