Markforged Onyx FR is a flame-retardant chopped-carbon-fiber-filled polyamide compound supplied for fused filament fabrication on the Markforged X7 extrusion platform. The spooled filament carries a machine-readable material profile that sets extrusion temperature, build chamber preheat, and toolpath parameters for the X7 closed-loop motion system. In standalone form, Onyx FR is applied to enclosure panels, connector housings, and interior brackets where a UL 94 V-0 classification at 3.0 mm nominal thickness is a design control point. The same compound functions as the matrix for continuous glass, carbon, or aramid fiber reinforcement inserted between Onyx FR shells. Published datasheet values include an ultimate tensile strength of 36 MPa in accordance with ASTM D638-14, a tensile modulus of 3.1 GPa, a flexural strength of 55 MPa in accordance with ASTM D790-15, and a flexural modulus of 2.6 GPa. Density is 1.2 g/cm³, and heat deflection temperature is 145 °C at 0.45 MPa in accordance with ASTM D648-16.

On the Markforged X7, Onyx FR is extruded through a 0.4 mm orifice with a sealed material path that limits ambient moisture ingress during printing. The X7 closed-loop system measures deposited layers with an in-process laser scan and reports dimensional deviation. This inspection step is relevant for flame-rated components because wall thickness variation can shift the UL 94 rating from the nominal 3.0 mm to a lower local value. A part programmed with 3.0 mm nominal walls may still contain thin regions at corners or around holes if toolpath overlap and extrusion width are not controlled. Because the flame-retardant additive package is dispersed in the nylon matrix alongside short carbon fiber, the printed material occupies a different position in the Markforged polymer portfolio than standard Onyx. The design decision is not merely a substitution of one filled polyamide for another; the flame-retardant chemistry changes the bending response, the notch sensitivity, and the qualification route.

What Happens to the Tensile and Flexural Response When the Flame-Retardant Package Is Added?

The addition of the flame-retardant package to chopped-carbon nylon does not lower ultimate tensile strength dramatically relative to standard Onyx. Onyx FR is specified at 36 MPa ultimate tensile strength, while standard Onyx is specified at 37 MPa. The tensile modulus, however, increases from 2.4 GPa to 3.1 GPa. This stiffer tensile response is consistent with a filled semicrystalline matrix in which the dispersed flame-retardant particulate phase restricts amorphous chain mobility before yield. The bending response reverses that trend. Flexural strength drops from 71 MPa for standard Onyx to 55 MPa for Onyx FR, and flexural modulus drops from 3.0 GPa to 2.6 GPa. The combination of higher tensile modulus and lower flexural strength indicates that the flame-retardant particles act as stress risers when the outer fiber of the bend specimen is placed in maximum tension. Elongation at break is typically below 3%; published datasheets list a tensile strain at break near 2.4% when tested to ASTM D638-14. These values place Onyx FR in the stiff, low-elongation class of engineering thermoplastics, not in the ductile regime of unfilled nylon 6/66.

Heat deflection temperature remains 145 °C at 0.45 MPa under ASTM D648-16, the same as standard Onyx. This is significant for short-term thermal exposure in enclosure applications where the material may be specified for operating temperatures below that limit. The rating does not constitute a continuous-use temperature rating; long-term oxidative aging and creep must be evaluated separately for the printed geometry. The flame-retardant package also contributes to a more notch-sensitive failure mode in thin sections. Sharp internal corners, thread-forming bosses, and press-fit inserts can reduce load-carrying capacity more than the nominal datasheet values would suggest.

