Making carbon fibre rarely starts with carbon. Most often the starting point is an acrylic polymer yarn, polyacrylonitrile (PAN), which is first heated in air to roughly 180–300 °C, then to over 1,000 °C without oxygen. The yarn loses almost everything that is not carbon and becomes a carbon filament. These filaments are then gathered into tows, woven, impregnated with resin and cured in a mould. Here is each step, explained simply.
The crossed pattern you see on a woven carbon fibre case is the visible result of this whole chain: thousands of filaments grouped into tows, interlaced, then locked in a clear resin.
In short
- Most carbon fibres start life as a PAN yarn, stabilised in air and then carbonised under inert gas.
- The carbonisation temperature sets the fibre type: high strength at around 1,200–1,400 °C, high modulus up to 3,000 °C.
- A rigid part only appears with resin: fabric or prepreg, a mould, then curing under vacuum or in an autoclave.
What is carbon fibre made from?
Pure carbon cannot be spun: it does not melt at ambient pressure and does not dissolve. To obtain a carbon filament, you therefore first make a filament from another material, called a precursor, then use heat to remove everything that is not carbon, without losing the shape of the yarn.
Three precursors are described in the technical literature: polyacrylonitrile (PAN), viscose (rayon) fibres and pitch, a residue from petroleum or coal. This guide follows PAN, the route described for high-strength and high-modulus fibres, which offers a good carbon yield: according to the Indian Institute of Technology course published on NPTEL, PAN precursors give a carbon yield of 50 to 55%. In other words, about half the mass of the original yarn leaves as gas during production.
PAN is a polymer close to the one used in acrylic textiles. It is spun into thousands of continuous filaments, stretched to align its long molecules along the axis of the yarn. This alignment matters: it is what will later give the carbon fibre its stiffness along its length.
How does an acrylic yarn turn into carbon?
The transformation is continuous: the tows run through a series of furnaces at increasing temperatures. There are three stages.
1. Stabilisation (or oxidation)
The PAN yarn is first heated in air, at roughly 180–300 °C. The molecules reorganise and take up oxygen: the yarn turns black and, above all, infusible. Without this step, it would melt or burn in the following furnaces. The reaction releases heat, so the temperature must rise slowly to avoid a runaway.
2. Carbonisation
The stabilised yarn then enters furnaces filled with inert gas, without oxygen, so it cannot burn. The same NPTEL course describes two zones: up to 600 °C, very gentle heating (less than 5 °C per minute), then from 600 to 1,300 °C, where heating can be faster. Hydrogen, nitrogen and oxygen escape as gases, and the remaining carbon atoms arrange themselves into sheets close to those of graphite.
3. Graphitisation (for some fibres)
To obtain even stiffer fibres, the temperature is pushed to around 3,000 °C, under tension and still under inert gas. Above 2,000 °C, nitrogen is no longer used as the protective gas, because it would react with the carbon: other inert gases are used.
The final temperature determines the type of fibre obtained:
| Fibre type | Treatment temperature | Carbon content |
|---|---|---|
| High strength | 1,200 to 1,400 °C | 92 to 96% |
| High modulus | 1,800 to 2,500 °C | about 99% |
| Ultra-high modulus | 2,800 to 3,000 °C | about 99% |
Source: NPTEL, “Manufacturing of Carbon Fibres from PAN Precursors”. The stiffer the fibre (“high modulus”), the more energy it requires, which partly explains the cost difference between grades.
Why is the fibre surface-treated and sized?
Straight out of the furnaces, carbon fibre is very smooth and chemically unreactive. Yet it is only useful once bonded into a resin. Two operations prepare that bond.
Surface treatment. Japanese manufacturer Toray explains in its carbon fibre selector guide that this treatment “adds oxygen functional groups and etches the surface of the fiber”. The resin therefore grips the filament better.
Sizing. A very thin protective coating, the sizing, is then applied, chosen to be compatible with the resin that will be used: epoxy, polyester, vinyl ester… Sizing also protects the filaments during weaving. Toray specifies that a light sizing makes the tow easier to spread, while a heavier sizing better protects the fibre during demanding handling.
Finally, the fibre is wound onto bobbins. At this stage it is still only a black, flexible yarn that can be bent: stiffness will come from the resin.
What do 3K, 12K and “twill” mean?
