# Investigating microstructural features and tensile properties of 3D-printed co-polyester reinforced with carbon fibres

Lotfi Hedjazi, Sofiane Belhabib, Jaianth Vijayakumar, Elodie Boller, Sofiane Guessasma

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Lotfi Hedjazi, Sofiane Belhabib, Jaianth Vijayakumar, Elodie Boller, Sofiane Guessasma. Investigating microstructural features and tensile properties of 3D-printed co-polyester reinforced with carbon fibres. Composites Part C: Open Access, 2025, 17, 10.1016/j.jcomc.2025.100604. hal-05494703

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# Investigating microstructural features and tensile properties of 3D-printed co-polyester reinforced with carbon fibres

Lotfi Hedjazi<sup>a</sup>, Sofiane Belhabib<sup>b</sup>, Jaianth Vijayakumar<sup>c</sup>, Elodie Boller<sup>c</sup>, Sofiane Guessasma<sup>d, *</sup>

## ARTICLE INFO

### Keywords:

- Additive manufacturing
- Co-polyester-carbon fibre
- Microstructure
- Tensile performance
- X-ray micro-tomography

## ABSTRACT

This study investigates the 3D printing of carbon fibre-reinforced copolyester (COP-CF) composites using fused filament fabrication (FFF) technology, with a focus on the influence of printing parameters on mechanical performance and microstructure. We explore the effects of different printing angles (0° to 90°) on the tensile behaviour, pore connectivity, and microstructural characteristics of 3D-printed COP-CF specimens. Synchrotron X-ray microtomography is employed to analyse the internal structure of printed parts, revealing insights into porosity distribution and fibre alignment. Our results indicate that a 45° printing angle yields the highest mechanical performance, with a tensile strength improvement approaching 70 MPa and a Young's modulus nearing 1 GPa, attributed to filament alignment in the loading direction and optimal load transfer. Additionally, the elongation at break reaches approximately 10%, indicating a balance between strength and ductility. The study also highlights the role of process-induced porosity and its impact on mechanical properties. Additionally, the design and testing of a 3D-printed curved hook demonstrate the material's potential for functional applications under mixed-mode loading conditions effectively at a 45° printing angle—outperforming other angles by a factor of 1.71. The findings underscore the importance of printing angle and microstructure control in optimizing the mechanical performance of 3D-printed COP-CF composites for technical applications.

## 1. Introduction

Cop-polyesters (COPs) consist of repeating ester functional groups (—COO—) within the polymer backbone, formed through the polycondensation reaction of diacids (or their derivatives) and diols. In copolyesters, at least two different types of diacid or diol monomers are used resulting in hydrocarbon chains or aromatic groups that induce the heterogeneous arrangement of repeating units. This particular arrangement gives the material its distinctive properties such as polyethylene terephthalate glycol (PETG) [1]. COPs are widely used in various applications due to their excellent mechanical properties, chemical resistance, and clarity [2]. They are commonly employed in the plastic packaging industry for producing bottles, containers, and films, offering durability and transparency that meet food and beverage safety standards [3]. In the medical field, copolyesters are utilized to create disposable medical devices and components, benefiting from their biocompatibility and ease of sterilization [4]. For instance, in tissue

engineering, copolyesters are studied thanks to their transparency and hardness. Chiriac et al. [5] studied the bioresorbable copolyester poly (DL-lactide-ε-caprolactone) use for nerve repair. Their study concludes on the advantage of the copolyester transparency for adequate positioning of nerve ends as well as resorption, and semi-permeability while the same study points out the lack of softness as an issue when used close to hand joints.

In addition to biomedical applications, copolyesters are also used in consumer goods, such as electronics, automotive interiors, and household products, where their heat resistance and toughness are essential [6]. The material's ease of processing through injection moulding, extrusion, and 3D printing further enhances its suitability for complex designs and functional prototypes in diverse industries [7–9].

Copolyesters have gained significant attention in additive manufacturing (AM) due to their versatility, ease of processing, and excellent mechanical and thermal properties [10,11]. These materials are particularly well-suited for fused filament fabrication (FFF) and

<footnote><sup>a</sup> ESTP Campus de Troyes, 1 Rue Fernand Sastre, 10430 Rosières-prés-Troyes, France</footnote>

<footnote><sup>b</sup> Nantes Université, IUT, 2 Av. du Professeur Jean Rouzel, 44470 Carquefou, France</footnote>

<footnote><sup>c</sup> ESRF, 71 Avenue des Martyrs, 38000 Grenoble, France</footnote>

<footnote><sup>d</sup> INRAE, Research Unit BIA UR1268, Rue Geraudiere, F-44316 Nantes, France</footnote>

<footnote>* Corresponding author.
E-mail address: sofiane.guessasma@inrae.fr (S. Guessasma).</footnote>
other extrusion-based 3D printing methods, offering high strength, toughness, durability, flexibility, odour neutral processing and temperature resistance [12,13]. For example, Pedrini et al. [14] explored additive manufacturing (AM) to evaluate the printability of scaffolds and the cytocompatibility of 3D-printed Poly(L-co-D,L-dactic acid-co-trimethylene carbonate). Their study highlighted the role of trimethylene carbonate in enhancing mechanical flexibility and its impact on polymer degradation through changes in transformation temperature after printing. The findings demonstrated the material's potential for biomedical applications, further supported by the successful adhesion of mesenchymal stem cells to the Poly(L-co-D,L-dactic acid-co-trimethylene carbonate) scaffolds.

Ahlinder et al. [15] investigated the printability of high-molecular-weight copolyesters for medical applications. They utilized an extrusion-based process to fabricate scaffolds from L-lactide, poly(ε-caprolactone-co-L-lactide), and poly(L-lactide-co-trimethylene carbonate). The study emphasized the critical importance of controlling printing parameters to prevent thermal degradation. The authors proposed that such control could be achieved by conducting extrusion trials to determine the rheological properties of the copolyesters.

