DE102019216959B4 - Powder coating process - Google Patents

Abstract

powder coating process,
wherein powder is removed as removed powder from a container (1) in which the powder (3) is present with an initial particle size distribution of powder particles,
the removed powder is sprayed onto a workpiece in a spraying process,
Powder that is not deposited on the workpiece during the spraying process is recovered as recovered powder and
the recovered powder is fed to the container (1),
characterized in that
when removing the powder (3) from the container (1), a particle size distribution of the removed powder based on a particle size distribution of the recovered powder and/or a mixture of the recovered powder with powder (3) remaining in the container (1) and/or a mixture of the recovered powder with powder (3) remaining in the container (1) and with powder supplied into the container (1), which has the initial particle size distribution, is adjusted by means of sifting so that a mixture of the recovered powder supplied to the container (1) with the powder (3) remaining in the container (1) has a particle size distribution that corresponds to the initial particle size distribution or that a particle size distribution of the powder deposited on the workpiece corresponds to the initial particle size distribution.

Description

The invention relates to a method for powder coating, wherein powder is removed from a container in which the powder with an initial particle size distribution is present, the removed powder is sprayed onto a workpiece, powder not deposited on the workpiece is recovered and the recovered powder is fed to the container, wherein when the powder is removed from the container, a particle size distribution of the removed powder is adjusted by means of sifting so that the mixture of the recovered powder with the powder remaining in the container maintains a substantially constant initial particle size distribution and/or that powder deposited on the workpiece has the initial particle size distribution. The application also describes a vortex chamber classifier that can be used in the process and a fluidization container with which the process can be carried out.

The present invention relates to the field of powder coating, which is used, for example, in powder painting. The ability to recover and reuse powder that has not been deposited on the object to be coated, the so-called overspray powder, is one of the main advantages over liquid coatings, such as liquid paint finishes. However, one of the problems with powder coating is that the properties of the powder in circulation differ from those of the fresh powder. In most cases, the fine powder, which does not deposit well on the workpiece, accumulates in the circulating powder and leads to a reduction in application efficiency. To ensure a constant layer thickness, the powder output from the spray guns must be increased manually. This manual adjustment not only leads to uncontrollable losses in quality, but also to high material losses due to overcoating. The application and cabin conditions, as well as the selected process for recovering the overspray powder and the powder material itself, play an important role.

The DE 10 2010 049 808 A1 relates to a method and a device for monitoring properties of fluidized powder. The DE 198 08 765 A1 relates to a powder coating system and method for feeding and mixing powder in a coating system. The US 6,361,605 B1 relates to a device for applying powder coating material to large objects. The DE 42 39 611 A1 relates to a fluidized bed classifier for classifying a bulk material mixture into coarse material and fine material.

1 shows the typical changes in the particle size spectrum during powder circulation. Application parameters such as the nozzle geometry of a powder gun used for coating, an arrangement of the powder gun in relation to the substrate, the powder mass flow, the process speed of the painted object or the powder gun, the high voltage, the flow conditions in the coating booth, the recovery method (for example filter or cyclone recovery) and the powder coating material itself.

In 1 In step 111, fresh powder is supplied with an initial particle size distribution, which is shown as a histogram in the diagram at 111. The particle size is plotted from left to right and the number of particles in the corresponding size interval is plotted upwards. The fresh powder with the particle size distribution 111 is fed to a powder paint supply of a powder coating process. The supplied powder is sprayed and has the size distribution shown hatched in the foreground in diagram 112. The original size distribution of the fresh powder is in the background of all diagrams 1 presented for reference.

Some of the sprayed particles are deposited on the workpiece. Larger particles are often separated better than smaller particles, so that the particle size distribution on the workpiece is slightly shifted towards larger particles compared to the fresh powder.

In the context of this document, the terms particle and particulate should be used synonymously.

Due to this shift, however, the overspray powder has a size distribution as shown, for example, in 114, in which the particle sizes are slightly shifted towards smaller particles than in the fresh powder 111. A portion 115 of the overspray powder is deposited in the system and can be used for recovery be carried out. The particles that precipitate tend to have a large particle size. Another part of the overspray 116 is sucked off. The extracted particles tend to be smaller in size than in the fresh powder. A rough understanding of the behavior of particle sizes shown in diagrams 113, 114, 115 and 116 is obtained by considering that very large particles are less responsive to electromagnetic interaction and airflow and are primarily subject to gravity, while very small particles are less responsive to gravity and electromagnetic interaction and mainly follow the air flow. Medium-sized particles are particularly suitable for electromagnetic separation and therefore tend to be deposited the most. Since it is mainly the small particles that follow the air flow, they dominate in the extracted overspray, while in the deposited overspray the very large particles dominate due to the main effect of gravity.

