Effects of the Infill Density on the Mechanical Properties of Nylon Specimens Made by Filament Fused Fabrication

Round 1

Reviewer 1 Report

Regarding style and presentation, the authors could consider the following comments/suggestions:

References are not numbered according to the journal’s instructions (in order of appearance)

Since the number of equations in the text is significant, I believe that numbering the equations and employing the corresponding numbers to explain mathematical development will significantly help the reader in following through the mathematical analysis

Fig. 4 in its current position does not provide any significant information and seems a bit out of place. It should probably be moved later in the text, preferably in the last section, where some short discussion of the corresponding diagram could also be added, or else there is no point in its presence.

In some cases, terminology should also be checked, in order to comply with relevant bibliography. In the caption of Fig. 1, for example, the term ‘filament’ should be probably used instead of ‘coil’ and ‘addition material’ should probably be replaced by ‘support material’.  

To facilitate the replication and expansion of the experiments some further data would also be useful.

How many specimens for each infill value were investigated?

What were the values of Nu, NL, Nint and bTR?

Since the measured weight of the test specimens is a crucial parameter for calculating AACT% and interpreting the results, it would also be useful to provide the mean measured weight (mass) in relation to the infill value AT%, possibly in an additional separate column of Table 3.

Finally, the dimensional measurements of cross sections that support the claim in line 229 (that the cross section of the sample increases as the infill value increases) should also be given.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this article, the authors investigate the filling degree influence in the mechanical properties of FDM nylon parts. The work in interesting and useful, but the quality of the paper itself (both presentation and technical content) needs major improvement before it could be considered for publication. The authors also need to more clearly communicate the intellectual contribution of the work. Please note that all comments are meant to help the authors and improve the paper and are not meant to reflective negatively on the authors personally. 

1. The title is confusing and should be changed. I suggest "Effects of the infill density on the mechanical properties of fused deposition modeling nylon" or something similar 

2. The introduction section, especially the discussion of nylon and its uses, needs more references

3. The choice of references used by the authors is generally poor (except for 10-13). Since this is an international journal, I would expect that the important papers in the field would be referenced, most of them in strictly-English language journals. I don't really see any references to IJAMT, Rapid Prototyping Journal, some of the Sage IMECHE journals, JMMP, Materials & Design, Polymer Testing, Solid Freeform Fabrication Conference, or other sources that have very important fundamental papers in the field and should not be ignored in a literature review. The journal Additive Manufacturing has some good papers, but be careful because this journal also has many low-quality papers. Many, many papers in these journals are open-access and most of the rest have authors that are still active in research and are more than happy to share papers if asked. There is nothing wrong with using Russian-language sources as long as they are supported by the fundamental literature in the field, but this is almost entirely in English-language journals and conferences. A significant percentage of the additive manufacturing body of work in the journals/conference mentioned above was written by Russian authors. 

4. There are several papers which explore nylon FDM properties and the focus has not just been on nylon SLS. I recommend that the authors expand their literature review on the topic.

5. The authors need to state explicitly in the intro paragraph what the new and original contribution of this work is. There have been numerous previous works on impacts of infill density for FDM materials, including those for nylon. A more extensive literature review will aid in this. The authors state what the purpose of the work is, but not how it is novel and a good candidate for publication in an archival journal. 

6. Line 70-71: The authors need to give much more information about the drying process used for the nylon, both of the filament before the printing and of the samples before testing. The entire study is dependent on this parameter, so it is vital that must detail be given for it. If the authors needs to dedicate an entire section to this, it is recommended that they do so in order to put the results in context. 

7. Table 1: What does "acceptable softening" mean? Need to quantify that term. The glass transition temperature (where the polymer material can start to flow) of both printed and molded nylon is 158C

8. Figure 1: Please make sure that the authors have authorization to use this figure. I have seen this figure before in several places and there are much better ones available to use. I recommend that the authors either draw a new figure or find one in an open-access journal (there are tons of FDM papers with process diagrams in MDPI journals Materials, JMMP, Polymers, and others) and contact the corresponding author of a paper to ask for a high-quality figure to use in your paper. The authors have the right to use figures from CC-BY (open access license) papers in their own as long as they cite the original paper. 

9. Table 2: In my experience, using a 0,3 mm nozzle with FDM nylon is not a good idea, as it can clog easily and cause breaks/voids in the material structure. Was there any clogging or filament motor clicking observed during printing? This will also influence the mechanical properties of the printed parts. 

10. Since the authors are using Russian polymer testing standards to test the materials, the equivalent international standards from ASTM and ISO also need to be discussed, with the authors mentioning the differences between them and the Russian ones. This is because the vast, vast majority of the literature on this type of work relies on ASTM and ISO standards and so the results may not be understandable to an international readership without this context. It would also be very helpful to have an English translation of the GOST standard(s) used attached with the paper as supplemental materials so others can easily replication your results if they wish. 

11. Figure 3: The authors need to show pictures of the actual samples used (both for the readers to evaluate the quality of the print and the results of the tests). The authors also need to calculate the shell-infill ratio for each of the sample configurations, as the anisotropic material will behave very differently based on this (i.e., the shell is several times stronger than the infill)

12. Line 137-138: Tensile testing at 25 mm/min is extremely fast for this kind of test, potentially introducing dynamic elements into the test. Please explain the reason for this. Typically tests are done at 1-2 mm/min to best simulate static conditions without introducing any vibration or shock into the material. 

13. Figure 4: Please improve this figure greatly. Reset the axes to zero and make it more clear what the axes represent. Is the x-axis elongation or deflection (these are NOT the same, since you used dogbone samples and the material fractured at different rates based on the cross section). Also state whether this is a representative sample (i.e. just one test) or an average of several. 

14. Please provide some images (either microscope or high-quality photographs) of the break surfaces of the samples

15. The infill calculation/calculation method provided are very useful and I think the main contribution of this work. They should be given more prominence in the abstract. 

Overall, I think this paper could be promising once the authors consider the points raised here and perform a careful and thorough revision, including adding many more figures (especially photographs of the actual samples and equipment).  

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for your thorough revision. All my comments were adequately addressed and I have no further concerns with the manuscript.