The development of two-phase or more as feedstock materials for additive manufacturing (AM) has been a major route for enhancing material performance, enabling multifunctional composites, and expanding the range of industrial applications. Sam-Dalir et al. [16] investigated recycled glass fibre reinforcement in polypropylene, showing that material extrusion processing enabled 40% fibre loading, yielding filaments with 112 MPa tensile strength and printed parts reaching 23 MPa. Ghabezi et al. [17] optimized the processing of waste polypropylene (PP) reinforced with basalt fibres for AM, improving printability and mechanical properties, with tensile and flexural strengths peaking at 54 MPa (15% fibre) and 34 MPa (30% fibre), respectively.

The use of carbon fibre as a reinforcement material in AM has gained significant momentum due to its ability to enhance the mechanical properties of 3D-printed components including thermoplastics such as copolymers.

Ghabezi et al. [18] investigated the upcycling of carbon fibre and PP waste through extrusion and 3D printing, demonstrating that carbon fibres significantly enhanced the stiffness of printed composites. However, fibre contents exceeding 15 wt% led to clustering and reduced ductility, particularly in elongation at break.

Carbon fibre-reinforced composites offer superior strength, stiffness, and lightweight characteristics, making them ideal for applications in aerospace, automotive, and high-performance sporting goods [18]. In AM, short, chopped, or continuous carbon fibres are integrated into polymer matrices, such as polylactic acid (PLA), nylon, or polyether ether ketone (PEEK), to produce parts with tailored properties [19]. Continuous carbon fibre reinforcement, in particular, has shown promise in creating load-bearing structures with strength-to-weight ratios comparable to those of traditionally manufactured metal parts [20]. Advances in 3D printing technologies, such as fused filament fabrication (FFF) [21] and direct ink writing (DIW) [22], have enabled precise control over fibre orientation and distribution, further optimizing mechanical performance.

Successful attempts were reported to use carbon and glass fibre reinforcement capabilities with polyester matrices for instance using thermoset or Hand lay-up approaches [23,24]. In additive manufacturing, expectations in terms of higher stiffness and higher toughness for polyester-reinforced feedstock materials compared to pure copolymers [25] are advanced especially to obtain technical filaments and functional parts with high standards for dimensional accuracy and stability [19]. Despite their merits, the use of carbon fibre reinforced copolyester (COP-CF) feedstock material in additive manufacturing [26] is still sparse, and most of the literature focus on polyamide. Acrylonitrile butadiene styrene, and polylactic acid (PLA) [27]. For instance, Maqsood et al. [26] investigated carbon fibre-reinforced composites

produced through the fused filament fabrication method. The incorporation of carbon fibres significantly enhanced tensile strength and stiffness under tensile loading conditions. Furthermore, composites printed with continuous carbon fibres exhibited superior flexural properties.

In this study we investigate the 3D printing of COP-CF using fused filament technology, with a focus on analysing the microstructural factors influencing the mechanical performance of 3D-printed composites. By combining synchrotron X-ray phase-contrast microtomography as a non-destructive technique we assess the internal structure of printed parts under varying printing conditions. Furthermore, the results highlight the potential for designing consumer products made from COP-CF, showcasing how the material can achieve tailored properties by addressing both structural and mechanical requirements.

## 2. Materials and methods

### 2.1. Feedstock materials and additive manufacturing

For this study, we use a commercially available COP-CF thermoplastic filament from ColorFabb, which combines Eastman Amphora™ 3D styrene-free matrix with 20 wt% carbon fibres. This filament is designed for prototyping applications that demand both high surface quality and excellent mechanical performance. According to the supplier's recommendations, the printing parameters include an extruder temperature of 240-260 °C, a bed temperature of 60-70 °C, and a printing speed of 40-70 mm/s. The COP-CF filament is processed using a Raise3D Pro2 Plus printer, following the printing conditions detailed in Table 1. These conditions are applied to fabricate 3D-printed dogbone-shaped specimens (Fig. 1a, b) for analysis. Key standards are adopted such as ASTM F2792 for AM terminology and ISO/ASTM 52,900 for general principles such as material selection, printer calibration, layer resolution, build orientation, and post-processing procedures. The ISO 527-1/-2 standard was used for sample preparation according to a previous work to optimise material consumption with regards to tensile specimen [28]. In all processing conditions, the specimens are oriented longitudinally, with the build direction parallel to the thickness of the specimens. The printing angle is adjusted between 0° and 90° (Table 1, Fig. 1a, b). This angle refers to the orientation of a printed layer within the plane of construction relative to the reference axis attached to the building platform [29].

### 2.2. X-ray-micro-tomography

Before tensile testing, we examine the microstructure of 3D-printed COP-CF samples produced under varying printing angles (θ = 0°, 15°, 30°, and 45°). We use synchrotron X-ray microtomography at the ESRF beamline BM05 in Grenoble, France (Fig. 1b). The measurements are carried out using photon energy of 97 keV, a sample-detector distance of 1396 mm. For a tomogram, we acquire a total of 5000 radiographic images with 360° rotation, 101 reference images, 100 dark images. The detector size is fixed to 2048 × 800 pixels, exposure time of 0.015 s, and

