Patent Publication Number: US-2006005766-A1

Title: Ultrasonic standing wave spraying arangement

Description:
The invention relates to an ultrasonic standing-wave atomizer arrangement for producing a paint spray mist for painting a workpiece, with a sonotrode, with a component arranged lying opposite the sonotrode, a standing ultrasonic field being formed in the intermediate space between the sonotrode and the component in the case of operation, and with a paint-feeding device, by means of which paint can be fed into the vicinity of a maximum of the sound particle velocity of the ultrasonic field.  
      For painting workpieces, in particular in mass painting as frequently encountered in the automobile industry, at present the generally known high-rotation atomizers are preferably used. In the case of high-rotation atomization, the paint is passed through the interior of a metal bell and in this way reaches the front side of the latter, facing the workpiece. The metal bell is usually driven by a compressed-air turbine and rotates at up to 80 000 revolutions per minute. The centrifugal forces acting in this case then cause the paint to reach the front-side edge of the bell, to break away there in fine droplets. This achieves the effect that the droplet size of the paint spray mist required for adequate quality of a coat of paint lies in the range from 10 μm to 60 μm.  
      Considerations of the fundamentals which have become generally known indicate that, in principle, paint can also be atomized by means of ultrasonic standing-wave atomization. Following these considerations of the principles concerned, however, average droplet sizes during atomization of between 100 μm. and 200 μm. have been measured, with some instances of still larger drops occurring. However, large drops of this kind adversely influence the quality of the coat of paint in such a way as to make use in painting technology unattractive.  
      It has been proposed how an ultrasonic standing-wave atomizer arrangement for producing a paint spray mist for painting a workpiece can be designed to achieve smaller droplet sizes. For example, specific designs of the sonotrode and of the component, shut-off elements or multi-piece rings, which improve the quality of the paint spray mist produced and consequently allow comparatively small droplet sizes to be achieved, have become known. A disadvantage is that only comparatively low delivery rates of paint can be atomized by the arrangement that has become known.  
      On the basis of this prior art, it is the object of the invention to provide an ultrasonic standing-wave atomizer arrangement for producing a paint spray mist with which it is possible to increase the atomized amount of paint, that is the rate of paint, and at the same time to maintain a selected range of droplet sizes occurring.  
      This object is achieved by the ultrasonic standing-wave atomizer arrangement according to the invention for producing a paint spray mist for painting a workpiece with the features specifed in Claim  1 .  
      The ultrasonic standing-wave atomizer arrangement according to the invention, of the type stated at the beginning, accordingly has a paint-feeding device, which has in the region of the standing ultrasonic field at least two pieces of pipe for discharging paint. Moreover, at least two of the pieces of pipe are arranged in the region of a selected maximum of the sound particle velocity of the standing ultrasonic field. According to the invention, it is therefore provided that a selected maximum of the sound particle velocity of a standing ultrasonic wave is used for the purpose of atomizing a comparatively large amount of paint into paint droplets. This is so because it has been found that, in particular in the case of ultrasonic standing-wave atomizer arrangements of a simple construction, a selected maximum of the sound particle velocity is often particularly well formed in the standing ultrasonic field, for example in the case of standing ultrasonic fields with an uneven number of sound particle velocity antinodes, the middle sound particle velocity antinode. That is to say that this maximum is particularly stable, with a comparatively high sound particle velocity. These particularly good atomizing properties of the selected maximum are used according to the invention for increasing the amount of paint to be atomized or the flow of paint through the paint-feeding device and it is provided that at least two pieces of pipe for discharging paint are arranged in the region of the selected maximum. Consequently, the amount of paint to be atomized can be increased in an advantageous way. An advantageous design of the ultrasonic standing-wave atomizer arrangement according to the invention is achieved if the component is a further sonotrode. In this way, the atomizing capability of the standing ultrasonic field can be increased. Moreover, a more stable ultrasonic field can be formed in this way.  
      A further advantageous refinement of the subject-matter of the invention provides that the distance between the pieces of pipe in the region of the selected maximum is so great that sheets of paint that are separate from one another are formed for each piece of pipe. For technical vibration-related reasons, a sheet of paint is respectively formed in any case on the pieces of pipe, extending from the paint outlet point. If the distance between the pieces of pipe has been chosen to be great enough that the sheets of paint can form separately from one another without influencing one another, the region in which droplets of different sheets of paint collide and in this way can recombine to form larger droplets is avoided in any case. The quality of the paint spray mist is improved with the proposed arrangement.  
