38 Focus | Additive manufacturing FiberFlex 40D demonstrates that over 105 cycles for the pump, and experimental characterisation of the prototype pump, there is an order of magnitude lower base pressure than a state-of-the-art, single-stage, miniaturised diaphragm vacuum pump. The technology can also be used for pumps in space systems. Printing completely airtight layers in the flexible tube is necessary to get the working vacuum pumps. Trial and error found that an extrusion multiplier of 1.2 and an extrusion width of 0.4mmwere suitable for printing the airtight tubes with a radius of only several millimetres. The cross-section of the tube has been designed to overcome the limitation faced by traditional peristaltic pumps when used to create andmaintain vacuum. The starting tube design had an outer diameter of 9mmwith 1 mmwall thickness. The design has two symmetrical notches to prevent dead volumes from forming when the tube is compressed and to reduce stress concentration in thematerial. The tubes are printed vertically, for two key reasons. First, printing them without any overhangs makes it possible to achieve much cleaner prints without any supports inside the tube. Second, printing individual layers perpendicular to the tube’s length axis results in a more durable structure during compression, because the weakest bond in structures printed using material extrusion is between separate layers. FiberFlex 40D is also used to print the support structure for the vertical tubes, close enough to the tube to prevent it from bending but also far enough to prevent the support from bonding to the tube. After the print, the flexible support structure is simply peeled off by hand. Materials Other materials are being tested for uncrewed systems that operate at high speeds. Current 3D metal printing has been described as using papyrus in a modern printer – it works, but not as well as it could. Researchers are therefore developing alloys that are optimised to undergo both 3D printing and are suitable for use in hypersonic flight. Not every material, or every shape, can withstand travelling at five times the speed of sound and faster. AM offers far more design flexibility than traditional manufacturing here though, as it allows designers to create precise geometries that aren’t otherwise possible, particularly for features such as cooling pipes incorporated directly into the structure of a vehicle. This requires newmetal alloys. Inconel 718 for example, which was developed in the 1950s specifically for forging, has a mixture of elements such as nickel, chrome and iron to give it maximum strength. But it also has some elements that cause the material, when processed using AM, to become brittle. The newmaterialswill have to operate with temperatures of up to 2760 oC, so heat resistance is one of themost critical features of the newalloys. Thematerials will also need to be resistant to oxidation. Needless to say, testing thesematerials is a major challenge. Ceramics are more heat-resistant than metal, so hypersonic vehicles are often coated with specialised ceramics that can withstand ultra-high temperatures. But the vehicles are made largely of metal, which is less brittle and better able to bear the mechanical loads at hypersonic speeds. That means there has to be metal underneath the ceramic. Improving the temperature capabilities of the metallic structures offers significant design and performance benefits with the ceramics. This high-temperature expertise and cooling design can also be applied to the engines to boost their performance. Welding pen Joining these 3D-printed metal components together is also a challenge. One new technique is a metal 3D-printing pen that can continuously print metal in a 3D space. It can freely and continuously print metal with freedom in the direction of the welding torch’s movement in the 3D June/July 2023 | Uncrewed Systems Technology The notched cross-section of this peristaltic pump actuator means the pump needs less than half the force of other types to fully seal (Courtesy of MIT)
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