In the overspray recovery, the precipitated particles 115 and the suctioned-off particles 116 of the overspray are mixed. If a cyclone is used for recovery, this often leads to a loss of particularly small particles, as can be seen in diagram 118. The recovered overspray has a particle distribution such as that shown in Figure 117. Optionally, the overspray can be treated further, so that a size distribution 119 that deviates from 117 can result. But the effect is the same, even if the size distribution 117 is fed directly to the powder coating supply. In the powder coating supply 120, the recovered overspray powder 119 is mixed with the fresh powder 111. Since the recovered overspray powder 119 has a particle size distribution that is shifted toward smaller particle sizes, the mixture of the recycled particles with the fresh powder also has a particle size distribution that is shifted somewhat toward smaller particles.

The in 1 The process shown can lead to the particle size in the cycle shown constantly decreasing. This particularly has the effects described above on the quality of the coating.

In order to avoid the separation of the recovery powder and thus the circulation powder from drifting towards smaller particle sizes, the prior art sometimes even operates without recovery, i.e. at a loss, and the storage container that feeds the spray guns is kept exclusively with fresh powder. This results in a high loss of material and time and therefore considerable additional costs.

In 2016, 1.8 million tons of powder coating were produced worldwide with a production value of $7,885 million. The losses during powder application are estimated, among other things. due to drifting away in the circuit despite recovery to approx. 4%. In 2016, this corresponded to a loss of around 72,000 tonnes of powder coating worldwide with a production value of around $316 million. What cannot be recorded is the proportion of powder coating that is additionally applied for “safety reasons” to maintain the minimum required layer thickness and which most likely far exceeds the statistically recorded losses.

The proportion of powder coating on industrially painted surfaces was around 10% in Europe in 2015. Worldwide, the share of powder coatings in the total amount of VOC-free and low-VOC coatings was around 7-8% in 2016. Powder coatings still have considerable potential for improvement. The failure to exploit the potential is largely due to the insufficient production reliability of the powder coating process, especially when high demands are placed on decorative coating quality and layer thickness consistency. This is particularly true with frequent color changes. In addition to particle separation, contamination and agglomerate formation are other causes of high powder loss. Unfavorable process parameters can lead to an undesirable shift in the particle size distribution in the circuit and thus to changes in the application efficiency and the average layer thickness.

A simulation program was developed at the Fraunhofer Institute IPA with which the circulatory behavior of the powder could be simulated through simple powder spray tests. It has been shown that the powder does not refine itself arbitrarily in the cycle, but that after a certain number of cycles, an equilibrium state is asymptotically established between the separated and recovered powder, in which the powder particle size spectrum deposited on the workpiece ultimately corresponds to that of the fresh powder, i.e of the starting powder corresponds. This behavior is in 2 shown. Here the time is plotted in minutes on the horizontal axis and the median value of the particle size distribution in micrometers is plotted on the vertical axis. The top curve corresponds to the powder deposited on the workpiece, the middle curve to the sprayed powder and the bottom curve to the overspray. The solid curves are the results of the simulation, while the points indicate experimental values. It can be seen that the median values of the particle size distribution behave asymptotically, and the asymptote of the median value for the powder deposited on the workpiece corresponds to the value of the fresh powder, which is the γ-intercept of the middle curve. In the equilibrium state, the powder deposited on the workpiece corresponds to the fresh powder.

In order to achieve a moderate reduction in the particle size of the circulating powder and thus in the application efficiency, a sifted powder can be used in which the proportion of fine particles is reduced. However, this leads to material loss for the powder manufacturer. Another option is to use low-separation spray nozzles.

Another way to solve the problem described is to use powder storage containers that are as small as possible to feed spray guns. Keeping the fill level in the storage container as constant as possible through continuous metering leads to small fluctuations in the layer thickness on the workpiece, since only small but constant amounts of powder are in the circuit.

The problem here, however, is that the equilibrium state always only occurs after a certain amount of time; in previous technologies, at the earliest after about 25 minutes. Since the powder is changed frequently in production, for example when changing colors in powder coating, in practice a stationary state is almost never achieved in the circulation system. For example, colors are often changed every half hour. In this case, the next color change takes place before the stationary state is reached. The stationary state must then re-establish itself. In practice, the layer thickness must therefore be adjusted manually by changing the conveying air volume flow.

The object of the present invention is to provide a method for powder coating with which a stationary state of the particle size distribution of the powder deposited on the workpiece can be achieved immediately or at least after a significantly shorter time than in the prior art.

The object is achieved by the method according to claim 1. The respective dependent claims specify advantageous developments of the powder coating method according to the invention.

According to the invention, a process for powder coating is specified. The powder coating can be, for example, powder painting. Powder is removed from a container. The removed powder is hereinafter referred to as removed powder. The powder is present in the container with an initial particle size distribution of the powder particles. The powder is supplied with the initial particle size distribution by the manufacturer, for example.

If we are talking about a particle size distribution, this can refer to the median or average value of the particle sizes, or a discretized size distribution. In the case of a discretized size distribution, the range of particle sizes occurring is divided into at least two intervals and the number of particles or the proportion of particles in the total amount of all particles whose size lies in the corresponding interval is determined for each interval. For example, an optional subdivision into five classes is common, which are called fine grain for the smallest particles, medium fine grain for the next largest particles, medium grain for the next largest particles, medium coarse grain for the next largest particles and coarse grain for the largest particles.

