Abstract: This paper discusses the significance of paint film thickness control, the factors affecting paint film thickness, and the adjustment methods of corresponding parameters when using robots to spray paint. Keywords: Robot spraying; paint film thickness; thickness control 1 Introduction With the development of the domestic passenger car industry, more and more mechanical spraying has replaced manual operations. In this trend, the proportion of robot spraying is also increasing. For example, the 6-cup or 9-cup system that was commonly used in the body painting is also being replaced by robot spraying. Robot spraying is also widely used for the exterior parts of automobile bodies. For example, more than half of the production of domestic car bumper painting uses robots. Robot spraying maintains the adaptability of manual spraying to complex surfaces and has accuracy and repeatability. This paper will discuss the various factors affecting the final coating thickness during robot construction and provide some ideas for the control and adjustment of film in production. 2 Significance of film thickness control For coating construction, coating thickness is the most important control factor in the coating process. Its significance is: (1) Preventing coating defects caused by inappropriate film thickness. According to my experience, more than half of the appearance defects of coatings in on-site production are caused by improper control of paint film thickness. Some common coating defects such as sagging, thin paint layer, and exposed base color are directly related to the loss of control of film thickness, and some defects are also indirectly related to this. For example, if the first layer of primer for bumper spraying is not thick enough, it will lead to a decrease in the adhesion of the entire coating. At the same time, when the thickness of the primer does not meet the requirements, its conductivity will also decrease, which will cause a decrease in the transfer rate of paint when the first coat of color paint is electrostatically sprayed, and finally lead to insufficient color paint. (2) Helping to adjust appearance indicators. Common paint film appearance indicators such as gloss, color difference, orange peel, DOI, etc. all require film thickness control as the basis. The above indicators are significantly affected by film thickness, especially the topcoat film thickness. Therefore, in the whole coating quality control, it is necessary to regard film thickness as the most important control factor. (3) Cost control. In addition to the quality cost reflected by the impact of film thickness control on coating quality, about half of the main cost of coating is occupied by paint. Precise film thickness control not only helps stabilize coating quality but also saves paint. Statistics show that when using the same equipment for spraying, the amount of paint consumed differs by more than 25% depending on whether the film thickness is precisely controlled. Currently, the robot spraying used in China mainly consists of Japanese Iwata or Mitsubishi robots. These devices were introduced earlier and have poor control precision. New coating lines generally use multi-axis robots such as ABB, FANUC, MOTOMAN, and DURR. This paper mainly focuses on ABB robots for discussion. 3 Factors affecting film thickness In robot spraying, the coating thickness can be calculated using the following formula: Dry film thickness = (flow rate × paint volume solids content × paint transfer rate) / (gun speed × spray width) (1) Flow rate, which is the volume of paint flowing out of the spray gun nozzle per unit time during spraying. In robot spraying, this data is directly determined in the BRUSH (brush) parameter table. In some older robot spraying systems, the flow control is not linked to the robot system, and the flow rate cannot be changed at any time during a spraying program. Most new robots' flow control systems are directly controlled by the robot's IPS system, making flow control more precise and convenient. For example, in ABB robot painting flow control, there are two categories based on whether the flow control is closed-loop. One is closed-loop control for equipment with high flow accuracy. In closed-loop control, two common equipment configurations are used: One is using a metering gear pump, where the volume of paint obtained per revolution is constant. The robot's IPS system controls the pump's speed to achieve quantitative paint supply. In this system, the paint's power comes from the pressure generated by the gear pump. The second is a closed-loop system composed of a flow meter and a throttle valve. In this system, the paint pressure comes from the paint supply system. The flow signal obtained by the flow meter is transmitted to the robot's IPS system and compared with the calibrated value. When there is a deviation in flow, the signal is fed back to the throttle valve, and the valve's opening degree is adjusted. Using the second method requires high stability of the paint supply pressure. The robot painting system provides multi-stage methods for correcting flow deviations. For example, on the ABB TR5002 painting robot, there are two ways to adjust system deviations. First, the paint characteristics can be set in the ROBOT PARAMETRE section of the robot's settings. This allows for different settings for each paint system, such as correcting for the influence of paint viscosity and relative density on flow rate. Second, the BRUSH setting can be used in the TEACHPADENT. For example, if the