<table><caption>**Table 1**

Printing conditions for COP-CF composite material.</caption><thead><tr><th>Type</th><th>Parameter</th><th>Value</th></tr></thead><tbody><tr><td rowspan="10">Fixed</td><td>Infill (%)</td><td>100</td></tr><tr><td>Nozzle diameter (mm)</td><td>0.6</td></tr><tr><td>Layer height (mm)</td><td>0.3</td></tr><tr><td>Layer height, first layer (mm)</td><td>0.25</td></tr><tr><td>Bed temperature (°C)</td><td>60</td></tr><tr><td>Printing speed (mm/s)</td><td>50</td></tr><tr><td>Printing temperature (°C)</td><td>250</td></tr><tr><td>Frame width (mm)</td><td>1.2</td></tr><tr><td>Orientation (-)</td><td>flat</td></tr><tr><td>Varied</td><td>Printing angle, θ (°)</td><td>0, 15, 30, 45</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The investigation utilizes a dogbone-shaped geometry and a standardized tensile testing protocol, featuring: (a) definition of the printing angle θ (b) a sliced view of the COP-CF 3D-printed specimen showing two different printing angles, (c) the X-ray microtomography setup at the BM05 beamline (European Synchrotron Radiation Facility, ESRF) for 3D microstructural analysis, and (d) the tensile testing apparatus with optical recording for deformation tracking.</figcaption></figure>

a voxel size of 3.04 µm. Tomograms are reconstructed using a filtered back-projection algorithm combined with Paganin phase retrieval to enhance phase contrast.

To cover a large area of the sample at the defined voxel size, two consecutive tomography acquisitions are performed along the sample's height, effectively doubling the imaged volume. The resulting tomogram is a 3805 × 3805 × 1400 to 3807 × 3807 × 1400 voxels, cover a volume of approximately 11.6 × 11.6 × 4.3 mm³. Image analysis is carried out using ImageJ software (NIH, USA), involving brightness calibration, grey-scale conversion to 256 levels, segmentation, 3D rotation, and morphological filtering with opening and closing operations. Additional processing includes background removal, phase content quantification, anisotropy assessment, and size distribution measurements.

## 2.3. Mechanical characterisation

Both the as-received filaments and the 3D-printed COP-CF parts are separately characterized. Tensile testing is conducted on all specimens using a Zwick/Roell universal testing machine equipped with a 10 kN load cell (Fig. 1c). The tests are performed until material failure, with a displacement rate of 5 mm/min. This rate is chosen based on standard testing protocols commonly used for the mechanical characterization of polymer and composite materials, such as those outlined in ASTM D638 for tensile testing. This rate provides a balance between minimizing dynamic effects and allowing sufficient time to capture detailed deformation behaviour, especially in materials like 3D printed composites or flax fibre-reinforced specimens. Additionally, this rate has been widely adopted in the literature for similar materials, enabling meaningful comparisons with previous studies including previous works by the
authors [30].

For each condition, three to four replicates are tested. Engineering constants, including tensile strength, Young's modulus, yield stress, and elongation at break, are calculated as averages from force-displacement curves, using a custom-fitting algorithm developed in the MATLAB environment. Additionally, deformation sequences are captured with a Photonline high-speed optical camera (Phantom V7.3), recording the entire loading process at a resolution of 800 × 600 pixels and a frame rate between 50 and 400 fps (frames per second).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Tensile behaviour of as-received COP-CF filament. (a) typical loading sequence showing typical deformation stages, (b) engineering strain – stress tensile response for three replicates.</figcaption></figure>

# 3. Results and discussion

## 3.1. Mechanical properties of as-received and 3D printed COP-CF

Fig. 2a presents a typical sequence of an as-received COP-CF filament loaded to rupture under tensile conditions. The filament exhibits limited stretchability with no significant necking, indicative of quasi-brittle behaviour. This brittleness is attributed to the 20 wt% of carbon within the filament, which affects the elongation at break of the blend. Fig. 2b provides further quantification of the tensile behaviour for three replicates, showing a typical elastic-plastic trend with a limited plastic region, and limited response dispersion. The tensile strength is below 70 MPa, and the elongation at break is approximately 10 %. Slight jaggedness in the curve suggests potential damage mechanisms, such as interfacial debonding. Table 2 summarizes the extracted engineering constants associated with the loading response of the as-received filament. Young's modulus is slightly below 1 GPa, and the difference between yield stress and tensile strength is as high as 17 MPa. Additionally, the gap between tensile strength and rupture stress is approximately 9 %.

Fig. 3 illustrates the impact of printing parameters on the tensile behaviour of 3D-printed COP-CF. Fig. 3 depicts the deformation sequence of COP-CF samples printed at angles between 0° and 45°, highlighting the moments of rupture and maximum extension prior to failure. Before loading, no significant defects are observed, except in regions where the resolution is insufficient to achieve a true 100 % infill—this is particularly evident in the sample printed at a 45° angle. The apparent surface irregularities are primarily due to light reflections rather than actual material defects.

The images show limited extension before rupture, reflecting a predominantly quasi-brittle behaviour. Cracking occurs rapidly, with minimal visible damage accumulation leading up to failure. At maximum load, the onset of cracking is detectable through subtle changes in the grey level, indicating localized damage accumulation.

The deformation sequences do not clearly reveal a distinct influence of the printing angle, as all samples exhibit similar extensions prior to rupture. The rupture patterns also lack significant crack deviation due to material structuring (Fig. 3a). Instead, they show a tendency for jagged crack paths (Fig. 3b), likely influenced by the filament layup. The insensitivity of the cracking trajectory to the printing angle is attributed to the limited stretchability of the as-received filament. This restricted extension prevents the development of strong shear deformations needed to support crack deviation.

In all cases, unstable cracking predominates especially for large printing angles (Fig. 3c-b), occurring primarily under an opening mode. Furthermore, no evidence of crack branching is observed, which might otherwise arise from competing transverse cracks.

