Composites materials have gained ground in aerospace because of their light weight, high strength, fatigue resistance and the ability to use manufacturing processes that can produce integrated structures with reduced parts count and assembly time.
Composites typically have two constituents: a reinforcement material imparting mechanical and electrical properties that can be cellulose, carbon or glass fibres; and a matrix material that surrounds and supports the fibres and which is normally a thermoplastic or thermoset polymer. In aircraft primary structures, the material of choice is carbonfibre/epoxy.
Working with composites rather than metals requires companies to master different manufacturing processes.
Rather than sheets and billets, the raw material arrives as rolls of fabric or spools of fibre, as in the textile industry, and it comes in two main classes: dry or pre-impregnated with resin – pre-preg - requiring refrigerated storage and special handling.
Pre-preg composites manufacturing typically involves cutting the material to shape and laying up the layers in a mould, followed by vacuum bagging and autoclave curing.
Increasingly lay-up is being automated using tape-laying machines, with filament winding or tow placement used for more complex shapes where the fibres are placed directly on to a mandrel by computer-controlled head.
Process developments are focused on moving to dry material to avoid the need for refrigeration and to reduce cure times. GKN Aerospace and Bombardier have invested in resin transfer moulding (RTM). This produces moulded surfaces on both sides of the part, low levels of resin voids and possible labour reductions with automation. But RTM requires preforms, where dry fabric is pressed to the mould shape with a binder chemical.
GKN is also focusing on developing resin film infusion (RFI), which is seen as potentially lower cost than pre-preg processes while still delivering high fibre volumes and automation.
RFI can also avoid the need for autoclaves, which represent production bottlenecks.
Source: Flight International
