Markforged HSHT FG 3D Printed Composite is a continuous glass fiber reinforcement material within the Markforged composite material family, supplied as a spooled continuous fiber filament for fused filament fabrication platforms equipped with a dedicated fiber extruder. The compatible production platforms include the Markforged Mark Two and the Markforged X7, where HSHT FG is co-deposited with Markforged Onyx nylon micro-carbon matrix rather than printed as a standalone thermoplastic. Manufacturer-published longitudinal tensile properties for HSHT FG include a tensile strength of 600 MPa and a tensile modulus of 21 GPa when tested in accordance with ASTM D638-14. The unreinforced Onyx matrix alone is specified at 36 MPa tensile strength and 2.4 GPa tensile modulus. Heat deflection temperature for HSHT FG is specified at 150 °C under ASTM D648. Standard Markforged Fiberglass is specified at 105 °C under the same method. The material is therefore positioned for fiber-reinforced tooling and fixture applications in which continuous fiber stiffness and elevated-temperature resistance are required without the electrical conductivity of carbon fiber.

What Distinguishes HSHT FG from Standard Fiberglass and Carbon Fiber?

Compared with standard Fiberglass, HSHT FG incorporates a high-temperature fiber sizing and matrix-compatible resin chemistry intended to reduce property loss above 100 °C. The difference in manufacturer-published heat deflection temperature between HSHT FG and standard Fiberglass is 45 °C. This gap is relevant for thermoforming tools, autoclave support fixtures, and heated inspection gauges where short-cycle thermal stability controls part-to-part repeatability. Standard Fiberglass is specified at a tensile strength of 590 MPa and a tensile modulus of 21 GPa, placing HSHT FG only slightly higher in tensile strength but substantially higher in elevated-temperature resistance. Carbon Fiber continuous reinforcement is specified at 800 MPa tensile strength and 55 GPa tensile modulus, approximately 2.6 times the tensile modulus of HSHT FG. Carbon Fiber remains the preferred reinforcement for deflection-critical parts, but its electrical conductivity can create current paths or galvanic coupling when tooling contacts metallic components in wet or energized environments. HSHT FG retains the dielectric behavior of glass fiber, making it applicable near electrostatic-sensitive devices and live electrical assemblies. The selection decision normally weighs thermal resistance, stiffness, electrical isolation, and cost as competing constraints rather than treating HSHT FG as a drop-in substitute for carbon fiber.

At tool surface temperatures above 120 °C, the performance envelope of HSHT FG becomes matrix-dominated in off-axis loading, interlaminar shear, and through-thickness compression. The Onyx matrix has a manufacturer-published heat deflection temperature of 145 °C, but continuous fiber composites can exhibit measurable creep and stress relaxation below that threshold because the nylon matrix interface begins to soften under sustained mechanical load. Tooling evaluations on Markforged X7 systems indicate that vacuum-form tools printed from HSHT FG can support sheet surface temperatures in the 130–140 °C range for short-duration cycles. Published data for this specific configuration is limited for continuous exposure above 150 °C. Potential failure modes in elevated-temperature service include interlaminar delamination at fiber terminations, surface blistering from moisture in the nylon matrix, and dimensional drift during repeated heating and cooling. These limitations are driven primarily by the thermoplastic matrix rather than by the glass fiber itself. For this reason, HSHT FG is typically assigned to short-run tooling and fixture applications with controlled thermal cycling, not to continuous high-temperature production tooling.

Mechanical Property Reporting and Test Specimen Geometry

Published mechanical property values for HSHT FG are obtained from printed test specimens with continuous fiber oriented in the 0° direction. The tensile test method is ASTM D638-14, and heat deflection temperature is reported under ASTM D648. These values do not represent isotropic material behavior. Printed continuous fiber composites are strongly anisotropic, with the highest properties in the fiber direction within the X-Y build plane. Z-axis tensile response is governed by the Onyx matrix and is approximately 36 MPa in unreinforced form. Design verification must therefore account for the difference between in-plane fiber-dominated properties and out-of-plane matrix-dominated properties. A part that is stiff in bending along the fiber plane may delaminate at matrix-dominated stress levels when loaded normal to the layers. The table below summarizes representative manufacturer-published longitudinal data for the matrix and continuous fiber materials.

