Review Reports - Effect of Flax Fiber Content on the Properties of Bio-Based Filaments for Sustainable 3D Printing of Automotive Components

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.The use of fully bio-based composite filaments (PA10.10 + flax) for FDM, targeting automotive structural parts, aligns well with sustainable manufacturing trends.Systematic and Comprehensive Experimental Design: The study covers the entire workflow from material preparation and characterization (thermal, rheological, mechanical) to 3D printing and performance validation.In-depth Method Comparison: The comparison between conventional twin-screw extrusion and the METEOR elongational mixer regarding fiber dispersion, porosity, and mechanical properties adds depth.

2.Introduction: The background on FDM and natural fibre composites is adequate. However, the statements on page 2 (e.g., "This is the first review...") are inconsistent with the paper's identity as a research article and should be deleted or rewritten to avoid misleading readers.

Discussion Section could be more profound:
The reasons for high porosity in METEOR-processed samples could be further explored (e.g., related to device design, venting/de-gassing).
The limitations of using untreated fibres (interfacial compatibility) could be analysed further in the context of existing literature.

3. Fibre Length and Porosity Data (Table 4): The statistical significance is unclear (e.g., sample size of only ~15 fibres). More systematic statistical analysis of fibre length distribution is recommended.
Rheological Analysis: The criterion for determining the "Linear Viscoelastic Region" is not explicitly stated. Please supplement with the strain sweep graph or clarify the determination method.
3D Printing Parameter Optimization (Table 5) is somewhat brief. Consider adding more key parameters (e.g., nozzle temperature, build plate temperature, cooling strategy).

4. Figure 2 shows optical microscopy images but lacks scale bars. Please add them.
Legends for Figures 4 and 5 are not entirely clear. Please explicitly label which curve corresponds to which sample directly in the figures or captions.
Figure 8: "Young's Models" should be corrected to "Young's Modulus".
Appendix A.1 Table Title/Content: Contains obvious errors like "ber ber" which need correction.
5. Language and Formatting
The language is generally fluent but contains minor grammatical inaccuracies and awkward phrasing. Examples:
"This finding could be linked to the slightly increased screw speed, die temperature and elongational mixing." (Sentence structure is loose; consider rephrasing).
"The explanation given was that the rotational speed allows the blend to be less viscous..." (Suggest: "...reduces viscosity due to shear-thinning behavior...").
Professional English editing is recommended for final polish.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Comment 1 - The use of fully bio-based composite filaments (PA10.10 + flax) is aligned with sustainable manufacturing. The study is comprehensive and compares METEOR vs. twin-screw extrusion.

Response:

We sincerely thank Reviewer 1 for their positive and encouraging assessment. We would like to clarify that the objective of this study is to compare the performance of filaments produced using the elongational mixing technology METEOR® with those produced using a conventional single-screw extrusion process dedicated to filament manufacturing. Prior to filament extrusion, the PA10.10/flax blends were compounded using a twin-screw extruder to ensure consistent fiber distribution, at CEA. This workflow is now explicitly illustrated in the revised manuscript (see updated experimental workflow Figure 1).

Comment 2

2.1 Introduction not consistent / “first review” statement

Statements such as “This is the first review…” are inappropriate for a research article.

Response:
Corrected.
The entire paragraph resembling review-style content has been removed and replaced with an original, research-focused introduction. The revised version:

Removes all review-type statements
Clarifies the research gap
Strengthens scientific justification
Improves flow and supports claims with references

(See updated Introduction and highlighted parts)

2.2 Discussion must be more profound

Discuss porosity causes in METEOR and limitations of untreated fibers.

Response:
A significantly expanded Discussion section has been added, now including:

Quantitative porosity differences between METEOR and Single Screw Extruder (see Table4)
Explanation of METEOR-induced porosity due to low-pressure zones, air entrapment, moisture release, incomplete wetting, and pressure oscillations
Literature-supported discussion linking fiber hydrophilicity, degassing, and flow mechanisms
Detailed discussion of untreated fibers and interfacial adhesion issues

Relevant references (Gallos, Pickering, Deb, Le Duigou) were added.

Comment 3 — Fibre length statistics; rheology; printing parameters

3.1 Fibre Length:

We expanded the analysis using a larger dataset (on around 100 fibers per condition). Fiber-length histograms and standard deviations were added in Table 4 and SI (Figure 3).

3.2 Rheology (Linear Viscoelastic Region):

We have added a dynamic strain sweep curve (in the Supporting Information, Figure S4), a time sweep conducted at 210 °C for 15 min (Figure 6), and we have clarified the determination of the Linear Viscoelastic Region (0.05% strain) in Section 3.5 and in the Supporting Information (SI).

