Analysis of Film Forming Law and Characteristics for an Air Static Spray with a Variable Position of the Plane
Round 1
Reviewer 1 Report
Authors of the article used CFD to the simulation of static air spraying with displacement posture. Using the Euler model, authors investigated the effect of different spray heights and angles on the quality of the coating. I suggest to accept the paper for publication in Coatings after following major corrections.
Comments:
- Does the model use the turbulent dispersion force of gas and liquid phases as well as the interphase force of gas and liquid phases presented in equation 2. If so, which?
- The description of the numeric grid is missing. How did you verify the mesh sizes and numbers? The authors should provide the independence of the results from the mesh. It is worth providing parameters determining the quality of the mesh (e.g. minimum orthogonal quality, skewness or maximum aspect ratio)
- The article lacks a description of Eulerian liquid film model.
- It would be useful to present the boundary conditions in Figure 1.
- Figure 4 lacks a description of the velocity value.
- Figures of the Fluent program are difficult to read (Figures 3, 6 and 9).
- On pages 10 and 11, the numbering of the figures is incorrect.
Author Response
Response to Reviewer Comments
Point 1: Does the model use the turbulent dispersion force of gas and liquid phases as well as the interphase force of gas and liquid phases presented in equation 2. If so, which?
Response 1: The q in Equation 2 means: when q = g or d, g and d represent gas and liquid phases, respectively. Therefore, when q=g, and respectively represent the gas-phase turbulent dispersion force and the interphase force, and the same is true when q=d.
The interphase force is the sum of the drag force Fd, the lifting force Fl and the virtual mass force Fv. This study ignores the lifting force Fl and the virtual mass force Fv, and only considers the drag force caused by the speed difference between the two phases.
The description in the text has been modified. The revised position is on line 111 to 119 on page two.
Point 2: The description of the numeric grid is missing. How did you verify the mesh sizes and numbers? The authors should provide the independence of the results from the mesh. It is worth providing parameters determining the quality of the mesh (e.g. minimum orthogonal quality, skewness or maximum aspect ratio)
Response 2: Based on your suggestion, I evaluate the mesh quality according to the following criteria.
The average element quality of the grid is 0.89, the minimum orthogonal quality is 1.9, and the skewness is 0.23. Increasing the number of grids cannot significantly improve the calculation accuracy.
The revised content is in lines 198 to 201 on the fifth page of the article.
Point 3: The article lacks a description of Eulerian liquid film model.
Response 3: The description of the wall liquid film model has been added in lines 150-175 on page 4 of the article.
Point 4: It would be useful to present the boundary conditions in Figure 1.
Response 4: Boundary conditions have been added to Figure 1.
Point 5: Figure 4 lacks a description of the velocity value.
Response 5: The velocity value has been added to Figure 4.
Point 6: Figures of the Fluent program are difficult to read (Figures 3, 6 and 9).
Response 6: The picture resolution has been modified to be higher.
Point 7: On pages 10 and 11, the numbering of the figures is incorrect.
Response 7: We have modified it in the text.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper presents the simulation study regarding air spraying technology. The Paper seems interesting due to the fact that simulations were validated by the experimental research.
My comment on the paper:
- There is a wide variety of "spray" technologies and many types of feedstock materials used for coatings deposition. The paper should introduce to them by shortly mention the typical technologies and materials likewise given in the following papers:
APS ceramic coatings deposition https://doi.org/10.1016/j.surfcoat.2021.126979
air electrostatic deposition of polymer paint coatings https://doi.org/10.24425/bpasts.2021.137519
relatively novel suspension spraying of ceramics https://doi.org/10.3390/coatings11080879
HVOAF spraying of complex composites 10.1016/j.matdes.2021.109566
And many more. Please add such a paragraph. - Are your research results can be implemented for other spray systems including the suspension deposited coatings?
- It is known that spraying distance and torch velocity are crucial for the fabrication of high-quality coatings, you can mention it in introduction. There are many papers studying the effect of spray distance and torch velocity (etc) on the operation properties of coatings. Not only CFD but also ANN methods are used, likewise https://doi.org/10.1016/j.surfcoat.2019.124988 https://doi.org/10.1016/j.surfcoat.2012.11.006 https://doi.org/10.1016/j.surfcoat.2009.04.023 https://doi.org/10.1016/j.surfcoat.2021.126979.
- Please emphasize the novelty of your work in the introduction, what are the gaps in knowledge?
- Provide much more details regarding the material type and properties of deposited coatings that were used for simulations (density, material properties etc)
- Please explain clearly the type of "air technology" and the materials which can be deposited (in section spraying experiment). Now only in the title, it is mentioned about "painting" which is not explained in detail in the paper.