Fig. 4 quantitatively illustrates the effect of printing angle (θ) on the tensile behaviour of 3D-printed COP-CF. For θ = 0°, the tensile response is the lowest compared to other printing angles. While there is notable variability in how the printing angle influences tensile performance, previous findings often report higher tensile responses for specimens printed with a -45°/+45° layup. When the printing angle is set to 15°, the tensile response improves, suggesting that layup orientations closer

<caption>**Table 2**</caption>

Mechanical properties of as received and 3D-printed COP-CF as a function of printing angle. E<sub>Y</sub>: Young's modulus, σ<sub>Y</sub>: Yiled stress, σ<sub>T</sub>: tensile strength, σ<sub>R</sub>: rupture stress, ε<sub>R</sub>: elongation at break.

<table><thead><tr><th>θ (°)</th><th>E<sub>Y</sub> (MPa)</th><th>σ<sub>Y</sub> (MPa)</th><th>σ<sub>T</sub> (MPa)</th><th>σ<sub>R</sub> (MPa)</th><th>ε<sub>R</sub> (%)</th></tr></thead><tbody><tr><td>As-received</td><td>945 ± 28</td><td>53.5 ± 10.4</td><td>70.2 ± 1.7</td><td>64.2 ± 0.5</td><td>10.7 ± 1.5</td></tr><tr><td>0</td><td>807 ± 16</td><td>35.1 ± 0.6</td><td>39.9 ± 0.6</td><td>36.6 ± 1.2</td><td>6.1 ± 0.3</td></tr><tr><td>15</td><td>842 ± 14</td><td>36.9 ± 0.2</td><td>41.8 ± 0.2</td><td>39.0 ± 0.4</td><td>5.9 ± 0.2</td></tr><tr><td>30</td><td>887 ± 6</td><td>41.8 ± 0.6</td><td>43.1 ± 0.6</td><td>42.6 ± 0.4</td><td>5.2 ± 0.2</td></tr><tr><td>45</td><td>955 ± 8</td><td>40.8 ± 0.7</td><td>44.8 ± 0.7</td><td>43.6 ± 1.3</td><td>5.2 ± 0.2</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Optical monitoring of deformation sequences in 3D-printed COP-CF as a function of the printing angle (θ). The arrows indicate damage accumulation sites that are origin to cracking departure: (a) 0° (b) 15°, (c) 30°, (d) 45°.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Tensile behaviour of 3D-printed COP-CF as influenced by printing angle (θ).</figcaption></figure>
to the loading direction, such as 60°, enhance load-bearing capacity. This adjustment results in improved mechanical resistance compared to the 0° printing angle. At a printing angle of 30°, the layup becomes more oriented toward both transverse and longitudinal directions, allowing filaments aligned at approximately 75° to contribute more effectively to mechanical resistance. This orientation is expected to promote mixed-mode fracture, characterized by two primary crack angles (–15° and 75°); however, crack instability remains predominant.

For a 45° printing angle, a significant proportion of filaments - approximately half - effectively contribute to load transfer along the loading direction, yielding the highest mechanical performance. As shown by previous results based on X-ray micro-tomography observations and finite element computations [31,32], the orientation of the printed filaments relative to the loading direction is a key factor influencing mechanical performance. As illustrated in Fig. 1a, a 45° printing angle results in a filament arrangement where approximately half of the strands are oriented in alignment with the loading direction. This alignment enables more effective load transfer through the structure, enhancing the overall mechanical response of the printed part.

This observation highlights the role of uniaxial deformation of the filaments in achieving superior tensile behaviour. In the case of COP-CF material, the reinforcing effect of carbon fibres, particularly their orientation along the filament layup, is likely the primary factor driving the trends observed in Fig. 3.

Beside these differences, all tested samples exhibit elastic characteristics with a minimal plastic phase. The predominantly elastic-like behaviour is attributed to the filament intrinsic properties, which is weighted by the nature of filament arrangement.

A quantitative assessment of the impact of printing angle on the performance of 3D printed COP-CF composites is discussed in Table 2, through typical engineering parameters such as Young's modulus ($E_Y$), yield stress ($\sigma_Y$), tensile strength ($\sigma_T$), rupture stress ($\sigma_R$), and elongation at break ($\epsilon_R$).

Further analysis of the overall impact of the printing angle reveals an approximate 18 % increase in stiffness as the printing angle rises. However, this improvement is accompanied by a proportional decrease in elongation at break, which reduces by around 15 %. Other engineering properties follow a similar trend to Young's modulus, with increases of 16 %, 12 %, and 19 % observed for yield stress, tensile strength, and rupture stress, respectively, within the same range of printing angle variation.

To further quantify the relationship between the printing angle and the engineering constants, an automated fitting routine was utilized. This routine, which evaluated over 240 functions, was designed to identify a predictive pattern using a minimal number of fitting parameters.

$$E_Y(\text{MPa}) = 807 + 5.84 \times \sqrt{\theta^*(\circ)} + 0.05 \times \theta^*(\circ); R^2 = 0.99 \quad (1)$$

$$\sigma_Y(\text{MPa}) = 34.6 + 0.26 \times \theta^*(\circ) - 5.48 \times \theta^*(\circ)^3; R^2 = 0.90 \quad (2)$$

$$\sigma_T(\text{MPa}) = 40 + 0.11 \times \theta^*(\circ) - 0.0002 \times \theta^*(\circ)^2; R^2 = 1.00 \quad (3)$$

$$\sigma_R(\text{MPa}) = 36 + 0.22 \times \theta^*(\circ) - 2.66 \times 10^{-5} \times \theta^*(\circ)^3; R^2 = 0.98 \quad (4)$$

$$\epsilon_R(\%) = 6.15 - 0.002 \times \theta^*(\circ) + 3.56 \times 10^{-5} \times \theta^*(\circ)^3; R^2 = 0.94 \quad (5)$$

Where $R^2$ is the correlation factor.

