Lehvoss LUVOCOM 3F TPU CNT 9902 BK is a carbon nanotube-modified thermoplastic polyurethane compound formulated for extrusion-based additive manufacturing. The “3F” designation places the grade within the LUVOCOM 3F product family for fused filament fabrication and fused granular fabrication; “TPU” identifies the thermoplastic polyurethane matrix; “CNT” denotes carbon nanotube filler; “9902” is the grade code; and “BK” indicates black pigmentation. The material is intended for flexible electrically functional printed parts where unfilled TPU remains insulating. Published grade-specific numerical property values are not reproduced in this document because the technical datasheet is the authoritative source and may be revised. Class-level behaviour of carbon nanotube-filled TPU composites and processing boundaries derived from industrial additive manufacturing practice are described below.

What effect does carbon nanotube addition exert on melt rheology and extrusion stability?

Carbon nanotube incorporation into a TPU matrix modifies the melt from a relatively Newtonian or mildly shear-thinning response to a more strongly shear-thinning response at printing shear rates. This occurs because high-aspect-ratio CNT particles orient under shear and release entanglements in the melt, reducing viscosity at increasing rates. The practical consequence is that the compound may flow through a nozzle at high shear rates but exhibit elevated viscosity at low shear in the melt reservoir or gear zone. Melt volume-flow rate should be recorded according to ISO 1133-1:2022 using the temperature and load specified on the manufacturer’s datasheet. The measurement is used for incoming feedstock lot consistency and moisture-degradation screening, not as a direct predictor of printability.

At a volumetric flow rate of 10 mm³/s, a 0.4 mm diameter nozzle produces an apparent wall shear rate of approximately 1.6 × 10³ s⁻¹; a 0.2 mm diameter nozzle exceeds 10⁴ s⁻¹. These shear rates are usually insufficient to fully disperse CNT agglomerates, but they can orient CNTs, causing anisotropic electrical conductivity in printed strands. Surface resistance measured parallel to the raster may therefore differ from the perpendicular direction by one or more orders of magnitude. This anisotropy must be characterised using IEC 62631-3-2 electrode configurations rather than inferred from a single point measurement.

The lower processing temperature boundary is set by incomplete melting of the TPU hard segments and excessive melt pressure at the nozzle; the upper boundary is set by thermomechanical degradation of urethane linkages and oxidative damage to the carbon nanotube network. Polyurethane hard segment dissociation can begin above approximately 230 °C depending on hard segment composition, and residence time at these temperatures is therefore a critical parameter. High-shear dispersion in a single-screw extruder with an L/D ratio of 24:1 to 30:1 is commonly used for pellet-fed fused granular fabrication, but melt filtration should be considered for nozzle diameters below 0.4 mm because undispersed CNT agglomerates can accumulate and produce extrusion instability. For filament-based FFF, direct-drive extrusion with a hardened drive gear is preferred because the melt is more viscous than unfilled TPU and can require higher motor torque. Elongational viscosity at the die entry also rises with CNT loading, which can increase pressure drop but reduce die swell compared with unfilled TPU. This lower die swell may require recalibration of the extrusion multiplier when changing from a non-CNT TPU to this grade.

Pre-drying, moisture uptake and storage thresholds for flexible conductive compounds

TPU is hygroscopic, and residual moisture at melt processing temperatures causes hydrolysis of ester or urethane groups, reducing molecular weight and interlayer adhesion. Residual moisture should be below 0.02 wt% as measured by Karl Fischer titration according to ISO 15512:2019. Drying in a desiccant dryer at 80 °C for 3 h to 4 h is a common class-level recommendation for Shore 85A to 95A TPU, but carbon nanotube filler can increase the effective diffusion path length for water removal. If the material is dried from a pellet or filament coil, the drying time may need to be extended until the residual moisture target is reached rather than relying on fixed time alone. A drying air dew point of -40 °C or lower is widely used for moisture-sensitive TPU extrusion grades.

Moisture diffusion through flexible TPU is faster than through semi-aromatic rigid polymers. A spool or pellet container opened at 23 °C and 50% relative humidity can exceed the moisture limit within hours for exposed surface layers, while internal spool layers equilibrate more slowly. This gradient means that drying time determined by surface moisture alone can be misleading; total residual moisture after drying should be confirmed by Karl Fischer analysis on material taken from the core of the feed container. Below 60% relative humidity, open feed systems can be operated with a dry air purge. Above 60% RH, exposure time should be limited and the material returned to a sealed moisture barrier container with desiccant. If the printer is located in an uncontrolled room with RH above 60%, a heated dry feed box is used to maintain the dry state. Feedstock that has absorbed moisture above the limit may show nozzle foaming, bubbles in printed strands, and reduced dielectric consistency. Product-specific drying instructions for LUVOCOM 3F TPU CNT 9902 BK should be taken from the LEHVOSS technical datasheet, especially when the material is supplied as filament wound on a spool, because spool geometry increases the moisture diffusion path.

