The Markforged Smooth TPU 3D Printing Polymer is a thermoplastic polyurethane feedstock supplied in the model designation Smooth TPU 95A for material-extrusion systems that accept Markforged sealed spooled media. The grade is classed as a low-modulus elastomer rather than a rigid structural thermoplastic, and its Shore hardness, 95A, places it above typical soft TPU grades while retaining high strain capacity. The polymer is supplied as a flexible monofilament; the low flexural rigidity relative to Onyx or Nylon White requires specific spool tension, feed path, and idler settings that differ from those used for rigid filaments. Moisture control is critical because the urethane backbone hydrolyzes at melt temperatures above approximately 220 °C when free water is present.

What Are the Documented Mechanical Properties and Test References for Smooth TPU 95A?

Table 1 provides representative values for the dried elastomer. Values should be treated as engineering references rather than universal lot minima, and certification data should be consulted for exact limits.

Specimen conditioning before mechanical evaluation follows ISO 291 class 23/50 or an equivalent controlled atmosphere at 23 °C and 50% RH for at least 24 h. Tensile testing at 500 mm/min crosshead speed is normal for this grade because the viscoelastic response at low strain rates under-represents the stiffness observed under impact-like deformation. Elongation values above 500% are sensitive to grip slip; pneumatic grips with serrated faces and initial jaw separation around 50 mm reduce premature failure. A clip-on extensometer should be inspected for knife-edge indentation of the soft specimen surface because a small nick can initiate a tear at a fraction of the monotonic elongation at break.

Within the Markforged product line, the difference is not solely a Shore hardness shift. Onyx derives its stiffness from discontinuous micro-carbon-fiber reinforcement in a semicrystalline nylon matrix, while Smooth TPU 95A derives its compliance from a phase-separated segmented polyurethane network. That morphological difference changes the failure mechanism from yield-dominated deformation in rigid nylon to elastomeric tearing and buckling in the TPU.

Moisture Uptake, Drying, and Feed Zone Stability

Thermoplastic polyurethane elastomers are segmented block copolymers in which diisocyanate and short-chain diol hard segments form hydrogen-bonded domains within a polyether or polyester soft-segment matrix. The 95A hardness grade contains enough hard-segment volume to resist cold flow at room temperature, but the polar urethane and soft-segment linkages remain hygroscopic. At 23 °C and 50% RH, equilibrium water uptake can approach 0.2% by mass; at 80% RH, the same value may double. A spool left unsealed in a humid environment should be considered non-printable until residual moisture is measured below 0.03% by mass. Drying at 80 °C for 4 h to 6 h in a desiccant dryer with dew point below -20 °C is typical for 95A TPU feedstocks. Convection ovens are less effective because inner layers of the spool do not reach required temperature without continuous dry-air circulation. After drying, the spool should transfer directly to a sealed feed box or dry cabinet maintained below 10% RH.

On production-scale FFF equipment, the primary failure mode is filament buckling between the feed gear and the hot-end inlet. The column strength of the flexible filament is low, and idler pressure above approximately 3 N compresses the filament into an oval cross-section, increasing friction and causing feed jams. The hot-end barrel should use a shallow PTFE liner transition and a sharp melt-zone start to prevent melt creep backward into the cold zone. Nozzle temperatures in the 230 °C to 250 °C range and build-plate temperatures between 45 °C and 65 °C are common for 95A TPU. Lower bed temperatures reduce first-layer adhesion to polyimide or polypropylene-based build surfaces, while higher temperatures allow lower layers to sag under retained heat. The part-cooling fan should remain off or at minimum speed for the first 3 to 5 layers to preserve interlayer coalescence; after that, a low fan speed is used only where bridging or overhang geometry demands it.

When Flexible TPU Replaces Rigid Nylon in Markforged Production

Substitution of Smooth TPU 95A for Onyx or Nylon White is justified only where recoverable deformation, impact absorption, or low compressive stiffness dominates the requirement. Onyx is a micro-carbon-fiber-filled nylon with a tensile modulus commonly reported near 1.4 GPa; Smooth TPU 95A has a tensile modulus roughly two orders of magnitude lower. The result is a product that should not be used in structural brackets, motor mounts, or dimensioned flatness-critical fixtures. Conversely, a TPU component can tolerate repeated bending, snap-through assembly, and drop-impact loads that would crack or yield a rigid nylon matrix. The product is not qualified as a continuous-fiber matrix, and published data for the use of Smooth TPU 95A with Markforged continuous carbon fiber or fiberglass reinforcement are limited.

Table 2 compares the product with Markforged rigid feedstocks in the properties most relevant to material selection.

Qualified equipment for this filament is limited to Markforged FFF platforms with sealed spool bays. The material is not sold as a generic open filament; feed-path geometry, spool calibration, and print-file parameter sets are machine-specific. This differs operationally from open-market TPU feedstocks because off-the-shelf print profiles for 95A TPU may not preserve part dimensions or interlayer strength on other machines.

Application-specific endurance testing under cyclic deformation is required because monotonic tensile elongation alone does not predict crack propagation. In some polyurethane formulations, strain-induced crystallization in the soft segment can locally harden a flexure region, shifting the hinge stiffness over repeated cycles. A design that operates at a maximum principal strain below 50% of the monotonic elongation at break can still initiate a tear at a sharp internal corner. Notches should be radiused to no less than 2 mm unless prototype batch testing under the intended displacement amplitude demonstrates otherwise. Wall thickness transitions should be tapered rather than stepped because FFF weld lines act as stress concentrators in the z-axis.

Tear growth resistance is governed by weld-line coalescence and hard-segment ordering.

Tear strength values obtained on compression-molded sheet by ISO 34-1 do not fully transfer to FFF printed bodies because adjacent extrusion roads create weld-line anisotropy. Weld-line coalescence depends on the temperature of the previously deposited road, the print speed, and the rate of hard-segment ordering as the melt cools. At print speeds below 20 mm/s, the deposited road remains above the hard-segment ordering temperature long enough for chain interdiffusion, which improves z-direction tear resistance. At higher speeds, surface freezing creates a pseudo-skin that limits inter-road diffusion and creates preferred tear paths. Tensile bars should be printed with long axes at 0°, 45°, and 90° to the build-plane raster direction to quantify anisotropy. The measured tear strength of printed specimens is typically lower than injection-molded specimens of the same chemistry, and the difference must be characterized rather than ignored.

Chemical resistance trials should be conducted under ASTM D543-21 immersion or wipe-exposure conditions before production release. The polyurethane backbone is susceptible to strong polar solvents; methyl ethyl ketone, acetone, and tetrahydrofuran produce swelling, surface tack, and loss of tensile strength. Chlorinated solvents such as dichloromethane are more aggressive and can extract soft-segment fractions. Resistance to non-polar aliphatic hydrocarbons and aqueous solutions is usually better; however, aromatic hydrocarbons and certain phosphate esters can extract low-molecular-weight processing lubricants and shift Shore hardness upward. No implicit food-contact status exists; any application involving food, skin contact, or flame resistance must be supported by formal compliance certificates such as FDA 21 CFR 177.1680 when applicable, REACH SVHC declarations, and RoHS directive certificates. Electrical surface resistivity and charge dissipation are not specified for this elastomer without an additive package, so static-charge retention in powder-free or dust-free assembly environments should be evaluated.