Methods of composite manufacture
There are many ways to process FRP composites and the method of processing needs to be considered in the early stages to decide what is best for each application.
The properties of the finished part are affected not just by the properties of the component materials, but also by the way the fibres are incorporated. Better fibre alignment and higher pressure in processing will improve fibre volume fraction, leading to better mechanical properties.
The cost of materials, mould tooling and equipment, the required finished properties and the number of parts to be produced will all be factors in deciding the process.
This section briefly looks at some of the ways FRPs can be processed, however we recommend that this knowledge is backed up with further education on the topic.
This is a skilled manual process which requires low capital investment and is widely used for low-volume products, such as boats and bespoke products.
The reinforcement (as a woven or chopped strand fabric) is carefully laid into a mould and the matrix (resin) is applied and spread with a roller or brush. This is then left to cure at ambient temperature for several hours or overnight.
To get a good surface finish, a gelcoat can be added to the mould before the reinforcement is placed into it. The top surface is the side which is face down in the mould.
Resin is fed through a tube and mixed with catalyst in a hand-held spray gun. The fibre (fed from a bobbin) is chopped into the resin stream as it is sprayed onto the mould. This is then left to cure at ambient temperature.
This is a quick and simple method, but leads to a low fibre volume fraction with randomly oriented fibres. Applications include custom parts in low to medium volumes, such as baths, swimming pools, storage tanks.
Dry fabric plies (layered pieces) are laid up into the mould and covered with a film, or vacuum bag, sealed at the edges. The fabric is compacted under vacuum pressure as resin is drawn through from a reservoir. A flow layer may be needed between the fabric and the bag, especially for less permeable carbon fibre fabrics. This is good for large parts such as boat hulls, wind turbine blades or bridge structures.
Resin Transfer Moulding (RTM)
Dry fabric is laid into a two (or more) part mould which is then closed in a heated press. Resin is injected under pressure (e.g. 10 to 20 bar) until the fabric is impregnated and the tool is heated to cure the resin. The flow may be assisted by a vacuum (then referred to as vacuum assisted – VARTM).
Increasing the pressure reduces the cycle time, and the term high pressure RTM (HP-RTM) is used where the pressure is up to 150 bar in the mixing head and from 30 to 120 bar inside the mould, depending on part size and geometry. This process requires matched metal tooling, and is well-suited for mass production of 100 to 10,000 units/year.
A variant of vacuum infusion and RTM which combines the benefits of low cost tooling and equipment, with the simple impregnation process by resin pumping of RTM. The upper part of the mould is lightweight and can be flexible, in some cases using a silicone ‘bag’, made by spraying a silicone compound on a pattern. Aimed at low production rate parts.
Prepreg fabrics (pre-impregnated with resin) are cut into plies (layered pieces) which are laid up, usually by hand, and smoothed onto the mould tool surface. This is the most popular process for high performance carbon fibre applications in aerospace and motorsport. Some manufacturers are now using automated tape-laying or automated tow placement rather than hand laid prepreg.
The part can be cured in several ways:
Autoclave: The laid-up part is vacuum bagged and consolidated under vacuum, then cured in an autoclave (pressurised oven) at around 120-180°C and 2-6 bar.
Oven: Vacuum bagged parts may be cured in an oven, with just the vacuum pressure applied. Low temperature prepregs are available, formulated to cure at 60-100°C at vacuum pressure. This is good for large parts where a big enough autoclave is not cost-effective.
Hot press: Oven and autoclave processing typically takes several hours. For a shorter cycle time, the prepreg plies may be laid up into matched metal tooling and cured in a press under high pressure, with rapid heat transfer. The part can then be cured and demoulded in just a few minutes, but the tooling is expensive. It may be post-cured in an oven.
Fibre tows are drawn through a liquid resin bath and wound onto a rotating cylindrical mandrel in a variety of orientations. This is often used to make pipes and tanks.
Multi-axis filament winding can be used to create pressure vessels or other shapes by winding the tow around a mandrel with shaped ends. The mandrel may then remain as an inner lining.
Moulding compounds have excellent electrical and fire protecting characteristics and ‘Class A’ surface finish from mould. In-mould flow means that fixtures can be incorporated in the part, reducing assembly costs.
Bulk moulding compound can be processed by injecting into a mould at high pressure or by compression moulding. Applications include automotive components, electrical equipment, household appliances.
Sheet moulding compound is compression moulded, and can be used in conjunction with woven / unidirectional fabrics or prepregs. Applications include light and tough vehicle body panels, electrical cabinets, shower trays.
For a constant cross section product, several continuous processes are used:
Pultrusion: Multiple rovings and/or fabric are pulled from reels through a resin bath into a heated die where the resin hardens and the shape is formed. The profile is pulled through the machine and cut to required lengths by an automated saw. Typical products are structural sections, cable trays, strips, bars or tubes.
Pullwinding: As pultrusion, but a roving is wound onto the cured profile as it emerges from the die. This prevents the profile from splitting and increases flexural strength.
Continuous sheet: Rooflights and flat sheet products can be made by spraying resin onto a moving carrier film and chopping fibres onto the resin. As this moves down into an oven, it may be shaped by passing over a profiled former. It is then trimmed to length.
Continuous filament winding: As filament winding, but continuous for making long pipes.
Thermoplastic composites are usually produced by commingling polymer fibres with reinforcing fibres and heating in a mould so the polymer melts to form the matrix. In a hybrid process known as overmoulding, a short-fibre filled thermoplastic compound is injected onto a thermoplastic prepreg in a single mould. This can eliminate the need to add fixings, etc afterwards.
There are some thermoplastics that can be processed like thermosets, because they react and polymerise in the mould, but they can still be re-melted afterwards.
*Photo courtesy of Lamplas*