Mechanical Performance of rPET Filament Obtained by Thermal Drawing for FFF Additive Manufacturing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the paper, authors showed interesting approach to produce rPET filaments, but I have some general and more specific comments/suggestions.

General comments

Title: I did not understand where you are using "thermoforming" for production of filaments? In the line 215 you mentioned extrusion… In general thermoforming (also known as vacuum forming) of plastics is different process compared with this one described in your paper. In filament preparation you are using cutting, heating, bending, and "welding" of PET stripes … Maybe consider changing the title.

In your paper you were focused on orientation of the layer printing on the properties on 3D printed specimens. But without any analysis, due to the anisotropy of the parts produced with MEX technologies, you can conclude that orientation which is orthogonal on load direction during testing will be the wors possible scenario (in your case 0 degree). There is only sense to use this information for comparison with the rest of the results (other orientations).

It is also already known that with polymer material recycling, most of mechanical properties will decrease, so it was also expected. But you did not try to adjust 3D printing parameters in order do enhance rPET properties to get better results. I understand your research as first step, so I would recommend in the next step 3D printing parameters optimisation.

 

Specific comments

Figures 1 and 2 - I will recommend you to use more recently data about production od plastics. In attached file you can find data about plastic production for 2023.

Figure 3 – For which year is presented diagram? And I cannot find data and the diagram presented in Figure 3 in the reference you mentioned. I can not agree that plastic bags are the most common oceans and sees polluter. I have a lot of results from research from this field and there are at least two more common polluters: cigarette buts and plastic bottles. Therefore, my suggestion is to reconsider to do some more research in this topic or to remove the image.

FDM/FFF/MEX - If you are using term Fused Deposition Modeling (FDM) – that it is presumed that you are working with Stratasys equipment – because this is a patented 3D printing procedure from this company. In all other cases you are dealing with Fused Filament Fabrication (FFF). Therefore, those two terms are not synonyms. If you want to avoid this kind of misunderstanding, you can use more general term defined with ISO/ASTM standard 52900:2021, where all 3D printing procedures with filaments are defined as Material Extrusion (MEX).

Line 151 – Delete a part "I used" and change it with "was used".

Figures 15 and 16 – Are the shapes of printed "towers" defined with certain standards?

Chapter 2.3 – There is no info about infill density?

Line 332 – It is not clear at this point in the paper did you orient printing on single layer (XY plane) or you orient complete specimen in Z direction – direction of printing? Later you explained more precise but here you are not precise.

Table 2, 3, 4 – Resistance – change it to Tensile resistance.

Generally, in the paper you are using in some cases decimal point and in other decimal comma. Please unify this.

 

Comments for author File: Comments.pdf

Author Response

In the paper, authors showed interesting approach to produce rPET filaments, but I have some general and more specific comments/suggestions.

General comments

Title: I did not understand where you are using "thermoforming" for production of filaments? In the line 215 you mentioned extrusion… In general thermoforming (also known as vacuum forming) of plastics is different process compared with this one described in your paper. In filament preparation you are using cutting, heating, bending, and "welding" of PET stripes … Maybe consider changing the title.

Response 1 – Thank you for pointing this out. We agree with this comment and therefore we have reviewed the title. Changes in the text are marked (highlighted in red)

In your paper you were focused on orientation of the layer printing on the properties on 3D printed specimens. But without any analysis, due to the anisotropy of the parts produced with MEX technologies, you can conclude that orientation which is orthogonal on load direction during testing will be the wors possible scenario (in your case 0 degree). There is only sense to use this information for comparison with the rest of the results (other orientations).

Response 2 – Thank you for pointing this out. We disagree with this comment, since the mechanical behaviour of specimens having 0-degree orientation depend mostly of the adhesion between layers, while the mechanical behaviour of specimens having 90-degree orientation depend mostly of the extruded material itself. On the other hand, the mechanical behaviour of specimens having 45-degree orientation combines both influences. We made this information clearer in the document (highlighted in red)

It is also already known that with polymer material recycling, most of mechanical properties will decrease, so it was also expected. But you did not try to adjust 3D printing parameters in order do enhance rPET properties to get better results. I understand your research as first step, so I would recommend in the next step 3D printing parameters optimisation.