According to the invention, the removed powder is sprayed onto a workpiece in a spraying process. For example, the powder can be sprayed onto the workpiece using a spray gun or spray nozzle. The process according to the invention can be carried out in all powder coating systems. For example, it can be used for electrostatic powder coating, where the particles are electrostatically charged and the workpiece is set to a potential so that the particles are electrostatically moved to the workpiece and adhere to the workpiece.

According to the invention, powder that was not deposited on the workpiece during the spraying process is recovered as so-called recovered powder or overspray powder. Such powder can be vacuumed powder and/or powder deposited in the apparatus. The recovered powder is fed to the container so that there is a mixture of the starting powder and the recovered powder in the container.

According to the invention, when the powder is removed from the container, a particle size distribution of the removed powder is set so that a mixture of the returned recovered powder with the powder remaining in the container has a size distribution of the particles that corresponds to the initial particle size distribution, or set so that a particle size distribution of the powder deposited on the workpiece corresponds to the initial particle size distribution. The particle size distribution of the mixture or the deposited powder can correspond to the initial particle size distribution if the average size of the particles is the same or substantially the same, if the median of the particle sizes of the mixture and the initial particles is or is substantially the same and/or if the proportions of the particles is the same or essentially the same in size classes into which the particles are divided. For comparison, the particle sizes can be divided, for example, into two or three classes, particularly preferably into five classes. It should be emphasized that two classes can be sufficient for carrying out the process, since even with a division into just two classes, i.e. a class of fine particles and a class of coarse particles, it can be seen when the particle size shifts towards small particles. The use of multiple classes allows more precise control of the process.

The particle size distribution of the removed powder is adjusted by sifting. The sifting is preferably a non-selective sifting that does not remove any particle sizes but merely shifts the particle size distribution. For example, if small particles are deposited less well than coarser particles in the specific coating process, the particle size distribution can be shifted towards smaller particles through screening.

In principle, any method that changes the size distribution of particles in the desired direction can be used as a screening method. In the simplest case, the particle size distribution of the removed powder can be adjusted using the sifting process on the basis of the particle size distribution of the recovered powder. If the particle size distribution of the recovered powder is known, it can be concluded how returning the powder to the container would change the particle size distribution in the container.

According to the invention, the particle size distribution of the removed powder is adjusted based on a particle size distribution of the recovered powder and/or a particle size distribution of a mixture of the recovered powder with powder remaining in the container and/or a particle size distribution of a mixture of the recovered powder with powder remaining in the container and with in the Powder supplied to the container, which has the initial particle size distribution. Powder with the initial particle size distribution can be supplied to the container to maintain the coating process. The particle size distribution can then take place in the finished mixture and/or before the fresh powder, which has the initial particle size distribution, is added. The recovered powder can also be measured before mixing with the powder remaining in the container. Which of these options is used can depend on where the given equipment structure allows a favorable measurement. Since all of the particle sizes mentioned are known or can be measured, they can be converted into one another.

If the powder separation situation is such that finer particles are separated in smaller proportions than coarser particles, screening can advantageously shift the particle size distribution towards finer particles. The overspray powder then has a finer particle distribution than the starting powder, while the proportion of larger particles in the storage container increases because finer particles are preferably removed by means of sifting. If the composition of the powder deposited on the workpiece corresponds to the composition of the starting powder, the particle size distribution of the mixture of the powder remaining in the container and that of the returned overspray powder essentially adds up to the starting size distribution. Since this state is achieved within one pass of the powder, the desired stable state can be set immediately or at the latest when the first overspray powder is returned.

In an advantageous embodiment of the invention, the sifting can take place in that the removed powder is removed from the container via a turbulence chamber which is at least partially arranged above the powder in the container and is open in the direction of the powder. If the powder is in the non-turbulent state, the upper boundary surface of the powder advantageously limits this turbulence chamber downwards. The turbulence chamber is advantageously closed on the sides and upwards with the exception of incoming and outgoing pipes.

In this embodiment of the invention, swirling gas, for example air, can be blown onto or into the powder in the swirling chamber, whereby the powder is whirled up. Tests have shown that blowing the fluidizing gas onto the powder as well as into the powder achieves the desired effect.

Advantageously, the fluidizing gas can be blown onto or into the powder via a nozzle which is directed onto the surface of the powder at rest or which is immersed in the powder.

Preferably, the powder can be removed from the storage container via a suction pipe that ends in the swirling chamber. The suction tube can then suck in powder that has been whirled up by the fluidizing gas. Since particles of different sizes react differently to the fluidizing gas, the fluidized powder has a different size distribution than the non-fluidized powder. For example, particles of smaller size can follow the fluidizing gas more easily and can therefore be whirled up more easily by it. The whirled up powder can therefore have a size distribution that is shifted towards smaller particle sizes than the non-fluidized powder in the container. By adjusting parameters of the swirl chamber, the particle size distribution of the powder can be adjusted.