- What is the name of the materials used for a deposition? Was it self-made or commercial?
- What are the possible applications of those spray technology, write about it.
- fig. 12 is unclear - the spray gun, substrate and other important issues should be marked by arrows/labels etc.
- conclusions should include the information about the possible application of the obtained research results in specific industries.
Author Response
Thank you for your questions and suggestions, which are of great help to us. Based on your suggestions, we have made changes in the manuscript. The response to the question has been uploaded to the attachment.
Response to Reviewer Comments
Point 1: There is a wide variety of "spray" technologies and many types of feedstock materials used for coatings deposition. The paper should introduce to them by shortly mention the typical technologies and materials likewise given in the following papers:
APS ceramic coatings deposition https://doi.org/10.1016/j.surfcoat.2021.126979
air electrostatic deposition of polymer paint coatings https://doi.org/10.24425/bpasts.2021.137519
relatively novel suspension spraying of ceramics https://doi.org/10.3390/coatings11080879
HVOAF spraying of complex composites 10.1016/j.matdes.2021.109566
And many more. Please add such a paragraph.
Response 1: Spraying technology used for coating deposition is mainly divided into air spraying, airless spraying, thermal spraying and electrostatic spraying.
The relevant introduction is added to lines 32 to 46 on the first page of the article.
Point 2: Are your research results can be implemented for other spray systems including the suspension deposited coatings?
Response 2: The research object of this article is an air spray gun. The atomization principle of the air spray gun is that the paint is broken into fine droplets by the impact of the high-speed airflow and then adheres to the wall surface. This paper is based on computational fluid dynamics to study the changing law of paint deposition when spraying height and spraying angle change. Further work will establish a predictive model of paint deposition based on the changing law of paint deposition. Therefore, the research results in this article are only applicable to air spray gun spraying.
Point 3: It is known that spraying distance and torch velocity are crucial for the fabrication of high-quality coatings, you can mention it in introduction. There are many papers studying the effect of spray distance and torch velocity (etc) on the operation properties of coatings. Not only CFD but also ANN methods are used, likewise https://doi.org/10.1016/j.surfcoat.2019.124988 https://doi.org/10.1016/j.surfcoat.2012.11.006 https://doi.org/10.1016/j.surfcoat.2009.04.023 https://doi.org/10.1016/j.surfcoat.2021.126979.
Response 3: As far as air spraying is concerned, the torch speed is mainly affected by the atomization pressure. Spraying distance and spraying speed are very important to the quality of coating deposition.
The relevant description is added to lines 48 to 51 on the second page of the article.
Point 4: Please emphasize the novelty of your work in the introduction, what are the gaps in knowledge?
Response 4: When spraying complex curved surfaces, spraying distance and spraying inclination are critical to the quality of coating deposition. The current research has not conducted in-depth analysis by establishing a mathematical model of the influence of spraying distance and spraying angle on coating deposition. This paper studies the film formation law and characteristics of static spraying with spray gun under the influence of spraying height and spraying angle at the same time. Based on simulation and experimental data, this paper summarizes the variable range of spray height and spray angle during air spraying. It lays a theoretical foundation for the establishment of a prediction model for the thickness distribution of the spray gun spraying with variable poses.
The relevant changes are in lines 73 to 83 on the second page of the article.
Point 5: Provide much more details regarding the material type and properties of deposited coatings that were used for simulations (density, material properties etc)
Response 5: The paint used in the spray experiment is water-based paint. Therefore, the material of the deposited coating used for the simulation is set to the liquid phase. Use liquid water for the liquid phase, reset the diameter particles of the liquid phase to 20um, viscosity to 0.1kg/(m s), and density to 1.2×103kg/m3.
The relevant description is in lines 204 to 206 on page 5 of the article.
Point 6: Please explain clearly the type of "air technology" and the materials which can be deposited (in section spraying experiment). Now only in the title, it is mentioned about "painting" which is not explained in detail in the paper.
Response 6: Air spray gun was used in the experiment. Deposition material is a commercial coating with water as solvent or dispersion medium, commonly known as water paint. The coating is deposited on the surface of the workpiece by air atomization.
This article studies static spraying with air spray guns. Mainly study the effect of spraying height and spraying angle on coating deposition. The title of the article is incorrect and has been slightly revised.
The relevant changes are in lines 355 to 358 on page 11 of the article.
Point 7: What is the name of the materials used for a deposition? Was it self-made or commercial?
Response 7: The material used for deposition is water-based paint, not self-made, commercial.
Point 8: What are the possible applications of those spray technology, write about it.