In direct-drive FFF systems, the feed path from the drive gear to the melt zone is short enough to reduce filament buckling. Flexible TPU filaments of Shore 85A to 95A exhibit low column strength; Bowden tube lengths above 300 mm can produce compressive buckling at high print speeds. Carbon nanotube addition may increase filament stiffness relative to unfilled TPU, but the improvement is grade-specific and does not eliminate the need for correct idler pressure. Idler pressure should be set to the minimum value that prevents filament slip. Excessive idler force deforms the filament cross-section and creates uneven melt feeding. Filament diameter should be checked with a calibrated micrometer and ovality greater than 0.05 mm can produce visible extrusion pulsation. Unfilled TPU and carbon black-filled TPU may tolerate higher ovality, whereas CNT-filled grades can amplify diameter variation because the higher melt viscosity reduces the self-compensating effect of nozzle backpressure.

When carbon nanotube-filled TPU replaces carbon black in flexible ESD components

Carbon nanotube and carbon black differ primarily in aspect ratio and percolation behaviour. In published TPU composite literature, CNT percolation thresholds are commonly reported between 0.1 wt% and 3 wt% under optimal dispersion, while carbon black-filled TPU often requires 8 wt% to 20 wt% to achieve a conductive network, depending on carbon black structure and surface area. This difference allows CNT grades to preserve more of the TPU elongation at break and Shore hardness profile at a given electrical conductivity class. However, CNT dispersion is more sensitive to agglomeration. Undispersed agglomerates can act as stress concentrators and create point-to-point surface resistivity differences in printed parts. Dispersion quality can be inferred indirectly from pressure rise across a melt filter or from optical microscopy of microtomed printed sections. CNT also tends to provide more stable surface conductivity after flexural cycling than carbon black, but this behaviour is class-level and must be verified using ASTM D7774 or a custom bending fixture with in-situ resistance measurement.

Electrical surface resistance should be measured on conditioned specimens according to IEC 62631-3-2 or ASTM D257 at 23 °C and 50% relative humidity, with electrode geometry, voltage, and contact force reported. For ESD applications, IEC 61340-5-1 classifies materials as conductive below 10⁴ Ω and dissipative below 10⁹ Ω; a specific printed part must be tested in its final infill density and contact configuration because surface resistance is structure-dependent. LUVOCOM 3F TPU CNT 9902 BK is positioned for flexible conductive and static-dissipative parts, but the manufacturer’s datasheet should be consulted for grade-specific resistivity and mechanical property values.

Interlayer fusion governed by nozzle residence time, melt pressure and chamber thermal history

Layer adhesion in TPU extrusion depends on chain diffusion across the interlayer weld interface. The interface temperature must remain above the hard segment softening point long enough for polymer chain reptation. For CNT-filled TPU, the higher melt viscosity can slow chain diffusion, while higher thermal conductivity may increase heat transfer to the previous layer. The net effect is not monotonic; it depends on nozzle temperature, print speed, layer height, and chamber temperature. Excessively high part-cooling fan speeds rapidly quench the surface and produce a skin that reduces interlayer strength. A heated chamber can extend the welding window, but the chamber setpoint must remain below the softening point of the printed part to avoid slumping.

Printing conditions commonly used for filled flexible TPU include nozzle diameters from 0.4 mm to 0.8 mm, layer heights between 0.1 mm and 0.25 mm, and print speeds below 40 mm/s depending on part geometry. These are class-level ranges, not product-specific settings. Mechanical validation should follow ISO 527-2 or ASTM D638-14 on specimens printed in X-Y and Z orientations, because the Z-direction tensile strength of an FFF part is typically lower than the X-Y direction and is the limiting value in end-use loading. No published interlayer strength data for LUVOCOM 3F TPU CNT 9902 BK are available in open literature; qualification on the target printer is therefore required.

Solvent exposure and chemical incompatibility are operational boundaries. TPU can undergo stress cracking in contact with ketones, chlorinated solvents, and some ester-based plasticizers. LUVOCOM 3F TPU CNT 9902 BK should not be exposed to amine-based additives or high-pH cleaning solutions at elevated temperatures because these may degrade urethane linkages. For end-use parts in industrial environments, chemical compatibility testing should follow ISO 22088 or ASTM D543 on printed specimens. No public chemical resistance dataset for this specific CNT-filled grade has been published.

For ESD-safe grippers, flexible conductive gaskets, or sensor housings printed from this compound, electrical qualification must be performed on the printed geometry rather than on a compression-moulded plaque. Air voids, infill density, raster angle, and contact pressure change the measured resistance. Contact resistance between the printed surface and a metal electrode is a function of surface roughness and normal force; measurement reports should record force and electrode area. Printed parts intended for static control should be tested according to IEC 61340-5-1 after conditioning at 23 °C and 50% relative humidity unless the end-use environment requires a different humidity condition. Published application-specific datasets for LUVOCOM 3F TPU CNT 9902 BK are limited, so process validation on the actual FFF or FGF machine and end-use dimensional tolerance verification remain necessary before series production.