Coextrusion of Conductive Filament in Composites

• Coextrusion of conductive filament is an additive manufacturing process that deposits a resin‐preimpregnated CCF alongside carbon-black-loaded PLA to form multifunctional composite strain gauges.
• The method employs dual-nozzle 3D printing with precise temperature and layer height controls to ensure effective fiber wetting and formation of redundant conductive pathways.
• The resulting composites demonstrate high gauge factors, sub-0.1% noise, and reliable piezoresistivity, proving ideal for advanced structural health monitoring applications.

Coextrusion of conductive filament refers to the additive manufacturing process in which a continuous carbon fiber (CCF) bundle, pre-impregnated with thermoset resin, is simultaneously deposited with a conductive thermoplastic, such as carbon-black-loaded polylactic acid (PLA), to fabricate composite structures exhibiting both high mechanical performance and self-sensing capability. In the context of 3D-printed strain gauges, coextruding a conductive filament matrix around the CCF enables the formation of robust electrical connections, reliable piezoresistive behavior, and improved signal-to-noise characteristics, in contrast with conventional insulating thermoplastics. This technique is central to recent developments in multifunctional composites, particularly in applications demanding structural health monitoring and in situ strain sensing (Drilea et al., 14 Dec 2025).

1. Process Architecture and Coextrusion Parameters

The coextrusion process employs a dual-feed composite 3D printer, exemplified by the Anisoprint A4 “Composer,” which integrates a standard thermoplastic extruder (“plastic nozzle”) for PETG or conductive PLA, and a fiber-specific extruder (“fiber nozzle”) for feeding the CCF-prepreg. The conductive filament of choice, Protopasta Conductive PLA, consists of a PLA matrix with 20–30 wt % carbon-black filler, conferring a bulk resistivity of approximately 30 Ω·cm.

Key coextrusion parameters for CCF–conductive PLA beams include:

• Plastic nozzle temperature: 225 °C (Protopasta Conductive PLA), 240 °C (PETG)
• Fiber nozzle temperature: equal to plastic nozzle
• Heated bed: 60 °C
• Layer heights: 0.15 mm (microlayer, pure plastic), 0.30 mm (macrolayer, fiber composite)
• Extrusion widths: 0.65 mm (PETG), 0.78 mm (composite)
• Print speeds: 50 mm/s (plastic), 6 mm/s (composite)
• Flow multiplier: 1.0
• Perimeter strategy: 1 outer plastic, 2 inner plastic, 1 reinforced (CCF) outer, no inner
• Infill: 0 %
• Part orientation: CCF-composite layers placed on the top and bottom faces, loaded under three-point bending

Successful deposition at the stated speeds implies a PLA melt viscosity η(T) below 10³ Pa·s at the selected temperature, sufficient to wet and encapsulate the resin-rich fiber bundle without excessive drag.

2. Filament Composition, Rheological Interaction, and Wetting

Protopasta Conductive PLA is composed of a PLA matrix and carbon-black particles, giving it moderate conductivity. The CCF (Anisoprint CCF 1.5K) constitutes approximately 1,500 carbon monofilaments (diameter ~7 μm each), pre-impregnated with bisphenol-A epoxy. The resultant CCF composite exhibits Young’s modulus $E_{c} \approx 56.6$ GPa, tensile strength ~774 MPa, and compressive strength ~237 MPa.

During coextrusion, the thermoplastic must flow around and partially infiltrate the epoxy resin that encapsulates the CCF bundle. Although precise melt rheology was not reported, robust coextrudate formation at 6 mm/s demonstrates the adequacy of the PLA's flow and its ability to fill inter-fiber voids. The partial infiltration of PLA during print establishes multiple direct contacts with individual fiber monofilaments, essential for generating a parallel network of conductive paths.

3. Microstructural Characterization and Current Pathways

Micrograph analysis of the transverse cross-section reveals:

• Two CCF-composite strands (width approx. 0.78 mm), symmetrically situated about the mid-plane
• Each CCF bundle resides in a resin pocket, yet the conductive PLA pervades interstitial regions due to incomplete encapsulation by the epoxy, contacting numerous fiber monofilaments directly
• The bulk structure contains outer and inner PETG (or conductive PLA) perimeters encasing the composite layers.

This arrangement generates parallel current pathways: fiber-to-fiber, fiber-to-PLA, and PLA-to-PLA, mitigating the risk of catastrophic electrical failure in the event of localized fiber breakage. The cross-sectional structure can be textually schematized as:

[ PETG ][ conductive PLA ⊃ CF+epoxy ⊂ conductive PLA ][ PETG ]

where “⊃…⊂” denotes regions where the conductive PLA matrix contacts exposed CCF monofilaments.

