In this Explainer we are going back to basics to explain the fundamentals and highlight some common processing methods.
What is a composite?
A composite is any material made from two or more distinctly different components, where each component maintains its physical identity. For example, concrete is a composite, as it contains aggregate and cement. In this Explainer, we are focussing specifically on polymer matrix composites.
What is a matrix?
The matrix is the part of the composite which is being reinforced so, in this case, it is the polymer, which can be either thermoplastic or thermosetting.
Thermoplastic matrix composites are formed by heating (until the polymer softens) and then moulding. They can be re-melted and remoulded, making them good candidates for recycling.
A thermosetting matrix undergoes a chemical change during the forming process, known as curing. This means that, once formed, thermosets cannot be softened and re-moulded by subsequent heating.
The role of the matrix is to facilitate load transfer from one fibre to the next, to hold the fibres in place and prevent them from buckling, and to protect the fibres from potentially damaging environmental conditions.
You can find out more about polymer matrices here.
What is a reinforcement?
In this case, we are talking about reinforcement fibres, which may be glass, carbon, natural fibres or thermoplastic fibres.
Reinforcement fibres are available in a number of different formats offering a range of fibre lengths and orientations. These formats include continuous roving or tow, spun yarns, woven fabrics, multi-axial textiles, aligned or random fibre mats, or chopped fibre.
The role of the fibres is to provide strength and stiffness to the composite.
There are multiple ways in which reinforcement and matrix can be combined. Deciding upon the appropriate one requires consideration of a number of factors; the performance requirements of the final component, the specific choice of fibre and matrix, and the target production volumes. This is a topic we discuss in more detail elsewhere. For now, though, we are just going to provide a brief overview of a few of the more common composites processing methods.
A layer of dry fibre reinforcement (typically chopped strand glass) is placed in a single sided tool.
Resin (typically unsaturated polyester) is then poured over and worked into the fibres by hand, using a consolidating roller and/or brush.
The process is repeated until the desired part thickness is reached.
No need for specialist equipment.
Can be used for very large components.
Possibility of uneven consolidation (use of a vacuum bag can help).
Poor surface finish on the back side of the part.
Many traditional “fiberglass” components are produced in this way – boat hulls, storage tanks, tank covers, kiosks, electrical enclosures etc.
Layers of reinforcement (typically continuous filament glass mat, or woven or multi-axial glass fabrics) are laid in a mould with a peel ply, breather cloth and vacuum bag sealed over the top (vacuum bagging techniques are explained in more detail here).
Resin inlet tubes are strategically placed around the part, in order to ensure the reinforcement can be fully wet out before the resin cures.
Vacuum is applied to firstly remove air from system, and then to draw resin through the reinforcement.
Once the part is completely wet out, the vacuum port is closed off and the part left to cure.
Low tooling costs.
Higher fibre volume fractions can be achieved than with hand layup.
Even consolidation on all part surfaces.
Can be used for very large parts.
Lengthy set-up process.
Lots of consumable waste.
Prone to failure – leaks in the vacuum bag lead to loss of consolidation pressure and resin flow.
Wind turbine blades.
Prepreg is the term for reinforcement fabrics which have been pre-impregnated with resin, which is then partially cured – resulting in a tacky material which can be stored (typically in a refrigerator or freezer) until required.
Prepregs are produced either by passing reinforcement fabrics through a resin bath (solvent dip method) or by lowering reinforcement fabrics into a resin film (hot melt method).
Layers of prepreg can then be used to make a composite laminate using compression moulding, autoclave processing or vacuum consolidation.
Clean and easy to use,
Can achieve high volume fractions.
Vacuum or autoclave processing provides even consolidation on all surfaces.
Prepregs typically have short shelf out-life (so freezer storage required)
Expensive equipment required (autoclaves or matched metal tooling).
Maximum part size is limited by the size of press or autoclave available.
Aircraft wing skins.
Aircraft interior components.
High end sports car monocoques.
Compression moulding of organosheets
It is also possible to produce prepregs using thermoplastic matrices, to give so-called thermoplastic prepregs (also known as organosheets).
These organosheets can also be formed by the application of heat and pressure , which would typically be achieved by compression moulding, using a heated press.
Rapid cycle times are possible.
High quality surface finish on both sides of the part.
Expensive equipment and tooling required.
Certain shapes are not possible, without special modifications to the tool and/or process.
Maximum part size is limited by the size of the press.