      It is particularly advantageous if the paint outlet openings of the at least two pieces of pipe in the region of the selected maximum of the sound particle velocity of a standing ultrasonic wave are arranged on a straight line, and if the straight line is perpendicular to an imaginary centre line which passes through the centroids of the opposing sound faces of the sonotrode and of the component. In the case of an arrangement of this type, the distance between the paint outlet points on the pieces of pipe and the sonotrode or the component are respectively of approximately the same size. A particularly advantageous position, seen in the X direction, is achieved in the region of the maximum of the sound particle velocity.  
      The advantage mentioned above may also be achieved if three pieces of pipe are arranged in the region of a selected maximum of the sound particle velocity of a standing ultrasonic wave, and if these pieces of pipe or their paint outlet openings are arranged in a triangle. An arrangement in an equilateral triangle is particularly favourable. It is a further improvement if that area which is determined by the triangle is perpendicular to an imaginary centre line which passes through the centroids of the opposing sound faces of the sonotrode and of the component. In this case, too, the effect is in turn achieved that, seen in the X direction, the paint outlet openings are situated in the region of the maximum of the sound particle velocity.  
      It has also been found that the atomizing operation or the atomizing rate can be improved by choosing the specific maximum such that it is closer to the sonotrode than to the component. There is then the possibility of the so-called capillary wave turbulence effect, that is to say the effect which keeps the paint droplets away from the sonotrode as a result of the vibrations of the latter and in this way assists the atomization process.  
      Further advantageous refinements of the subject-matter of the invention can be taken from the dependent claims. 
    
    
      The invention, its advantages and further improvements of the invention are explained and described in more detail on the basis of the example embodiments specified in the drawings, in which:  
       FIG. 1  shows a first ultrasonic standing-wave atomizer arrangement,  
       FIG. 2  shows a second ultrasonic standing-wave atomizer arrangement,  
       FIG. 3  shows a third ultrasonic standing-wave atomizer arrangement,  
       FIG. 4  shows a fourth ultrasonic standing-wave atomizer arrangement,  
       FIG. 5  shows a fifth ultrasonic standing-wave atomizer arrangement,  
       FIG. 6  shows a sixth ultrasonic standing-wave atomizer arrangement.  
    
    
       FIG. 1  shows a first ultrasonic standing-wave atomizer arrangement  10  according to the invention in an isometric representation. The coordinates are indicated by the directional arrows for the X, Y and Z directions in a system of Cartesian coordinates. Moreover, the representation is intended to be only of a schematic character, with the result that the actual relative sizes cannot be taken from this figure.  
      A first sonotrode  12  is arranged lying opposite a first reflection body  14 . In this figure, the sonotrode  12  is schematically represented by a cylindrical basic body  16  and a sound body  18 , which protrudes from the end face of the cylindrical basic body  16  facing towards the reflection body  14 . The sound body  18  and the basic body  16  have an approximately cylindrical form. The opposing end faces of the sound body  18  and of the first reflection body  14  are to be referred to as the first sound face  20  for the end face on the sound body  18  and as the second sound face  22  for the end face on the reflection body  14 . The first sound face  20  and the second sound face  22  are concavely formed, that is to say their form corresponds approximately to a portion of the surface of an imaginary hollow sphere. To illustrate this form, a first dotted line  24  and a second dotted line  26  have been drawn on the first sound face  20 . The point of intersection between the first line  24  and the second line  26  lies exactly centrally on the first sound face  20 . Lines corresponding to the first line  24  and the second line  26  are also shown on the second sound face  22 , without however being provided more specifically with reference numerals. Also shown through the point of intersection of the first line  24  with the second line  26  and also the corresponding lines of the second sound face  22  is a centre axis  28 , which runs exactly in the direction of the X coordinate.  
      Shown in the intermediate space between the first sound face  20  and the second sound face  22  is a first piece of pipe  30 , a second piece of pipe  31  and a third piece of pipe  32 , the free ends of which are arranged exactly midway between the sound faces  20 ,  22 . That is to say that the pieces of pipe  30 ,  31 ,  32  are arranged next to one another, the free ends all lying in one plane, which is defined by the centre axis  28  and the second line  26 . Moreover, all the free ends can be joined by an imaginary straight line. The longitudinal axes of the pieces of pipe  30 ,  31 ,  32  are arranged parallel to the Y direction and are connected by their ends remote from the ends to a paint-feeding device  29  (not represented any more specifically in this figure), which provides the required amount of paint to be atomized by the first ultrasonic standing-wave atomizer arrangement  10 . However, the idea of the invention also includes the option of each of the pieces of pipe  30 ,  31 ,  32  being respectively connected to a separate paint-feeding device  29 . This is in any event also to be intended by the paint-feeding device  29  described here.  
      The other end of the pieces of pipe  30 ,  31 ,  32  therefore ends as it were in “free space”, without which the connection to the paint-feeding device  29  would be represented.  