To adjust the particle size distribution, in particular a volume of the turbulence chamber above the surface of the powder in the non-turbulence state can be changed and/or a distance within the turbulence chamber lying end of the suction tube from the surface of the powder in the non-fluid state, a speed of the fluidizing gas blown onto the powder and / or a distance of the nozzle from the surface of the powder in the non-fluid state. Since these parameters can be regulated based on the particle size distribution of the recovered powder, possibly mixed with powder and/or fresh powder remaining in the container, the particle size distribution of the powder deposited on the workpiece, or the particle size distribution of the removed powder, it is not necessary to specify specific values of these Calculate parameters. Rather, it is sufficient to empirically adapt the corresponding parameters to the specific particle size distributions. For example, if there is a finer particle size distribution in the powder deposited on the workpiece than in the starting powder, the particle size distribution of the removed powder can be shifted to larger sizes via the parameters of the swirling chamber mentioned. However, if there is a shift towards larger particles on the workpiece, the proportion of fine particles can be reduced by the aforementioned parameters of the swirl chamber.

The particle size distribution of the recovered powder can be used to adjust the screening of the removed powder. The particle size distribution of the recovered powder can advantageously be measured or simulated. A measurement of the particle size distribution is possible, for example, using differential pressure measurement or using a laser diffraction measurement method, as described, for example, in “Modern pharmaceutical technology, Cornelia M. Keck, Rainer H. Müller 2009, chap. 3.1 p. 50 Laser Diffractometry”. In the case of the laser diffraction measurement method, particle size distributions can be determined by measuring the angular dependence of the intensity of scattered light of a laser beam that penetrates a dry dispersed particle sample. Large particles scatter light at small angles relative to the laser beam, while small particles result in large scattering angles. In the case of differential pressure measurement, inexpensive measuring devices are available in which a hose can be immersed directly into the powder fluid. With particle refinement and the resulting poorer fluidizability up to the formation of channels in the powder fluid, the differential pressure decreases. In this way, the particle size distribution can be determined from the differential pressure.

A simulation of the particle size distribution of the deposited powder or the overspray powder can be done, for example, using a CAD model of the workpiece. In such a simulation, the deposition of powder with a known particle size distribution onto the modeled workpiece can be simulated and the particle size distribution of the overspray powder can be simulated.

In an advantageous embodiment of the invention, the particle size distribution of the powder present in the container can also be measured, with differential pressure measurement also being advantageous here, but a laser diffraction measuring method can also be used. The particle size distribution of the removed powder can then be adjusted if the measurement of the particle size distribution in the container indicates a change in the particle size distribution of the powder present in the container. The screening of the removed powder can also be regulated based on this size.

In an advantageous embodiment of the invention, the fluidizing gas can have a speed of greater than or equal to 3 m/s, preferably greater than or equal to 20 m/s, preferably greater than or equal to 40 m/s and/or less than or equal to 80 m/s, preferably less than or equal to 60 m/s, preferably less than or equal to 50 m/s, are blown onto the surface of the powder in the storage container.

Advantageously, a volume flow of the fluidizing gas that is blown onto the surface of the powder in the container can be greater than or equal to 0.5 m ³ /h, preferably greater than or equal to 1 m ³ /h, preferably 2 m ³ /h and/or less or equal to 5 m ³ /h, preferably less than or equal to 3 m ³ /h.

In an advantageous embodiment of the invention, the particle size distribution of the removed powder can be measured and, based on the measurement, the screening can be adjusted so that the particle size distribution of the removed powder assumes the desired distribution. Since the change in the size distribution as a result of the change in the parameters of the turbulence chamber is immediately apparent in the measurement, the parameter used to adjust the sifting can easily be adjusted so that the desired particle size distribution of the removed powder results in the measurement. Advantageously, the particle size distribution of the removed powder is then adjusted so that powder with the initial particle size distribution is deposited on the workpiece and/or that the spraying process results in a particle size distribution of the recovered powder, which, when mixed with the powder remaining in the container, leads to a particle size distribution of the mixture , which corresponds to the initial particle size distribution.

Advantageously, the screening of the removed powder does not sort out particle sizes from the mixture of particles, but only shifts the proportion of particle sizes. If the particle sizes are divided into size classes, the particle size distribution of the removed powder preferably contains particles of all size classes that are also present in the starting powder.

As described, the sifting can be based on the particle size distribution of the recovered powder and/or the powder deposited on the workpiece and/or the powder in the container and/or a mixture of the recovered powder with powder remaining in the container and/or a mixture of the recovered powder powder remaining in the container and with powder fed into the container and having the initial particle size distribution. The sifting can be done by setting the parameters of the vortex chamber sifter. In a simple case, the association between the measured particle size distribution and the corresponding parameter of the vortex classifier or between the measured particle size distribution and the particle size distribution of the removed powder can advantageously be carried out via a table, an empirical function or a neural network, the table being the empirical Function or the neural network links the particle size distribution of the recovered powder, possibly mixed with powder and/or fresh powder remaining in the container, of the separated powder and/or the powder in the container with the corresponding parameter of the screening or the particle size distribution of the removed powder.

