The Markforged Carbon 3D Printed Composite is a continuous carbon fiber-reinforced polyamide system manufactured on Markforged fused filament fabrication platforms, most commonly the Mark Two, X7, and X7 Field Edition. The matrix is Onyx, a micro carbon fiber-filled polyamide with a density of 1.2 g/cm³, an ultimate tensile strength of 36 MPa, and a tensile modulus of 2.4 GPa under ASTM D638-14. The reinforcement is a continuous carbon fiber tow deposited from a separate second nozzle; the final part is a thermoplastic laminate with toolpath-defined fiber orientation rather than a homogeneous polymer. The Eiger build preparation software controls the number of carbon fiber rings, isotropic fiber fill regions, shell thickness, and layer height. Default layer height for the Onyx matrix is 100 µm on Mark Two and X7 systems. The X7 build volume is 330 mm × 270 mm × 200 mm; the Mark Two build volume is 320 mm × 132 mm × 154 mm. Because fiber orientation is a process variable, mechanical properties are anisotropic and must be reported with respect to the print X, Y, and Z axes.

Unreinforced Onyx should not be confused with the continuous carbon fiber composite. Onyx alone is a micro carbon fiber-filled polyamide; it provides higher stiffness than unfilled polyamide but remains matrix-dominated. The continuous carbon fiber configuration achieves vendor-published longitudinal ultimate tensile strength of 800 MPa and longitudinal tensile modulus of 60 GPa under ASTM D638-14. The density of the carbon fiber-reinforced configuration is approximately 1.4 g/cm³. This results in a higher specific modulus than 6061-T6 aluminum, whose density is 2.7 g/cm³ and tensile modulus is 68.9 GPa under ASTM E8/E8M-21. The comparison is valid only along the continuous fiber axis; transverse properties are governed by the Onyx matrix and are significantly lower.

What Mechanical Performance Does ASTM D638-14 Report for Continuous Carbon Fiber Reinforcement in Onyx?

Vendor-published data for unidirectional continuous carbon fiber with Onyx matrix report an ultimate tensile strength of 800 MPa and tensile modulus of 60 GPa under ASTM D638-14. Under ASTM D790-17, flexural strength is 540 MPa and flexural modulus is 51 GPa. The flexural modulus is lower than the tensile modulus because the compression side of the bend specimen behaves differently from the tension side; the Onyx matrix at the compression face yields before the tensile fibers fracture. In uniaxial tension, the carbon fiber tow carries most of the load once the matrix yields, producing the high longitudinal modulus. If the specimen is printed with the fiber oriented perpendicular to the load, the tensile strength drops to the level of the Onyx matrix, approximately 36 MPa, and the modulus returns to approximately 2.4 GPa. This anisotropy is a design characteristic of continuous fiber composites rather than a processing defect.

Layer adhesion remains matrix-limited. In a tensile test, printed coupons may fail in the gauge section or at the tab/radius interface depending on the fiber path and shell design. Vendor datasheets use specific Eiger toolpath settings; users who alter fiber fill percentage or shell count should generate their own design allowables. The fiber volume fraction is not user-adjustable as a direct percentage; it is a consequence of tow size, layer height, number of fiber rings, and toolpath density. Published data for transverse tensile modulus of the continuous carbon fiber configuration is limited, so the 60 GPa longitudinal value must not be substituted into isotropic material models.

The data show that continuous carbon fiber-reinforced Onyx is not a direct geometric substitute for 6061-T6 aluminum. The composite reaches 87% of the aluminum tensile modulus along the fiber axis at 1.4 g/cm³ compared with 2.7 g/cm³ for aluminum, but the transverse modulus is matrix-dominated. For stiffness-critical brackets, the design must route continuous fiber along principal stress trajectories. Eiger software permits alternating fiber angle layers to approximate quasi-isotropic in-plane behavior, but through-thickness mechanical properties remain below in-plane values. Published data for Z-direction tensile strength of this composite is limited; bolted joints, inserts, and through-thickness loading points require independent testing before production release.

Moisture uptake is a primary processing variable because the Onyx matrix is polyamide. Unsealed filament spools exposed to relative humidity above 60% require desiccant drying before printing. Improperly dried filament produces steam at the nozzle, creating internal voids and reducing interlayer adhesion between the fiber tow and the matrix. Printed parts also absorb moisture from water-based coolants and humid shop air. Mechanical testing should be conditioned per ASTM D618-21, typically 40 h at 23 °C and 50% relative humidity, before comparing results with vendor datasheets. If parts are to be used in continuous contact with water-glycol coolants or alkaline cutting fluids at pH above 10, long-term exposure testing is required because polyamide matrix degradation and fiber wicking can occur.