Eqs. (1) to (5) typically stem from fitting experimental data to empirical or semi-empirical models. To assess the accuracy of the fittings, statistical measures are considered such as the coefficient of determination ($R^2$): indicating how well the model explains the variability in the data. A high $R^2$ value (close to 1) suggest that the equation provides a good fit to the experimental data, supporting its validity for predictive or analytical use. Conversely, lower accuracy may imply limitations in the model or the need for more refined characterization.

Examination of Eqs. (1) to (5) demonstrates the fitness of the selected equations.

The analysis of the relationship between the printing angle and the engineering constants through Eqs. (1) to (5) indicates that the majority of the observed correlations are nonlinear.

The data suggests a positive correlation between tensile strength ($\sigma_T$) and density ($\rho$) of the material (Eq. (3)). The as-received material, with the highest density of 1.4 g/cm³, also exhibits the highest tensile strength at 70.2 MPa. In contrast, the 3D printed samples, with lower densities ranging from 1.065 to 1.075 g/cm³, show significantly reduced tensile strengths between 39.9 and 44.8 MPa.

Density differences among the printed samples are relatively small, and a general trend cannot be observed. However, the data indicates a clear inverse correlation between porosity content ($f_p$) and tensile strength ($\sigma_T$). As porosity increases, tensile strength decreases, which is expected due to the weakening effect of voids within the material's structure. The as-received material, with the lowest porosity (2.94 %), shows the highest tensile strength at 70.2 MPa. In contrast, the 3D printed samples exhibit significantly higher porosity levels (ranging from ~19 % to ~22 %) and correspondingly lower tensile strengths (between ~39.9 and 44.8 MPa). While the tensile strength among the printed samples shows a slight increase (from 39.9 to 44.8 MPa), this trend appears even as porosity slightly increases, suggesting that other factors—such as filament orientation or interlayer bonding—may partially offset the weakening effect of porosity in specific cases.

Still, the overall trend is consistent: materials with higher porosity content generally exhibit reduced mechanical performance, particularly in terms of tensile strength, due to the stress concentration and crack initiation sites introduced by pores. A more quantitative assessment (e. g., regression analysis) could help model this relationship more precisely, but the inverse correlation is clearly evident from the presented data.

### 3.2. Microstructure of as-received and 3D printed COP-CF composites

To better understand the mechanical performance ranking of 3D-printed COP-CF, an analysis of the induced microstructure is performed. Considering the significant impact of the printing angle on the mechanical outcomes and the central hypothesis regarding the alignment of carbon fibres within the COP-CF filament, the study focuses on the 3D microstructure of the as-received filament.

Fig. 5a presents 2D views of the as-received filament before the printing process, highlighting both cross-sectional and longitudinal perspectives. The longitudinal section measures 1242 × 607 pixels, corresponding to a physical area of 3.78 × 1.85 mm. The cross-sectional view is 626 × 607 pixels, representing an area of 1.85 × 1.90 mm.

Since X-ray micro-tomography grey level distributions are linked to density contrast, the presence of darker grey levels suggests the existence of voids. The distinction between COP and CF phases is not clearly apparent, as only subtle changes in grey levels within the matrix are observed. According to the CES database, unfilled copolyesters have a density ranging from 1.2 to 1.22 g/cm³, while the material specifications provided by the supplier indicate a slightly higher density of approximately 1.27 g/cm³. In contrast, carbon fibres exhibit a slightly higher density, on the order of 1.75 g/cm³. The 2D cross-sectional view reveals both lighter and slightly darker grey levels, representing COP and CF phases, respectively. Due to the similar densities of the two phases, the exact morphology of the carbon fibres is not clearly captured, although their fibrous structure can be inferred within the filament. This low contrast is consistent with findings in the literature for various composites reinforced with carbon fibres, such as PEEK-CF and PLA-CF [26, 33,34].

The porous structure is segmented by applying a threshold of 176 on a scale of 256 grey levels and flooding the external air regions with a typical grey level of 128.

Fig. 5b shows a perspective view of the porosity within the as-
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** X-ray microtomography results of the as-received COP-CF filament before the printing process: (a) Orthogonal views highlighting the ultrastructure of the COP-CF composite filament, (b) 3D representation of the porous structure within the filament and (c) Size distribution for both CF and porosity within the filament.</figcaption></figure>
<caption>**Table 3**</caption>

Structural attributes of 3D-printed COP-CF as a function of printing angle. $\rho$: printed part density, $f_p$: porosity content, $\tau_p$: Pore connectivity, $\tau_c$: Carbon fibre connectivity, $D_p$: average pore size.

<table><thead><tr><th>θ (°)</th><th>ρ (g/cm³)</th><th>f<sub>p</sub> (%)</th><th>τ<sub>p</sub> (%)</th><th>τ<sub>c</sub> (%)</th><th>D<sub>p</sub> (μm)</th></tr></thead><tbody><tr><td>As-received</td><td>1.4*</td><td>2.94 ± 0.20</td><td>1.00 ± 0.34</td><td>74.98 ± 2.84</td><td>31.64 ± 0.06</td></tr><tr><td>0</td><td>1.081±0.005</td><td>19.17±0.14</td><td>68.82 ± 1.70</td><td>78.11 ± 0.54</td><td>39.41 ± 0.11</td></tr><tr><td>15</td><td>1.081±0.010</td><td>20.15±0.11</td><td>68.37 ± 2.67</td><td>74.37 ± 1.61</td><td>41.55 ± 0.30</td></tr><tr><td>30</td><td>1.084±0.013</td><td>20.35±0.03</td><td>69.82 ± 0.18</td><td>71.12 ± 12.61</td><td>44.47 ± 1.70</td></tr><tr><td>45</td><td>1.078±0.009</td><td>21.82±0.16</td><td>72.97 ± 1.36</td><td>68.42 ± 5.18</td><td>45.06 ± 0.27</td></tr></tbody></table>