3.3 3D Printing Parameters:

Table 5 has been expanded to include nozzle and bed temperatures. In addition, chamber conditions, humidity control, cooling strategy, and adhesion strategy have been fully detailed in Section 3.7. Moreover, both qualitative and quantitative information related to the 3D printing of the Fiat 500 dashboard fascia such as dimensional accuracy, maximum warpage, wall thickness, and overall print quality have been incorporated into the revised manuscript, and figure 10 (a) and (b). Look at lignes 457-461 “The printed component exhibited a maximum warpage of 1.8 mm across its 165 × 180 × 45 mm geometry, corresponding to a dimensional deviation within ±0.7 mm relative to the CAD design. The printed walls were 3 mm thick, and no delamination or fibre-induced surface defects were observed, confirming the compatibility of the formulation with FDM processing.”

Comment 4 — Figures and tables corrections

Scale bars added to all microscopy images (Figure 4 and Figure 5).
Legends of Figures 4 and 5 rewritten for clarity.
“Young’s Models” corrected to “Young’s Modulus” (Figure 8 and 9).
Appendix A.1 table cleaned; “ber ber” removed.
All figures have been carefully reviewed, revised where necessary, and enhanced to ensure higher visual clarity and publication-quality standards.

Comment 5 — Language & style

The full manuscript underwent careful editing:

Improved grammar and sentence structure
Clarified complex sentences
Removed redundant phrasing
Ensured coherence across sections

Examples given by the reviewer were corrected (e.g., “slightly increased screw speed…” and shear-thinning explanations).

Conclusions:

All reviewer comments have been carefully addressed.

The manuscript has undergone major structural, stylistic, and scientific improvements:

Fully rewritten Abstract and Introduction
Strong, quantitative Discussion
Enhanced Results (porosity, rheology, fibre morphology, 3D printing)
Improved figures, tables, and captions
Expanded methodological transparency
Integrated sustainability and circularity aspects
Corrected references and formatting

We believe these changes significantly improve the scientific quality, clarity, and relevance of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript requires revision through the following comments to enhance the scientific justification, data interpretation, correction of inconsistencies, and process–structure–property relationship.

1) The title of the manuscript is long and descriptive; the authors should consider revising the title. For example, “Effect of Flax Fibre Content on the Properties of Bio-Based Filaments for Sustainable 3D Printing of Automotive Components"

2) The authors can refine the abstract by highlighting the quantitative outcomes and including more keywords.

3) The authors should cite all figure numbers in sequential order. For example, Line 303 Figure 1c replace with Figure 3c. All figure axes, lines, and axis titles must be clearly drawn.

4) In the literature review, the authors need to support with references for the statements mentioned. For example, the paragraph from lines 47-58 lacks literature support.

5) The authors should consider rewriting the materials and methods section with a flowchart highlighting the methodology and reducing the number of subdivisions.

6) The manuscript reports that METEOR increases the porosity; the authors need to provide clear justification for porosity due to air entrapment or moisture release during mixing, etc.

7) The authors need to discuss the fibre length reduction with suitable justification and also include the interconnection between the screw speed and fibre breakage.

8) The Line 403, the statement " As Anticipated," should be supported by references.

9) The demonstration of the Fiat 500 fascia lacks dimensional accuracy measurements, warpage quantification, and printing parameters.

10) The authors need to clarify the viscosity decreases at 15% even though the fiber concentration is higher. This is due to the occurrence of degradation. Justify it.

11) The authors need to specify the standard for measurement taken and support it with a reference.

12) The authors must include the recyclability and correlate it to the scope of sustainability.

13) The paragraph from lines 72-83 closely resembles wording commonly found in previously published review articles on natural fibre-reinforced FDM filaments. ensure the originality.

14) The authors should add more relevant references to support the literature review and discussion, and correct the unnecessary number “12” at Line 83, ensuring all citations follow a consistent style. Remove duplicate references, for example, Haque 2023 repeated twice.

Comments on the Quality of English Language

The language needs to be improved.

Author Response

We sincerely thank Reviewer 2 for their positive and encouraging assessment.

Comment 1- Title too long

Response:
Title changed to:

“Effect of Flax Fiber Content on the Properties of Bio-Based PA10.10 Filaments for Sustainable 3D Printing of Automotive Components.”

