Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

62 Focus | Composites tow fabrics, and consist of only a few thousand fibres. Recently the use of thin-ply technologies, where the carbon fibres are distributed in much thinner tapes than in regular reinforcing materials, has become possible. These thin plies have a very low weight, with carbon available at as little as 30 gsm (grammes per square metre), glass fibre at 35 gsm and quartz fibres down to 17 gsm. Thin plies have a much higher surface-to-volume ratio for a given weight of reinforcement, providing a greater area for load transfer between matrix and reinforcement. The small size and closer packing of reinforcement fibres within a thin-ply material greatly improve the homogeneity of the composite material and the fibre distribution. This improved distribution also helps to reduce the presence of manufacturing defects such as resin-rich pockets or dry areas. The disadvantage with thin plies is in their processing (or draping) – the thinner and lighter tows have very little inherent rigidity or form, so are difficult to place correctly. To get around this limitation, automatic placement machines ensure a consistent lay-up. Typically, such machines work with thin adhesive- backed tapes that can be placed to form a 2D flat pattern which is then draped into a conventional 3D mould. Other composite lay-up techniques include wet lay-up, where the reinforcing fabric is placed into a mould and liquid resin manually applied. Another option is to lay-up the fabrics in the dry condition, then force resin into the component using vacuum pressure, a technique commonly called Resin Infusion Moulding (RIM). This seals the part within a plastic cover, before drawing out all the air using a vacuum pump; one or more resin inlet valves are then opened, allowing the vacuum pressure to draw resin into the fabric. The position of the inlet valves can be important in ensuring an even infiltration to avoid dry areas or voids where the resin has been unable to penetrate. Once the resin has been infused, the mould tool can be placed in an industrial oven to cure the resin if required. Taking this process a step further, many composite parts are processed using autoclaves. Essentially large pressure cookers, autoclaves are able to subject the composite component to much higher temperatures and pressures (several atmospheres rather than the 1 atm available in RIM). This higher pressure is very effective in ensuring complete infiltration of the resin, while the control over temperature can be used to manage the curing process. Care needs to be taken however when curing a composite with foam or honeycomb cores to avoid crushing the core material under the autoclave pressure. On the face of it the resin-curing process is quite simple – achieve a high enough temperature to activate the chemical reaction, and allow the reaction to be completed. However, the exothermic nature of this reaction means it is important to avoid any overheating. While this can probably be achieved quite easily on a single component in an oven, the expense of running an autoclave means it will rarely be switched on to produce just a single component. It is therefore vital to control the temperature distribution within the autoclave’s volume, so as to achieve the optimum curing parameters for all the parts inside. Part geometry can also have an influence on the processing parameters required. While thin regions may cure quickly, thick sections will naturally take longer and have differing temperatures through their thickness, so it’s essential to ensure that the centre of the part is November 2014 | Unmanned Systems Technology Automated tape-laying machine (Courtesy of NTPT) Autoclave monitoring and control are vital to producing high-quality components (Courtesy of Premier Autoclaves)