Coating thickness control of spraying robot

Posted on Oct 20th, 2015

1 Introduction
With the development of the domestic passenger car industry, more and more mechanical spraying replaces manual work. In this trend, the proportion of robot spraying is also increasing. Such as the original body spray in the general use of the 6 - or 9 - station system, there have been replaced by the trend of robot spraying. Automotive body parts are also widely used in robot spraying, such as domestic car bumper spray in more than half of the output using the robot. The robot spraying not only keeps the manual spray to adapt to the complex surface, but also has the accuracy and repeatability. This paper will discuss the various factors that affect the final film thickness during the construction of the robot.

2 The significance of film thickness control
For the coating construction, the coating thickness is the coating process the most important control factors, its significance is:
      (1) to prevent defects due to improper film thickness caused by the coating. According to the author's experience, the appearance of the coating in the field of more than half of the appearance of defects due to improper control of the film thickness. Some common coating defects such as sagging, paint thin, exposed background and other direct control with the film thickness control out of control, there are some defects are also indirectly related to this. For example, the bumper coating of the first layer of sticky film thickness is not enough, will lead to the decline in adhesion of the entire coating, while the primer thickness of less than the requirements of the conductive effect will decline, which will cause the first Paint The use of electrostatic spraying when the transfer rate of paint decreased, resulting in insufficient paint.
      (2) to help adjust the appearance of indicators. Common paint film appearance indicators such as gloss, color, orange peel, DOI and so need to film thickness control as a basis. The above indicators are significantly affected by the film thickness, especially the thickness of the film thickness, therefore, in the entire coating quality control, the film thickness as the most important control factor is necessary.
      (3) cost control. In addition to the mass cost of film thickness control on the quality of the coating, about half of the major cost of painting is occupied by the coating. Precise film thickness control not only contributes to the stability of the coating quality, but also helps the paint to save. Statistics show that the use of the same equipment spraying, whether the precise control of the film thickness of the paint consumed by more than 25%.

At present in the domestic use of robots sprayed mainly in Japan Iwata or Mitsubishi robot, the introduction of these equipment earlier, poor control accuracy; new coating line commonly used ABB, FANUC, MOTOMAN, DURR and other multi-axis robot, in this paper is mainly Discuss based on ABB robots.

