About Carbon Fibre
What are composites?
Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure.
Our main area of interest is carbon fibre reinforced composites. The composite is broadly categorised into two main elements.
- The reinforcement – typically a textile made from high strength/stiffness fibre e.g. carbon fibre
- The matrix – the glue that hold the layers of textile together eg. epoxy resin
There are many different ways of bringing together reinforcement and matrix, either prior to or during the moulding process.
Once the matrix is added and cured, the part will be solid, shape stable and ready to be removed from the mould.
At Sigmatex our focus is the design, development and manufacture of textile reinforcements using carbon fibre and other high performance materials.
Sigmatex uses a vacuum resin infusion process in its wet lab to produce small samples for characterisation. The process used at Sigmatex to create a composite involves fabric being layered and then sealed. A vacuum is applied to suck all the air out of the material then the carbon is infused to create a composite material.
What is carbon fibre?
Carbon fibre is a continuous filament material that is formed when a polymer precursor is carbonised in a vacuum at high temperature. Carbon fibre filaments – which are much thinner than a human hair – are supplied to Sigmatex as a bundle of between 1,000 and 48,000 filaments (the filaments in carbon are recognised by the letter K, 1K = 1000 filaments). Carbon fibre precursor raw materials are either Pitch based or Polyacrylonitrile (PAN) based.
- Higher thermal and electrical conductivity
- Typically more ‘brittle’ – not as suitable for critical structural applications
- More than 96% of the total market for carbon fibres
Tow count is the number of filaments in the carbon fibre bundle. It is usually measured in thousands of filaments, eg. 3k = 3,000 filaments.
Fibre codes vary, however the usual format for codes includes manufacturers’ designation and number of filaments. The tex or weight per unit area is used for calculating the weight of fabrics 12k = 800 tex (800g per 1000m) or 0.8g/m. Decitex is also used for fibres and relates to weight in g/10000m.
- Lay up of fibres in multiple directions and at zero crimp
- Three insertion heads, angles can be altered +45° to -45° or angles between, each station has been adjusted to each of these angles, where there is a ‘creel clash’, one position will be fed from a pre-spread tape.
- 0° can be inserted at three locations on the NCF machine.
Our dobby looms typically control 10 warp shafts, which when combined with complex drafting sequences can create elaborate weave patterns. The looms are capable of multiple weft insertion which also enables hybrid fabrics to be manufactured and designed. Although the majority of woven fabrics used in composites are plain, twill and satin, the ability to tailor constructions for end user applications is paramount.
Sigmatex has developed a range of materials and geometries using its state of the art 3D weaving technology. The structures differ from conventional woven textiles in the fact that they are multi-layered structures with fibre in the x, y & z direction.
Classification of 3D structures
Solid, Constant Cross Section, Hollow Structures & One Piece Woven Components
Solid structures are woven in such a way that the materials when removed from the weaving loom have multiple layers all interconnected. The method of connecting these layers and the design used is very diverse with millions of iterations possible. Whilst the number of designs is very large the different weave styles can be characterised into a few design styles, each offering different properties when combined with the resin matrix.
- Multilayer Orthogonal Structures
- Multilayer Angle Interlock Structures
- Multilayer Layer to Layer structures
Constant Cross Section Structures
Constant cross section profiles are woven in continuous lengths and can be woven in multiple widths, offering advantages in cost and output performance. Multiple layer ‘T’ Piece, ‘H’ Beam & Pi shapes are some of the shapes manufactured.
Hollow structures are designed to have sections that are hollow when the design is removed from the loom. Tethers link the different layers in various formats and all are interwoven to form the structure. Different types include Shell Structures, Honeycomb and tethered multilayer configurations.
One Piece Woven Components
One piece woven components can be designed and woven using this technology, which can be nested for optimised materials usage and can contain multiple elements. These components can be very complex as with blade type aero structures or nodal structures for joining applications. The Sigmatex team will work with customer CAD teams to design the optimised part using 3D technology and design.
Method for Designing 3D Structure
SIgmatex has developed a method for designing 3D woven structures to ensure the components manufactured are to tolerances specified by customers. The method incorporates various design protocols from initial customer CAD geometry through to finished woven component. Typical Design Process flow includes:
- Determination of material and geometry
- Combining weave parameters and dimensional requirements
- Thread path determination
- Combining all elements of weave file
- Finished output file
Whilst traditional jacquards can deliver single end control, the extent of the design parameter is either end lifted to upper shed or lowered to bottom shed. Sigmatex has worked with our technology partners to provide improved control of the fibre and weaving shedding geometry, which has improved fibre control and reduced fibrillation of the weaving fibres.
Electronic control makes it possible to include elements such as:
- Rate of lift
- Height of lift
- Relative timing of lift
All uniquely to each warp thread.