An Effect of Coating Parameters to Dry Film Thickness in Spray Coating Process Mutcharin Choikhruea, Suksan Prombanpongb, and Pinet Sriyothac Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi (KMUTT), Bangkok, 10140, Thailand a
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Keywords: Optimization; Spray coating process; Dry film thickness
Abstract. In cookware manufacturing industry, spray coating process plays an important role in function and appearance quality of a product. Typically, Teflon (PTFE) is used for interior coating of a product to increase corrosive resistance and makes the product become nonstick. In the spray coating process, there are three important coating variables: spray time, nozzle size in terms of nozzle rotation, spindle rotational speed which affects dry film thickness (DFT). The objective of this paper is therefore to study a relationship of these variables to dry film thickness and then determine the optimal condition so that the minimum spray time is used whereas the required DFT is met. To achieve this goal, the experimental design technique and analysis of variance (ANOVA) is employed in this study. It is found that the condition of spray time, nozzle rotation, and spindle rotational speed at 1.0 sec, 620 degree, and 530 rpm respectively provides the required DFT at 12.67 micron. Introduction Spray coating process is process to adhere coating material on the substrate surface by an air atomizing spray gun. A high volume low pressure (HVLP) spray gun is normally used in the spray coating process where a coating material with air pressure deposits on a target surface. The coating layer is developed due to the surface tension and adhesion force between the coating material and a substrate surface. The main objectives include protective surface on the substrate or create beautiful surface to add value to the product. As such, the coating material consumption and the control of film characteristics will be the important concerns that critically affect the processing time and costs. There are few research studied factors affecting the spray coating efficiency [1,2,3]. Research work of Winnicki et al, and of Steenkiste and Gorkiewicz revealed the performance of the coating process through a measurement of coating thickness on the substrate [4,5]. Poonkwan et al investigated an optimization Teflon spray coating for nonstick application [6]. Luangkulab and Prombanpong determined optimal spray coating parameters to dry film thickness in order to minimize material consumption [7]. Murugan et al and Hong et al studied to find the optimal parameter for coating process with the thermal of high velocity oxy-fuel (HVOF) in terms of spray gun distance and found that the spray gun distance effect to surface hardness of the coating process [8,9]. In addition, Kang et al studies the effects of flow rate of Argon with
thermal in the plasma spray process [10].A variety of research studies have been conducted in the coating process; however, none attempted to study an effect of work rotational speed to spray performance in the spray coating process. Thus, the aim of this research is to study an effect of spray time, nozzle size and work rotational speed to DFT in the spray coating process. Methodology The experimental design technique is employed to study the effect of spray parameters to DFT. The concerned variables are spray time, nozzle rotation, and spindle rotational speed which two levelsof process parameter is shown in Table 1. The spray time range is 1.0 and 1.2 second whereas nozzle rotation is 540 and 630 degree. Note that the nozzle size inside a spray gun cannot directly measure; therefore, the knob rotation in degree will be used to represent the nozzle size. Then the spindle rotational speed which is used to spin a product while being sprayed is 530 and 750 rpm. Hence, the experiment will be conducted with regard to the 23 factorial designs with DFT as the process response. The experiment is randomly performed in five replications with a total of 40 experiment sets. The obtained results are analyzed by the analysis of variance (ANOVA) in order to statistically determine the degree of significance. Table 1 Level of process variables in this study Variable Spray time (second) Nozzle rotation (degree) Spindle rotational speed (rpm)
Min 1.0 540 530
Max 1.2 630 750
Statistical Analysis The experimental data will be processed by MINITAB software program for variance analysis. The normal distribution, constant variance and independently distributed (Randomization) tests are performed to statistically justify the adequacy of experimental results. It is found that the p-value obtained from the analysis was 0.439, 0.423 and 0.449 respectively and all of them were in the 95% confidence interval as shown in Table 2. After the data is verified, the analysis of variance (ANOVA) is employed to determine the effects of these three parameters on dry film thickness. The spray time, nozzle rotation, spindle rotation speed and their interaction are factors that can induce the significant change to DFT. The criteria used to determine the significant factors are the p-value. If the p-value is less than 0.05, this corresponding value is considered significant to the response value. The obtained results of p-value in the experiment are tabulated in Table 3.It is found that spray time, nozzle rotation and spindle rotational speed as well as an interaction of spray time and spindle rotation speed have an effect to DFT value.