Comment 2- Abstract should highlight quantitative outcomes

Response:

The abstract has been fully rewritten to provide clearer quantitative insights into the material performance and printability. The revised version now includes:

Tensile modulus improvements (e.g., +84% at 15 wt% flax)
Quantified warpage of the printed demonstrator (maximum 1.8 mm)
Dimensional accuracy of the Fiat 500 fascia (±0.7 mm)
Key FDM processing parameters (nozzle temperature, bed temperature, print speed, chamber conditions)
Description of the observed pseudo-percolation behaviour in the rheological response of fiber-filled PA10.10
Additional quantitative and qualitative printability indicators
An expanded set of keywords to improve indexing and visibility

These modifications strengthen the clarity and scientific value of the abstract in accordance with the reviewer’s recommendations.

Comment 3- Figures must be numbered in sequence

Corrected throughout the manuscript. All axes, labels, and titles were redrawn for clarity.
All figures have been carefully reviewed, revised where necessary, and enhanced to ensure higher visual clarity and publication-quality standards.

Comment 4- Literature support missing (lines 47–58)

All claims in the revised Introduction are now supported by appropriate references. The original, review-style content has been removed and replaced with a concise, research-focused introduction. The revised version:

Eliminates review-type statements
Clearly identifies the research gap
Strengthens the scientific rationale
Enhances the logical flow and supports all claims with relevant end added references

(See updated Introduction with highlighted changes.)

Comment 5- Rewrite Materials & Methods with flow chart

Rewritten into a more concise form.
A methodological flowchart has been added (See Figure 1).

Comment 6- Porosity justification for METEOR

Discuss porosity causes in METEOR and limitations of untreated fibers.

Response:
A significantly expanded Discussion section has been added, now including:

Quantitative porosity differences between METEOR and Single Screw Extruder (see Table4)
Explanation of METEOR-induced porosity due to low-pressure zones, air entrapment, moisture release, incomplete wetting, and pressure oscillations
Literature-supported discussion linking fiber hydrophilicity, degassing, and flow mechanisms
Detailed discussion of untreated fibers and interfacial adhesion issues

Relevant references (Gallos, Pickering, Deb, Le Duigou) were added.

Comment 7- Fibre length reduction, screw speed and fibre breakage

The manuscript has been revised to include a dedicated and expanded discussion on fiber length evolution during compounding and filament extrusion (revised Section 3.4, lines 297-309 in the updated version).

The newly added text explains:

Shear-induced fiber breakage occurring during conventional single-screw extrusion, particularly due to high shear gradients near the screw flight and die entrance.
The influence of screw speed on fiber integrity and dispersion: higher rotational speeds increase shear stress and fiber fragmentation, whereas excessively low speeds lead to insufficient mixing and poor fiber wetting.
The role of the METEOR® elongational mixer, which applies controlled convergent–divergent deformation and therefore better preserves fiber length by minimizing transverse shear forces while improving fiber distribution.
Quantitative fiber length measurements: we expanded the analysis using a larger dataset (on around 100 fibers per condition). Fiber-length histograms and standard deviations were added in Table 4 and SI (Figure 3).
Additional literature references (Depuydt et al., Pickering et al., Gallos et al.) supporting the correlations between shear rate, fibre morphology, and mechanical performance.

This expanded discussion clarifies the process–structure relationship between fiber brakage, screw speed, and the resulting mechanical properties of the biocomposite filaments.

Comment 8 - “As anticipated” must be supported

Corrected. The sentence now includes appropriate justification and reference.

Comment 9- Automotive demonstrator missing dimensional measurements

Section on 3D printing expanded.

Comment 10 - Viscosity decrease at 15% flax

The manuscript has been revised to provide a clearer mechanistic explanation for the observed decrease in viscosity at 15 wt% flax. The updated discussion (Section 4) now details the combined factors responsible for this effect:

Polymer chain degradation, supported by TGA results (Figure S2, in SI) showing a slight reduction in thermal stability at higher fiber loadings, and by the measured decrease in intrinsic viscosity. This degradation likely arises from longer residence times and moisture release from untreated flax fibers.
Increased porosity at 15 wt%, which disrupts melt continuity and reduces the effective load-bearing cross-section of the composite during oscillatory shear, thereby lowering apparent viscosity.
Fiber agglomeration and incomplete wetting observed in microscopy analyses, which further reduce the homogeneity of the melt and promote localized slippage, leading to an apparent viscosity drop despite higher fiber concentration.
A note that similar trends have been reported in the literature for natural fiber reinforced thermoplastics when porosity and degradation outweigh the expected reinforcement effect.

These explanations have been fully integrated into both the rheology section and the general Discussion to provide a coherent interpretation of the phenomenon.

