Insights into the Processing of Recycled Carbon Fibers via Injection Molding Compounding

Round 1

Reviewer 1 Report

The authors presented a systematic study on manufacturing and properties of recycled carbon fiber materials using injection molding. The paper is well-written in terms of research needs, methodology and comparison of mechanical properties with other materials. Therefore, I highly recommend the paper to be published in the journal of composite science. 

Author Response

Thank you very much for your review. We are proud and very happy to see that you approve of this paper.

Reviewer 2 Report

In this paper, the authors describe the methodologies adopted to recycled fiber-reinforced polymer. The paper includes fiber preparation and direct compounding via injection molding. My comments are the following:

• What is an injection molding compounding process? The concept of it is not straightforward from the title and the abstract. I would recommend changing the title and abstract. Then, add the definition of it and references in the introduction.
• 26-31: I suggest explaining the contribution of each reference rather than using [1-5] all agglomerated.
• 32-33: How are discontinuous fiber technologies well suited?
• 42: `Studies (e.g. [2,8-11])` this is not an acceptable form of citation. Need to explain what the contribution of each reference is. This comment applies to most of the reference used throughout the manuscript.
• How did you measure the fiber length?
• Section 2 is not easy to follow. I recommend reporting only essential information and describing the material (resin and fiber) preparation in a more linear form. Section 2.5 should not be there to summarize, but it should be clear what the steps are.
• Section 2 lacks experimental approach information. How many samples did you produce and test?
• What is the goal of the preliminary investigation (Section 3.2)?
• Result graphs need error bars (cf. Fig. 7).
• 430-432: what is the single-stage process? How were the settings calculated? What are the desired results?
• Section 3.4: what is the uncertainty of the fiber length measurements?
• 478-488: this paragraph would be more appropriate for the Introduction.
• 507-509: this needs to explain. Please compare with specific recycling routes. What is BMC?
• Please check grammar, typos, and style to correct minor English errors.

I recommend accepting with major revisions. The article needs to be rewritten as a scientific paper rather than a technical report. The introduction, methodology, and results and discussion, sections all need rewriting and restructuring.

Author Response

Thank you very much for your review. The comments were helpful. We updated the text according to your comments and used the track changes function of Microsoft Word to highlight our changes. A detailed view of our actions is given below.

• What is an injection molding compounding process? The concept of it is not straightforward from the title and the abstract. I would recommend changing the title and abstract. Then, add the definition of it and references in the introduction.

Injection molding compounding (IMC) is a single-stage process for the processing of plastic materials developed by KraussMaffei in the 1990s and taken up by others later on. It combines the compounding and the injection molding process in one machine.

We added more details about the IMC and tried to be more precise in our description of the process steps. See 46-58.

• 26-31: I suggest explaining the contribution of each reference rather than using [1-5] all agglomerated.

We added a passage where the references are listed in more detail and explain what each reference contributes to this paper. See 32-37.

• 32-33: How are discontinuous fiber technologies well suited?

We added a statement that discontinuous fiber technologies are well suited for the processing of recycled fibers since these fibers, after processing, become in fact discontinuous. This is also supported by the examples and applications shown in the referenced literature. See 38-39.

• 42: `Studies (e.g. [2,8-11])` this is not an acceptable form of citation. Need to explain what the contribution of each reference is. This comment applies to most of the reference used throughout the manuscript.

We have rewritten most of the references to better explain each contribution to this paper and why it is listed here. If a single statement needs to be supported by multiple sources, we did not distinguish between different contributions because we think that this emphasizes the statement. Also, if different references are listed for a single process, we think that a list of sources is appropriate to emphasize that several groups are working on this topic.

• How did you measure the fiber length?

The carbon single fiber length was determined according to the standards DIN 53803 and DIN 53808-1. For this purpose, 150 individual fibers were randomly and carefully removed from the fiber sample by hand and measured under a magnifying glass using a ruler. This procedure has been added to the manuscript (368-394).

• Section 2 is not easy to follow. I recommend reporting only essential information and describing the material (resin and fiber) preparation in a more linear form. Section 2.5 should not be there to summarize, but it should be clear what the steps are.

We tried to better describe the approach we took and the experimental setup. We agree that this section might not be easy to follow, which is why we added a short summary of all the process steps at the end of section 2. Also, a short summary of the process steps is now given at the beginning of section 2 with references to the different sections, see 183-187

• Section 2 lacks experimental approach information. How many samples did you produce and test?

Seven to ten test specimens were tested and afterwards statistically evaluated. We have added this information to the paper. See 383-384.

• What is the goal of the preliminary investigation (Section 3.2)?

As far as we know, carbon fibers have never before been processed in a single-stage process using a direct fiber feed module. It is therefore not known which settings of mass throughput and speed have to be selected to achieve the desired fiber content and whether the calculation method, used for glass fibers, can also be applied to carbon fibers. This will be clarified in the preliminary investigations. We tried to better describe the goals of these investigations, see 439-444.

• Result graphs need error bars (cf. Fig. 7).

Figure 7 shows the median and maximum value of the measurements and therefore error bars are not necessary.

• 430-432: what is the single-stage process? How were the settings calculated? What are the desired results?

The single-stage process is the IMC-process. This needed to be clarified in the text. Thank you! We further elaborated on how exactly the IMC works to improve comprehensibility, see first comment.

• Section 3.4: what is the uncertainty of the fiber length measurements?

Due to the complex and resource-intensive measurements, only representative samples were measured to determine the fiber length. The actual fiber length of the individual test samples is not evaluated. However, the measurements are only intended to demonstrate the possibility of fiber length measurement to this extent and to show a trend in the fiber length and fiber length distribution. We referred to this uncertainty in the text, see 541-543.

• 478-488: this paragraph would be more appropriate for the Introduction.

We have added this paragraph to Section 1. However, we believe it is important for the interpretation of the results to mention it here as well.

• 507-509: this needs to explain. Please compare with specific recycling routes. What is BMC?

We further elaborated on the results and compared our results to results summarized by Pimenta [2]. We also gave a short explanation, why we compared to these processes specifically. See 548‑553.

• Please check grammar, typos, and style to correct minor English errors.

The manuscript was professionally proofread before submission. If any errors have remained in the manuscript, we would appreciate it, if you please point them out.

I recommend accepting with major revisions. The article needs to be rewritten as a scientific paper rather than a technical report. The introduction, methodology, and results and discussion, sections all need rewriting and restructuring.

We have improved the structure and have rewritten parts of the paper to distinguish it from a technical report. A professional proof-reading of the paper was conducted to improve the overall language and style.