 Response 3 – Thank you for your recommendation. We included this comment as a future work in the conclusions section (marked in red)

Specific comments

Figures 1 and 2 - I will recommend you to use more recently data about production od plastics. In attached file you can find data about plastic production for 2023.

Response 4 – Thank you for pointing this out. We agree with this comment and therefore we have reviewed this information with more recent information. Changes in the text are marked (highlighted in red)

Figure 3 – For which year is presented diagram? And I cannot find data and the diagram presented in Figure 3 in the reference you mentioned. I can not agree that plastic bags are the most common oceans and sees polluter. I have a lot of results from research from this field and there are at least two more common polluters: cigarette buts and plastic bottles. Therefore, my suggestion is to reconsider to do some more research in this topic or to remove the image.

Response 6 – The authors thank you for this comment, the diagram is not in the cited study, rather built from the data presented in the study. Namely, from the second paragraph of page 485 of the cited study “Of the 112 litter categories used in our analysis, 10 types of products accounted for three-quarters of all litter items found globally (Fig. 2 and Supplementary Fig. 3). The top ten products, in descending order, were bags (14%±8% across environments), plastic bottles (12%±5%), food containers and cutlery (9%±13%), wrappers (9%±14%), synthetic ropes (8%±11%), fishing-related items (for example, strings, threads, buoys; 8%±8%), plastic caps and lids” . However, as the reviewer pointed out, cigarette filters and cigar tips are also very common, although in this study “cigarette filters and cigar tips were excluded from our analysis due to their small size and inconsistent sampling across inventories (Methods).”. We included this exclusion in the article (highlighted in red).

FDM/FFF/MEX - If you are using term Fused Deposition Modeling (FDM) – that it is presumed that you are working with Stratasys equipment – because this is a patented 3D printing procedure from this company. In all other cases you are dealing with Fused Filament Fabrication (FFF). Therefore, those two terms are not synonyms. If you want to avoid this kind of misunderstanding, you can use more general term defined with ISO/ASTM standard 52900:2021, where all 3D printing procedures with filaments are defined as Material Extrusion (MEX).

Response 6 – Thank you for pointing this out. We agree with this comment and therefore we have changed across the manuscript (marked in red)

Line 151 – Delete a part "I used" and change it with "was used".

Response 7 – Thank you for pointing this out. We have changed this phrase accordingly (marked in red).

Figures 15 and 16 – Are the shapes of printed "towers" defined with certain standards?

Response 8 – The authors thank you for this comment, these towers are common non standardized methods of calibration, each tower seeks to look at specific defects in the print. For instance, temperature towers have bridges, as it is a characteristic that is severely affected by temperature, and is related to print quality. As this is not standardized, we included more details within the document (marked in red).

Chapter 2.3 – There is no info about infill density?

Response 8 – The authors thank you for this comment, this information was included (marked in red).

Line 332 – It is not clear at this point in the paper did you orient printing on single layer (XY plane) or you orient complete specimen in Z direction – direction of printing? Later you explained more precise but here you are not precise.

Response 9 – The authors thank you for this comment, this information was included (marked in red).

Table 2, 3, 4 – Resistance – change it to Tensile resistance.

Response 10 – Thank you for pointing this out. We have changed accordingly to “Tensile Strength” in tables 2,3 and 4 (marked in red).

Generally, in the paper you are using in some cases decimal point and in other decimal comma. Please unify this

Response 11 – Thank you for pointing this out. We have changed in all cases to decimal comma.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors presented a study concerning the 3D printing of a self-recycled PET. The topic is interesting and well fits with the journal’s scope. However, there are issue to be addressed before publication. Hereafter, I will point out the major problems:

-         Data presented in the introduction are not so up to date, and not even so consistent. First, it was reported that in 2019 the global plastic production was of 450 million tons, and then in 2022, 400 million tons. Is there a decreasing or an increasing production trend, then?

-         In the introduction, it’s just very briefly cited previous studies in this regard (“several studies have explored the potential of recycled filaments in 3D printing, with promising results. Recycled filaments exhibit mechanical properties comparable to virgin filaments, making them a viable alternative”), but more quantitative and specific results could be reported.

-         There are too many figures, most of which are not so many meaningful (for example, all those presented in the introduction could be avoided, since illustrates data that are already reported in the text. All the figures of each component may be compacted in one single picture pointing out the fundamental components.)