In an advantageous embodiment of the invention, the container can be a fluidizing container. On the one hand, such a container can have a container and, on the other hand, a fluidizing base arranged in the container. A powder volume can be present in the container above the fluidizing base, in which powder to be fluidized can be arranged or is arranged. In such a fluidization container, air or another gas can be passed from below the fluidization base into the powder present above the fluidization base, whereby the powder is fluidized.

Advantageously, the fluidizing container can have a swirling chamber which is arranged above the powder and which is open to the powder volume. The fluidizing container can then, for example as part of the swirling chamber, have a suction pipe which has an open end within the swirling chamber above the powder volume. The powder volume can be the volume that is occupied by the powder when powder is present above the fluidization base and fluidization is taking place but no turbulence occurs.

The fluidizing container can also have a gas nozzle with which fluidizing gas can be blown onto the powder. For this purpose, the gas nozzle in the swirling chamber or in the powder volume can have a nozzle opening, which can be directed, for example, in the direction of the fluidizing base. In a simple case, the nozzle can be a hose, preferably with a constant inner diameter, one opening of which then functions as a nozzle. Such a hose can, for example, have an inner diameter of 8 mm, although other inner diameters are also possible. In an advantageous embodiment, a cap can be placed on the opening facing the powder, which has a plurality of nozzle openings, for example 8 nozzle openings, which face the powder. These nozzle openings can each have a diameter of 1 mm, for example.

Advantageously, a distance of the open end of the suction tube from the powder volume can be greater than or equal to 10 mm, preferably greater than or equal to 20 mm and/or less than or equal to 50 mm, preferably less than or equal to 30 mm.

The swirl chamber can form a swirl chamber classifier together with the suction pipe and the gas nozzle. A vortex chamber classifier is specified below, which can be advantageously used in the method described. Such a swirl chamber classifier can have a swirl chamber that is open to one side and can be arranged with its open side over powder in a container, for example a fluidizing container. The swirl chamber classifier can also have a nozzle with which swirling gas can be blown towards the open side. A distance of the nozzle from the open side of the swirl chamber can be advantageously varied if the particle size distribution is to be adjusted via the distance from the nozzle.

The swirl chamber classifier can also have a suction pipe that has an opening inside the swirl chamber and extends out of the swirl chamber through a wall. The suction tube can also be displaceable in the direction of its length or in the direction of the opening of the turbulence chamber if this distance is to be used to adjust the particle size distribution during sifting.

In an advantageous embodiment, the swirling chamber can be a dip tube that is open in the direction of the powder and has its open side can be immersed in the powder. Such a dip tube can, for example, have a cylindrical cross section. The said suction pipe can then, for example, run coaxially to the dip tube, but end within the dip tube. The swirling air can, for example, be introduced laterally through the dip tube into the dip tube and be guided in the dip tube to the nozzle, which can then be oriented, for example, parallel to the cylinder axis of the dip tube.

The method according to the invention can be carried out particularly advantageously with the fluidizing container described and/or the vortex chamber classifier described. The fluidizing container can advantageously contain a vortex chamber classifier as described.

The invention will be explained below by way of example using a few figures. The same reference numbers indicate the same or corresponding features. The features shown in the examples can also be implemented independently of the specific example and can be combined between the examples.

• 1 den Effekt der Pulververfeinerung in einem Kreislauf von Beschichtungspulver,
• 2 der Verlauf der Partikelgröße über die Zeit in einem solchen Kreislauf,
• 3A und 3B ein beispielhaftes Verhalten von Partikelgrößen in einem Kreislauf von Beschichtungspulver,
• 4 ein erstes Beispiel eines Fluidisierbehälters mit einem Wirbelkammersichter,
• 5 ein weiteres Beispiel eines Fluidisierbehälters mit einem Wirbelkammersichter, und
• 6 eine beispielhafte Anlage, mit der das erfindungsgemäße Verfahren ausführbar ist, und in der die Fluidisierbehälter der 4 und 5 zum Einsatz kommen können.

It shows

• 1 the effect of powder refinement in a coating powder cycle,
• 2 the course of particle size over time in such a cycle,
• 3A and 3B an exemplary behavior of particle sizes in a coating powder cycle,
• 4 a first example of a fluidization container with a vortex chamber classifier,
• 5 another example of a fluidization container with a vortex chamber classifier, and
• 6 an exemplary system with which the method according to the invention can be carried out, and in which the fluidization containers 4 and 5 can be used.

3A and 3B show an example of the behavior of particle size distributions in a coating process with powder recycling.

3A On the left shows the particle size distribution of a powder in a storage container. It is assumed that there is a powder quantity of 100 g in the storage container. The powder is classified here into five classes, fine grain, medium fine grain, medium grain, medium coarse grain and coarse grain. Of the 100 g in the storage container, 9 g should be fine grain, 24 g medium fine grain, 42 g medium grain, 21 g medium coarse grain and 4 g coarse grain.

It should now be assumed that 66 g, i.e. two thirds of the powder in the storage container, will be sprayed. Powder is removed from the storage container and viewed according to the invention. In the example shown, inspection shows that of the 66 g of sprayed powder, 8 g is fine grain powder, 18 g is medium fine grain, 28 g is medium grain, 10 g is medium coarse grain and 2.66 g is coarse grain. The sifting is carried out in such a way that the particle size distribution of the sprayed powder shifts towards the finer grain classes compared to the powder in the storage container. In particular, the proportion of the fine grain and medium fine grain classes becomes larger than in the storage container.