Table 2 P-value of model adequacy checking Model Adequacy Checking Normal Distribution Equal variance Randomization
P-value 0.439 0.423 0.449
Table 3 Analysis of variance (ANOVA) results fordry film thickness Source
DF
SS
Adj. MS
F
P-value
Spray time Nozzle rotation
1
21.934
21.9336
119.68
0.000
1
73.984
73.9840
403.70
0.000
Spindle rotational speed
1
3.994
3.9942
21.79
0.000
1
1.989
1.9892
10.85
0.002
32
5.802
0.1813
39
108.315
Spray Time* Spindle rotational speed Error Total
S = 0.428095, R-Sq = 94.08%, R-Sq (pred) = 92.27%, R-Sq (adj) = 93.40%
Results The effects of spray time are shown in Fig. 1 where the increase in these variables increases the dry film thickness. This finding is logical to an effect of nozzle rotation that a more amount of coating materials passing through the nozzle will increase DFT. However, the more nozzle rotation, the larger size of particles which could affect the uniformity and texture of the coating layer. Hence, it can be noted that a trade-off between these two factors is to be considered for yielding the best coating performance. An increase of spindle rotational speed of spray coating process increases coating film thickness and coating efficiency. This can be explained that a faster spindle rotation speed will increase coating rate; therefore the coating efficiency is enhanced. Main Effects Plot for DFT Data Means Spray Time
Average dry film thickness (micron)
14
Nozzle Rotation
13 12 11 1.0
1.2
540
Rotation Speed
14 13 12 11 530
750
Fig. 1 Main effect plots of variables on average dry film thickness
630
Both of spray time and spindle rotational speed have direct effect to the dry film thickness as shown in Fig. 2. It is obvious that in the same spray time, DFT increases with spindle rotational speed. Likewise, at the same spindle rotational speed, DFT increases with the spray time. In addition, at 1.2 sec spray time DFT increase at faster rate than 1.0 spray time while the rotational speed is increasing. Interaction Plot for DFT Data Means Average dry film thickness (micron)
14.0
Spray Time
1.0 1.2
13.5
13.0
12.5
12.0
11.5 530
750
Rotation Speed
Fig. 2 Effect of interaction factor between spray time and rotation speed of spindle on average dry film thickness The process optimization is also analyzed to attain the dry film thickness ranging between 10-15 µm. It is found that the optimal condition is the spray time at 1.0 sec, nozzle rotation 620 degrees and spindle rotation speed of 540 rpm as shown in Table 4. Table 4 Optimal parameter of each variable Variable Spray time (s) Nozzle rotation (degree) Rotation speed of spindle (rpm)
Optimal Parameter 1.0 620 530
The regression analysis, provides an empirical equation to predict DFT as shown in Eq. 1. Y= -0.95055 - 5.56955X1 + 0.0302222X2 - 0.019427X3+ 0.0202727X1*X3
(1)
Where Y is dry film thickness and X1, X2, X3 are spray time, nozzle rotation and spindle rotational speed respectively. The one-sample t-test method is performed to compare the actual result and the calculated one. Using the optimal condition, the calculated dry film thickness (DFT) obtained from Eq. 1 is 12.67 µm whereas the actual result is 13.03 µm.
Conclusion and Discussion The effect of spray time, nozzle size and work rotational speeds to DFT are studied in this research employing the experimental design technique. The analysis of variance is used to determine the relationship among the interested variables. It is found that all three abovementioned variables play a crucial role to the thickness of coating material. In addition, an interaction of spray time and work rotational speed also affects DFT. An increase of spray time, nozzle speed (degree of nozzle rotation), and work rotational speed increase DFT. Another contribution of this research is the optimal condition at lower spray time. It is found that at 1.0 spray time in order to obtain required thickness, the nozzle rotation will be 620 degree with work rotational speed at 530 rpm. The lower spray time, the shorter cycle time. Thus, the higher productivity can be obtained. References [1] P.J. From, J. Gunnar and J.T. Gravdahl: Optimal Paint Gun Orientation in Spray Paint Applications-Experimental Results. Transaction on Automation Science and Engineering. Vol. 8(2011), p. 438-442 [2] M. Saremi and Z. Valefi: The Effects of Spray Parameters on the Microstructure and Thermal Stability of Thermal Barrier Coatings Formed by Solution Precursor Flame Spray (Spfs). Surface and Coatings Technology. Vol. 222(2013), p. 44-51 [3] A. Kout, and H. Muller: Parameter Optimization for Spray Costing. Advances in Engineering Software.Vol. 42(2009), p. 1078-1086 [4] M. Winnicki, A. Małachowska and A. Ambroziak: Taguchi Optimization of the Thickness of a Coating Deposited by LPCS. Archives of Civil and Mechanical Engineering. Vol.14 (2014), p. 561-568 [5] T. Van Steenkiste, D.W. Gorkiewicz: Analysis of Tantalum Coatings Produced by the Kinetic Spray Process. Journal of Thermal Spray Technology. Vol 13 (2) (2004), p. 265–273 [6] O. Poonkwan, V. Tangwarodomnukun and S. Prombanpong: Optimization of Teflon Spraying Process for Non-Stick Coating Application. Industrial Engineering. (2015), p. 833-839 [7] S. Luangkularb, S. Prombanpong: Material Consumption and Dry Film Thickness in Spray Coating Process. Proceedings of the 47th CIRP Conference on Manufacturing Systems. Vol. 17 (2014), p. 789-794 [8] K. Murugan, A. Ragupathy, V. Balasubramanian and K. Sridhar: Optimizing HVOF Spray Process Parameters to Attain Minimum Porosity and Maximum Hardness in WC-10Co-4Cr Coatings. Surface and Coating Technology. Vol. 247 (2014), p. 90-102 [9] S. Hong, Y. Wu, B. Wang, Y. Zheng, W. Gao and G. Li: High-Velocity Oxygen-Fuel Spray Parameter Optimization of Nanostructured WC–10Co–4Cr Coatings and Sliding wear Behavior of the Optimized Coating. Materials and Design. Vol. 55 (2014), p. 286-291 [10] J.J. Kang, B.S. Xu, H.D. Wang, C.B. Wang: Influence of Spraying Parameters on the Microstructure and Properties of Plasma-Sprayed Al2O3/40%TiO2 Coating. Proceedings of Heat Treatment and Surface Engineering. Vol. 50(2013), p. 169-176