-         All stress and strain curves should be retraced, since axis name are not in English

-         The characterization of the 3D printed specimens is quite limited, since it basically consists only of mechanical characterization. How were the thermal or viscoelastic properties affected by this re-cycling?

-         The discussion section is not referred to previous studies in this direction or also with virgin PLA. How does the properties change? These observations should be supported by references, which are now totally missing in section 4, thus resulting in a restrained total amount of references (19).

-         Another general aspect that was not discussed in this present version, it’s the number of mechanical recycling that can be performed. This, I believe that it is a very significant aspect.

Author Response

The authors presented a study concerning the 3D printing of a self-recycled PET. The topic is interesting and well fits with the journal’s scope. However, there are issue to be addressed before publication. Hereafter, I will point out the major problems:

-         Data presented in the introduction are not so up to date, and not even so consistent. First, it was reported that in 2019 the global plastic production was of 450 million tons, and then in 2022, 400 million tons. Is there a decreasing or an increasing production trend, then?

Response 1 – Thank you for pointing this out. We agree with this comment and therefore we have reviewed this information. 450 million tons were a prevision. 400 million tons was the actual consumption Changes in the text are marked (in green)

-         In the introduction, it’s just very briefly cited previous studies in this regard (“several studies have explored the potential of recycled filaments in 3D printing, with promising results. Recycled filaments exhibit mechanical properties comparable to virgin filaments, making them a viable alternative”), but more quantitative and specific results could be reported.

Response 2 – The authors thank you for this comment. As mentioned in the introduction, the potential of recycled filaments is briefly addressed, and the results obtained in this study show that the mechanical properties of the rPET filament produced here are quite comparable to those of the commercially available PETG also analysed. The rPET filament exhibited performance levels similar to those of commercial PETG, thus supporting the viability of rPET as an alternative material for 3D printing applications." Other works and results were included in Material and Methods.

-         There are too many figures, most of which are not so many meaningful (for example, all those presented in the introduction could be avoided, since illustrates data that are already reported in the text. All the figures of each component may be compacted in one single picture pointing out the fundamental components.)

Response 3 – The authors thank you for this comment. Figures 1-4 were removed, since they illustrate data that are reported in the text. All the figures of each component were compacted in one single picture (new Figure 2)

-         All stress and strain curves should be retraced, since axis name are not in English

Response 4 – Thank you for pointing this out. It was our distraction and Stress-strain curves were changed.

-         The characterization of the 3D printed specimens is quite limited, since it basically consists only of mechanical characterization. How were the thermal or viscoelastic properties affected by this re-cycling?

Response 5 – Thank you for pointing this out. This is an initial study to prove the possibility of using this method of recycling to produce filaments with sufficient quality to replace commercial filaments. Further characterizations will be the aim of future works. In our opinion, this would increase furthermore the number of pages of an already large article. Therefore, we include this comment as future works in conclusions.

-         The discussion section is not referred to previous studies in this direction or also with virgin PLA. How does the properties change? These observations should be supported by references, which are now totally missing in section 4, thus resulting in a restrained total amount of references (19).

Response 6 – The authors thank you for this comment, similar studies were indeed lacking and were now included. A comparison with similar studies was discussed. The comparison with virgin PLA was not made as PLA is not the focus of this study, however a comparison with virgin PETG was made and discussed within the study. Changes in the text are marked

-         Another general aspect that was not discussed in this present version, it’s the number of mechanical recycling that can be performed. This, I believe that it is a very significant aspect.

Response 7 – The authors thank you for this comment. This recycling method is designed to generate filament from PET bottles, which can then be used to produce 3D-printed parts. However, 3D-printed parts, even if shaped like bottles, cannot be recycled back into filament using this method. This limitation arises because 3D-printed parts produced via FFF lack the uniform wall thickness and material homogeneity required for this recycling process.

While it is theoretically possible to recycle 3D-printed parts through conventional bottle recycling channels, producing new PET bottles that could then be processed into filament, the responsibility for ensuring material quality in such cases would lie with the bottle manufacturers. This often involves incorporating virgin material to preserve the necessary mechanical properties.

