Eight basic concepts in automotive paint application

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In this post, I will share with you the eight basic concepts related to automotive paint application, explaining each one one by one! : Q+ J* j) e" Q+ n; f) {+ m; p4 n3 \( ]: x* Q& y' o6 A2 s - T$ |; a6 J2 ? 1. Atomization 8 p# M) X/ ? a' ~( p# v \% m% K 2. Atomization Energy (AE); 7 Z- L3 L1 V% a) Q! D/ N! _ a. Bell – Cup Edge Speed, Shaping Air; 3 {% Z$ t# P+ j0 X1 J' h b. Gun – Atomization Air & Fan Air;+ o# ?+ b" m' q! i3 f # t& a, Y5 f- G; G( j4 w3. Coatings Flow Rate (CF);2 S3 H/ K) m* D) H# a * m+ w0 @& H4 ?5 B4. Critical Atomization Ratios – the ratio of Atomization Energy to Coatings Flow Rate;* u5 p/ y! _) V$ X : V0 E% d( v3 L, I! Q+ u4 U3 B 5. Evaporation Rate Behavior;( Y" a5 Y R4 I0 D: C ! b2 A' R4 z/ \; 6. Foil Solids (Measured %NV);' J( A0 U/ H& v. W' P a. Foil Solids (%NV @ Spray) b. Foil Solids (%NV @ Film) c. Typical (%NV) Data Collection 7 z4 D6 \5 D5 o/ _- e- ~9 j 4 ^& v& j1 B9 Z' z( U 7. Atomized Droplet Size & Distribution;” J/ h6 h/ W! J+ d0 ~, O8 ^6 \ 8. Mass Film Deposition (MD) – the rate at which droplets are deposited; The first concept: Atomization – this occurs during the paint spraying process! Nebulization is the process of breaking down controlled liquid paint into fine droplets under the action of machinery or air pressure! In this process, this force not only helps to disperse the liquid coating but also determines the speed and direction at which each small liquid particle reaches the substrate. The properties of the coating formula, such as viscosity, rheological properties, and the combination of solvents, all affect the decomposition behavior of the coating, which in turn influences the atomization effect! Second concept: Atomization energy. Atomization energy is the mechanical force used to break up and disperse the coating fluid. This blasting process is a controllable one, with its operation regulated by adjusting various spraying parameter values. There are a total of four such parameter values: the spray gun and the swirl cup each have two parameters, which serve as the driving forces for the atomization process. Spray gun atomizer, Bell Atomizer, swirl cup atomizer. Atomization pressure: % e* q/ A8 u4 W2 o2 N; d. Swirl cup rotation speed: ! E$ z$ s9 v. J N, n3 Z% d. Air pressure at the fan surface: $ _3 P' d8 e* D$ d9 q* n5 L: X p, J. Pressure adjustment at the fan surface: 7 K9 X4 Y7 G0 a7 g% ~0 t. As for the specific effects of these variables: as the atomization pressure of the spray gun, the air pressure at the fan surface, and the rotation speed of the swirl cup increase, the particle size of the nebulization particles becomes smaller. The greater the fan pressure of the swirl cup, the larger the average particle size of the atomized particles becomes. Third concept: coating flow rate. The coating flow rate is easy to understand; it represents the amount of liquid that flows per unit of time, and it is the parameter that can be most easily controlled during the spraying process ； From a mechanical perspective, the flow rate of a coating is related only to the following two variables: flow pressure and backflow pressure ； Flow pressure refers to the force that drives fluid movement within a coating circulation system. The backflow pressure is the reaction force resulting from the flow driving force generated by the converging structure of the nozzle; these two forces together ensure the formation of a stable fluid pressure with slight fluctuations. Fourth concept: the ratio of flow rate to atomization energy. During the coating application process, we use certain parameters to describe it. For air spray guns, we often use values such as 17/26 and 200 cc/minute to specify the gun’s parameters; this means that the atomization pressure is 17 PSI, the air pressure in the fan area is 26 PSI, and the coating flow rate is 200 cc per minute. Using a standardized ratio notation, these same parameters can be expressed as a ratio of 1:1.53 ； 11.76. In other words, for every 11.76 CC of fluid, 1 psi of atomizing air is used, along with 1.53 psi of fan air. (The same applies to the impeller: 30k/28psi/100cc/min means that the impeller’s rotation speed is 30,000 RPM, the air pressure at the fan outlet is 28 psi, and the flow rate is 100 cc/min.) ； Expressed in terms of the denominator, it is: (1.07K over 1 over 3.57). For a fluid with a volume of 3.57 cc, 1.07 revolutions per second are used, along with 1 psi of air pressure. In the ratio of flow rate to atomization energy, if the weight of CF in the (AE)CF ratio is high, it will result in poor atomization performance and larger atomized particles! Conversely, if the CF weight is low, it will result in an uneven atomization effect: the atomized particles are small, the particle size distribution is uneven, and there is a possibility that a dense film cannot be formed. / E1 X L0 E5 h$ T1 m7 q$ Q The fifth concept: solvent evaporation rate. Evaporation rate refers to the speed at which the paint releases its volatile components into the surrounding air. The evaporation rate of each liquid varies, and this is the theoretical basis for selecting solvents in coatings in order to achieve the desired dryness level of the coating film. Everyone has a pretty good understanding of the principle behind the evaporation of solvents themselves! However, there is very little research on the effect of the atomization process on evaporation! The evaporation rate of the paint’s solvents is not only related to the paint formula, but also closely tied to the spraying environment! In other words, it is related to the ambient temperature, relative humidity, and the air flow rate over the wet film surface. _" U’ g. C) R! V3 D# B1 ^6 e There are two distinct phases of volatilization during the spraying process: 5 Q+ v( w” y W6 F2 AA. The volatilization rate during spraying refers to the