Process analysis: robot spray paint film thickness control

With the development of domestic passenger car
industry, more and more mechanical spraying has replaced manual work. In this trend, the proportion of robot spraying is also increasing. Such as the original common use in body spraying 6 cup station or 9 cup station system, there is also a tendency to be replaced by robot spray. Robot spraying is also heavily used in car body overrance, as in more than half of the production of domestic car bumper spraying. Robot spraying not only maintains the adaptation of manual spray to complex surfaces, but also has precision and repeatability. This paper will discuss various factors that affect the thickness of the final coating film during the construction of the robot, and provide some ideas for the control adjustment of the film in production.The significance of film thickness control For coating construction, coating thickness is the most important control factor in the coating process, and its significance lies in: (1) to prevent coating defects caused by inappropriate film thickness. Experience has shown that more than half of the appearance defects of coatings in field production are caused by improper thickness control of lacquer coatings. Some common coating defects such as flow hanging, thin paint layer, dew color and so on directly related to membrane thickness control out of control, and some defects are also indirectly related to this. For example, the first layer of the bumper sprayed with a viscose primer film thickness is not enough, will lead to a decrease in the adhesion of the entire coating, while the film thickness of the primer does not meet the requirements of its conductive effect will also be reduced, which will cause the first color paint when the use of electrostasis spray paint paint transfer rate decreased, and finally lead to insufficient color paint.(2) to help adjust the appearance indicator. Common paint film appearance indicators such as gloss, color difference, orange peel, DOI, etc. need to take film thickness control as the basis. The above indicators are obviously affected by the film thickness, especially the paint film thickness, therefore, in the whole coating quality control, the film thickness as the most important control factor is necessary.(3) cost control. In addition to the quality cost reflected in the impact of film thickness control on coating quality, about half of the main cost of coating is occupied by coatings. Accurate film thickness control not only contributes to the stability of coating quality, but also contributes to the savings of coatings. Statistics show that the use of the same equipment spraying, whether to accurately control the thickness of the film its consumption of paint difference of more than 25%. At present, the use of robot spraying in China is mainly Japanese Iwata or Mitsubishi robots, these devices introduced earlier, poor control accuracy; The new coating line is generally used ABB, FANUC, MOTOMAN, DURR and other multi-axis robots, in this paper is mainly based on ABB robots to discussDry film thickness ( flow × coating volume solid content × paint transfer rate) / (walking gun speed × spray width) (1) flow, i.e. the volume of paint flowing from the mouth of the gun at the time of spraying. In robot spraying, this data is determined directly in the BRUSH parameter table. In some older robot sprays, flow control is not linked to the robot system and cannot change the flow at any time in the middle of a spray program. Most of the new robot's flow control system is directly controlled by the robot's IPS system, making the flow control more accurate and convenient. For example, in the flow control of ABB robot spraying, there are two types according to whether the flow control is closed.First, the flow accuracy of high equipment using closed-loop control, in closed-loop control, commonly used equipment configuration has two:First, the use of metering gear pump, that is, the volume of the pump per turn is constant, the robot 1PS system control the speed of the metering pump to achieve quantitative paint, in this system, the paint power from the pressure generated by the gear pump.2 is controlled by a closed-circuit system consisting of a flow meter and a throttle valve, in which the pressure of the coating comes from the paint supply system, and the flow signal obtained by the flow meter is transmitted to the robot IPS system for comparison with the measured value, and when the flow is biased, the signal is fed back to the throttle valve, which is adjusted by changing the throttle valve opening and closing. The stability of the paint supply pressure is required when controlled using the second scheme. The robot spraying system provides a multi-stage method for correcting flow deviations. For example, on the ABBTR5002 spray robot, there are two ways to adjust the deviation of the system.First, coating characteristic settings in ROBOTPARAMETRE in robot setupallow for different settings for each coating system, such as correcting flow to be affected by the viscosity and relative density of the coating.the second is through the BRUS setup in TEACHPADENT. If the BRUSH is set to 200 and the actual measured flow is 220, the BRUSH ratio can be set to 200/220 x 91%, so that the actual spray flow becomes 200. It is important to note that this setting automatically reverts to 100% after powering on. The selection of flow range in spraying is mainly affected by two links: metering pump and atomizer. Bottlenecks in both devices determine the range of traffic that can be obtained in the end. For example, when using a 6cc metering pump, because the pump's controlled speed range is 0 to 150r/min, it is rated at 0 to 900mL/min. At the same time, the atomizer also has different flow limits. For example, the upper limit of the ABB robot screw cup ROBOBEL625 is 400mL/min, so in this equipment configuration, the maximum flow rate can only be 400mL/min. Similarly, the low flow rate of the pump is too slow to achieve the accuracy it deserves when using the metering pump. Another factor to be concerned about is that when air is sprayed, the size of the flow affects the atomization effect of the coating. According to the experience of robot spray bumper, 20% to 70% of the approved flow rate of air spray gun selection is more suitable, and 20% to 100% flow rate is selected for the rotary cup.