Although the number of mechanical recycling cycles a material can undergo is an interesting topic for further exploration, it falls outside the scope of this study due to the constraints of the described method. The authors acknowledge the importance of this consideration for future research in broader recycling contexts.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors show a set-up capable of cutting PET bottles into stripes, forming them through a nozzle into 1.75 mm filament and printing and testing specimens for stress-strain-measurements from this.

The idea to do this is not new, and there are a lot of design files and descriptions for how to set up such a system out there in the maker community. In addition, there are several papers out there doing detailed investigation on print quality and tensile testing of filament produced via this route, and some of these papers even do a comparison to other/virgin materials.

Despite this, your work adds some more information to the current state of research by testing different printing directions and comparing the printed part to original plastic cut from the bottle. Hence, this increases the value of this work and is of some interest for the reader.

I recommend accepting the paper after a major revision. In the following, I list several things to consider during this revision. I hope, this helps to improve the paper.

 

The paper should be shortened. Especially the introduction

 

The figures (photographs as well as diagrams) should be redone to show more details and be better readable and of higher quality. I did some detailed comments regarding this in the list below.

 

Wouldn’t it be more accurate to choose a commercially available rPET filament for comparison instead of PETG, as the latter is a different material from the chemical point of view? Please add one comparison to a part printed from a commercially available rPET or commercial available PET filament.

 

Figure 2 (and others in the introduction): Are you allowed to reprint the figures in your article?

 

Figure 5 is not needed, as figure 13 shows a better picture of it, including the bottle and the cutting.

 

Figure 7: Please do not use a picture of the manufacturer or distributor of the nozzle and heating block. Please make a photograph of your nozzle after adjusting it to your needs.

 

Figure 8 to 10: There is no need of this as a figure. Please add an expressive photograph of the parts in your setup if they are needed to describe your work.

 

Line 187 ff: The needed width of the plastic strips depends on the wall thickness of the bottle. Please write down the wall thickness of your bottles. And what was the wall thickness in “similar machines” in the studies you investigated?

 

What is the cross-section of the strips cut to a width of 8.75 mm multiplied by the wall thickness, and does this correspond to the cross-section of a commercial 1.75 mm filament?

 

Is there a stretching during production of the filament? Then this should be taken into account when comparing cross-section of strips and filament, too.

 

Line 216: “molding temperature at 140°C“: I assume this is a typo and should be 240 °C?

 

Figure 16 and 20: I cannot investigate the print quality based on the figure given. Can you please add additional sub-views, showing magnified details?

 

Printing and mechanical testing: How many samples were printed and tested per material and orientation? Please give this information in the text.

 

Figure 23 and following: Please increase the size of the figure and label it in English.

 

Table 2: The table's caption states “at 0°”, but I assume this should be 45° in this case?

 

There are several typos or British English words in the document such as e.g. tonnes, ammount, util, prioritise, diretions, behaviour

Author Response

The authors show a set-up capable of cutting PET bottles into stripes, forming them through a nozzle into 1.75 mm filament and printing and testing specimens for stress-strain-measurements from this.

The idea to do this is not new, and there are a lot of design files and descriptions for how to set up such a system out there in the maker community. In addition, there are several papers out there doing detailed investigation on print quality and tensile testing of filament produced via this route, and some of these papers even do a comparison to other/virgin materials.

Despite this, your work adds some more information to the current state of research by testing different printing directions and comparing the printed part to original plastic cut from the bottle. Hence, this increases the value of this work and is of some interest for the reader.

I recommend accepting the paper after a major revision. In the following, I list several things to consider during this revision. I hope, this helps to improve the paper.

 

The paper should be shortened. Especially the introduction

 

The figures (photographs as well as diagrams) should be redone to show more details and be better readable and of higher quality. I did some detailed comments regarding this in the list below.

Response 1 – The authors thank you for this comment. The paper was largely shortened, mainly Figures were combined.

Wouldn’t it be more accurate to choose a commercially available rPET filament for comparison instead of PETG, as the latter is a different material from the chemical point of view? Please add one comparison to a part printed from a commercially available rPET or commercial available PET filament.

Response 2 – The authors thank you for this comment. However, this work was the resume of a Master dissertation that was concluded. Furthermore, this would further extend the already large article.

 

Figure 2 (and others in the introduction): Are you allowed to reprint the figures in your article?

