Among fused-filament applications requiring repeated compressive strain, unfilled thermoplastic polyurethane is processed at layer heights between 0.12 mm and 0.20 mm using a direct-drive extruder with a constrained filament path. The Clariant natural TPU filament, an unpigmented monofilament, is dried at 80 °C for 4 h in a desiccant dryer to residual moisture below 0.03% before extrusion. An extrusion multiplier of 1.00 to 1.05, four perimeter walls, and top/bottom shell thickness of 0.6 mm are applied to lattice midsole prototypes. The absence of carbon black and inorganic pigments removes a melt-viscosity variable in lot-to-lot processing, but the natural grade has no UV-absorbing pigment package; outdoor exposure therefore demands separate stabilization. For athletic footwear component qualification, tensile specimens cut from printed panels are conditioned for 24 h at 23 °C and 50% RH and tested according to ASTM D638-14. Hardness is checked with a Shore A durometer per ASTM D2240-15, with readings taken at 3 s after presser foot contact. Under EU market access, the footwear component is assessed against REACH Annex XVII Entry 50 for skin-contact articles; if filler compounds or color masterbatches are introduced at the print stage, phthalate restriction under Entry 51 must be re-evaluated.

On production lines using gantry-style fused-filament machines without an enclosed chamber, edge curling of thin insoles is reduced by lowering the part cooling fan to 20% for the first 2 mm of build height and holding the build plate at 55 °C. Thin-walled lattice nodes are mechanically trimmed rather than vapor-smoothed because solvent polishing of TPU does not produce consistent surface fusion; residual tool marks are removed by low-temperature tumbling at 15 °C. Printed footwear components used in service are not direct replacements for expanded TPU foam midsoles because the fused-filament part depends on cell-wall buckling rather than gas-filled bead expansion. Cushioning energy return is compared using ASTM F1976-20 if the finished shoe makes impact attenuation claims.

What limits interlayer fusion when printing ankle-foot orthosis shells with 0.8 mm extruders?

Interlayer fusion in an ankle-foot orthosis shell becomes strongly dependent on melt viscosity when the melt path is shortened by a 0.8 mm brass nozzle. The larger nozzle delivers a wider bead but reduces backpressure and requires a print speed between 12 mm/s and 20 mm/s to prevent under-extrusion. Clariant natural TPU filament must be dried to below 0.02% residual moisture because ester-based polyurethane reacts with entrained water at melt temperature, producing bubbles and a lower-viscosity melt that reduces Z-axis tensile strength. A heated bed at 50–60 °C and a first-layer extrusion width of 0.9 mm improve contact area. For an external orthosis used against skin, the finished article falls under medical device assessment in the European Union; skin-contact compatibility is verified following ISO 10993-1:2018 and cytotoxicity per ISO 10993-5:2009. In the United States, a Class I external orthotic device is documented under 21 CFR 890.3475. Typical Shore hardness for rigid orthotic TPU blends is Shore 95A to Shore 64D; published specific hardness data for the Clariant natural unfilled grade is limited, so feedstock hardness should be confirmed from the lot certificate of analysis. Post-annealing at 80 °C for 2 h reduces residual stress but can shrink the part 0.3–0.8%, so scaling factors must be developed per build plate zone. Bonding to polymeric reinforcement layers uses polyurethane hot-melt adhesives; amine-based epoxy accelerators should be avoided when the TPU is ester-based because residual amines accelerate chain scission under humid service. For melt-flow control, lot acceptance is done by melt volume-flow rate per ISO 1133-1:2022; however, low-shear melt index does not fully predict large-nozzle extrusion stability because TPU is shear-sensitive.

For legacy pneumatic seals and gaskets where original tooling is unavailable, fused-filament replacement parts are printed from Clariant natural TPU at 0.10 mm layer height. The application is limited to low-pressure circuits below 8 bar and continuous service below 60 °C. To achieve pressure tightness, the part is built with six perimeter walls and 100% rectilinear infill, alternating 0° and 90° toolpaths; the extrusion multiplier is set at 1.03 to fill inter-bead voids without excessive material accumulation at corners. Drying at 80 °C for 6 h is used after the filament has been exposed to more than 4 h at 50% RH. The bed temperature is held at 50 °C and the cooling fan is disabled for the first three layers to improve adhesion to polyetherimide. Chemical compatibility is assessed according to ISO 1817:2015 by measuring volume and hardness change after immersion in the actual pneumatic or low-pressure fluid at 70 °C for 72 h. Printed flange gaskets, rod wipers, and dust boots for linear slides are the immediate replacement parts. This process is not suitable for high-pressure hydraulic cylinders because mineral oil swelling of polyester TPU can exceed 10% volume change and cause extrusion from the gland; polyether TPU is preferred for water-glycol fluids, but published data for the specific natural Clariant grade is limited.