Response 8: Robot offline programming and spraying technology can be applied to many fields, such as the coating of ships, aerospace, automobiles, and household appliances. The spray objects are often the surfaces of products with complex free-form features such as fighter fuselages and car bodies.
The relevant changes are in lines 26 to 28 on page 1 of the article.
Point 9: fig. 12 is unclear - the spray gun, substrate and other important issues should be marked by arrows/labels etc.
Response 9: Labels have been added and other experimental equipment is attached in Figure 12.
Point 10: conclusions should include the information about the possible application of the obtained research results in specific industries.
Response 10: Air spraying is widely used in automobile, aviation, aerospace, shipbuilding and other manufacturing fields. The spray objects are often the surfaces of products with complex free-form features such as fighter fuselages and car bodies.
The relevant changes are in lines 389 to 391 on page 12 of the article.
Author Response File: Author Response.docx
Reviewer 3 Report
This paper presents a study of coating formation during air spray painting. For this purpose, an Euler-Euler formulation was applied, to simulate the spraying process and analyse the resulting coating thickness for different spraying heights and dip-angles, with a stationary spray-torch. Additionally, experiments were conducted to reproduce the simulation results. Unfortunately, I can not suggest this paper for publication due to the following issues:
- The authors showed a CFD model, calculating the spray-paint thickness with different input parameters (spray-hight and dip-angle). However, the contribution of this manuscript is ambiguous: it could potentially add understanding of (i) a new coating model to be exercised for different input parameters or (ii) novel insights into the physics of the spray-painting process. However, Chen et al [12] already showed an Euler-Euler model to calculate the thickness of spray-paint, in that case with an even more complex robotic path and curved surfaces. Furthermore, the physics of the influence of the dip-angle and spraying height on the coating thickness is known.
- It is unclear what Fig.12 is showing. According to the text, dip-angle and spray hight are input parameters of the simulation but are the X- and Y-axis of this figure too. The explanation in section 4.4. is hard to comprehend.
- More details have to be discussed on how input parameters of the simulation were found. For examples, it was assumed, that the droplets are of a const. diameter (20 micrometers). I suppose, this is an important input parameter and has to be measured if results are compared to experiments.
- It is ambiguous what the 'spray quality' is. This parameter was not defined in the manuscript.
- Some pictures have low resolution - details or additional annotations are unclear (esp. Fig.3, Fig.6, Fig.9).
- It is hard to compare the subfigures of Fig.7 (and Fig.8) - an improved data representation might be necessary
- Parts of the manuscript are hard to read, especially the abstract, second paragraph of the introduction, lines 190-198, and line 285-292. Furthermore, repeated words and phrases in the same sentence are difficult to comprehend (e.g. line 13, line 60, line 193).
Author Response
Thank you for your questions and suggestions, which are of great help to us. Based on your suggestions, we have made changes in the manuscript. The response to the question has been uploaded to the attachment.
Response to Reviewer Comments
Point 1: The authors showed a CFD model, calculating the spray-paint thickness with different input parameters (spray-hight and dip-angle). However, the contribution of this manuscript is ambiguous: it could potentially add understanding of (i) a new coating model to be exercised for different input parameters or (ii) novel insights into the physics of the spray-painting process. However, Chen et al [12] already showed an Euler-Euler model to calculate the thickness of spray-paint, in that case with an even more complex robotic path and curved surfaces. Furthermore, the physics of the influence of the dip-angle and spraying height on the coating thickness is known.
Response 1: When spraying complex curved surfaces, spraying distance and spraying inclination are critical to the quality of coating deposition. The current research has not conducted in-depth analysis by establishing a mathematical model of the influence of spraying distance and spraying angle on coating deposition. This paper studies the film formation law and characteristics of static spraying with spray gun under the influence of spraying height and spraying angle at the same time. Based on simulation and experimental data, this paper summarizes the variable range of spray height and spray angle during air spraying. It lays a theoretical foundation for the establishment of a prediction model for the thickness distribution of the spray gun spraying with variable poses.
Point 2: It is unclear what Fig.12 is showing. According to the text, dip-angle and spray hight are input parameters of the simulation but are the X- and Y-axis of this figure too. The explanation in section 4.4. is hard to comprehend.
Response 2: Figure 12 shows the spraying experiment platform. The equipment and tags have been added in Figure 12. After setting the tool coordinates and workpiece coordinates of the six-degree-of-freedom spraying robot, the spraying height and inclination of the spray gun can be adjusted by the spraying robot.
Section 4.4 states that when spraying complex curved surfaces, adjusting the appropriate spray height and spray angle can obtain a better quality coating. This paper uses a large amount of simulation data to summarize the spraying height of high-quality coating film when the spraying angle is 0° to 30°.
