Digital Manufacture of a Continuous Fiber Reinforced Thermoplastic Matrix Truss Core Structural Panel Using Off-the-Tool Consolidation

Round 1

Reviewer 1 Report

Manuscript ID: JCS-1899525

“DIGITAL MANUFACTURE OF A CONTINUOUS FIBER REINFORCED THERMOPLASTIC MATRIX TRUSS CORE STRUCTURAL PANEL USING OFF-THE-TOOL CONSOLIDATION”

In this work, the authors proposed a new manufacturing solution to truss core sandwich panels. The developed specimens were compared with compression moulded samples in terms of compressive and shear characteristics. The description of the developed technique is very thorough, but the experimental results should be better illustrated.

In my opinion, this contribution can be considered for publication in Journal of Composites Science after Major revision.

Following suggestions:

-pay attention to title format;

-The abstract is too long. Please revise the length of the abstract according to Journal template. (200 words maximum is required.)

-Furthermore, the background section of the abstract appears to be too long, and the obtained results are slightly outlined. Please, provide a right balance between the background, the methods and description of results in the abstract.

-Line 32, please check the “23” in the keywords;

-the introduction should be shortened. It is very long. Furthermore, each sentence should be supported by references. From 34 line to 72 line, a single reference ([1]) is present;

- Line 54,   the section (1.1. Subsection ) does not have a title;

-“Results” section should be improved. Both in the case of compression and shear testing a graphical comparison among each developed type of specimens should be provided in terms of representative stress-shear curves;

-the “ 5.1. Performance related to Digital Manufacturing Process Variables” paragraph, present in the Discussion section, describes findings and effect of processing temperature on sample characteristics. Such paragraph should be placed in the “Results” sections, and representative graphs of data should be added.

-Line 728-729, Please, explain better what “ the improvements were implemented” means

-please, take care of the scientific written style (not intended as English editing or grammar but straightforward, specific, and concise)

Author Response

Thank you for your comments:

• Title format adjusted
• Adjusted the abstract for length and content.  Reduced background and added specific results
• Corrected formatting in "Keywords"
• Added correct name for 1.1 Subsection
• Load deflection curves were added as figures 21 and 22 to show the 5 repetitions and linearity in the load range stated.
• Concluding remarks are adjusted to be more specific in relation to extruder temperature and programmed cooling control.

Reviewer 2 Report

This paper studies digitally manufactured Navtruss core sandwich panels and various processing variable. Please see following comments.  

1.       Please revise Fig.2 by adding description of core components for better clarity. Similarly for Fig. 3

2.       Authors writes that there is improved adhesion between the facesheets and the truss core elements for DM4. Please explain further.

a.       Is there a limit on temperature increment of nozzle for matrix flow as shown in case of PET for improved modulus (DM4)?

3.       How modulus could be impacted if a higher nozzle temperature is combined with programmed cooling?

Author Response

Thank you for the comments:

• Figures 2 and 3 have been replaced in hopes that these more directly address concerns with clarity and description of core components
• The improved adhesion comment relates to reduced viscosity
• A limit on upper nozzle temperature?  Ultimately, a temperature is reached at which the thermoplastic matrix (PET) will start to thermally decompose (pyrolyze).  Brief comments have been added (line 685) and two citations, one of which shows a 2% degradation temperature for PETG of 366C and a second that shows measurable degradation at 320C.  The extruder temperature is not a direct T measurement of the polymer temperature as the measurement is somewhat remote.  Also, higher temperatures can lead to too much viscosity reduction and loss of dimensional fidelity.

Reviewer 3 Report

see the attachment 

Comments for author File: Comments.pdf

Author Response

Thank you for your comments:

• The abstract has been modified - reducing background content and including more specific results
• The typographical error in the Keywords has been corrected.
• The word pultrusion, which occurred once in the manuscript, was changed to be consistent with the description as extrusion throughout the manuscript. Even though the commingled material does flow through the heated nozzle, the geometry of the tow at exit does match the cross-section geometry of the nozzle (die in pultrusion) and is not consolidated in a fashion matching that of a pultruded cross-section.  Thus, it seemed best to remove the one instance of the term to minimize confusion.
• Section 1.1 was correctly renamed.
• The raw material manufacturers are cited as 33, 34 - Jushi and Polystrand.
• It is not clear why did authors used angle of 45°.  I did not specifically make changes to the manuscript regarding this.  However, the manuscript was meant to focus on the Navtruss geometry as described in the literature and as such utilized 45 degree truss segments.  The processing system described in the manuscript itself is capable of virtually any combination of x,y,z motion, so various angles can be generated.  Discussing the differences would required discussing the tow cross-section geometry generation in detail.  This would not add to the current work as the same geometry, manufactured by compression molding and by tow placement are compared.  Truss segments configured at 45 degrees are common as the fibers are positioned to best resist shear stresses generated in plate bending.
• “…the upper facesheet was more poorly consolidated than the lower facesheet”.  A brief comment on this is added near line 450 of the revised manuscript to indicate that with increasing upper facesheet thickness, the properties are expected to approach those of the lower facesheet.