In the 3A The table shown shows to the right of the sprayed powder values how many grams of the powder of each class is applied to the workpiece and further to the right shows how many grams of the sprayed powder is overspray powder. In the column to the right of this, the remainder of the corresponding portion in the storage container is shown. It can be seen that fine grain and medium fine grain are deposited on the workpiece in smaller proportions than medium grain and medium coarse grain. Accordingly, the proportions of these size classes in the overspray powder are larger. Those portions of the classes that were not sprayed remain in the storage container, so that the sum of the amount in the remaining powder in the storage container plus the amount of sprayed powder results in the amount of the corresponding class in the original powder in the storage container.

3B On the left side again shows the remaining powder in the storage container like the column on the right 3A , then to the right of it shows the amount of overspray powder recovered, corresponding to the “Overspray” column in 3A , and to the right of it the composition of re-dosed powder, which has the particle size distribution of the starting powder. In the example shown, two thirds of its contents were removed from the storage container and it was assumed that half of these two thirds would be deposited on the workpiece and the other half would be recovered as overspray powder. If the overspray powder is now mixed with the remaining powder in the storage container and a third of the original volume is added to the storage container, the amount in the storage container will again be 100 g.

The right side of the 3B then shows the composition of the powder composed of the three parts described in the storage container after recovery. It can be seen that the proportions of the different particle sizes, i.e. the particle size distribution, match again with the original particle size distribution as shown on the left 3A is shown. This agreement results from the fact that the sprayed powder in 3A does not have the particle size distribution of the powder in the storage container, but rather a particle size distribution that is shifted towards finer grain sizes. This shift just compensates for the also in 3A The effect that can be seen is that a smaller proportion of finer powder is deposited on the workpiece than coarser powder.

It can be seen that the screening of the removed powder, which here is the sprayed powder, can be adjusted by determining the particle size distribution of the powder on the workpiece, of the overspray powder or of the residual powder in the storage container. The determination can be made via measurement or via simulation. Such a simulation can be carried out, for example, using a CAD model of the workpiece. The particle sizes can be measured, for example, using differential pressure measurement or using laser diffraction measurement methods.

4 shows an example of a fluidization container with a vortex canal sifter. The fluidizing container has a container 1 in which a fluidizing base 2 is arranged. The container 1 has a rectangular cross section in the plane of the figure and is open in a direction referred to here as the top. The fluidizing base 2 is arranged in the lower region of the container 1 and lies parallel to a lower side of the container 1 at a distance from it. In the example shown, powder 3 is arranged in a powder volume and is fluidized by passing gas, for example air, into the powder 3 from below through the fluidizing base. A speed of the fluidizing gas that is passed through the fluidizing base 2 can be, for example, 0.009 m/s. The value of the speed is optional and the speed can be adapted to the powder, the geometric conditions and the desired properties of the fluidized powder 3.

The fluidizing container also has a dip tube 4 as part of the swirl chamber classifier, the dip tube 4 serving here as a swirl chamber 4. The swirling chamber 4 here has a cylindrical shape and its lower end is immersed in the powder 3. The swirling chamber 4 is open at the bottom, so that the powder 3 enters the swirling chamber 4. The powder 3 is then present inside the turbulence chamber 4 in a non-turbulence state with a coplanar surface to the powder outside the turbulence chamber 4.

The swirl chamber classifier also has a tube 5 with which swirl gas can be introduced into the swirl chamber 4. The tube 5 enters the interior of the swirling chamber 4 in the radial direction through the wall, then runs downwards in a 90 ° curve and is opened downwards towards the fluidization base. If air is introduced into the tube 5, it exits downwards and hits the surface of the powder 3 in the fluidization container and in the swirling chamber. In the example shown, the fluidizing gas can be introduced into the pipe 5, for example, with a volume flow of 2 m ³ /h. This value can be varied in order to adjust the particle size composition of fluidized powder 6 inside the swirling chamber 4.

The swirling chamber 4 has a suction tube 7 in its interior, which enters the swirling chamber from above coaxially with the swirling chamber and is opened downwards, the opening of the suction pipe 7 being located inside the swirling chamber and above the surface of the powder 3 in the non-swirled state. Swirled powder 6 can be sucked off via the suction pipe 7 and fed via a line 8, for example, to an electrostatic powder spray gun 9. The particle size distribution of the removed powder, which is the powder flowing in the suction pipe 7 to the powder spray gun 9, can be adjusted using various parameters, for example the following parameters, with exemplary values of the parameters being given in brackets.

