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88 conditions, it becomes imperative to use techniques that control the amount of resin more accurately. Direct coating machines, and to a greater extent film-coating machines, can be used for this. The latter involves creating a resin film with a particular aerial weight, which is then run through a machine with the UD fibres or bidirectional fabric. This achieves most of the advantages of prepregs – sheets of material in a ready-to-use form, in which the fabric, mat or other composite form has been pre-impregnated with resin, before being stored and kept in a preserved state – but at potentially lower cost and higher accuracy of the resin’s weight and mechanical properties due to machine control over the distribution of the resin. Resin transfer moulding, in which the fabric is clamped between two moulds and resin is injected into the cavity – sometimes with the use of a vacuum to help infuse the resin – offers another solution for impregnation and curing. Following the impregnation of the fibres, curing can be achieved in yet other ways. Autoclave curing under intense pressure and heat continues to be considered the standard method for curing large components and surfaces with relatively simple geometries. For smaller parts needed in large quantities, such as propellers or nose cones, the growing availability of ‘snap- curing’ offers a way for epoxies to be cured in as little as four minutes, compared with the hours that some prepregs can take. However, these rapid-press systems are only cost-effective if a lot of the same parts need to be produced. It can be difficult to produce the metallic moulds and parts this process needs, although the man-hours invested in doing do will be worth it. For applications requiring more intricate structures without loss of strength, unmanned vehicle designers can opt for bladder-moulded parts. This involves laying prepreg into a metal mould’s cavity, and the use of a (possibly disposable) bladder to fill up and ‘push’ the composite prepreg against the walls of the metal mould. After placing the bladder into the composite material’s cavity and closing it, the metal tool – most often aluminium, steel or Invar alloy – is put in a press that typically runs at between 80 C and 140 C. The tool is heated, and that in turn heats, softens and begins to cure the prepreg. At the same time, air is introduced into the bladder to drive the prepreg into the sides of the tool. The overall duration of the process varies from 10 minutes to several hours, depending on the properties of the material being used. After curing, the tool is removed from the press and cooled. The composite part is removed, and the bladder can be peeled out or left inside the component if it is not intended for re-use. For UAV designers wanting airframe body components with strength and stiffness but also specific intricate shapes, potentially down to small sizes such as springs, bladder moulding is a useful option. However, the resulting tool tends to be hollow. Alternatively, a technique known as compression moulding can achieve similar levels of complexity and unique geometries while including a core to fill the centre of the tool to provide added strength. Compression moulding follows a similar process to bladder moulding but the core, having been previously thermo- formed or CNC-machined (to give the outline of the shape of the composite part’s inner walls beforehand), takes the place of the bladder inside the prepreg. The core can be a structural foam or made from materials such as aluminium or phenolic honeycomb. The selection of the core can be critical owing to fire, smoke or toxicity requirements. For unmanned vehicles, the use of a core should not bring the risk of propagating an onboard fire, should one break out. Equally important to all forms of composite manufacturing is accurate design of the tooling to achieve the desired shape. The quality and lifespan of the tool is also worth considering – compression moulding tends to result in ‘flash’, where material is extruded sideways slightly through the physical gap after the tool has been closed. February/March 2018 | Unmanned Systems Technology Focus | Composites The material of the moulding tool should be carefully selected. Aluminium is inexpensive and widely available compared with steel but will deteriorate more quickly (Courtesy of Rockwood Composites)