Comment 11 - Standards used must be referenced

All relevant standards are now properly cited in Section 2:

ISO 527-1 (Tensile testing)
ISO 178 (Flexural testing)
ISO 4287 (Surface roughness measurement)

Comment 12 - Recyclability and sustainability

A dedicated paragraph (4.5. Perspectives for Recyclability and Sustainability of PA10.10/Flax Biocomposites) has now been added to the Discussion section addressing the recyclability and sustainability relevance of PA10.10/flax biocomposites, supported by appropriate references.

The revised text highlights:

The recyclability potential of PA10.10/flax systems, as both components are bio-based and compatible with mechanical reprocessing routes commonly used for polyamides.
Published evidence indicating that bio-based polyamides retain a substantial portion of their mechanical properties after multiple reprocessing cycles, making them suitable for circular manufacturing approaches.
The inherently lower carbon footprint of PA10.10 (derived from castor oil) and flax fibres compared to fossil-based feedstocks, consistent with the sustainability goals of the automotive sector.
The role of natural fibres in reducing material density and improving specific properties, contributing to lightweighting strategies and reduced environmental impact.

Although recyclability and biodegradation tests were not performed in the present study, they are planned as part of the next phase of the European project. Future work will evaluate:

The effect of multiple reprocessing cycles on fibre length retention and fibre breakage,
The evolution of thermal, rheological, and mechanical properties,
The impact of reprocessing on filament quality and 3D printability,
And the end-of-life behaviour (mechanical recyclability and biodegradation potential) of PA10.10/flax composites.

These planned investigations will help establish the full circularity potential of the developed biocomposites and strengthen their positioning as sustainable materials for additive manufacturing.

Comment 13 - Lines 72–83 resembled review content

Removed and replaced with an original, concise, research-focused introduction.

Comment 14 - References and duplicates

Duplicate references removed (e.g., Haque 2023).
Citation style harmonised.
Added >10 new references supporting processing, fibre hydrophilicity, viscosity behaviour, and elongational mixing.

Conclusions:

All reviewer comments have been carefully addressed.

The manuscript has undergone major structural, stylistic, and scientific improvements:

Fully rewritten Abstract and Introduction
Strong, quantitative Discussion
Enhanced Results (porosity, rheology, fibre morphology, 3D printing)
Improved figures, tables, and captions
Expanded methodological transparency
Integrated sustainability and circularity aspects
Corrected references and formatting

We believe these changes significantly improve the scientific quality, clarity, and relevance of the manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The research of Isnard et al is focused on preparation and characterization of composite filaments based on bio-based polyamide (PA10.10) reinforced with flax fibers and processable by 3D printing for automotive applications. In particular, authors explored the effects of fiber content and processing methods on mechanical, thermal, and rheological properties, aiming to identify an optimal composite formulations.

The results of the experiment, performed with relevant characterization techniques, are clearly reported and thoroughly discussed in the manuscript.

The research, perfectly in line with the current industrial trend of developing new materials supporting circular economy and sustainability goals, is also valuable for the practical validation of the studied composites through the 3D printing of a real automotive component: a Fiat 500 dashboard fascia.

Despite the favorable premises and the scientific relevance of the contents, the following minor revisions are recommended to further improve the quality of the text.

The expression reported on line 154 notoriously determines the degree of crystallinity and not the rate of crystallization as erroneously reported on line 153. Please correct.
The text between lines 357 and 379, regarding the rheological behavior of materials derived from time-sweep tests at 210°C, is repetitive. Given that the only test variable is duration, it is recommended that this text be rephrase to make it easier to read.
Similarly, it is suggested to review the text between lines 385 and 389.

Author Response

We sincerely thank Reviewer 3 for their positive and encouraging assessment.

Comment 1 — Crystallinity wording (lines 153–154)

Corrected.
The sentence now properly states “percentage of crystallinity.” (See Ligne 180 and Table 1)

Comment 2 — Rheology time-sweep section repetitive

Rewritten for clarity with:

Condensed explanation
Focus on main interpretation
Removal of redundant sentences

Comment 3 — Lines 385–389 need review

Rephrased for clarity and precision.

Conclusions:

All reviewer comments have been carefully addressed.

The manuscript has undergone major structural, stylistic, and scientific improvements:

Fully rewritten Abstract and Introduction
Strong, quantitative Discussion
Enhanced Results (porosity, rheology, fibre morphology, 3D printing)
Improved figures, tables, and captions
Expanded methodological transparency
Integrated sustainability and circularity aspects
Corrected references and formatting

We believe these changes significantly improve the scientific quality, clarity, and relevance of the manuscript.