Carbon-Fiber Composites
Carbon-fiber composites put 5–7 µm carbon filaments — standard-modulus fibers such as T300 and AS4 at about 230 GPa and 3.5–4.4 GPa tensile, intermediate-modulus T800H and IM7 near 290 GPa and 5.5 GPa, high-modulus M55J and M60J at 540–590 GPa — into an epoxy or bismaleimide matrix. A cured quasi-isotropic plate comes out at about 1.55 g/cm³ with a stiffness close to aluminum at 60 % of the weight and almost no thermal expansion; a unidirectional laminate is about three times stiffer than steel per unit weight along the fiber. Onyx and other chopped-carbon nylons bring a fraction of that stiffness to printed parts that would otherwise be aluminum.
Popular carbon-fiber composite grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| Carbon Fiber | 600 | 650 | 1.55 | Fair | Machined CFRP plate and tube, the default |
| Onyx | 37 | 40 | 1.2 | Good | Printed stiff fixtures, end-effectors, housings |
| T300 | — | 3,530 | 1.76 | Poor | Standard-modulus reference fiber, cloth and UD |
| T700S | — | 4,900 | 1.8 | Poor | High-strength standard-modulus fiber for tubes |
| IM7 | — | 5,670 | 1.78 | Poor | Intermediate-modulus aerospace primary structure |
| M55J | — | 4,020 | 1.91 | Poor | High-modulus optical and satellite structure |
| AS4 | — | 4,433 | 1.79 | Poor | Standard-modulus aerospace fiber in 8552 prepreg |
Choose carbon-fiber composites for
Drone, robot and race-car arms, frames and chassis plates; camera, gimbal and instrument structure where deflection and thermal drift matter; tubes, booms and masts loaded in bending; machined plates that replace aluminum at half the weight and none of the thermal growth; printed fixtures and end-effectors in Onyx; and any part that lives or dies on stiffness per gram.
Look elsewhere when
The part is a thick 3D solid rather than a plate, tube or beam (machined 6061 or 7075 aluminum is cheaper and isotropic), it is loaded through the thickness or at bolts without inserts (laminates crush and delaminate), it sits wet against aluminum (galvanic corrosion of the aluminum — isolate it or use titanium fasteners), it must insulate electrically (G10), it needs impact and ballistic toughness (Kevlar, or a hybrid weave), or it must be formed, welded or reshaped after cure.
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Carbon-fiber composite by application
- Machined plates and brackets
- Carbon Fiber quasi-isotropic plate in 1–25 mm with twill or plain cloth faces; drill with bushings and bond or bolt through metal inserts. T300-class cloth is the default; a UD core under cloth skins when one direction carries the load.
- Tubes, booms and arms
- Carbon Fiber roll-wrapped or pultruded tube in T700S or standard-modulus UD; M40J or M55J when deflection rather than strength governs — a high-modulus boom is about twice as stiff for the same wall.
- Thermally stable structure
- M55J or M60J in a near-zero-expansion layup for optical benches, metrology frames and satellite trusses; the expansion coefficient can be tuned to zero, which no metal offers.
- Printed functional parts
- Onyx on its own for fixtures, jigs and housings; with continuous carbon reinforcement for end-effectors and brackets that approach aluminum stiffness in the reinforced direction.
Working with carbon-fiber composite
- Machining
- Cured carbon is abrasive and conductive: PCD or diamond-coated tools, high speed, low feed, sharp edges, and dust extraction that also keeps the conductive dust out of electronics. Plate holds ±0.1 mm routinely; drill against a backing plate with a brad-point or diamond drill to stop exit-side splintering, and compression-route through cuts so the top and bottom plies are both pushed inward.
- Design rules
- Load laminates in-plane and along the fiber. Through-thickness strength is about a tenth of in-plane, so bolts need bushings or bonded inserts, holes sit at least 3 diameters from an edge, and countersinks are avoided. Sheet thickness tolerance is about ±5–10 %; faces that need a precise thickness are machined. Carbon against aluminum in a wet environment corrodes the aluminum — isolate with a glass ply, sealant or titanium hardware.
