Aerospace composites processing

Most composite applications can be divided into:

(1) structural - continuous unidirectional fibres;

(2) semi-structural –continuous multidirectional fibres;

(3) non-structural - discontinuous fibres.

It can be instructive to divide aerospace composites structures processing, the processing techniques for aerospace composite components into three types:

(1) open mould (e.g. hand lay-up, filament winding);

(2) closed mould (e.g. vacuum bag moulding, RTM);

(3) continuous (e.g. pultrusion).

SMC aerospace composite components.

SMC (sheet moulding compound) composites can produce both large and small parts with a surface finish generally superior to all other composite processing methods, hence its attraction to the automotive industry for appearance-critical body parts. Tooling costs tend to be high, so production runs should be long. Raw materials suppliers are working with the equipment manufacturers in an attempt to reduce cycle times using faster cure cycles and increasing automation.

Resin transfer moulding

Resin transfer moulding (RTM) involves the injection of a low viscosity resin into a closed mould, usually under vacuum, containing the fibre reinforcement as a preform. Car manufacturers demonstrated the benefits of this method when they demontrated that the entire 90-piece front end of a car could be replaced by a 2-piece RTM structure. Production cycle times were estimated at less than 10 minutes and the RTM structure was lighter than the steel structure it replaced, as well as being stronger and stiffer. As with SMC, low viscosity, fast curing resins along with highly automated equipment have been developed in order to attract the automotive industry to the process.

For low volume production, it is possible to use low cost soft-tooling. For larger components, the tooling is frequently manufactured from composite itself. For larger production runs, aluminium tooling is generally used with built-in heating and cooling channels for faster cycle times. Semi-structural automotive components, with a fibre volume fraction approaching 60%, can be produced in large numbers using modified reaction injection moulding (RIM) equipment. The quality of surface finish with RTM is generally good and parts can be produced to tight tolerances. It is also possible to mould-in inserts and foam cores.

Pultrusion

In the pultrusion process, dry fibre reinforcements are passed through a resin bath and impregnated with resin before being drawn through a heated die, where the thermoset resin is cured. In general terms, pultrusion allows the manufacture of any profile that could normally be produced by extrusion of a thermoplastic. The applications for pultruded sections range from skis and ladders to structural sections for bridges and buildings. As a consequence of the fibre alignment, pultruded sections are stiff along their length but display poor mechanical properties in a transverse direction. This is often compensated for by over-wrapping the pultrusion with a woven reinforcing fabric to be impregnated with resin or prepreg, providing reinforcement in the transverse direction. It is also possible to pultrude woven cloth to prevent the need for further processing.

Polyester resins are generally used, although epoxies and phenolics are available for pultrusion. Pultruding with thermoplastic resins is also extremely attractive and is of considerable current interest.

Pultrusion is unique among the processing techniques for composites as it is capable of producing complex components on a continuous basis.

Prepreg moulding

Prepreg moulding is in many respects the next step up from the impregnation of resin by hand into layers of dry chopped-glass strand mat laid onto the mould. Using prepregs, the resin content and fibre orientation of the finished component can be more accurately controlled. Prepregs command a price premium and require ovencuring and vacuum bag or autoclave moulding to take full advantage of the properties. Prepreg moulding is particularly cost-effective in the manufacture of large one-off composite structures, such as ocean-going racing yachts, where performance requirements outweigh the costs. Moulding by autoclave is used in the aerospace and high-performance racing car industries, where the high temperatures and pressures achieved during moulding ensure that the optimum composite properties are achieved.

Aerospace composite structures

For the manufacture of stiff 'sandwich' structures in aerospace composites structures processing, where two laminate skins are separated by a low density core material such as PVC foam or aluminium or paper honeycomb, filmed adhesive can be first applied to the core material and the prepreg plies for the laminate are then placed onto this film adhesive, which provides the necessary resin to reticulate around the core material, thereby achieving a good skin-to-core bond.

Filament winding

Resin-impregnated fibres are wound onto a mandrel for the manufacture of components with cylindrical symmetry in the filament winding process. Pressure vessels, pipes and automotive drive shafts have all been produced by this technique. One European company manufactures commuter train carriages by a variant of the filament winding process. The mandrel can be any shape that does not possess re-entrant curvature, although it can be possible to remove the component from the mandrel before it has fully cured and reverse curvature can be produced by suitable forming operations.

Filament winding is usually computer controlled and the reinforcement can be oriented to precisely match the design loads. The fibres may be impregnated with resin before winding (wet winding), pre-impregnated (dry winding) or, more unusually, post-impregnated. High volume fractions of fibre are attained (60-80%). Only the inner surface of a filament wound structure will be smooth unless a secondary operation is performed on the outer surface.

Aerospace composite assemblies

using thermoplastic matrix composites

Many of the aerospace composites structures processing problems associated with polyester and epoxy thermosetting composites (e.g. limited shelf life of resins, long cure times and labour intensive) can be overcome by the use of thermoplastic resins. Thermoplastics need only to be warmed in order to flow in indirect forming processes such as stamping and hydro-forming. Thermoplastic matrix composites (TMCs) can also be produced into laminates by continuous tape laying techniques, into tubes by filament winding and rod by pultrusion. TMC prepreg material has an infinite shelf life and does not require refrigerated storage, unlike epoxy resin prepregs.

The material is dry and therefore easier to handle and therefore processes are easier to automate. Once the part has been formed, it is relatively easy to cool the part to room-temperature rigidity. While early TMC work concentrated on the more exotic thermoplastic resins such as PEEK, there is much interest into the use of commodity plastics such as polypropylene with continuous glass fibre reinforcement, particularly for semi-structural automotive, aerospace composite assemblies and construction components.