Fused Deposition Modeling (FDM)
Since the machines required for SLS manufacture tend to be very expensive and the depowder- ing process can be quite messy, it can be useful to turn to alternative 3D printing technologies for nonstructural parts. We have for many years operated our own, small FDM machine for making parts up to 330 mm x 100 mm x 100 mm in dimension, see Figure 18.4. In FDM, a plastic filament (generally ABS) is squeezed through a heated nozzle so that it emerges in a semiliquid form. This is then laid down onto a plastic platten in a chamber held just below the melting point of the plastic. The emerging plastic thus fuses to that already produced, and layers are built up at about 0.7 mm at a time. The major restrictions with this process is that overhanging structures can be made only if the resulting degree of overhang is limited (in practice a 45° angle can be achieved). If a greater overhang is required in the final part (such as in an arch or circular hole), a scaffold of sacrificial material has to be inserted to support the ABS filament and then this must be removed after printing. This is a tedious and not always successful, so when using FDM we typically design the part such that a build orientation can be achieved with no significant overhangs. On the more positive side, FDM does allow fully enclosed voids to be produced provided the overhangs are controlled. We have printed double-skinned parts in this way, which are linked by internal baffles at the appropriate angles. A selection of FDM-printed parts is shown in Figure 18.5. In all cases, the parts that result are highly orientation-dependent in terms of mechanical properties, being very weak in the layer-to-layer direction. This can be overcome by using such parts in a prestressed form by including tension rods to link and compress the FDM components. The fuselage of the aircraft in Figure 18.6 is made from ABS using FDM in this way.
It is possible to manufacture fuel tanks using 3D SLS printing, although this makes sense only when they also form an integral part of the main structure. If nylon is used for this
Figure 18.5 FDM-printed ABS fuselage parts.
Figure 18.6 Aircraft with FDM-printed fuselage and wing tips.
purpose, we find that, as manufactured, it is very slightly porous, so the finished tank must be internally sealed with an appropriate fuel-safe compound (we use Kreem Fuel Tank Liner). Before applying the sealer, it is important that a thorough mechanical cleaning is carried out to remove any residual powder from the manufacturing process. We load the tanks with bead blasting balls and shake them well before emptying the balls out and rinsing with a degreaser.
-  http://www.kreem.com/fueltankliner.html.