3 factors affecting the thickness of the film
        In the robot spray construction, the coating thickness can be calculated according to the following formula:
        Dry film thickness = (flow rate × coating volume solid content × paint transfer rate) / (gun speed × spray width)
        (1) flow rate, that is sprayed when the unit time from the gun mouth out of the paint volume. In robot spraying, this data is determined directly in the BRUSH (Brush) parameter table. Some old robots spray, flow control and robot system is not established in contact, can not be in the middle of a spray program at any time to change the flow. And most of the new robot flow control system directly by the robot's IPS system control, so that flow control more accurate and convenient.
        Such as ABB robot spray flow control, according to whether the flow control closed loop in two categories. First, the flow of high precision equipment using closed-loop control, closed-loop control, the commonly used equipment configuration in two ways:
        First, the use of metering gear pump, that is, the volume of the pump per revolution is constant, the robot 1PS system to control the speed of the metering pump to achieve quantitative paint, in such systems, the power of the paint from the gear pump pressure.
        Second, through the flow meter and throttle valve composed of closed circuit system to control, in this type of system, the pressure of the paint from the paint system, flow meter to get the flow signal to the robot IPS system and calibrated value for comparison, When the flow is biased, the signal back to the throttle, by changing the throttle opening and closing to adjust. The use of the second program to control the stability of the pressure on the paint requirements. The robotic spray system provides a multi-level method for correcting flow deviations.
        As in the ABB TR5002 spray robot, there are two ways to adjust the system deviation.
        This can be done by setting the coating properties in the ROBOT PARAMETRE in the robot, which allows different settings for each coating system, such as the ability to correct flow by the viscosity and relative density of the coating.
        Second, you can set the BRUSH in TEACH PADENT. If the BRUSH setting is 200 and the actual measured flow is 220, the BRUSH ratio can be set to 200/220 = 91%, so that the actual spray flow becomes 200. It should be noted that this setting is automatically restarted after the parameters automatically restored to 100%. The choice of flow range in spraying is mainly affected by two steps: metering pumps and atomizers. The bottleneck of these two devices determines the range of traffic that can be finally obtained.
        When the 6 cc metering pump is used, the rated flow rate is in the range of 0 to 900 mL / min because the controlled speed range of the pump is 0 to 150 r / min. At the same time, the atomizer also has a different flow cap. Such as ABB robotic swivel ROBOBEL625 upper limit of 400 mL / min, so in this equipment configuration, the maximum flow can only be 400 mL / min. Similarly, the low flow rate when using the metering pump pump speed is too slow, can not achieve the desired accuracy. Another factor to be concerned is that when the air is sprayed, the size of the flow affects the paint fog effect. According to the experience of the robot bumper bumper, air gun selection permit flow of 20% to 70% is more appropriate, spin cup selection of 20% to 100% flow.
      (2) Paint transfer rate, refers to the final attached to the surface of the paint coating from the spray gun in the proportion of the total flow, also known as painting rate. In fact, the history of the entire coating equipment can also be seen as a history of improving the rate of paint transfer, because it is the coating manufacturing costs and environmental protection are closely related to the two themes. The main parameters affecting the transfer rate include: atomizer type, electrostatic level, spray parameters, conductivity and so on. The use of what kind of spraying equipment is the first factor in determining the transfer rate, because the different equipment transfer rate has a significant difference.
        Some of the main atomizer transfer rate from small to large: ordinary air spray gun, electrostatic air spray gun, spin cup. They are in the spraying of metal or plastic parts when the paint transfer rate shown in Figure 1, Figure 2. Static electricity is the second largest factor affecting the transfer rate of paint, with or without static and electrostatic differences in the construction of the performance was very obvious. Because of the electrostatic spraying, the coating particles charge the coating to the workpiece adsorption, so need to reach the workpiece surface of the charged particles quickly transfer charge to maintain the workpiece surface and the pressure difference between the gun to ensure that the space between the two electric field strength for the paint transfer The rate is very critical. This adds another factor, that is, the ground state of the workpiece directly affects the transfer rate of the paint.
      This factor is particularly noticeable when spraying poorly conductive workpieces such as plastic bumpers. Experiments show that the use of ROBOBEL spray paint, the general grounding and good grounding product transfer rate difference of 10% to 20%. Such as ROBOBEL625 in the use of metal clip to spray the SVW2000 threshold bar of the measured transfer rate of about 70%, while the case of poor grounding only about 50%. For air spray guns, the impact of atomization air pressure on the transfer rate is also large, the atomization pressure is too large to cause the air sprayed to the spray surface after the rebound air flow increased, to prevent the follow-up small particles to reach the spray surface, resulting in decreased transfer rate The
      (3) solid content. Solid content parameters are usually the volume percentage and mass percentage of the two, the use of the film thickness is calculated using the volume percentage. In the construction of paint, often overlooked by this factor changes brought about by instability, due to other factors in the robot spray control, the impact of this factor than in the manual spray is even more prominent. The following factors may cause instability in the solids content of the coating during construction:
        ① different batches of paint solids content changes. The deviation caused by the solid content of the original paint control index is generally ± 2%, and the effect of this deviation is sometimes large. For example, a paint with a hiding power of 11μm has a solid content of (27 ± 2)%, and the deviation of the upper and lower limits is (29-25) / 25 = 16%. If the original 29% spray film thickness of 12μm, now 25% can only be sprayed to 12/29 × 25 = 10.3μm, obviously the film thickness is not enough. In this case, it is necessary to strictly monitor the solid content of the coating raw materials and require the supplier to give a smaller range of sensitive coatings.
        ② deviation of the original paint storage time is too long. General paint viscosity with the storage time will increase, and in the deployment of paint is often based on the viscosity of paint as a control index. This presents the change in the solid content of the coating before and after the original paint storage. such as. A paint in the storage after 6 months of viscosity increased by 10% (this rate is more normal, such as storage environment temperature is high, the viscosity increase even greater), in the same viscosity to adjust the need to join the thinner 6 months ago to increase, which reduces the deployment of a good coating of solid content, the other spray factors unchanged, the coating film thickness will be reduced.
        ③ non-standard paint operation and storage methods will lead to reduced solid content. Such as the original paint in the drum did not get fully stirred, high solids content of the ingredients remain in the bucket, which indirectly reduces the paint solids content. Also, the deployment of a good paint for a long time and the seal is not good lead to solvent evaporation, the solid content will increase.
      (4) take the gun speed. Take the ABB TR5002 spray robot as an example, the gun range is: 0 ~ 3000 mm / s. Production, the general selection of the speed of rotating cups for the 600 ~ 1000 mm / s, air gun selection speed of 800 ~ 1500 mm / s between. In theory, the spraying speed is inversely proportional to the film thickness, but in fact, because the spray parameters selected at different speeds indirectly affect the transfer rate, the lower gun speed is preferred under the premise of satisfying the beat. With regard to the effect of gunpowder on the transfer rate, it can be explained that the slow gun speed, the low flow rate of the coating used for the same film thickness, and the small amount of atomizing air are favorable for increasing the transfer rate. This is also true for rotary cups, which may be related to the time required for charge transfer. Tests show that the product is sprayed under the same conditions and the transfer rate is increased by 5% at a speed of 700 mm / s at 500 mm / s using a spin cup speed.
      (5) Spray width. Refers to the width of the paint sprayed by the atomizer on the surface to be sprayed. Spray width is affected by the following parameters: spray gun from the spray surface distance, atomization and fan parameters (air spray gun) or plastic air (spin cup). Single-head air spray gun spray shape is oval, rotating cup of the fog is round, double-headed spray gun according to the angle between the two nozzles, the shape is different, but basically also oval. From a spatial point of view, their fog is both conical or elliptical. So that when the spraying distance becomes shorter, the width of the ejection width is proportionally reduced. For air lances, the ratio of atomizing 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 width of the spray width which is indirectly affected by the adjustment of the atomization and the fan air value.