Figure 5 is not needed, as figure 13 shows a better picture of it, including the bottle and the cutting.

Figure 7: Please do not use a picture of the manufacturer or distributor of the nozzle and heating block. Please make a photograph of your nozzle after adjusting it to your needs.

Figure 8 to 10: There is no need of this as a figure. Please add an expressive photograph of the parts in your setup if they are needed to describe your work.

Response 3 – The authors thank you for this comment. Figures 1-4 were removed, since they illustrate data that are reported in the text. All the figures of each component were compacted in one single picture (new Figure 2). The authors thank you for this comment. Figures 5 and 13 were combined into the new Figure 1.

 

Line 187 ff: The needed width of the plastic strips depends on the wall thickness of the bottle. Please write down the wall thickness of your bottles. And what was the wall thickness in “similar machines” in the studies you investigated?

Response 4 – The authors thank you for this comment. Our bottles have 0.25 mm, and this information was added (marked in yellow). The wall thickness in “similar machines” range from  0.15 to 0.3 mm.

What is the cross-section of the strips cut to a width of 8.75 mm multiplied by the wall thickness, and does this correspond to the cross-section of a commercial 1.75 mm filament?

Is there a stretching during production of the filament? Then this should be taken into account when comparing cross-section of strips and filament, too.

Response 5 – The authors thank you for this comment. Considering the 8.75x025=2.2 mm2 cross-section of the original strip, one can conclude that stretching has occurred during filament production. This comment was introduced (marked in yellow). However, the final cross-section was measured optically, and the infill was increased to 122% to compensate the internal hole. 

 

Line 216: “molding temperature at 140°C“: I assume this is a typo and should be 240 °C?

Response 6 – The authors thank you for pointing out this distraction. It was changed accordingly (marked in yellow)

Figure 16 and 20: I cannot investigate the print quality based on the figure given. Can you please add additional sub-views, showing magnified details?

Response 7 – The authors thank you for this comment. The referred images were magnified.

Printing and mechanical testing: How many samples were printed and tested per material and orientation? Please give this information in the text.

Response 8 – The authors thank you for this comment. Three samples were printed and tested per material and orientation. This information was introduced (marked in yellow).

Figure 23 and following: Please increase the size of the figure and label it in English.

 Response 9 – Thank you for pointing this out. It was our distraction and Stress-strain curves were changed.

Table 2: The table's caption states “at 0°”, but I assume this should be 45° in this case?

 Response 10 – Thank you for pointing this out. It was our distraction and was corrected.

There are several typos or British English words in the document such as e.g. tonnes, ammount, util, prioritise, diretions, behaviour

Response 11 – Thank you for pointing this out. These typos were addressed.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors addressed all raised points

Author Response

The authors addressed all raised points.

Response 1 – Thank you for your constructive comments and critics that helped us to largely improve the original version of this paper.

Reviewer 3 Report

Comments and Suggestions for Authors

I thank the authors for their effort when taking my considerations into account.

I would suggest accepting the paper in the present form and have just one minor recommendation/issue:

 

Your response 7 said: "The authors thank you for this comment. The referred images were magnified."

I'm sorry, but the figures (now figure number 6 and 10) are still shown in the same magnification and were not magnified. Please double their size or add additional sub-views, showing magnified details. In addition, I would recommend this for figure 5, too. Perhaps you can add figure 5 and 6 into one picture? Same for figure 8 and 9, showing the cross section of the filament?

 

Author Response

I thank the authors for their effort when taking my considerations into account.

I would suggest accepting the paper in the present form and have just one minor recommendation/issue:

Your response 7 said: "The authors thank you for this comment. The referred images were magnified."

I'm sorry, but the figures (now figure number 6 and 10) are still shown in the same magnification and were not magnified. Please double their size or add additional sub-views, showing magnified details. In addition, I would recommend this for figure 5, too. Perhaps you can add figure 5 and 6 into one picture? Same for figure 8 and 9, showing the cross section of the filament?

Response – Thank you for your constructive comments and critics that helped us to largely improve the original version of this paper. Figure 6 was replaced, and details were included. In Figure 10, magnification details were included as suggested, hopefully according to reviewer comments. We did not compile figures 8 and 9 as we also increased the size of FIgure 8.