- Bonding and joining
- Structural epoxy and toughened acrylic adhesives on a lightly abraded, solvent-wiped surface give joints as strong as the laminate; bonded aluminum or titanium inserts carry thread loads. Nothing welds, and rivets crush the layup without a sleeve. Printed Onyx bonds with cyanoacrylate and epoxy and takes heat-set threaded inserts.
- Finishing
- Machined plate comes with a clear-coated or matte cosmetic face; cut edges are sealed with epoxy to stop moisture wicking and fibers lifting. Clear coat or paint for UV protection — bare epoxy chalks in sunlight. No plating or anodizing; the conductive surface can serve as an EMI ground through a bonded metal tab.
Other composite series
Carbon-Fiber Composites — frequently asked
What is carbon-fiber composite?
Carbon-fiber composites put 5–7 µm carbon filaments — standard-modulus fibers such as T300 and AS4 at about 230 GPa and 3.5–4.4 GPa tensile, intermediate-modulus T800H and IM7 near 290 GPa and 5.5 GPa, high-modulus M55J and M60J at 540–590 GPa — into an epoxy or bismaleimide matrix. A cured quasi-isotropic plate comes out at about 1.55 g/cm³ with a stiffness close to aluminum at 60 % of the weight and almost no thermal expansion; a unidirectional laminate is about three times stiffer than steel per unit weight along the fiber. Onyx and other chopped-carbon nylons bring a fraction of that stiffness to printed parts that would otherwise be aluminum.
Which grade of carbon-fiber composite should I use?
Carbon Fiber plate and tube — standard-modulus cloth or UD in epoxy — for machined brackets, arms, frames and chassis plates; it is the machinable form and the answer to almost every "stiff and light" request. Name the fiber only when the layup is being engineered: T300, T700S, AS4 and HTS40 are the standard-modulus workhorses with the best strain to failure; T800H, IM7 and IM8 add 20–25 % stiffness and more strength for aerospace primary structure; M40J, M55J and M60J are high-modulus fibers for optical benches and satellite structure where deflection or thermal stability governs, at lower strain and higher price; T1000G is the highest-strength fiber, for pressure vessels. Pair them with 8552 or 3501-6 epoxy for 120–150 °C service and 5250-4 bismaleimide for 200 °C. Onyx for printed end-effectors, fixtures and housings, with continuous-fiber reinforcement where stiffness matters; C/C carbon-carbon for brake discs, furnace fixtures and rocket nozzles above 1000 °C.
Which grade of carbon-fiber composite is the strongest?
Carbon Fiber has the highest yield strength in the series at 600 MPa; Cu-Graphite follows at 80 MPa. Values are for each grade's default condition.
Which grade of carbon-fiber composite is easiest to machine?
Cu-Graphite (machinability very good, 80/100), then Onyx (55/100).
Standard-, intermediate- or high-modulus fiber?
Standard modulus (T300, T700S, AS4, about 230 GPa) is the default: best strain to failure, most forgiving in impact and by far the widest availability in cloth and prepreg. Intermediate modulus (IM7, T800H, about 290 GPa) buys 20–25 % more stiffness and higher strength for aerospace primary structure. High modulus (M55J, M60J, 540–590 GPa) is for deflection- and thermal-stability-driven parts — optical benches, satellite trusses — where its brittleness and price are accepted. Most machined plate is standard-modulus cloth, and the layup matters more than the fiber grade.
Is Onyx really as stiff as aluminum?
On its own, no: chopped-carbon nylon has a flexural modulus of about 3 GPa, more than twenty times below 6061 aluminum, but it is stiff and tough for a printed plastic and holds size better than plain nylon. With continuous carbon fiber laid into the print, the reinforced direction reaches a stiffness comparable to aluminum on a per-weight basis, which is why it works for end-effectors and brackets. It remains a printed nylon between layers and in the unreinforced direction — design around that.