4. Electrical and Mechanical Models

Several quantitative models describe the electromechanical behavior of the coextruded system:

• Gauge factor (GF):

$GF = \frac{\Delta R}{R_{0} \, \epsilon}$

where $\Delta R$ is the resistance change, $R_0$ is baseline resistance, and $\epsilon$ is strain.

• Linear piezoresistivity (prior to damage):

$$R(\epsilon) = R_0(1 + k \, \bar{\epsilon}) \implies \Delta R = R_0 k \bar{\epsilon}$$

with $k \approx GF$ and $\bar{\epsilon}$ the average fiber strain.

• Parallel-resistance effect:

$$R_{\rm tot} = \frac{R_{\rm fibers} R_{\rm PLA}}{R_{\rm fibers} + R_{\rm PLA}}$$

As $R_{\rm PLA} \gg R_{\rm fibers}$ (by a factor ~$3,750$), $R_{\rm tot} \approx R_{\rm fibers}$ until fiber cracks increase $R_{\rm fibers}$ and current is diverted through the PLA.

• Residual thermal stress in the CCF post-print (bar model), compressive after cooling:

$\sigma_{\rm therm} \approx -200\,\text{MPa}$

• Two-wire contact resistance:

$R_{\rm meas} = R_{\rm gauge} + 2\,R_{\rm contact}$

where $R_{\rm contact} \approx \rho_{c} L_{c}/A_{c}$ is set by the silver-paint and screw interface.

5. Experimental Performance: Sensitivity, Noise, and Reliability

Table: Key Performance Metrics With and Without Conductive PLA

Property	Protopasta Conductive PLA	PETG (Non-conductive)
Baseline $R_0$—Contact Success (%)	100% (6/6 functional)	16% (1/6 functional)
Noise: $\Delta R/R$ std. dev.	< 0.1%	Up to ±3%
Initial GF ($<0.2\%$ strain)	$0 \pm 0.5$	$0 \pm 0.5$
Post–breaking-in GF (max.)	126 (tension, short beam)	Not reported

After post-print “breaking-in”—extensive flexural cycling up to ∼50 N to introduce micro-damage—the gauge factor rises sharply (see below). For Protopasta-CCF coextrudates, GF values reach 54–126 in tension and 25–54 in compression (across beam sizes), with linearity $R^{2} = 0.87$–0.97, and baseline noise remains below 0.1%. PETG-only matrices exhibit high impedance contacts, excessive noise (±5%), or complete failure in the majority of samples.

6. Failure Mechanisms and Piezoresistive Enhancement

Progressive fiber fracture, initiated by the superposition of residual compressive stress (–200 MPa) and applied bending (totaling ~–500 MPa locally) during “breaking-in,” systematically severs monofilaments within the CCF bundle. Each micro-crack disrupts fiber-to-fiber conduction, elevating the resistance under strain—analogous to crack-based piezoresistive sensors—thus drastically increasing the GF. Over-fracture, conversely, reduces continuity and sensitivity. The conductive PLA provides redundant current paths bridging across cracks, which suppresses noise transients and ensures continued gauge operation despite progressive internal damage. PETG matrices lack this safeguard, resulting in noisy or failed sensing when CCF continuity is interrupted.

7. Design Guidelines and Application Best Practices

To attain optimal self-sensing performance in coextruded CCF–conductive filament beams, the following design rules apply:

• Employ a conductive thermoplastic matrix (e.g., Protopasta conductive PLA–carbon black) for robust, low-noise electrical interfacing and redundant current pathways.
• Maximize strain response by orienting composite layers at the beam surfaces, remote from the neutral axis during bending.
• Apply dual slicing: 0.3 mm (macrolayer, composite) and 0.15 mm (microlayer, plastic) for volume control and optimal fiber wetting.
• Precisely control nozzle temperatures (within ±2 °C) to balance polymer viscosity against the structural integrity of the CCF resin.
• Execute post-manufacturing “breaking-in” by repeated large-amplitude flexural loading to generate micro-fractures and irreversibly boost the gauge factor.
• Machine or saw parallel terminal surfaces to expose CCF loops for silver-paint and screw connections, minimizing contact resistance.
• Adopt four-wire resistance measurement where possible to exclude the contact resistance associated with the standard two-wire configuration.

In conclusion, coextrusion of a carbon-black–conductive PLA around CCF allows the fabrication of 3D-printed structural strain gauges uniting high stiffness, robust electrical performance, and heightened piezoresistive sensitivity. These sensors achieve gauge factors up to 126 with sub-0.1% noise post–“breaking-in”, demonstrating the critical role of the conductive filament both in operational reliability and in enabling repeatable, high-resolution in situ structural diagnostics (Drilea et al., 14 Dec 2025).