Values are manufacturer-published longitudinal values. Heat deflection temperature is reported under ASTM D648. The dash indicates that a value is not reproduced in this comparison. Transverse and Z-axis values are significantly lower and are dominated by the Onyx matrix. Continuous fiber volume fraction in a printed layer is not a single fixed material constant; it depends on the number of concentric fiber rings, isotropic fiber fill density, layer height, and wall thickness settings assigned in Eiger software.

Elevated-Temperature Tooling Behavior Is Matrix-Dominated

When a tool printed from HSHT FG is cycled between 25 °C and 150 °C, the difference in coefficient of thermal expansion between continuous glass fiber and the Onyx nylon matrix produces thermal stress at the fiber-matrix interface. That stress can initiate microcracking at fiber ends and at feature boundaries where continuous fiber has been terminated. The effect becomes more pronounced as tool volume increases and as thermal cycling frequency increases. For tooling intended for repeated thermal cycling, the recommended practice is to place continuous fiber terminations away from high-moment regions and to avoid abrupt changes in fiber fill density across the tool face. No autoclave post-cure is required for HSHT FG parts to achieve manufacturer-listed mechanical properties; the material is used in the as-printed state after support removal and surface preparation.

Moisture uptake in the Onyx matrix is an additional operational boundary. At relative humidity above 60 %, nylon matrices absorb sufficient atmospheric moisture to reduce interlayer adhesion and increase dimensional variability during heated service. Spooled filament should be kept in the manufacturer-supplied dry container until use, and tool surfaces that will be exposed to humidity during storage should be sealed or preconditioned before thermal cycling. HSHT FG should not be specified for continuous immersion in water, strong acids, or aggressive solvent environments without validated barrier coatings, because the nylon matrix acts as the chemical resistance limit of the composite.

When Fiber Placement Is Constrained by Wall Thickness and Overhang

Continuous fiber deposition in HSHT FG requires a closed toolpath. Eiger software automatically places fiber rings and isotropic fiber fill only where the local geometry satisfies path-continuity and minimum-wall-thickness thresholds. Thin walls below the software-defined limit receive unreinforced Onyx only, so the nominal HSHT FG properties cannot be assumed in small ribs, thin gussets, or narrow gasket-groove walls. Overhangs and unsupported spans also interrupt fiber continuity because the continuous fiber is deposited in the X-Y plane and cannot bridge unsupported regions without underlying Onyx. The designer should treat unsupported regions as unreinforced matrix unless the software confirms otherwise.

Because continuous fiber is not deposited through the Z axis, bolted joints, threaded inserts, and clamping features must be oriented so that preload is not carried through unreinforced layer interfaces. An insert installed vertically in a horizontal plate loads the Z axis unless concentric fiber rings are routed around the insert bore in each layer. Eiger staggers fiber start and stop positions in adjacent layers to reduce crack propagation from fiber terminations, but the termination points remain local stress risers. In load-bearing areas, fiber ends should be positioned in low-stress zones rather than at maximum bending-moment locations. This is particularly important for end-of-arm tooling where robot acceleration and deceleration create reversing bending loads.

Typical production applications for HSHT FG include vacuum-form tools, thermoforming fixtures, end-of-arm tooling, heated inspection gauges, and assembly jigs used near live electrical systems. In thermoforming, HSHT FG tools are used for short-run production where sheet surface temperatures reach 130 °C and where metal tooling lead time or cost cannot be justified. Published data for this specific configuration is limited beyond 150 °C continuous exposure. In end-of-arm tooling, the fiberglass-reinforced Onyx tool mass is lower than steel or aluminum alternatives, while continuous fiber carries bending loads during high-acceleration robot moves. The dielectric character of glass fiber permits tooling to operate near energized equipment where carbon fiber would require insulation. Applications should not be extended to abrasive media, continuous high-humidity exposure, or direct solvent contact without surface sealing, because the glass fiber-reinforced Onyx surface has lower hardness than nitrided steel and will wear rapidly under abrasive flow or high-contact-force conditions.