Round 2

Reviewer 1 Report

Overall, the paper has been improved according to the suggestions. In my opinion, it is suitable to be published in its current form.

Author Response

Thank you for your positive assessment of the revised manuscript.

Reviewer 2 Report

Revised manuscript can be accepted for publication.

Author Response

Thank you for your positive review of the revised manuscript.

Reviewer 3 Report

please, see the attachment.

Comments for author File: Comments.pdf

Author Response

The Authors performed revision of the paper. Surprisingly, some of my recommendations were simply ignored. The manuscript still needs some additional effort to bring it to a level worthy of publication. I must therefore still recommend the revision of this paper.

1. The updates of the manuscript are very hard to be tracked as Authors haven’t included it in the text of the response. Simple “has been corrected/has been modified” is not good enough for high level journal.
   • My apologies.  Please see details that follow.
2. What is the country of origin of raw materials (Section 2.1)? Please, mention it properly.
   • I'm afraid that I have not seen raw materials described by country of origin.  The datasheets for the materials are cited, which indicate a variety of sources for the materials.  The materials that were used were sourced in the USA.  The companies have distribution points in many locations.
3. The Authors didn’t respond to my comment “What are the drawbacks of the presented method?”
   • A sentence related to the main challenge is included on line 170. “The principal challenge of this approach is related to tow consolidation and void removal, which must be accomplished as the tow exits the extrusion nozzle, without the aid of pressure generated between the nozzle and a tool surface.”
4. The Authors ignored my comment “I would recommend to combine several figures in a single figure with sub-figures…”
   • Figures 12, 13 & 14, showing the truss core segments being placed are combined into a single figure. (Figure 12(a), 12(b) and 12(c).)
   • Combining additional figures (such as figures 6, 7 ,8) would require combining subsections which the author believes would make it more difficult to follow the fabrication procedure.
5. The Authors ignored my comment on the length of the manuscript. It is still too verbose and lengthy. Text can be shortened by 20-30% without any harm to the manuscript quality.
6. In order to emphasize the importance of the subject, the Authors are encouraged to discuss composite materials that are also widely used in civil engineering along with sandwich panels. Please, refer to:
   10. https://doi.org/10.1016/j.tws.2021.108300
   11. https://doi.org/10.1088/1757-899X/747/1/012013

• • Starting Line 76.  Text modified to include reference to corrugated cores in a single direction that can be pultruded versus bi-directional cores which require more complex forms of manufacture.

“While the lattice structure can be produced from a thin isotropic material [4], or even via additive manufacture of unreinforced or short fiber reinforced polymers [5], placing con-tinuous fiber reinforcement within the lattice of the core promises an increase in perfor-mance with a decrease in weight of the resulting truss core sandwich panels.  Lattice cores, also often described as corrugated cores, can be created with the corrugations in a single direction, such as those that may be pultruded from fiber reinforced composites for civil infrastructure applications [6].  However, bi-directional corrugated lattice cores, with continuous fiber reinforcement, result in significantly greater manufacturing complexity.”

5. Ge, L.; Zheng, H.; Li, H.; Liu, B.; Su, H.; Fang, D.  Compression Behavior of a Novel Sandwich Structure with Bi-directional Corrugated Core.  Thin-Walled Structures 2021, 161, 107413.

6. Xin, H.; Mosallam, A.; Liu, Y.; Xiao, Y.; He, J.; Wang, C.; Jiang, Z.  Experimental and Numerical Investigation on In-plane Compression and Shear Performance of a Pultruded GFRP Composite Bridge Deck.  Composite Structures 2017, 180, 914–932.

7. The Authors ignored my comment on the references in the Introduction section. Selected references are quite old, which from the one point of view is good, since the authors cited necessary references to define a research problem, while from the other hand, lack of recent references may indicate an insufficiently performed literature review. Try to refer to some recent and up-to-date research papers, for example:
   10. https://doi.org/10.3390/polym14112246
   11. https://doi.org/10.1016/j.coco.2022.101281

• • Three citations have been added to the introduction (currently numbered [4], [5] and [6].  These were published in 2017, 2019 and 2021.  Of the 34 citations in the introductory content 23 of the citations date 2015 or more recent and 13 are in the past 5 years (since 2017).

Round 3

Reviewer 3 Report

comments were addressed. paper can be accepted