How polymer-matrix composites are made

Open and closed moulds

Every route has the same task: bring resin and fibre together in the shape of the part, then consolidate and cure it. The U.S. Department of Energy groups the routes into open forming, on a single mould surface, and closed forming, inside a closed tool, and notes that a part’s final properties depend on the materials, the form they start in and the process together.^[1]

Open moulding: hand lay-up and spray-up

In hand lay-up, resin is worked by brush or roller into dry fibre fabric laid on a mould and left to cure. In spray-up, chopped fibre and catalysed resin are sprayed into the mould together.^[1,2]

Both are cheap, and both pay for it. Hand lay-up depends on the laminator’s skill, and the thin resins it needs compromise properties; spray-up takes only short fibres and gives heavy, resin-rich laminates; and both carry health risks for the people doing them. Their use has narrowed to large consumer goods such as bathtubs and pools, and to repair; wet lay-up is used for many aircraft repairs.^[1,2,3]

Continuous fibre, continuous process: filament winding and pultrusion

Filament winding passes resin-wetted fibre tows onto a rotating mandrel. It is the predominant way to make pressure tanks and pipes, but it cannot lay fibre along the part’s axis.^[1,2,3]

Pultrusion pulls fibres through a resin bath and then through a heated die in which the resin cures, producing a continuous profile such as the constant-section spar of a wind blade — and only constant sections.^[1,2,3]

Liquid moulding: resin transfer moulding and vacuum infusion

In resin transfer moulding a dry fibre preform is clamped between matched mould tools and resin is injected into the closed cavity. It can make continuous carbon-fibre structural parts, but tooling strong enough for the pressure is expensive, and any dry spot the resin misses is scrap.^[1,2,3]

Vacuum infusion replaces the upper tool with a vacuum bag and lets the vacuum draw resin through the fibre. With one tool surface it can make bridge sections and rail carriages affordably; the price is a poor finish on the bag side and shapes that are nearly flat.^[1,2,3]

Moulding compounds: compression and injection moulding

Sheet and bulk moulding compounds are fibres pre-mixed with a thermoset resin. In compression moulding a charge, often of moulding compound, is placed in the mould and pressed, at up to 14 MPa, into its shape; it is fast and repeatable, and it is how many non-structural car parts are made from short glass fibre. Its moulds and presses are costly, and parts can suffer residual stress, delamination, warpage and fibre orientation set by the flow.^[1,2,3,4]

Injection moulding melts polymer pellets that already contain fibre and forces them into a metal mould. It is the high-volume route for fibre-reinforced thermoplastics, but the melt’s viscosity limits it to short fibres, the screw breaks them further, and their orientation in the part cannot be controlled, which keeps it to non-structural parts.^[1,2,3]

Prepreg, the autoclave, and curing outside it

A prepreg holds fibre in a resin advanced to a tacky, part-cured stage and stored frozen. Plies are laid up and cured under heat and pressure — in aerospace, in an autoclave, traditionally at 177 °C and typically 586 kPa — so that resin flow, degassing and consolidation are controlled.^[1,2,3]

Autoclave cures take hours and a great deal of energy, because the tooling and the vessel heat and cool with the part. Out-of-autoclave prepregs cure under vacuum alone at 93 °C or 121 °C, which also eases the mismatch between how tool and part expand and lets large stiffened structures be co-cured in one cycle.^[1,2,3]

The matrix decides the process

For carbon-fibre-reinforced polymers in 2015, almost every route was in wide use with thermoset matrices, while with thermoplastics injection moulding was widely used, resin transfer moulding and infusion were still at research stage and spray-up was not used at all. The reason is the one the fibres-and-resins article gives: a thermoplastic melt is too viscous to soak a preform the way a liquid thermoset does.^[1,2]

Commonly misunderstood