Composites are manufactured using one of the following techniques:
More detailed descriptions than those given below are available in 'Composites - a guide to best practice: Section 4. Manufacturing composites'.
The component is built up in a mould by applying several layers of reinforcement and wet resin, which is distributed by a roller, until the desired thickness is achieved. The resin is then cured, using applied heat if necessary, to produce the finished component.
This process is widely used in the marine industry to prepare glass-fibre reinforced polyester resins. Material costs are relatively low and the process is very flexible. However, it is labour intensive and suffers the drawback of high styrene emissions.
A spray gun is used to apply chopped fibre reinforcement and wet resin to a mould until the desired thickness of material is built up. The resin is then cured. The process is faster and cheaper than wet lay-up, but mechanical properties are lower.
The process is typically used for large, relatively simple structures such as bathtubs, boat hulls and storage tanks.
A charge of fibre and resin, either sheet moulding compound or bulk moulding compound, is placed in a preheated mould, which is then closed, and held under pressure until the resin is cured. The process can generate a Class A surface finish, and the similarity to the stamping process used for sheet metals has led to applications in the automotive industry.
The high investment in heated tooling means that the process is only suitable for medium to high volume production.
Bulk moulding compound is heated and injected into a heated mould, where it is held under pressure until the resin has cured. The process is used for relatively small components for which a short cycle time can be achieved. The high cost of tooling means that the process is suitable only for medium to high volume production.
Resin transfer moulding (RTM)
A fibre preform or fabric is placed in a heated mould. Reactive resin is mixed and injected into the mould under pressure. Pressure is maintained until the resin has cured and the part is removed. The process is suitable for complex, highly-loaded parts, and is used in a wide range of industries. Vacuum assisted resin transfer moulding (VARTM) is a variant of the process in which vacuum is applied to the closed mould, allowing resin to be injected under low pressure. The loads on the tooling are therefore lower, allowing cheaper, larger tools to be used to fabricate large structures such as boat hulls or wind turbine blades. A further variant is structural reaction injection moulding(SRIM). This uses highly reactive resins such that the mould does not need to be heated to cure the resin, although it often is heated in order to reduce cycle time.
Vacuum is used to cause a low viscosity resin to impregnate a fibrous preform. Most commonly, the resin is caused to flow over the surface of the preform, and then to impregnate through the thickness, minimising the distance the resin must travel through the preform. The process is well-suited to large components such as boat hulls or wind turbine blades. The tooling does not have to carry substantial loads during the process.
A fibre tow is passed through a resin bath and applied, under tension, to a convex mandrel. The mandrel is rotated and the fibre release is moved to lay down fibres in the desired geometry until the required thickness is achieved. The composite is then allowed to cure, using elevated temperature if necessary. The process can be automated for high volume production, and is used for tubular structures such as pipes and driveshafts, as well as more specialised structures such as pressure vessels or monocoque bicycle frames. In the last two applications, the mandrel will remain inside the component. In a variant of the process, pre-impregnated fibre tows or slit prepreg can be used, removing the need for a resin bath, but requiring a high-temperature curing stage.
Fibre tows are taken from a creel and fed through a resin bath, before being pulled through a heated extrusion die, which cures the resin to produce an extruded part with axial reinforcement and constant cross-section. Applications include gratings, ladder sections, bridge parts and handrails. The process can be automated and uses low-cost raw materials, making it suitable for high volume applications.
Prepreg (pre-impregnated fibre) consists of fibres, fabrics or mats impregnated with resin. Thermosetting prepregs include hardeners in the resin, and therefore have a limited shelf life and are usually stored under refrigeration. Before use, the prepreg is brought to room temperature to avoid condensation, and plies are cut to the required shape and orientation. These are stacked in a mould to the required thickness, using a roller to avoid entrapment of air. The laminate is sealed in a vacuum bag, which is evacuated, and the part is then cured, either in a conventional oven, or in an autoclave, under additional pressure. Autoclave processing gives less porosity and superior mechanical properties, but requires a slow cycle time, and expensive equipment, particularly for large components. Prepreg is used where high mechanical properties are required and the high cost of processing can be justified. Examples include aerospace structures, sporting goods and wind turbine blades.