An Optimization Of Spray Coating Process To Minimize Coating Material Consumption

In cookware industry, interior spray coating process is an important process to protect the cookware product from the corrosion. In this process, the coating material “TEFLON” is used to spray to cover all the part’s surface. If a pot or pan is not too deep, one spray gun is enough to spray to cover all the interior surface. However, for the high side-wall pot, two spray guns must be used to spray at two different areas. The first area is at the bottom and its corner. The second area is around the top rim of the pot or pan. Note that the spray pattern is of fan-type. Thus, the sprayed area will be covered by the mentioned two spray guns. Consequently, the large amount of coating material is consumed to meet the dry film thickness (DFT) requirement. In this paper, the optimization of spray coating process is studied aiming to minimize the material usage. The experimental design technique is applied to determine the optimal spray coating parameters. The parameters used in this study include angle, spray time, fan pattern, and air pressure. The relationship models of coating material volume and DFT are presented in this research. The optimal parameters of the two spray guns are presented. INTRODUCTION The cookware production comprises many processes starting from blanking to packing. The coating process is an important step before assembly. It makes the products strong, long lasting, beautiful and protects the surfaces. In the coating process, the spray gun is normally used to spray the coating material to a target surface. Basically, the coating process consist of triplelayer coatings: primer, middle, and top layer. The primer is the first layer of coating that is applied to the substrate for interfacing between the surface of the part and the middle layer. The main purpose of the primer layer is a preparation of the coating surface to ensure smoothness and good adherent of coating material to the part surface. The middle layer is the layer on the primer layer. The coating material of the middle layer is an actual coating material whose function is to increase corrosive resistant of a part. Finally, the top layer will be applied make a part look shinny and increase the efficiency of the coating material in the middle layer. Considering all three layers of the coating process, the middle layer is the most important which affects the durability of the cookware product. As a consequence, it increases the service life of products, while the primer is the substrate of coating and the top layer is the decoration purpose. Therefore, this research will focus on the middle layer coating. In the spray coating process, a coating material waste due to an overspray and bounch of the coating material are the main issue. Thus, the objective of this research is to determine optimal spray parameters to minimize the coating material consumption due to DFT specification. (Winnicki et al. 2014 and From et al. 2011) study the optimal parameters of spray coating process to meet DFT but not the coating material consumption. There are few researchers studied the optimal parameters affecting both the coating material consumption and DFT research (Song et al. 2008 and S. Hong et al. 2014). (Luangkularb and Prombanpong 2014) present the optimal parameters concerning both of the coating material consumption and DFT for the single spray gun. However, this paper demonstrates a determination of optimal parameters considering both of material consumption and DFT for the two spray guns. The 26 cm in diameter of a pan is the specimen in this study. The spraying area is divided two positions i.e. bottom and corner, and top rim of a pan. Thus, the first gun sprays to cover the bottom area of the part whereas the second spray gun aims to cover the side wall and rim of the pan. METHODOLOGY The experimental design technique is applied to obtain the optimal spray parameters to minimize the coating material consumption and to attain DFT. The concerned parameters include gun angle, spray time, fan pattern and air pressure. The two levels used for the experiment is presented in Table 1. Thus, at each spray gun, the Proceedings 31st European Conference on Modelling and Simulation ©ECMS Zita Zoltay Paprika, Péter Horák, Kata Váradi, Péter Tamás Zwierczyk, Ágnes Vidovics-Dancs, János Péter Rádics (Editors) ISBN: 978-0-9932440-4-9/ ISBN: 978-0-9932440-5-6 (CD) experiment is designed with regard to the 2 factorial design with two replications. Thus, a total of 16 experiments will be performed for each spray gun. The material consumption and DFT are measured as the response and the gun angle, spray time, fan pattern and air pressure are recorded as the independent variables. The data obtained from the experiment will be then analyzed using the analysis of variance (ANOVA) technique to determine the effect of these four independent variables on material consumption and DFT. The required DFT is in a range of 7.5-12.5 μm. The predictive relationship model for these two responses is then constructed to find the optimal parameters for the spray coating process. Table 1: Data Used in the Experiment Factor First spray gun Second spray gun min max min Max Gun Angle (degree) 60 70 35 40 Spray time (sec) 1.2 1.4 1.2 1.4 Fan pattern (rev.) 315 360 315 360 Air pressure (bar) 2.5 3.0 2.5 3.0 RESULTS AND DISCUSSION The result obtained from the experiment will be analyzed using MINITAB software. The normality, constant variance and randomization tests are performed. The analysis of variance (ANOVA) is subsequently conducted to determine the effect of variables on the coating material consumption (MC) and average dry film thickness (DFT). In addition, the process optimization is performed to minimize material consumption and also yield the DFT ranging in the specification. The results of statistical analysis of first spray gun and second one are as follows. Coating Process of First Spray Gun Statistical Analysis The model adequacy checking of the material consumption data shows that the p-value for the normality probability test, equal variance and randomization test equal to 0.722 0.487 and 0.153 respectively. For the DFT data, the p-value of the normality probability test, equal variance, and randomization test are equal to 0.437, 0.637 and 0.249 respectively (Table 2). The result indicates that the MC and DFT data are in the normal distribution and there is no deviation in a variance of each test. ANOVA is conducted and the results are obtained for MC and DFT responses as shown in Table 3 and 4. The p-value which is less than 0.05 is used as the criterion to determine the significance of the response. Table 2: P-value of Model Adequacy Model adequacy checking P-Value Material consumption Dry film thickness Normal distribution 0.722 0.437 Equal variance 0.487 0.637 Randomization 0.153 0.249 Table 3: Result of ANOVA Material Consumption for First Spray Gun Term P-Value Gun angle 0.002 Spray time 0.000 Fan pattern 0.000 Air pressure 0.000 Gun angle*Fan pattern 0.004 Spray time* Fan pattern 0.020 Spray time*Air pressure 0.000 Fan pattern*Air pressure 0.003 S = 0.547875 PRESS = 12.8071 R-Sq = 86.84% R-Sq(pred) = 76.61% R-Sq(adj) = 83.00% Table 4: Result of ANOVA Dry Film Thickness for First Spray Gun Term P-Value Gun angle 0.033 Spray time 0.000 Fan pattern 0.001 Air pressure 0.000 Gun angle*Fan pattern 0.007 Spray time*Air pressure 0.023 Fan pattern*Air pressure 0.000 S = 0.547875 PRESS = 12.8071 R-Sq = 86.84% R-Sq(pred) = 76.61% R-Sq(adj) = 83.00% Effect of Spray Condition Analysis The effects of gun angle, spray time, fan pattern and air pressure on the material consumption and dry film thickness are shown in Figure 1 and 2. The results indicate that the increment of gun angle, fan pattern and air pressure will decrease the material consumption and dry film thickness. However, the decrement in the spray time will decrease the material consumption and dry film thickness.