Rheological Properties of Wood–Plastic Composites by 3D Numerical Simulations: Different Components

Round 1

Reviewer 1 Report

The main merit of the paper is the coupling between experimental measurements and numerical simulations. The paper, though, must be improved in several aspects: in this form, it cannot be accepted.

Here is a short list of problems that must be solved.

1) English language is poor throughout the whole paper. Extensive English proofreading is necessary. Also the writing of the article is sloppy and careless.

2) Introduction, row 41. Temperature and natural fiber content was considered in a different article by the same authors, i.e. V. Mazzanti, F. Mollica, Rheological behavior of wood flour filled poly(lactic acid): Temperature and concentration dependence, Polymer Composites Vol. 40, E169-E176, January 2019.

3) Introduction. In rder to extrude highly loaded WPC lubricant is necessary, in fact also the formulation of the paper include 2% lubricant. The presence of lubricant makes it mandatory to consider issues of wall slip. This subject was studied in V. Mazzanti, F. Mollica, Pressure dependent wall slip of wood flour filled polymer melts, J. Non-Newtonian Fluid Mech. Vol. 247 178-187 (2017)

4) Materials and Methods. Usually the coupling of MAPE with stearic acid based lubricants is not very effective. I am not sure this WPC composition is a good recipe

5) Row 97. Correct r/min with RPM

6) Finite element model. Why didn't the authors not include the conical part of the die? I think it would be interesting, and i strongly recommend they do

7) In the free surface, issues of heat transfer should be considered. Please explain why you did not consider them

8) What is the flow rate superimposed on the numerical simulation?

9) Fig. 3a. The pressure that is represented is about 50 bar, it is quite low: it looks strange to me

10) Fig. 3c and 3d. Units of measurement of pressure is Pa, not Pa s. Perhaps the authors are picturing viscosity. In any case the figure is not clear

11) Fig. 4a. The velocity that is pictured with red color is 13m/s, i.e. about 50km/hour. I believe that might be a bit too much, don't you think???

12) Fig. 7b. Maximum pressure here goes up to 3000 bar. I think any extruder would just explode if such a pressure was reached. Maybe there are some mistakes, I don't think this value is reasonable.

Author Response

Dear Editor and reviewers,

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “The study on rheological properties of wood–plastic composites by 3D numerical simulation: different components” (ID:1151547). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1. Comment: English language is poor throughout the whole paper. Extensive English proofreading is necessary. Also the writing of the article is sloppy and careless.

Reply: Thanks so much for your usefully and carefully comments. The manuscript has been thoroughly revised and edited by a native speaker, so we hope it can meet the journal and your highly standard.

2. Comment: Introduction, row 41. Temperature and natural fiber content was considered in a different article by the same authors, i.e. V. Mazzanti, F. Mollica, Rheological behavior of wood flour filled poly(lactic acid): Temperature and concentration dependence, Polymer Composites Vol. 40, E169-E176, January 2019.

2Reply: Thank you for your valuable suggestion. We have cited related literature about Rheological behavior of wood flour filled poly (lactic acid): Temperature and concentration dependence in the proper place of the revised manuscript

3. Comment: Introduction. In rder to extrude highly loaded WPC lubricant is necessary, in fact also the formulation of the paper include 2% lubricant. The presence of lubricant makes it mandatory to consider issues of wall slip. This subject was studied in V. Mazzanti, F. Mollica, Pressure dependent wall slip of wood flour filled polymer melts, J. Non-Newtonian Fluid Mech. Vol. 247 178-187 (2017)

Reply: Thank you for your rigorous consideration. In view of the complex rheological characteristics of WPC, usually no slip between the melt and the wall will be assumed in order to simulate convergence. The same assumption was made after adding lubricants to the WPC formulation, such as in the reference “Zhang, J.; Rizvi, G.M.; Park, C.B.; Hasan, M.M. Study on cell nucleation behavior of HDPE–wood composites/supercritical CO₂ solution based on rheological properties. J. Mater. Sci. 2011”.

4. Comment: Materials and Methods. Usually the coupling of MAPE with stearic acid based lubricants is not very effective. I am not sure this WPC composition is a good recipe

Reply: Thank you for your comment. The performance of this WPC composition is better, and it is widely used in WPC extrusion molding.