When cable jacket repair demands cold-temperature impact resistance without cross-linking

Unpigmented TPU filament is applied to field-repair of polyurethane-sheathed trailing cables and robotic cable carriers where a thermoset heat-shrink sleeve is not permitted. The printed sleeve is designed as a spiral overlap with 2.5 mm wall thickness and is fused to the original jacket by hot-air welding at 320–360 °C air temperature and 40 mm/min travel speed. The Clariant natural TPU filament is printed with a 0.6 mm nozzle, 0.25 mm layer height, and 1.5 mm retraction at 20 mm/s on a direct-drive extruder. In high-humidity environments, drying at 90 °C for 6 h is required; residual moisture above 0.03% creates microvoids at the hot-air weld interface because steam pressure disrupts the melt pool. The repaired section is not assumed to restore a regulated insulation rating unless dielectric withstand is tested according to IEC 60243-1:2013 or ASTM D149-20. For mechanical chafing protection, weld tensile elongation is checked per ISO 527-2:2012. Natural TPU contains no carbon black, so UV exposure in surface mining or offshore applications requires a UV-stabilized overcoat or specification of an aliphatic polyurethane-based grade. Low-temperature impact resistance is assessed with ISO 974:2000 or ASTM D746-20; published sub-zero data for this specific unfilled filament configuration is limited. Avoid cyanoacrylate adhesive directly on thin printed walls because local stiffening creates a stress concentration at the printed-weld boundary; use two-component polyurethane adhesive or hot-air welding. The final sleeve is allowed to cool to 23 °C before flexural testing so that residual thermal stress does not bias elongation values.

Soft gripper fingers and pneumatic bellows printed in Shore 95A TPU

Soft robotic end-effectors use unfilled TPU because the material maintains low compression set after repeated bending and delivers high elongation at break when printed without pigment agglomerates. The natural colorant-free Clariant filament eliminates pigment agglomerates that can nucleate microcracks at thin bellows walls. Printing uses a 0.4 mm nozzle, 0.16 mm layer height, and two perimeter walls to keep convolution thickness below 0.8 mm; infill is suppressed in bellows regions to retain compliance. Nozzle temperature is set at 240 °C, bed at 50 °C, and cooling fan at 25% only after the first two layers. Retraction is disabled or limited to 0.5 mm at 10 mm/s to avoid chewing in the extruder gear. For airtight actuator shells, the surface is post-processed with a waterborne polyurethane dispersion; leak testing is performed by positive-pressure decay from 0.5 bar to 0.45 bar over 30 s. Robotic integrators testing collaborative grippers refer to force and pressure limits in ISO/TS 15066:2016; the material is not a substitute for energy-absorbing covers that meet ISO 10218-2:2011 clauses on robot system design. Dimensional repeatability for unfilled TPU extruded at constant melt pressure is generally within ±0.15 mm when the filament is dried to 0.02%; published data for the Clariant natural grade is limited, so batch-specific capability must be established before large-scale production. For food-handling suction cups, migration testing must be performed under EU 10/2011 and 21 CFR 177.1680; unpigmented TPU reduces the extraction profile compared to colored grades, but printing aids and nozzle wear introduce migration variables. The printed gripper is limited to operation below 60 °C to avoid compression set; cyclic actuation beyond 100,000 cycles should be validated by dynamic fatigue testing.

Sports protective equipment and impact-damping lattice liners

Unfilled unpigmented TPU filament is used for protective pads, shin guard liners, and equestrian vest inserts where conformal damping is required. The Clariant natural TPU lattice is built with a 0.6 mm nozzle, 0.30 mm layer height, and 45% gyroid infill to produce controlled cell wall collapse before densification. A three-perimeter shell and 5 mm top and bottom layers prevent lattice cell burst under point impact. For skin-contact sportswear, compliance with REACH Annex XVII Entry 50 and 51 is verified for polycyclic aromatic hydrocarbons and phthalates; in the United States, CPSIA Section 108 restricts certain phthalates in children’s products, and California Proposition 65 requires assessment of listed substances when exposure exceeds safe harbor levels. Sold as personal protective equipment, the printed insert is tested under EN 1621-1:2012 for limb protectors; published instrumented impact attenuation data for this specific natural TPU filament is limited, so end-use certification is required. Because natural TPU has no UV stabilizer package, outdoor sports equipment is coated or replaced after seasonal exposure; mechanical property retention is verified by tensile testing per ASTM D638-14 after accelerated weathering under ISO 4892-2:2013. Printing large pads on a heated bed at 55 °C reduces edge curling; a polyvinylpyrrolidone adhesive or PEI sheet is used and the build plate is allowed to cool below 35 °C before part removal. Support material is not applicable because TPU generates stringing; excess material is removed by knife trimming and low-temperature tumbling at 15 °C.