Dimensional stability is moisture-dependent. Polyamide expansion from water absorption can change critical dimensions by tens of micrometres over a 300 mm part. For metrology fixtures and CMM fixturing, parts should be thermally soaked and humidity-conditioned before inspection. The vendor does not publish a single coefficient of linear thermal expansion for continuous carbon fiber configurations because the value is lower along the fiber and higher across the fiber. When a fixture requires dimensional tolerance tighter than ±50 µm over the build area, post-machining or a different tooling material may be required. Continuous carbon fiber cannot be treated as a direct replacement for Invar or carbon-epoxy tooling board without verification of the relevant dimensional tolerance.

Process failure modes observed on production printers include fiber tow breakage at concave corner transitions, fiber misregistration on unsupported overhangs, and nozzle clogging from degraded Onyx if the filament is not stored in a desiccated state. The Markforged X7 includes an in-process laser-based dimensional inspection system that checks external feature positions during the build; it cannot detect internal fiber waviness or voids. For load-critical parts, industrial computed tomography or ultrasonic scanning is used to image internal reinforcement continuity. Machining and drilling of the printed composite require diamond-coated or solid carbide tools because exposed carbon fibers are abrasive to high-speed steel. Standard high-speed steel tooling exhibits rapid edge wear and can pull fibers from the matrix.

Moisture Uptake, Dimensional Stability, and Process Failure Modes

The Heat Deflection Temperature of Onyx under 0.45 MPa is 145 °C per ASTM D648-18. The continuous carbon fiber configuration remains matrix-dominated in HDT testing, but HDT is a short-term thermal index and does not establish continuous use temperature. Polyamide matrices can lose stiffness and undergo oxidative degradation at sustained temperatures above 100 °C. For flame retardant applications, the standard Onyx/carbon fiber system is not a rated FR system; users must select the FR variant and verify its mechanical properties separately. Chemical compatibility should be tested per ASTM D543 or ISO 22088 for the specific service fluid, especially when hot water, strong acids, or alkaline degreasing agents are present.

Compared with chopped carbon fiber-filled thermoplastics such as Onyx itself or carbon-filled polycarbonate, the continuous carbon fiber system provides an order-of-magnitude increase in tensile modulus along the fiber axis. Onyx has a tensile modulus of 2.4 GPa; continuous carbon fiber increases this to 60 GPa, a factor of 25×. Compared with unfilled ABS at a tensile modulus of approximately 2.0 GPa or unfilled polyamide at 2.0 GPa to 3.0 GPa, the continuous fiber part is significantly stiffer along the fiber path. However, all fused filament fabrication parts retain layered surfaces and lower through-thickness strength than bulk polymer or metal. If the load path is out-of-plane, a machined metal or compression-molded composite laminate is preferred.

Selective laser sintering of glass-filled polyamide produces isotropic parts without continuous fiber, but flexural modulus rarely exceeds 4 GPa for polyamide-based SLS grades. The Markforged carbon composite reaches 51 GPa flexural modulus along the fiber axis, but SLS parts may be more appropriate for complex geometries with isotropic loads. The two processes occupy different design spaces, and the selection depends on whether the load distribution is directionally controlled or fully three-dimensional.

When Carbon Fiber-Reinforced Onyx Replaces Machined 6061-T6 Aluminum in Low-Rate Production Fixtures

The substitution is most defensible when the part is stiffness-limited along a single axis, mass-sensitive, and not exposed to temperatures above 80 °C under continuous load. At 1.4 g/cm³, the printed composite has approximately 52% of the density of 6061-T6 aluminum at 2.7 g/cm³. Along the fiber axis, the composite provides 60 GPa tensile modulus compared with 68.9 GPa for aluminum. For a simple tension member, the composite reaches 87% of the aluminum stiffness at roughly half the mass. In bending, the comparison is less favorable because the flexural modulus of the composite is 51 GPa; using the same cross-section as aluminum will result in larger deflection under equal load. Ribbing, shell thickening, or reorientation of the fiber path is required to recover bending stiffness.

The composite also changes failure behavior. 6061-T6 aluminum yields at 276 MPa and work-hardens before fracture; the carbon fiber composite fails in a more brittle manner once the fiber strain limit is exceeded. Where ductile overload warning is required, the composite is not an equivalent replacement. Linear-elastic strain at failure estimated from the vendor-published tensile strength and modulus gives approximately 1.3%; published elongation data for this specific configuration is limited, and users must not assume metallic energy absorption characteristics. If overload monitoring or high-strain deformation is part of the design requirement, an aluminum alloy or a toughened laminate may be more suitable.

The Markforged X7 platform includes in-process laser-based dimensional inspection that measures select external features during the build. The Mark Two does not include this metrology system, so final inspection on a calibrated CMM or vision system is required for critical dimensions. The FX20 is a larger-format platform with a build volume of 525 mm × 400 mm × 400 mm and is primarily intended for high-temperature polymer composites such as ULTEM 9085 and carbon fiber-reinforced PEKK, rather than the standard Onyx/carbon fiber system. Users must select the platform based on build volume, in-process metrology requirements, and material family. Published data for continuous carbon fiber-reinforced Onyx in high-temperature or chemical immersion service is limited; each application requires independent qualification under the expected service conditions.