5. Comment: Row 97. Correct r/min with RPM

Reply: Thank you for your comment, we have completed correct r/min with RPM.

6. Comment: Finite element model. Why didn't the authors not include the conical part of the die? I think it would be interesting, and i strongly recommend they do

Reply: We appreciate for your valuable comment. In this paper, we simply consider the numerical simulation of WPC melt in the slit die. The conical part to the finite element model will be studied in the next paper.

7. Comment: In the free surface, issues of heat transfer should be considered. Please explain why you did not consider them

Reply: Thank you for this very insightful comment. In this paper, the melt flow field information of WPC is under the isothermal environment. But we will consider the heat transfer in the future study.

8. Comment: What is the flow rate superimposed on the numerical simulation?

Reply: Thank you so much for your careful check. We have added the flow rate superimposed in Materials and Methods, row 333 (Revised manuscript).

9. Comment: Fig. 3a. The pressure that is represented is about 50 bar, it is quite low: it looks strange to me

Reply: We are appreciative of your suggestion. In the process of WPC extrusion molding with wood fiber content of 40wt%, the pressure of the extruder is generally stable between 40 and 50 bar, which is a more reasonable value for WPC.

10. Comment: Fig. 3c and 3d. Units of measurement of pressure is Pa, not Pa s. Perhaps the authors are picturing viscosity. In any case the figure is not clear

Reply: Thank you for your carefully review. The units of the pressure have been corrected in the revised manuscript.

11. Comment: Fig. 4a. The velocity that is pictured with red color is 13m/s, i.e. about 50km/hour. I believe that might be a bit too much, don't you think???

Reply: Thank you for your carefully review. The units of the velocity have been corrected in the revised manuscript.

12. Comment: Fig. 7b. Maximum pressure here goes up to 3000 bar. I think any extruder would just explode if such a pressure was reached. Maybe there are some mistakes, I don't think this value is reasonable.

Reply: We gratefully thanks for the precious time the reviewer spent making constructive remarks. We re-test and fit the rheological data of WPC with wood fiber content of 40wt% to 80wt%. And re-simulate and calculate the corresponding flow field information. For the pressure field of highly filled WPC, especially the filling amount is more than 70wt%, the simulated value is still on the high side. We think it may be related to the assumptions before the simulation. The existence of hypothetical conditions allows the simulation to be completed in an ideal state, such as assumed no slip, incompressibility and other factors, which may cause the simulated value to be higher than the actual value. The refitted data and cloud graphs are all marked in red.

Thanks again.

Sincerely yours,

Lichao Sun

Author Response File: Author Response.docx

Reviewer 2 Report

The content of the manuscript falls within the scope of the journal. However, there is a need to make some corrections.

Specific comments:

1. Delete "The study on" in the title
2. Rephrase Line 18-19 on the abstract for clarity. "....could be predicted calculated by...."
3. Include the objectives of the study in the last paragraph in the Introduction section 
4. Discussion Section: Discuss the results in the relation to relevant previous studies. There is a need to add more literature.

Author Response

Dear Editor and reviewers,

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “The study on rheological properties of wood–plastic composites by 3D numerical simulation: different components” (ID:1151547). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1. Comment: Delete "The study on" in the title.

Reply: Thank you for the title suggested. The precedent version of the title has been replaced in the revised manuscript.

2. Comment: Rephrase Line 18-19 on the abstract for clarity. "....could be predicted calculated by....".

Reply: Thank you for your valuable suggestion. Thank you for underlining this deficiency. This section was revised and modified according to the information shown in the work suggested by the reviewer.

3. Comment: Include the objectives of the study in the last paragraph in the Introduction section.

Reply: Thank you for your rigorous consideration. We have added the information required as explained above.

4. Comment: Discussion Section: Discuss the results in the relation to relevant previous studies. There is a need to add more literature.
5. Reply: Thank you for your comment. We have added more references in the proper place of the revised manuscript.

Thanks again.

Sincerely yours,

Lichao Sun

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The paper is ok now.

Reviewer 2 Report

The authors have addressed the comments, only minor spell checks are needed. 

1. Line 18: The sentence should read: "..... and viscosity distributions of the WPC....."
2.  Line 18: Delete the word "calculated"
3.  Line 317: Replace the word "he" with the word "The"