• v_FI: Fluidisierluftgeschwindigkeit [m/s](0,009)
• L_R: Länge Tauchrohr 7 [m](0,22)
• D_QR: Durchmesser Tauchrohr 7 [m](0,02)
• D_QR: Durchmesser Schlauch [m] (0,01)
• DpFl: Differenzdruck Pulver-Fluid
• im Fluidisierbehälter 1 [Pa](300-500)

Setting parameters:

• v _FI : Fluidizing air speed [m/s](0.009)
• L _R : Length of immersion tube 7 [m] (0.22)
• D _QR : Dip tube diameter 7 [m](0.02)
• D _QR : Hose diameter [m] (0.01)
• DpFl: Differential pressure powder-fluid
• in the fluidization tank 1 [Pa](300-500)

• Dps: Differenzdruck im Schlauch [Pa]
• Dp_R: Differenzdruck im Tauchrohr 7 [Pa]
• v_RL: Luftgeschwindigkeit Tauchrohr 7 [m/s]
• v_RP: Pulvergeschwindigkeit Tauchrohr 7 [m/s]
• v_W: Luftgeschwindigkeit in Pulverwolke in der Verwirbelungskammer 4 [m/s]
• V̇_R: Luftvolumenstrom Tauchrohr 7 saugseitig [m³/h]
• V̇_LS: Luftvolumenstrom im Pulver-Schlauch [m³/h]
• V̇_P: Pulverausstoß aus Sprühpistole 9 [g/min]

The 4 also shows further exemplary parameters of the fluidizing container 1 as follows:

• Dps: Differential pressure in the hose [Pa]
• Dp _R : Differential pressure in the dip tube 7 [Pa]
• v _RL : air speed in the dip tube 7 [m/s]
• v _RP : powder speed dip tube 7 [m/s]
• v _W : Air speed in powder cloud in the swirl chamber 4 [m/s]
• V̇ _R : Air volume flow, dip tube 7, suction side [m ³ /h]
• V̇ _LS : Air volume flow in the powder hose [m ³ /h]
• V̇ _P : Powder output from spray gun 9 [g/min]

5 shows another example of a fluidization container. The container 1, the fluidizing base 2 and powder 3 are as in 4 arranged as described. Im in 5 In the example shown, the swirl chamber 4 is designed as the upper region of the container 1, which is closed at the top here by means of a lid 10. The suction pipe 7 ends here again with its lower opening inside the swirling chamber 4 at a distance, here for example 20 to 30 mm, from the surface of the powder 3 in the non-swirled state. The suction pipe 7 exits through the cover 10 and leads the whirled up powder 6 via a further line 8 to the electrostatic spray gun 9.

Also in 5 a tube 5 is provided for supplying swirling air, which in turn ends in a nozzle inside the container 1, which is directed in the direction of the fluidizing base 2. Im in 5 In the example shown, the nozzle is immersed in the powder 3 in the non-turbulent state. Both the arrangement in 4 as well as the arrangement in 5 , that is, both a nozzle that ends above the powder 3 and a nozzle that dips into the powder 3, deliver suitable results.

The process parameters are in 5 Example shown corresponds to those in 4 chosen.

6 shows an example of a system in which the method according to the invention can be carried out, and in which, for example, the in 4 and 5 Fluidizing container shown can be used. A fluidizing container 1 with a turbulence chamber 4, into which a suction pipe 7 enters and into which turbulence air is blown via a pipe 5, serves as a storage container for powder. The swirl chamber 4, the suction pipe 7 and the air supply 5, which together form a swirl chamber classifier, can be used to adjust the particle size distribution of the removed powder that is passed through the pipe 8. The regulation can take place with a control device 12. All measurement results of the method according to the invention and all parameter settings can generally be carried out using the control unit 12.

For electrostatic application, the electrostatic spray gun 9 is at a potential that is applied by the control electronics 12 in the example shown here. A workpiece 13 to be coated is grounded, so that charged particles of the powder 6 move onto the workpiece 13 due to the electrostatic attraction caused by the electrostatic spray gun and adhere there. The coating takes place here in a coating chamber 14, from which overspray powder can be sucked downwards via a line 15. The extracted overspray powder can then be fed via line 15 to a separator system for powder recovery 16, in which the powder is separated. The separated powder is then fed back into the fluidizing container 1 via a line 17 from the separator system 16. Fresh powder, i.e. powder with the initial particle size distribution, is also metered into the fluidizing container 1 from a fresh powder supply 18. Im in 6 In the example shown, the fluidizing container 1 has a differential pressure measuring device 19 with which the density of the powder fluidized in the fluidizing container can be measured. Since this fluid density correlates with the particle size distribution, the particle size distribution of the powder 3 in the fluidizing container 1 can be deduced from the differential pressure measurement. If the particle size distribution of the powder 3 in the fluidizing container 1 changes, the particle size distribution of the extracted powder 6 can be changed by adjusting the parameters of the vortex chamber classifier, so that the particle size distribution in the fluidizing container 1 assumes the initial particle size distribution again. The measurement of the particle size in the fluidizing container 1 using the differential pressure measuring device 19 is just one example here. Alternatively, the particle size distribution of the removed powder 6 can also be measured and/or the particle size distribution of the overspray powder. The measuring methods described can in turn be used.