BRUSH setting is 200 but the actual measured flow rate is 220, the BRUSH ratio can be set to 200/220 = 91%, thus making the actual spray flow rate 200. It should be noted that this setting automatically reverts to 100% after restarting. The selection of the flow rate range during spraying is mainly affected by two components: the metering pump and the atomizer. The bottlenecks of these two devices determine the final achievable flow rate range. For example, when using a 6cc metering pump, because the pump's controlled speed range is 0–150 r/min, its rated flow rate is 0–900 mL/min. Simultaneously, atomizers also have different upper limits for flow rate. For example, the upper limit of the ABB robot rotary cup ROBOBEL625 is 400mL/min, so in this equipment configuration, the maximum flow rate can only be 400mL/min. Similarly, if the flow rate is too low, the pump speed will be too slow when using a metering pump, and the required accuracy cannot be achieved. Another factor to pay attention to is that the flow rate affects the atomization effect of the paint when spraying with air. According to the experience of robot spraying of bumpers, it is more appropriate to use 20% to 70% of the permissible flow rate for air spray guns and 20% to 100% for rotary cups. (2) Paint transfer rate refers to the proportion of paint that finally adheres to the surface of the product to the total flow rate of paint sprayed from the spray gun, also known as the paint application rate. In fact, the development history of the entire coating equipment can also be regarded as a history of improving the paint transfer rate, because it is closely related to the two themes of coating manufacturing cost and environmental protection. The main parameters affecting the transfer rate include: atomizer type, electrostatic level, spraying parameters, conductivity, etc. The type of spraying equipment used is the first factor that determines the transfer rate, because different equipment has obvious differences in transfer rate. The transfer rates of some major atomizers, from lowest to highest, are: ordinary air spray gun, electrostatic air spray gun, and rotary cup. Their paint transfer rates when spraying metal or plastic parts are shown in Figures 1 and 2. Static electricity is the second largest factor affecting paint transfer rate, and the difference between the presence and level of static electricity is very noticeable during application. Because paint particles become charged during electrostatic spraying, causing the paint to adhere to the workpiece, it is crucial that the charged particles reaching the workpiece surface quickly transfer their charge to maintain the voltage difference between the workpiece surface and the spray gun. Ensuring the strength of the electric field between them is critical for paint transfer rate. This adds another factor: the grounding status of the workpiece directly affects the paint transfer rate. This factor is particularly evident when spraying workpieces with poor conductivity, such as plastic bumpers. Tests show that when using ROBOBEL to spray paint, the transfer rate differs by 10% to 20% between products with normal grounding and those with good grounding. For example, the measured transfer rate of ROBOBEL 625 when spraying SVW2000 door sill strips grounded with metal clips is around 70%, while it is only around 50% with poor grounding. For air spray guns, the atomizing air pressure has a significant impact on the transfer rate. Excessive atomizing pressure will cause the air to rebound after being sprayed onto the surface, increasing the airflow and preventing subsequent small paint particles from reaching the surface, thus reducing the transfer rate. (3) Solid content. Solid content parameters are usually available in two forms: volume percentage and mass percentage. Volume percentage is used when calculating film thickness. In coating construction, the instability caused by changes in this factor is often overlooked. Due to the precise control of other factors in robot spraying, the influence of this factor is more prominent than in manual spraying. The following factors may cause instability in the solid content of the paint during construction: ① Changes in the solid content of different batches of paint. The deviation caused by the solid content, which is the control index of the original paint, is generally within ±2%, and the impact of this deviation can sometimes be significant. For example, for a paint with a hiding power of 11μm, the solid content of the original paint is between 27%±2%, so the deviation of the upper and lower limits is (29～25)/25=16%. If the original 29% concentration resulted in a 12μm film thickness, a 25% concentration might only achieve 12/29×25=10.3μm, which is clearly insufficient. In this case, strict monitoring of the solids content of the paint raw materials is necessary, and suppliers should be required to provide a smaller range for sensitive paints. ② Deviations caused by prolonged storage of raw paint. Generally, paint viscosity increases with storage time, and viscosity is often used as a control indicator when mixing paints. This leads to changes in the solids content of the mixed paint before and after storage. For example, if a paint's viscosity increases by 10% after 6 months of storage (this is relatively normal; if the storage temperature is high, the increase will be even greater), more thinner needs to be added to adjust to the same viscosity than before 6 months ago. This reduces the solids content of the mixed paint, and