(2) paint transfer rate, which refers to the proportion of the total flow of paint from the spray gun that is eventually attached to the surface of the product, also known as the paint rate. In fact, the whole history of coating equipment can also be regarded as a history of increasing paint transfer rate, because it is closely related to the two topics of coating manufacturing costs and environmental protection. The main parameters affecting the transfer rate include: type of atomizer, electrostasis, spray parameters, conductivity and so on. What kind of spraying equipment is used is the first factor that determines the transfer rate, because there are obvious differences between different equipment transfer rates. Some of the main atomizer transfer rate from small to large: ordinary air spray gun, static air spray gun, rotary cup. They are seen in the paint transfer rate when spraying metal or plastic parts.electrostasis is the second most important factor affecting the transfer rate of coatings, the difference between static electricity and electrostasis is very obvious in construction. Because when electrostasis is applied, the paint particles are charged causing the coating to adsorption to the work piece, so it is necessary to reach the charge particles on the surface of the work piece first to quickly transfer the charge, maintain the voltage difference between the surface of the work part and the spray gun, and ensure that the spatial electric field strength between the two is critical to the coating transfer rate. This adds another factor, that is, the ground state of the work piece directly affects the transfer rate of the coating.this factor is particularly evident when spraying work parts that are not electrically conductive, such as plastic bumpers. Experiments show that when US ROBOBEL is sprayed with color paint, the transfer rate of generally grounded and well grounded products varies by 10% to 20%. For example, ROBOBEL625 in spraying using metal clamp ground SVW2000 threshold bar of the measured transfer rate of about 70%, and poor grounding in the case of only about 50%. For the air spray gun, the atomized air pressure on the transfer rate is also a large impact, atomization pressure over the General Assembly caused the air sprayed to the spray surface after the rebound air flow increased, preventing the subsequent small paint particles to reach the sprayed surface, resulting in a decrease in the transfer rate.(3) solid content. Solid content parameters usually have volume percentage and mass percentage, and the volume percentage is used when calculating film thickness. In coating construction, the instability caused by this change is often overlooked, which is more prominent than in manual spraying due to the precise control of other factors in robot spraying. Several factors may cause instability in the solid content of the coating during construction:(1) changes in the solid content of different batches of paint.the solid content of the original paint control index is generally ±2%, the effect of this deviation is sometimes very large. For example, a color paint with a covering force of 11 m, the solid content of the original paint is between 27% ±2%, so that the deviation of the high and low limit is (29 to 25)/25 x 16%. If the original use of 29% sprayed film thickness at 12 m, now 25% can only spray to 12/29×25 x 10.3 m, obviously the film thickness is not enough.this situation, the solid content index of coating raw materials needs to be strictly monitored and suppliers are required to give a smaller range of sensitive coatings.(2) the deviation caused by the long storage time of the original paint.the viscosity of the general coating will increase with the extension of storage time, and the deployment of coatings is often based on the viscosity of the coating as a control indicator. This shows the original paint before and after storage of the paint solid content changes. Like what. A coating after 6 months of storage viscosity increased by 10% (this range is more normal, such as storage ambient temperature is high, viscosity increase is even larger), in the adjustment to the same viscosity need to add more diluent than 6 months ago, which reduces the solid content of the well-provisioned coating, other spray factors remain unchanged, the coating film thickness will be reduced.(3) irregular paint operation and storage will result in a reduction in solid content.if the original paint is not fully stirred in the barrel, the solid content of the components remain in the barrel, which indirectly reduces the solid content of the coating. In addition, the solid content will increase after the coating has been placed for too long and the seal is not good enough to cause the solvent to evaporate.(4) walking gun speed.ABBTR5002 spray robot as an example, the gun speed range is: 0 to 3000mm/s. In production, the general rotary cup selection speed is 600 to 1000mm/s, air gun selection speed is between 800 to 1500mm/s. In theory, the spray speed is inversely related to the thickness of the film, but in fact, because the spray parameters chosen at different speeds willindirectly affect the transfer rate, so under the premise of meeting the spray beat, the lower gun speed is preferred. Regarding the effect of gun speed on transfer rate, it can be explained that the gun speed is slow, the amount of paint used to obtain the same film thickness is low, and the corresponding atomized air is small, which is beneficial to improve the transfer rate. The same is true for rotary cups, which may be related to the time it takes for the charge to shift. Tests showed a 5% increase in the transfer rate at 500mm/s compared to 700mm/s when spraying the product under the same conditions.