From filaments to tows
A carbon tow brings together thousands of parallel filaments. The number followed by K gives their count in thousands: a 3K tow has 3,000 filaments, a 12K tow has 12,000. The Toray guide lists, for example, its T300 fibre in 1,000, 3,000 and 6,000 filaments per tow, and classes tows of 1,000 to 6,000 filaments as “small tows”; standard tows produced in the United States have 12,000, 24,000 or 36,000 filaments.
Small tows give a fine, regular weave, the one you see on visible parts. Large tows are better suited to industrial parts, where the pattern is rarely on show.
From tows to fabric
The tows are then woven on looms, like a textile. The “Prepreg Technology” document from Hexcel, a maker of composites for aerospace, describes the main weave styles:
- Plain: one over, one under, like a chequerboard. A stable fabric, but one that follows curved shapes poorly.
- Twill (2/1, 3/1 or 2/2): each tow passes over two or three others, drawing the diagonal lines typical of carbon. An average compromise between stability and the ability to follow curves.
- Satin: very few interlacings, a supple fabric that follows complex shapes well.
There are also unidirectional sheets, in which all the fibres run in the same direction, used when a part works mainly in one direction. The “classic” carbon pattern, the one that changes with the light, is most often a twill. It is also one of the clues described in our guide to telling real carbon fibre from an imitation.
How does the fabric become a rigid part?
Carbon fabric on its own is flexible. To make a part, it is combined with a resin (most often an epoxy) which, once hardened, holds the fibres in place and transfers loads to them. This fibre-plus-resin pairing is what is called a composite.
Prepreg
In industry, the fabric is often supplied already impregnated with a precise amount of resin: this is prepreg. Hexcel states that it is stored in a freezer at −18 °C, typically for 12 months, and that a roll can take up to 48 hours to return to room temperature before opening. According to the same document, carbon prepreg composites reach about 60% fibre by volume.
Lay-up, vacuum bagging and curing
Prepreg plies are cut and then stacked in a mould, sometimes alternating fibre orientation: this is the lay-up. The assembly is covered with technical films and a bag, then the air is drawn out. Vacuum bagging presses the plies together with a pressure of at most 1 bar, and the part is cured in an oven. For the most demanding parts, the oven is replaced by an autoclave, a vessel that adds pressure: Hexcel’s diagram shows pressures of 1 to 10 bar.
Prepreg epoxy resins typically cure at 120 °C or 180 °C. Hexcel associates the first with a service temperature of around 100 °C, and the second with 130 to 155 °C. After cooling and demoulding, the part is trimmed, sanded if necessary, then possibly varnished.
Fibernium’s tip: on an accessory, what matters to you is not the name of the process but the visible result. A real carbon weave shows depth and reflections that move when you tilt the object; a printed pattern stays flat. Look at the piece under low-angle light before you decide.
Is forged carbon made the same way?
The fibre itself is the same: it comes out of the same furnaces. What changes is the forming. Instead of a fabric with neatly aligned tows, forged carbon uses short fibres or fragments laid out at random, mixed with resin and pressed in a mould. The marbled pattern comes from this random arrangement, so no two pieces are ever identical. We go into the differences in look and use in our article forged carbon or woven carbon fibre.
For the story of carbon, from the racetrack to everyday objects, read from the racetrack to your pocket.
This guide describes the industry’s general process. On our product pages, we describe the material of each product: FIBRE is a genuine woven carbon fibre plate, FORGE and BRUTE are genuine forged carbon, MAILLE combines carbon and aramid. The GIVRE and HYBRIDE finishes, from the “Carbon style” range, are made of fibreglass, not carbon. To find these materials:
Frequently asked questions
Is carbon fibre pure carbon?
Almost. According to the NPTEL course, high-strength fibres contain 92 to 96% carbon, and high-modulus fibres about 99%. A finished part, however, is a composite: it also contains resin.
Why is carbon fibre expensive to produce?
The process chains several high-temperature furnaces, under inert gas, with slow stabilisation. About half the mass of the PAN precursor is lost along the way. Then the fibre still has to be woven, impregnated, moulded and cured.
What does 3K mean on a carbon fabric?
That each tow in the fabric brings together 3,000 filaments. A 12K tow contains 12,000. The figure tells you about the fineness of the weave, not the quality of the fibre.
What is an autoclave?
A heated, pressurised vessel in which vacuum-bagged composite parts are cured. The extra pressure presses the plies together better and reduces bubbles, which is mainly useful for thick parts or parts with a high fibre content.
Is this the process used for Fibernium cases?
This guide describes the general manufacture of carbon fibre and composites. The product pages state the material of each case; they do not detail the industrial process, so we do not make any claim about it here.