assuming other spraying factors remain unchanged, the paint film thickness will decrease. ③ Improper paint mixing and storage methods can lead to a reduction in solids content. If the original paint is not fully stirred in the packaging bucket, the high solid content component remains in the bucket, which indirectly reduces the solid content of the paint. Also, if the prepared paint is left for too long and not properly sealed, the solid content will increase after the solvent evaporates. (4) Gun speed. Taking the ABB TR5002 spraying robot as an example, the gun speed range is 0～3000mm/s. In production, the speed of the rotary cup is generally selected as 600～1000mm/s, and the speed of the air spray gun is selected as 800～1500mm/s. Theoretically, the spraying speed is inversely proportional to the film thickness, but in reality, since the spraying parameters selected at different speeds will indirectly affect the transfer rate, the lower gun speed is preferred under the premise of meeting the spraying cycle. The effect of gun speed on the transfer rate can be explained as follows: the slower the gun speed, the lower the paint flow rate used to obtain the same film thickness, and the smaller the corresponding atomized air, which is beneficial to improving the transfer rate. The same applies to the rotary cup, which may be related to the time required for charge transfer. Tests show that, under the same conditions, when spraying products, the transfer rate is 5% higher when the rotary cup speed is 500 mm/s than when the speed is 700 mm/s. (5) Spray width. Refers to the width of the paint sprayed by the atomizer covering the sprayed surface. The spray width is affected by the following parameters: the distance between the spray gun and the sprayed surface, atomization and fan parameters (air spray gun) or shaping air (rotary cup). The spray shape of a single-head air spray gun is elliptical, the mist shape of a rotary cup is circular, and the shape of a double-head spray gun varies depending on the angle between the two nozzles, but it is basically elliptical. From a spatial perspective, their mist shapes are all conical or elliptical conical. Therefore, when the spraying distance becomes shorter, the spray width decreases proportionally. For air spray guns, the ratio of atomizing air pressure to fan air pressure has a linear effect on the spray width. Therefore, when modifying the corresponding spraying flow rate, it is necessary to consider the spray width indirectly affected by adjusting the atomization and fan air values. 4. Film Thickness Control For robotic coating construction, ensuring the stability of the production process is a priority. The above five factors affecting film thickness can be controlled and adjusted in different ways. (1) To ensure the stability of the solid content parameter of the paint, the following measures are recommended: ① Monitor the solid content of different batches of raw paint, especially for paints that are sensitive to film thickness, such as paints with high hiding power; ② Shorten the storage time of raw paint and use fresh paint as much as possible; ③ Avoid excessively high temperature in the paint storage environment; ④ Standardize paint mixing operations; ⑤ Different thinner formulations are used in different seasons, which can be adjusted by setting parameters through the robot IPS to avoid changes in flow rate. (2) The spraying speed is adjusted during the programming of the spraying trajectory. Once determined, it will not change much. Adjustments are only made in some specific situations, such as when spraying paints with particularly poor hiding power and the spray gun flow rate is close to the upper limit, it is more effective to reduce the speed. (3) The spray width is mainly determined during the programming. The later adjustments are mainly for some special planes, such as using a small width for narrow planes to effectively save paint. During the adjustment, attention should be paid to other factors affecting the spraying quality caused by changes in spray width. For example, when adjusting the width by spraying distance, the solvent content of the paint reaching the sprayed surface changes simultaneously, which may result in corresponding sagging or dry spraying. When adjusting by atomizing fan pressure, the atomization effect of the paint may be affected. (4) Paint transfer rate is generally not an adjustment factor in production. What needs to be paid attention to in production is the spraying quality accident caused by changes in transfer rate. It usually occurs when the paint layer becomes thinner due to a decrease in transfer rate. For example, the transfer rate decreases due to a voltage drop caused by equipment failure in the electrostatic spray gun. (5) Flow rate adjustment is the parameter most frequently used for adjustment in production. It should be noted that when adjusting the flow rate of the air spray gun, the values ​​of gas atomization and fan pressure will change accordingly, which will affect the transfer rate and ultimately the film thickness. 5 Conclusion Although film thickness is only affected by the five factors discussed above, and robotic spraying enhances our control over these factors, each of these factors is influenced by multiple factors within the entire coating system. Therefore, in actual production line construction, it is necessary to design an effective monitoring system for construction parameters based on the actual situation to ensure that film thickness is controllable and adjustable.