volatilization of volatile organic compounds at the moment of spraying. Since it is difficult to obtain empirical values for specific volatile components in the laboratory or on the customer’s production line, it was agreed that the ratio of flow rate to atomization energy has a direct impact on the volatilization rate during spraying. In other words, the higher the atomization energy, the lower the paint flow rate, and thus the higher the volatilization rate during spraying. The same holds true in reverse. , J0 E, ?! p0 r B. The evaporation rate of the solvent on the wet film: refers to the evaporation of the solvent remaining in the atomized particles! In the wet film state, this portion of solvent must pass through the wet film and evaporate into the surrounding air! The ratio of flow rate to atomization energy has only an indirect effect on the evaporation rate of solvents on the wet film, as it is the atomization effect that determines the amount of solvent remaining in the coating droplets! However, the environment in the spray flash-drying area has a direct impact on it! Online coating solid content of foils: Collecting and analyzing the solid content of foils is a very effective way to quantify the volatilization amount during spraying as well as that of the wet film. Experimentally, we can observe that the value of the solid fraction collected immediately after spraying can accurately quantify the volatility of the volatile components generated during the intense spraying process. It is a measurable system with a result-oriented ratio of flow rate to atomization energy! Furthermore, the collection of the solid content of the foil film during the flash drying of the wet film enables the characterization of solvent evaporation on the surface of the wet film during film formation! . q3 H& b- |0 |' x5 R Under relatively ideal conditions, for a typical coating line that uses a cup gun/air spray gun to apply two coats of paint, the following 8 values of on-line solid content can be measured: r1. Cup gun air spray: measured immediately after spraying with the cup gun. 2. Solid content after flash drying between the spin cup and the air gun: collected before spraying with the air gun. : o/ p9 }& n7 k3 h( X 3. Solid content of the wet film layer obtained by spin-coating (calculated as Solid Content 2 – Solid Content 1). 4. Solid content sprayed by air gun: Collected immediately after spraying with an air gun (only the layer sprayed by the air gun) ； . Total solid content of the sprayed coating layer: Collected immediately after spraying with an air gun (only for the air gun coating layer); (including both the spin cup coating and the air gun coating layer. The foil used to collect the solid content is placed in place before the entire spraying process and removed after spraying with the air gun. This value represents the sum of the solid contents of 1, 3, and 4). 6. Solid content after complete flash drying: The solid content before pre-baking of water-based paints or before applying the varnish to solvent-based paints ； 7. Total solid content of the wet film after flash drying (calculated as = Solid content 6 – Solid content 5)! v+ U(GF: t 8. Solid content of water-based enamel after pre-baking: before applying the varnish ； Concept 7: Atomized droplet size and distribution. The size of atomized droplets as well as their distribution are very important factors that determine the quality of the coating layer formed on the substrate’s surface, as well as the rate at which the solvents in the wet film evaporate after deposition. For example, too many large-sized atomization particles result in a very wet, thick film coating, thereby making the film-forming process uncontrollable; on the other hand, too many small-sized atomization particles produce a thin and uneven coating surface. Therefore, we can say that when the aerosol of the painted material after atomization consists of small particles with a controllable and narrow particle size distribution, then such spraying is conducive to the formation of a coating. Moreover, the evaporation of the volatile components occurs continuously, and the entire spraying process can be effectively controlled. Eighth concept: Coating film accumulation. Film thickness refers, of course, to the thickness of the coating after it has cured; whereas the term coating film accumulation here denotes the thickness of the coating as a function of time, as well as how the coating is formed. The formation of a coating is a process in which atomized particles bond together to form a cohesive film layer. The properties of coatings formed from very small, dry atomized particles differ significantly from those of coatings formed from large, wet atomized particles; they have different solvent evaporation rates and different covering powers, resulting in distinct coating appearances. Even coatings with the same film thickness may not achieve identical performance levels due to differences in the way the film layers are stacked! 8 {; v0 P' m2 {6 c5 {! @0 ~% j The accumulation of the coating layer is usually measured solely by the thickness of the coating, but in reality, it is more important to understand how the coating accumulates than to measure its actual thickness. In Concept 6 regarding the solid content of the foil, a series of spraying steps can effectively explain the conditions under which each coating layer is formed. T& f, {8 W: B : \0 l' {4 J! ~: x$ A1 I used to use this material as training material for new colleagues. Personally, I think these concepts are very useful for understanding the manufacturing process, especially regarding ESTA electrostatic spraying technology. Since the original text is in English, I decided to translate part of it so that more people could access it; however, it turned out to be quite difficult, as there are many concepts for which I don’t know how to find appropriate expressions in Chinese. 9 y# u ^& M R