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91 0.1 mm thick, making them ideal for weight-critical systems where thin parts without cores are desirable. To achieve this reduced thickness, the fibre tows are ‘unbundled’ and spread into a flatter, dispersed form, called ‘spread tow’ material, before pre- impregnation. Without this, the material would be at least 0.2 or 0.3 mm thick (as thick as multiple tows laid side by side). Working at the level of almost individual filaments as thin as 5 microns requires delicate and precise lay-ups. The use of automated tape laying machinery rather than manual lay-ups therefore becomes critical in order to take a 6000-filament tow and reduce it to tape no thicker than two or three filaments. Such systems are suitable for making a skin-like layer for UAV airframe bodies, to house the frames of the wings and fuselage, when paring down weight to the absolute minimum is critical. As UAV manufacturers seek to maximise their flight times and complements of onboard mission systems, it is increasingly critical to concentrate the aerial weight into the primary structural elements such as spars, and to a lesser extent ribs, inside the cross-sections of the wings. Thin plies can thus be used to construct these load-bearing components in a precise step-wise manner, layer by layer, at no more weight than is absolutely necessary to provide the required strength and stiffness for their intended mission applications. The wing covers can be thinner layers still, using lightweight resins such as Mylar or a polymer film. Recent years have also seen thin plies spread to unmanned vehicles outside of high-altitude missions. Wind-powered sea vehicles for example are seeing their sails replaced by ‘wings’ constructed in a similar fashion. Also, electric motor containment – in which the high speed of some motors requires the use of tighter ‘grips’ for the magnets on the shaft to stop them coming loose and flying off – is making use of carbon composites to provide extra strength. This form of containment is already used in automotive and motorsport applications, and is expected to be applied to unmanned automobiles and UAVs as the performance requirements of the motors increase, and the use of energy recovery systems in cars becomes more widespread. Thin plies are not impregnated or cured in particularly different ways from other polymer matrix composites. The key difference, as mentioned, lies with the automated tape laying (ATL) machinery. Ordinarily, prepregs of conventional weight are applied by hand. The material is rolled out on a table and cut before being hand-fitted into the mould (or any other tool being used in the production process). In an example of spread-tow ATL machinery, the tape is dispensed from an overhead gantry to apply strips of material to the working surface in the required orientations, positions and lengths. To prepare the tape for manufacturing the plies, multiple spools of tow are unwound and the bundles are pulled through a spreading unit, which re-arranges them into flat tape. It then passes through an impregnation unit that infuses the resin into the fibre, before being pulled into a completed Unmanned Systems Technology | February/March 2018 Composites | Focus The creation of ultra-thin composite tape, or ‘spread tow’ material, relies on using automated tape laying machinery to unbundle the tows of fibre (Courtesy of North Thin Ply Technology)