The relevant changes are in lines 355 to 358 on page 11 of the article.
Point 3: More details have to be discussed on how input parameters of the simulation were found. For examples, it was assumed, that the droplets are of a const. diameter (20 micrometers). I suppose, this is an important input parameter and has to be measured if results are compared to experiments.
Response 3: The input parameters of the simulation in this paper refer to the following two papers. The droplet diameter distribution data used in this paper are from the experimental measurement data of Hicks. The pressure of atomizing hole is slightly smaller than that measured by pressure gauge in spraying experiment. The simulation data and experimental data are consistent to verify the feasibility of the simulation model.
P G Hicks, D. W. Senser, K. C. Kwok, B. Y. H. Liu. Drop Transfer Efficiency in Air Paint Sprays. Engineering Society of Detroit Advanced Coatings Technology Conference,November 9-11, Dearborn, MI.
Gu. Xiong. LIU. Simulation study on the spray flow fieldof air atomized paint spray gun [D]. Hangzhou: Zhejiang University, 2012: 31–49G.
The relevant changes are in lines 204 to 205 on page 5 of the article.
Point 4: It is ambiguous what the 'spray quality' is. This parameter was not defined in the manuscript.
Response 4: So the standard to judge the uniformity of coating distribution is as follows : the maximum thickness of coating is between 20μm and 100μm. The coating has a complete molding boundary, such as round and oval. Film smooth, no hanging, bulge and depression.
The relevant changes are in lines 334 to 337 on page 10 of the article.
Point 5: Some pictures have low resolution - details or additional annotations are unclear (esp. Fig.3, Fig.6, Fig.9).
Response 5: The picture in the article has been modified.
Point 6: It is hard to compare the subfigures of Fig.7 (and Fig.8) - an improved data representation might be necessary
Response 6: Fig. 7 shows the cross section of coating thickness after spraying for 0.5 seconds. Figure 7 ( d ) shows that the edge of the coating is uneven, with bulges and depressions, which means that the coating is unevenly distributed.
Figure 8 describes the variation trend of maximum coating thickness with spray angle.
Point 7: Parts of the manuscript are hard to read, especially the abstract, second paragraph of the introduction, lines 190-198, and line 285-292. Furthermore, repeated words and phrases in the same sentence are difficult to comprehend (e.g. line 13, line 60, line 193).
Response 7: All relevant content has been revised. On lines 12 to 18, lines 78 to 83, lines 250 to 257, and lines 345 to 352 of the article.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Thank you for the comprehensive response to comments and questions included in the review. The previous version of the article was re-edited according to my comments and suggestions. Since the authors took all remarks included in my review into consideration and added new valuable comments into the text I recommend the presented manuscript for publication in Coatings.
Author Response
It is a great honor to receive your recognition of our work. We are also very grateful for your valuable comments.
Reviewer 3 Report
From your last response, the overal goal of this research is to 'establishing a mathematical model of the influence of spraying distance and spraying angle on coating deposition'. In the current state of the study, I still think that this manuscript should be a part of a larger study, including a more detailed mathematical modeling of the coating thickness in regards to the spraying parameters. Other studies looked at these spraying parameters separately, for example [16] which studied the influence of the coating thickness on the dip angle. In the current state, the study is too incremental, reporting the combination of the influence of the dip angle and spraying height.
Additional minor issues:
- According to the updated paper, the droplet diameter is used from Hicks, however, the manuscript still stated that a diameter of 20um was used - it is still unclear if 20um is a mean droplet thickness.
- The issue with Fig 7 and 8 is that the both x- and y-axis are changing so that a comparison is difficult. Maybe one solution would be to overlay some of those subfigures.
- Please carefully check your literature. There are issues, for example in [16], the first and last name of the authors are switched.
Author Response
Response to Reviewer Comments
Point 1: - According to the updated paper, the droplet diameter is used from Hicks, however, the manuscript still stated that a diameter of 20um was used - it is still unclear if 20um is a mean droplet thickness.
Response 1: The droplet size is based on Rosin-Rammler empirical distribution function.
Relevant changes are in lines 203 to 204 on page 5 of the article.
Point 2: - The issue with Fig 7 and 8 is that the both x- and y-axis are changing so that a comparison is difficult. Maybe one solution would be to overlay some of those subfigures.
Response 2: The pictures have been integrated in one picture. Fig 7 and 9
Point 3: - Please carefully check your literature. There are issues, for example in [16], the first and last name of the authors are switched.
Response 3: References have been checked and revised.