The present invention allows powder coatings to be carried out in an extremely space-saving manner with a stable powder circuit, thereby saving material and improving the quality of electrostatic powder coating. Color changes are not critical for powder coatings because the small sifter system is very easy to clean. Since it is not necessary to wait for the equilibrium to be established, it is possible to work with a stable particle size distribution of the powder to be applied without having to wait for the equilibrium state to be established.

The sifter principle described can also be used for non-selective sifting in the food sector, pharmaceutical sector, agricultural sector or in plastics production. In particular, the swirl chamber classifier described can be used here.

Claims (11)

Method for powder coating, wherein powder is removed as removed powder from a container (1) in which the powder (3) with an initial particle size distribution of powder particles is present, the removed powder is sprayed onto a workpiece in a spraying process, and is not deposited on the workpiece in the spraying process Powder is recovered as recovered powder and the recovered powder is fed to the container (1), characterized in that when the powder (3) is removed from the container (1), a particle size distribution of the removed powder is based on a particle size distribution of the recovered powder and / or a mixture of the recovered powder with powder (3) remaining in the container (1) and/or a mixture of the recovered powder with powder (3) remaining in the container (1) and with powder fed into the container (1), which determines the initial particle size distribution has, is adjusted by means of sifting so that a mixture of the recovered powder supplied to the container (1) with the powder (3) remaining in the container (1) has a size distribution of the particles which corresponds to the initial particle size distribution or that a particle size distribution of the on the workpiece deposited powder corresponds to the initial particle size distribution. Method according to the preceding claim, wherein the sifting takes place in that the removed powder is removed from the container (1) via a swirling chamber (4) which is at least partially arranged above the powder in the container (1) and is open in the direction of the powder, in which Fluidizing gas is blown onto or into the powder (3), and the powder is removed from the storage container via a suction pipe (7). which ends in the swirl chamber, wherein the particle size distribution of the removed powder is adjusted by using, as a parameter of the sifting, a volume of the swirling chamber (4) above the surface of the powder in the non-swirled state, and a distance of an end of the suction tube (7) located within the swirling chamber (4) from the surface of the powder in the non-fluidized state and/or a speed of the fluidizing gas blown onto the powder is adjusted. Method according to one of the preceding claims, wherein the particle size distribution of the recovered powder and/or the mixture of the recovered powder remains in the container (1). Powder (3) and / or the mixture of the recovered powder with powder (3) remaining in the container and with powder fed into the container (1) which has the initial particle size distribution is measured, preferably by means of a differential pressure measurement or a laser diffraction measurement method or a fluidizability of the powder (3) in the container (1). Procedure according to one of the Claims 1 or 2 , whereby the particle size distribution of the recovered powder is determined by simulating the powder deposition on a model, preferably a CAD model, of the workpiece. Method according to one of the preceding claims, wherein the particle size distribution of the powder (3) present in the container (1) is measured, preferably by means of differential pressure measurement, and the particle size distribution of the removed powder is adjusted if the measurement indicates a change in the particle size distribution of the powder in the container (1). present powder (3). Procedure according to one of the Claims 2 until 5 , wherein the fluidizing gas has a speed of greater than or equal to 3 m/s, preferably greater than or equal to 20 m/s, preferably greater than or equal to 30 m/s and/or less than or equal to 80 m/s, preferably less than or equal to 60 m /s, preferably less than or equal to 40 m/s and/or a volume flow of greater than or equal to 0.5 m ³ /h, preferably greater than or equal to 1 m ³ /h, preferably 2 m ³ /h and/or less than or equal to 5 m ³ /h, preferably less than or equal to 3 m ³ /h, is blown onto the powder. Method according to one of the preceding claims, wherein in order to adjust the particle size distribution of the removed powder, the particle size distribution of the removed powder is measured, and the sifting is set so that the particle size distribution of the removed powder is such that powder with the initial particle size distribution is deposited on the workpiece or After the spraying process, a particle size distribution of the recovered powder results which, when mixed with the powder remaining in the container, leads to a particle size distribution of the mixture which corresponds to the initial particle size distribution. Method according to one of the preceding claims, wherein the particle size distribution of the removed powder is shifted towards smaller particles by the sifting compared to the initial particle size distribution. Method according to one of the preceding claims, wherein a size range between a largest and a smallest particle size occurring in the initial particle size distribution is divided into size classes, which are size intervals of non-vanishing width, the particle size distribution of the removed powder containing particles of all size classes after screening. Method according to one of the preceding claims, wherein the screening is adjusted based on the particle size distribution of the returned powder and / or the separated powder and / or the powder in the container via a table, an empirical function or a neural network, the table being the empirical Function or the neural network links the particle size distribution of the returned powder and / or the separated powder and / or the powder (3) in the container with at least one parameter of the screening. Method according to one of the preceding claims, wherein the container (1) is a fluidizing container, comprising a container (1), a fluidizing base (2) arranged in the container, and a powder volume located in the container (1) in which powder is to be fluidized (2) can be arranged, further comprising a swirling chamber (4) which is arranged above the powder volume and is open to the powder volume, further comprising a suction tube (7) which has an open end within the swirling chamber (4) above the powder volume, and a gas nozzle with a nozzle opening arranged in the swirling chamber (4) or around the powder volume.