4 film thickness control
        For the robot coating construction, to ensure the stability of the production process is the need for priority control. The above five factors that affect the film thickness can be controlled and adjusted in different ways.
        (1) In order to ensure that the solid content of the paint is stable, the following measures are recommended:
        ① monitoring the different batches of solid content of the original paint, especially for film thickness sensitive coatings, such as high hiding power paint;
        ② shorten the storage time of the original paint, as much as possible the use of fresh paint;
        ③ to avoid the paint storage environment temperature is too high;
        ④ standard paint operation;
        ⑤ different season used in different diluent formulations, you can set the parameters through the robot IPS settings to avoid changes in traffic.
        (2) Spraying speed is adjusted during the preparation of the spray trajectory. Once it is determined, it will not change anymore. Only in some specific cases, if the paint is particularly poor, the spray gun is close to the upper limit. Low speed method is more effective.
        (3) Spray width is mainly determined at the time of programming, the latter part of the adjustment is mainly for some special plane, such as for the narrow plane using a small width can effectively save the paint. In the adjustment, it is necessary to pay attention to the other conditions that affect the spray quality due to the change of the spray, such as when the width of the coating reaches the sprayed surface when the width is adjusted by the spraying distance, the corresponding sagging or dry spraying may occur; When adjusted by atomizing the fan pressure, it may affect the aerosol effect of the coating.
        (4) Paint transfer rate is generally not as a factor in the adjustment of production, in the production of concern is due to changes in the transfer rate caused by spray quality accidents. Generally occurs due to the decline in the transfer rate caused by the thinning of the paint layer. Such as static spray gun due to equipment failure caused by the voltage drop caused by the transfer rate reduction.
        (5) flow adjustment is the most frequently used in the production of adjustment parameters. It should be noted that when adjusting the flow rate of the air gun, the values ​​of the atomization and fan pressure of the adjusted gas will change, which will also affect the transfer rate and, finally, the film thickness.
5 Conclusion
        Although the film thickness is affected only by the five factors discussed above, the robot's spray has strengthened our ability to control these factors, but because each factor in the middle is affected by multiple factors throughout the coating system, Therefore, in the actual production line construction, according to the actual situation, the design of effective construction parameters of the monitoring system to ensure that the film thickness controllable adjustable.