* According to Supplier.

received filament. Pore content analysis, based on a typical filament length of 3.82 ± 0.06 mm, reveals a volume content below 3 % (Table 3). Most of the pores are centred within the core of the specimen and few of them are present at the periphery. The filament diameter is slightly smaller than the theoretical value of 1.75 mm, with image analysis indicating a diameter between 1.65 ± 0.01 mm and 1.66 ± 0.01 mm. Additionally, surface roughness is evident in both the transverse and longitudinal views, primarily resulting from the extrusion process of the two-phase material. The aspect ratio ranges from 0.98 ± 0.002 and 0.99 ± 0.006, indicating that the filament is nearly cylindrical. However, the roughness affects the filament's circularity. Circularity, defined as

$$C_F(-) = 4 \times \pi \times A_F / (P_F)^2 \quad (6)$$

where $C_F$, $A_F$, $P_F$ are circularity, the area and perimeter of the filament, respectively.

According to Eq. (6), $C_F$ is found to be between 0.73 ± 0.02 and 0.75 ± 0.01, well below the ideal circular value of 1 due to the jagged circumference.

Table 3 shows a clear correlation between the printing angle ($\theta$) and the resulting density ($\rho$) of the 3D printed specimens. Compared to the as-received material with a density of 1.4 g/cm³, all printed samples exhibit lower densities, indicating the presence of porosity or reduced packing efficiency inherent to the additive manufacturing process. As the printing angle increases from 0° to 45°, the density slightly increases, before decreasing from 30° onwards. This suggests that higher printing angles lead to increased inter-filament voids or less efficient material deposition, particularly beyond 15°. However, the variation in density across different printing angles is minimal, and the measured value of 1.081 g/cm³ corresponds to a porosity level of approximately 23 %. This is consistent with the porosity content determined through image analysis, confirming the reliability of the density-based estimation.

To assess the percolation of porosity within the filament, the pore connectivity $\tau_p$ is examined. This structural parameter is calculated as the ratio between the largest connecting feature $V_p^M$ and the total porosity volume $V_p^T$, as follows

$$\tau_p(\%) = 100 \times V_p^M / V_p^T; \quad (7)$$

According to Eq. (7), a small pore connectivity is found (Table 3, Fig. 5b), reflecting the low number of pores and their globular morphology, which limits effective pore connectivity.

Segmentation of the CF phase is challenging because both CF and the edges exhibit similar light grey levels. The bright edges, associated with construction bias, typically fall within the 220–255 grey level range. To address this, a filtering step is performed to subtract the bright edges from the rest of the segmented image. The resulting CF phase content is 14.46 ± 0.38 %, slightly lower than the expected value of 15.36 %. This minor discrepancy can be attributed to the miscounting of CF near the edges. Given the high CF content, extracting shape descriptors from the segmented images is difficult. However, phase connectivity is measured and found to be significant due to the elongated shape of the CF fibres within the filament, (Table 3).

Fig. 5c shows the size distribution of both CF and porosity within the as-received filaments. The size distributions are obtained through

isotropic sieving technique, which does not account for differences in phase morphology. Both phases including CF and pores present monomodal size distributions with average size of 10.05 ± 0.06 µm, and 31.64 ± 0.06 µm, respectively.

The directionality analysis reveals that the orientation of the CF phase relative to the longitudinal direction is approximately 2.92°, with a dispersion of about 16°. This indicates that most fibres are aligned with the filament extrusion direction, while the high dispersion is primarily observed near the porosity areas, where the CF orientation is notably disrupted.

Fig. 6 provides a comparison of 2D slices in the orthogonal planes corresponding to the width and height directions of 3D printed COP-CF composites, using printing angles between 0° and 90°. The region of interest is 10 × 4 mm. In Fig. 6a, the microstructure highlights significant features related to process-induced porosity, which is common in Fused Filament Fabrication (FFF), particularly in the areas along the specimen's height. Both voids within the filaments and those generated during the deposition process contribute to a bimodal distribution of porosity in the 3D printed COP-CF composite. When examining the effect of the printing angle on process-induced porosity, no obvious differences are apparent (Fig. 6). However, a more detailed analysis reveals a slight correlation between pore content and printing angle (Table 3), which can be expressed as a linear function.

$$f_p(\%) = 19.16 + 0.04 \times \theta^*(\%); \quad R^2 = 0.75 \quad (8)$$

In addition to pore content, Fig. 7a illustrates the differences between the 3D printed composites by showing the pore size distribution for all printing angles. Eq. (8) shows that no significant trend is observed between the pore size distribution and printing angles, although the 0° printing angle exhibits the lowest defect extent compared to the other angles. The average pore size, regardless of the printing angle, is 40.17

± 4.92 µm, which is 1.27 times larger than the pore size in the as-received filament. The average pore size is linked to the printing angle according to a linear fitting function

$$D_p(\mu m) = 39.64 + 0.13 \times \theta^*(\%); \quad R^2 = 0.95 \quad (9)$$

According to Eq. (9) and Table 3, the average pore size increases as the printing angle rises.