Moisture uptake in the polyamide phase is a processing boundary and a mechanical variable. When the spool is exposed to ambient relative humidity above 40%, the matrix adsorbs water, plasticizes, and lowers the effective glass transition. The result on the X7 is often an increase in stringing between travel moves and a reduction in interlayer adhesion. Markforged supplies Onyx FR in a sealed spool package with desiccant; the printed part should be produced from material that has been dried according to the spool-specific material profile. Published equilibrium moisture data for Onyx FR at 60% RH is limited, but filled polyamide 6/66 compounds generally reach moisture uptake values in the range of 1–2 wt% at 50% RH. This level is sufficient to alter tensile modulus and print surface finish. The UL 94 V-0 classification is not a moisture-specific rating; parts conditioned to equilibrium moisture before burn testing may consume more energy during water desorption and can show different ignition times than bone-dry coupons.

UL 94 V-0 Acceptance Criteria and the Thickness-Dependent Classification

Onyx FR is classified as UL 94 V-0 at a nominal specimen thickness of 3.0 mm. The vertical burn test uses bar specimens measuring 125 mm × 13 mm × 3.0 mm. A burner flame is applied for 10 s, removed, reapplied for 10 s, and the afterflame and afterglow times are recorded. The V-0 classification requires that no individual afterflame time exceed 10 s, that the total afterflame time for a condition set of 5 specimens not exceed 50 s, and that the combined afterflame plus afterglow time after the second flame application not exceed 30 s for each specimen. Specimens may not burn or glow up to the holding clamp, and dripping particles may not ignite a cotton indicator placed below the test bar.

The rating thickness is critical. A part with nominal walls below 3.0 mm, or a part using sparse infill, cannot inherit the V-0 classification without additional testing. Thin ribs, snap features, and raised lettering can produce local sections that behave differently under vertical burn. When the minimum as-printed wall thickness is below 3.0 mm, the design should be validated by a recognized test laboratory at the minimum section rather than relying on the material datasheet rating.

For material substitution studies, the comparative datasheet values in the following table separate Onyx FR from standard Onyx. The differences are most pronounced in flexural strength and modulus and in the UL 94 classification.

Standard Onyx is not a flame-retardant grade; its UL 94 classification is listed as HB. The mechanical comparison shows that Onyx FR is not a direct upgrade in all properties. If the application is bending-dominated and flame retardancy is not required, standard Onyx may provide higher flexural strength and modulus at the same density. If the application must pass a vertical burn requirement at 3.0 mm, Onyx FR is the appropriate matrix, and the reduction in flexural strength must be accepted as part of the material substitution. Onyx FR also differs from Markforged Onyx ESD, which is formulated for electrostatic discharge control rather than flame retardancy. Selection between these grades is governed by the end-product requirement: flammability classification, surface resistivity, or mechanical performance under bending load.

Examining Print Orientation and Layer Interfacial Effects Under Structural Load

All datasheet tensile and flexural values for Onyx FR are generated on solid XY-oriented coupons. The fused filament fabrication process deposits each layer onto a previously cooled polymer surface; interfacial chain entanglement across the layer boundary is lower than in the bulk material. Short carbon fiber and flame-retardant particulate do not bridge the layer interface as effectively as continuous fiber. Therefore, a ZX tensile specimen will fail at a lower stress than the in-plane 36 MPa value. Published data for the exact ZX tensile strength of Onyx FR is limited, and this limitation should be reflected in preliminary stress calculations. A conservative design approach is to validate bonded and bolted joints with printed coupons rather than assuming isotropic behavior from the datasheet.

The X7 toolpath can increase shell count and use continuous fiber to bridge critical interfaces. For an Onyx FR part loaded in bending, the print orientation should align the maximum tensile stress with the in-plane direction rather than the z-direction. If a load must cross the layer boundary, a continuous fiber layer placed normal to the interface improves load transfer, but the flame retardancy of that interface remains a separate qualification concern. Parts with heavy continuous fiber content near the surface can exhibit different residual stress states after cooling because the continuous fiber constrains matrix shrinkage.

