Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
We use 3D printing extensively in the manufacture of bespoke parts for our UAVs. We use selective laser sintering of nylon (for airframe parts) and of stainless steel (for highly loaded parts such as engine bearers) and fused deposition modeling of ABS for nonstructural parts where complex shaping is needed (such as for wing tips). This renders the production of parts into essentially a CAD-based design task followed by outsourced manufacture direct from the CAD files. The companies that operate such machines have a fast turn-around providing high-quality parts with very good repeatability. Figure 18.2 shows a stainless steel selective laser sintering (SLS) printed engine bearer for a gasoline engine.
Selective Laser Sintering (SLS)
SLS works by fusing fine-grained powder with a laser that scans a bed of the working material. This creates a thin laminar structure. A wiper then covers the fused part with a further thin
Figure 18.2 3D SLS stainless steel gasoline engine bearer after printing and in situ.
layer of powder, which is then fused to the first, thus growing a three-dimensional object layer by layer, with the parts being supported by the lower layers as they are “grown.” At the end of the print, a large “cake” of powder is left, within which lies the fused part. By removing the unfused powder (typically by suction or blowing with compressed air), the desired parts are exposed. These then need cleaning internally to remove any unwanted powder before use, see Figure 18.3. With nylon, this is all that is required. With metal SLS, it is normal to start the process on a metal base plate that must be cut off from the finished part (typically by using a wire cutting machine). In either case, the finished parts have a slightly rough surface finish
Figure 18.3 3D SLS nylon manufacturing and depowdering.
Table 18.2 Typical properties of SLS nylon 12.
For isotropic materials, the shear modulus G, bulk modulus K, Poisson’s ratio v, and Young’s modulus E are related as 2G(1 + v) = E = 3K(1 — 2v); for the tabulated values, SLS nylon is not isotropic.
that has the grain size of the raw powder. This can be removed by polishing or filling if a particularly smooth surface is required. Parts can also be colored or plated, and metal inserts can be added into the nylon. Generally we do not bother with further surface treatment for our parts. Moreover, the slightly roughened surface we find highly suitable for gluing with epoxy resins if required, either to attach parts or to carry out repairs.
When using SLS, the only limitations on design freedom are, first, it must be possible to remove any unwanted powder so fully enclosed cavities cannot be made in this way; second, there is a minimum wall thickness that can be achieved (around 1 mm); and third, the maximum size of part is constrained by the dimensions of the build chamber in the machine (currently for the machines our suppliers use to sinter nylon this is 700 mm x 380 mm x 580 mm). See Table 18.2 for typical SLS nylon properties. In metal the build chambers are generally smaller (our supplier’s machines have a maximum chamber size of 250 mm x 250 mm x 325 mm) but slightly thinner wall thicknesses can be achieved (down to around 0.5 mm). In either process, the build chamber in which sintering occurs is kept at a high temperature to aid the process. This means that allowance must be made for the shrinkage that occurs on cooling; our suppliers deal with this by scaling our designs before printing so that we do not have to consider the effect. It is also the case that the orientation of the part during construction has slight influences on the finished part. Curved surfaces, if not subsequently polished, show the lines where layers of powder end, for example, and there are slight variations in material properties. In general though, very high quality parts with good structural properties result, allowing highly functional components to be made.
Figure 18.4 Small office-based FDM printer. Parts as they appear on the platten and after removal of support material.
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