Fig. 6b shows the same 2D views of the microstructure, in the plane normal to the specimen's height. Besides the process-induced porosity, these views highlight further the pore connectivity along the length of the 3D printed COP-CF composites, especially at the junction between the external frame and the core of the specimen. One can clearly differentiate between the printing angles as the layups produced reflect the filament directions -45°/+45°, -30°/+60°, -15°/+75°, 0°/+90° for $\theta=0°$, 15°, 30°, and 45°

Fig. 6c presents 2D views perpendicular to the width direction for all the considered printing angles. These views emphasize the extent of pores perpendicular to the building direction, similar to what is shown in Fig. 6b. As noted in the literature, this pore distribution is a key factor contributing to the reduced tensile performance of 3D-printed structures under loading along the building direction. While no significant differences are observed between the printing conditions, analysing the axial
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Comparison between the cross-section views of 3D printed COP-CF microstructures at different printing angles ranging from 0° to 45° (a) in-depth view, (b) Top view, (c) lateral view.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. X-ray micro-tomography results for all printing angles: (a) cumulative distribution of the pore size, axial porosity profiles in (b) depth, (c) length, (d) width directions.</figcaption></figure>

porosity profiles can reveal distinct trends.

To evaluate the anisotropic spatial distribution of process-induced porosity, axial porosity profiles are analysed for all printing angles along the depth direction, as shown in Fig. 7b These profiles quantify the porosity surface content within each slice. Specifically, for the porosity profile along the depth direction, this is expressed as:

$$f_j(\%) = \sum_{i,k=1}^{D_L \times D_W} \delta(\gamma_{ijk}) / \mu(\gamma_{ijk}) \quad (10)$$

Here, $f_j$ represents the porosity level along the depth direction (i.e., the building direction) at the j-th position with $D_L$ and $D_W$ denoting the total number of voxels in the longitudinal and lateral directions, respectively. $\gamma_{ijk}$ denotes the grey level associated with the voxel at coordinates i, j, k. For each voxel, the grey level is either 255 (representing the COP-CF phase), 128 (indicating the background), or 0 (corresponding to porosity). $\delta$ and $\mu$ are functions of the form

$$\delta(\gamma_{ijk}) = \begin{cases} 1 & |\gamma_{ijk} = 0 \\ 0 & |\gamma_{ijk} > 0 \end{cases} \quad (11)$$

and

$$\mu(\gamma_{ijk}) = \begin{cases} 1 & |\gamma_{ijk} = \{0; 255\} \\ 0 & |\gamma_{ijk} = 128 \end{cases} \quad (12)$$

Using Eqs. (10) to (12) allows the building of the axial profiles such as across the depth (Fig. 7b). A cyclic pattern is observed for all printing angles, indicating that the alternation between high and low densities is linked to the necking effect in this region. The layer height is set to half the nozzle diameter, which enhances cohesiveness across the depth. However, this adjustment is insufficient to fully eliminate the necking effect, as shown in Fig. 6. Pore peaks exceeding 20 % result from these discontinuities. The periodic alternation between low and high pore levels aligns with the layer height, i.e., 300 µm (Fig. 7b).

For $\theta = 0^{\circ}$, the porosity peaks reach approximately 23 %, while the baseline values are around 13 %. For $\theta = 15^{\circ}$, the periodicity is less distinct, with higher peaks (26 %) and lower valleys (17 %), indicating more pronounced fluctuations compared to the previous case. The gap between peak and valley values becomes even greater for $\theta = 30^{\circ}$, where porosity peaks reach 30 % and baseline values drop to approximately 11 %. All samples exhibit consistent variations in porosity levels, following a similar repeating trend. For $\theta = 45^{\circ}$, the peaks also reach around 30 %, but the valleys shift to higher values, approximately 14 %.

Fig. 7c illustrates the porosity profiles along the length direction. The results show a distinct separation between the trends and a lower variation in porosity levels compared to the depth profiles. This indicates
that porosity in this direction is less likely to cause significant me-
chanical performance loss compared to loading along the depth direc-
tion. The highest porosity profiles are roughly associated with the
largest printing angles. However, this contrasts with the mechanical
results, which suggest that the best-performing printing conditions
correspond to larger printing angles. The valley-to-peak values are as
follows: 19%–20%, 19%–21%, 18%–21%, and 21%–23% for θ= 0°,
θ= 15°, θ= 30°, θ= 45°, respectively.

Porosity profiles across the specimen's width (Fig. 7d) reveal the presence of interfilament porosity within the external frame. The porosity highest peaks of 35 % are observed at the junction between the external frame and the raster. Since the frame consists of two adjacent filaments along the length, two consecutive peaks are observed for all specimens. The trend appears consistent across all printing angles. Within the core of the specimen, the highest pore content is observed at the centre, reaching levels as high as 25 %. Only slight variations are noted, primarily due to the gaps between adjacent filaments within the raster.

Fig. 8 provides a perspective view of the porous network in all 3D-
printed composites. The porous structure aligns with the layups
created by the printing angle within the specimen’s core. High con-
nectivity is evident across all printing angles, though slight discontinu-
ities are noted near the junction between the raster and the external
frame. Additionally, significant pore connectivity is observed within the
frame, attributed to defects in the filament before printing. Quantitative
analysis of pore connectivity reveals minor variations, ranging from 68
% to 73 %, across different printing conditions. The highest connectivity
is achieved at θ = 45°, which corresponds to the relatively larger pore
content observed under the same condition. Compared to the pore
connectivity in the as-received filament, the printing process enhances
connectivity, as the layering process introduces a repeating pattern of
discontinuities.

3.3. *Technical part using 3D printed COP-CF composites*

To validate the optimal printing configuration, a curved hook was
designed, and printing trials were conducted for all printing angles.
While a hook typically experiences mixed-mode loading involving both
compression and tension, the goal was to determine whether a higher
load transfer in areas under tension correlates with the largest printing
angle. Fig. 9 illustrates the slicing process performed using IdeaMaker
software for a hook with bounding box dimensions of 69.5 mm × 43.8
mm × 3 mm. The thickness was chosen to ensure at least 10 layers could

be printed with a nozzle diameter of 0.6 mm and a layer height of 0.3
mm.