When Continuous Carbon Fiber Reinforcement Must Coexist with the Flame-Retardant Matrix

Onyx FR is not limited to short-fiber printing. The X7 toolpath can embed continuous glass fiber, continuous carbon fiber, or continuous aramid fiber between Onyx FR shells. The continuous fiber layers are deposited in a controlled orientation, and the fiber volume fraction is set by the number of fiber rings and the layer count. In this configuration, the tensile and flexural properties are dominated by the continuous reinforcement, while the Onyx FR shell provides the outer surface, dimensional reference, and flame-retardant polymer phase. Datasheets for continuous-fiber-reinforced Onyx FR are orientation-dependent; published values for unidirectional continuous carbon fiber configurations are available, but comparing them to injection-molded short-fiber compounds is not meaningful without matching the fiber volume fraction and print direction.

Flame behavior in continuous-fiber-reinforced sections is not automatically identical to unreinforced 3.0 mm Onyx FR. The continuous fiber bundles create resin-rich interfaces and anisotropic heat conduction that can change flame propagation along the fiber axis. Published data for the UL 94 V-0 classification of maximum continuous-fiber-filled sections is limited. If the flame rating is required on a part that carries continuous fiber, burn specimens should be printed from the same toolpath configuration, including fiber direction, fiber density, and shell thickness.

Electrically, continuous carbon fiber layers introduce substantially higher in-plane conductivity than the chopped-carbon-filled Onyx FR matrix. This difference matters for enclosures that must meet creepage and clearance distances in electrical safety standards. A grounding path or clearance strategy may need to change when continuous carbon fiber is present, even though the outer surface is Onyx FR. The material datasheet for the short-fiber matrix does not capture this system-level electrical behavior.

Qualification Routes Differ by End-Product Standard

Onyx FR is specified in electrical enclosure programs where the end-product standard requires a reduced flammability rating for internal plastic parts. The material’s UL 94 V-0 classification at 3.0 mm can be referenced in technical documentation for IEC 62368-1 or UL 508A evaluations, but it does not replace end-product electrical, thermal, or mechanical testing. The printed part must still be evaluated for creepage, clearance, arcing, and mechanical impact under the relevant clauses of the end-product standard. In rail and aerospace applications, flammability is often accompanied by smoke density and combustion gas toxicity limits. The UL 94 V-0 rating does not measure smoke density or combustion gas toxicity. If the end product requires smoke density testing, ASTM E662 or an equivalent method must be performed separately. No claim of low-smoke or low-toxicity performance is assigned to Onyx FR by the UL 94 classification alone.

In aerospace interior and ground vehicle applications, Onyx FR is used for brackets, covers, and ducting segments where nonmetallic flammability requirements are part of the component specification. The 145 °C heat deflection temperature at 0.45 MPa under ASTM D648-16 provides a short-term thermal reference, but it is not a continuous-use temperature. Parts exposed to sustained temperatures above 100 °C should be assessed for creep, oxidative embrittlement, and dimensional relaxation. Markforged does not publish a complete 14 CFR 25.853(a) compliance dataset for every Onyx FR geometry; therefore, aerospace interior qualification requires part-specific burn testing on as-printed sections with the same infill density and shell count as production hardware.

Material handling limits for Onyx FR include its moisture sensitivity, reduced flexural ductility compared with standard Onyx, and thickness-dependent flame rating. The material is not indicated for direct food contact, repeated steam sterilization, or long-term immersion in aggressive solvents without chemical compatibility testing. The nylon phase is incompatible with strong acids, some concentrated organic acids, and polar solvents that attack polyamide. Acetone and methylene chloride can cause swelling or stress cracking; applications involving these fluids should use a chemical compatibility test at the operating temperature. If the application requires electrostatic dissipation, Onyx ESD should be considered instead of Onyx FR, because the flame-retardant package is optimized for flammability rather than controlled surface resistivity. If the application is bending-dominated and has no vertical burn requirement, standard Onyx may provide higher flexural strength and modulus at the same density. Published data for the specific UL 94 V-0 performance of thin-walled, continuous-fiber-reinforced Onyx FR sections is limited; qualification burn testing remains the controlling verification method when wall thickness drops below 3.0 mm.