Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

86 For instance, carbon composite moulds are useful as they exhibit similar thermal coefficients to the carbon fibre parts they help to form, and can be used several hundred times. Aluminium will last longer, of the order of thousands of uses, but it is harder to repair than carbon moulds and has less desirable thermal qualities. Meanwhile, steel moulds can be used about a million times, although they are naturally far more expensive. Once the fibres are fitted and the resin applied, curing must take place. The most popular approach is autoclave processing, which involves wrapping uncured material around a positive mould before being bagged, vacuumed tight, and effectively pressure-cooked to consolidate so that the resin spreads out evenly, cross-links and cures. It is the most popular approach in the aerospace industry for various reasons. For one, the high pressures produced by such machines lend themselves well to producing composite parts with high Vf numbers. Also, of all curing tools, autoclaves produce the fewest voids (air bubbles inside the materials, no more desirable in carbon or glass composite than in engine metals) and high-end aerospace components must have less than 2% void content. Vacuum infusion has also been used widely, for building fibreglass boats, for some time, and more recent hydrofoiling boats have applied the technique to carbon fibre composite hull-forming. The process involves pumping liquid epoxy into large-scale structures of fabric and then vacuuming the materials until airtight, to achieve a very high Vf and even distribution of resin. UAV and USV manufacturers looking to make large vehicles for logistics or autonomous transport could benefit from using the process, although considerable experience is needed to carry it out correctly, and it may not always achieve the necessary compaction for some foam core composites. Compression and bladder moulding are two other popular methods for forming an accurate geometry of materials. The former combines positive and negative moulds to encase the fibre and resin, while the latter uses an inflated bladder to press them into the negative mould. That makes it less expensive than compression moulding, but in the long run it can be faster than traditional autoclave manufacturing. Lastly, filament-winding manufacturing processes can produce various widely used axisymmetric composite structures such as pressure vessels or tubes, and parts can be wound as pre-forms (fibre impregnated with resin as it is applied to the mould) or even with pre-preg tow or slit tape. Such processes have advanced in the past few years to achieve more even spreading of resin and smoother surfaces through minimised disturbance of fibre. These processes continue to be applied to other axisymmetric parts. In recent years, filament-wound composites have been used to make rotors in permanent magnet synchronous motors and transmission shafts in drivetrains that are lighter, provide better electrical insulation and are more corrosion resistant than some of their traditional metal counterparts. Bonding Methods for joining two or more composite parts, or for bonding a composite to a metal, vary greatly between suppliers. Any joint marks a point of inconsistency in terms of the mechanical, thermal and vibratory transmissions across a vehicle’s hull, so judging joint quality can be contentious. The most glaring hazard among these is that the thermal expansion coefficient of the bonding material is often dramatically different from those of the parts being bonded. Repeated expansion and contraction will therefore damage their quality over time at a far quicker rate than the relatively inert composites. Some adhesives can overcome these issues to an extent, but their expense tends to rise at a similar or faster rate as their thermal stability. For such reasons, more and more high-end suppliers prefer to use mechanical attachments wherever possible, despite the added complexity of designing mechanical interfaces into composites and the fact that some metals used in mechanical joints will be subject to greater corrosion than that faced by composites. December/January 2022 | Unmanned Systems Technology High-quality bonds of composite parts such as this co-moulded elastomer joint can be challenging to achieve (Courtesy of Composite Factory)