All printing conditions were aligned with those used for tensile specimens, including a printing temperature of 250 °C. The only difference between the dogbane trials and the hook printing was the use of an intermediate PLA platform to facilitate part anchoring and prevent warping. The printing was carried out with 100 % infill and the printing duration for each part is 15 min. Two replicates per condition are processed to ensure reproducibility of the testing result. Fig. 9b shows the printing process using a Raise3D Pro2 Plus printer, with the build direction aligned with the part thickness. This orientation ensures filament continuity within the plane containing the loading direction. The additional PLA printing platform provides a textured surface that enhances adhesion during printing due to the roughness created by the layering process, eliminating the need for support material between the part and the build platform.

A custom grip was designed to test the printed hooks under tensile
loading conditions, as shown in Fig. 10a. This setup induces bending in
the hook, with the force monitored as a function of vertical extension.
Fig. 10b reveals that all printed specimens exhibited the same failure
pattern: transverse cracking at the bend area.

Analysis of the load response, presented as force versus vertical displacement, confirms that the printing condition θ = 45° performs the best, even in this bending scenario where both compression and tension are present. The comparison conducted at the elastic stage of the mechanical response, where the material behaviour is most consistent and reproducible, indicates that after a 1 mm extension, the hook printed at 45° outperforms the other samples by 57 N, corresponding to an approximate 60 % increase in load-bearing capacity within the elastic regime. Despite nearly half of the filaments being misaligned with the loading direction, the load-bearing capacity of the remaining filaments is sufficient to achieve a rupture force of 328 ± 16 N, significantly higher than the 191 ± 36 N observed for the other printing conditions.

3.4. Discussion of main correlations between the microstructure and
observed mechanical performance of 3D printed COP-CF

The data from Tables 2 and 3 reveal a clear relationship between the
microstructural attributes and mechanical properties of 3D-printed
COP-CF. As porosity increases in the printed samples (19–22 %)
compared to the as-received material (2.94 %), key mechanical prop-
erties such as Young’s modulus, tensile strength, and elongation at break
decline significantly. However, the printing angle (θ) plays a crucial role

<figure><img src="image_10.png" /><figcaption>Fig. 8. X-ray micro-tomography results showing (a) 3D rendering of the porosity within the as-received filament, (b) porous network for all printing conditions.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Technical part made of 3D printed COP-CF composite. (a) Slicing step showing the filament trajectory as a function of the printing angle. (b) ongoing printing process using an intermediate printing platform made of PLA.</figcaption></figure>

in mitigating these effects—higher angles (e.g., 45°) improve carbon fibre alignment and connectivity, leading to better mechanical performance despite elevated porosity. Notably, the 45° sample exhibits the highest Young's modulus (955 MPa) and tensile strength (44.8 MPa).

Fig. 10 suggests a strong connection between the orientation of the

carbon fibres and the applied loading direction, which helps explain the higher tensile strength observed in both the standard printed parts and the demonstrator at a 45° raster angle. Unfortunately, the resolution of X-ray micro-tomography images do not allow retrieving the orientation of carbon fibres. The main hypothesis is that for all printing angles, the
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Technical part made of 3D printed COP-CF composite. (a) tensile loading performed on 3D printed hooks. (b) rupture patterns as a function of the printing angle. (c) load response as a function of the printing angle.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 11. X-ray micro-tomography interpretation of reinforcing effect based on carbon fibre content evolution along the length for all printing angles.</figcaption></figure>

carbon fibres tend to follow the laying down direction, which for the printing angle of 45° allows more reinforcing effect to be available in the direction of extension (Fig. 1).

Fig. 11 adds a partial evidence as it shows the content of carbon fibres along the longitudinal direction for all printing conditions. Initially, at 0 mm, the values are relatively high for all angles (ranging from approximately 19 % to 21 %), indicating possible surface effects. As the length increases, a U shape profile is observed, with values stabilizing around 11 %–14 %, depending on the angle. The values for 0°, 15°, and 30° show a more pronounced drop and stabilize at lower values (~11°–12°), while the 45° measurements remain slightly higher, indicating a more reinforcing effect in this direction, which is aligned with the loading direction.

## 4. Conclusions

This work reveals the critical impact of printing parameters on the mechanical and microstructural characteristics of 3D-printed COP-CF composites. The main conclusions are as follows:

* Printing parameters, especially printing angle, significantly influence the mechanical and microstructural properties of 3D-printed COP-CF composites.

* Printing at 45° improves load transfer and mechanical performance by aligning half of the filaments with the loading direction.

* X-ray microtomography reveals that fibre alignment and process-induced porosity are key factors in determining mechanical behaviour.

* The printing process increases pore connectivity compared to the as-received filament, with connectivity ranging from 68 % to 73 % across different conditions.

* A 3D-printed curved hook was successfully designed and tested, demonstrating the material's suitability for functional applications under mixed-mode loading.

These findings establish a strong link between printing parameters, microstructure, and mechanical performance, guiding the optimization of COP-CF composites for high-performance applications in aerospace, automotive, and consumer products.

## CRediT authorship contribution statement

**Lotfi Hedjazi:** Writing – review & editing, Software, Resources, Methodology, Investigation, Formal analysis. **Sofiane Belhabib:** Writing – review & editing, Visualization, Supervision, Software, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Jaianth Vijayakumar:** Writing – review & editing, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation. **Elodie Boller:** Writing – review & editing, Supervision, Software, Project administration, Methodology, Investigation, Formal analysis. **Sofiane Guessasma:** Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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