(5) spray width.refers to the width of the coating sprayed by the atomizer on the spray surface. The spray width is affected by the distance of the spray gun from the sprayed surface, the atomization and fan parameters (air spray gun) or the plastic air (cyclone cup). The spray shape of the single-headed air gun is oval, the fog shape of the rotary cup is round, and the two-headed spray gun is different in shape according to the angle of the two nozzles, but it is also basically oval. From a spatial point of view, their fog shapes are conical or elliptical. Therefore, when the spray distance becomes shorter, the spray width is proportionally reduced. For air spray guns, the ratio of atomized air pressure to fan air pressure has a linear effect on the width of the spray. Therefore, when modifying the corresponding spray flow, it is necessary to consider the spray width indirectly affected by the adjustment of atomization and fan air values.the film thickness for robot coating construction, ensuring the stability of the production process is a priority control. The above 5 factors affecting film thickness can be controlled and adjusted in different ways.(1) In order to ensure the stability of the solid content parameters of coatings, the following measures are recommended: (1) monitoring the solid content of different batches of raw paint, especially for coatings sensitive to film thickness, such as color paint with high cover;(2) to shorten the storage time of raw paint, as far as possible, the use of fresh paint;(3) to avoid the coating storage ambient temperature is too high;(4) standard paint operation;(5) different thinner formulations used in different seasons, the parameters can be adjusted by robot IPS settings to avoid traffic changes. (2) spray speed is adjusted in the process of spraying trajectory programming, once determined, it will basically not change, only in some specific cases to adjust, such as spraying paint with particularly poor masking force and spray gun flow close to the upper limit of the use of the method of speed adjustment is more effective. (3) spray width is mainly determined in the programming, the later adjustment is mainly for some special planes, such as the use of narrow plane small width can effectively save paint. Adjustments need to pay attention to other effects due to the change in spray width of the spray quality, such as when adjusting the width through the spray distance, the solvent content of the coating to the spray surface at the same time changes, the corresponding flow hanging or dry spray may occur; When adjusted by atomizing fan pressure, the atomization effect of the coating may be affected. (4) paint transfer rate is generally not a factor in the production adjustment, in the production need to be concerned about the transfer rate changes caused by spray quality accidents. Most commonly occur when the paint layer becomes thinner due to a decrease in the transfer rate. For example, the electrostasis gun due to equipment failure caused by the voltage drop caused by the reduction of the transfer rate. (5) flow is the most frequently used parameter in production. It is important to note that when adjusting the flow of the air gun, the value of the atomization and fan pressure of the gas adjusted together changes, which affects both the transfer rate and, ultimately, the film thickness. although the thickness of cintation paint film is only affected by the five factors discussed above, robot spraying makes us more able to control these factors, but because each of these factors is affected by the whole coating system, so in the actual production line construction, it is necessary to design an effective monitoring system of construction parameters according to the actual situation, to ensure that the film thickness can be controlled