      To allow better illustration of the processes taking place in the standing ultrasonic field between the first sound face  20  and the second sound face  22 , the profiles of five sound particle velocity antinodes of the standing ultrasonic wave have been shown in the intermediate space, the profiles being represented about the centre axis  28 , to be precise in the plane defined by the X direction and Y direction. In the example chosen, a first distance  34  between the first sound face  20  and the pieces of pipe  30 ,  31 ,  32  and a second distance  36  between the pieces of pipe  30 ,  31 ,  32  and the second sound face  22  are of the same size. It is consequently clear that the free ends concerned of the pieces of pipe  30 ,  31 ,  32  are also situated at only one maximum of the sound particle velocity, that is to say in the middle one of the five sound particle velocity antinodes. In the design of the first ultrasonic standing-wave atomizer arrangement  10  that has been chosen for this arrangement, a first distance  34  and a second distance  36  of 17 mm are obtained for an ultrasonic frequency of 24 kHz and five sound particle velocity antinodes. That is to say that adequate space is available for cleaning or directing air which is possibly used for assisting the atomization process or for directing the particles of paint. With such an arrangement of three pieces of pipe  30 ,  31 ,  32  in only one sound particle velocity antinode, that is in the region of a maximum of sound particle velocity, the advantageous effect is therefore achieved that particularly high rates of paint, in particular rates of paint of more than 200 ml/min, are readily achievable. Moreover, it is ensured that the distribution of the diameters of the drops of atomized paint remain in an acceptable range. The atomizing operation is only symbolically represented in this figure at the respective free ends of the pieces of pipe  30 ,  31 ,  32 , in that many small paint particles are indicated around an exaggerated atomization bubble.  
       FIG. 2  shows a second ultrasonic standing-wave atomizer arrangement  40 , which is intended to have substantially the same components as the first ultrasonic standing-wave atomizer arrangement  10 , for which reason the same reference numerals have been chosen for equivalent components. A major difference between the first ultrasonic standing-wave atomizer arrangement  10  and the second ultrasonic standing-wave atomizer arrangement  40  is that, unlike in the arrangement shown in  FIG. 1 , the arrangement of the pieces of pipe  30 ,  31 ,  32  no longer takes place midway between the sound body  18  and the first reflection body, but closer to the sound body  18 . The arrangement of the pieces of pipe  30 ,  31 ,  32  is chosen such that their paint outlet openings in turn come to lie at a selected maximum of the sound particle velocity of the standing ultrasonic wave, to be precise at the second maximum shown, as seen from the sound body  18 . That is to say therefore that a third distance  38  between the sound body  18  and the pieces of pipe  30 ,  31 ,  32  is less than a fourth distance  39 , which is determined as the distance between the pieces of pipe  30 ,  31 ,  32  and the first reflection body  14 . In the case of the arrangement shown here, it proves to be an advantage that the pieces of pipe  30 ,  31 ,  32  lie closer to the first sonotrode  12 . This is so because it has been found that the vibrations of the sound body  18  of the first sonotrode  12  stop the atomized paint droplets comparatively well from adhering to the sonotrode due to the vibration of the sound body  18  itself. Or to put it another way, the vibrations of the sound body  18  keep the paint droplets away from it.  
      In addition, the representation of the pieces of pipe  30 ,  31 ,  32  and the atomization bubbles indicated with the atomized paint particles show that the distance between the pieces of pipe  30 ,  31 ,  32  is chosen such that atomizing regions that respectively operate independently of one another form at the free ends of the pieces of pipe  30 ,  31 ,  32 , that is to say that sheets of paint that are separate from one another are formed for each piece of pipe  30 ,  31 ,  32 . This has the advantage that the regions in which the discharged paint is atomized into particles do not disturb one another. Consequently, the atomizing operation is improved and a comparatively high atomizing rate is achieved.  
       FIG. 3  shows a further advantageous possibility for refining the subject-matter of the invention, with a third ultrasonic standing-wave atomizer arrangement  50 , which is of a substantially similar construction to that of the first ultrasonic standing-wave atomizer arrangement  10 . To make it easier to compare between the components used, the same reference numerals have therefore been used in turn for comparable components.  
      A major difference between the arrangement in this figure and that in  FIG. 1  is that in this figure a fourth piece of pipe  42 , a fifth piece of pipe  43  and a sixth piece of pipe  44  are arranged exactly midway between the sound body  18  and the first reflection body  14 . Although the corresponding paint outlet openings of the pieces of pipe  412 ,  43 ,  44  are accordingly arranged in turn in the region of the central maximum of sound particle velocity, the paint outlet openings no longer lie in the plane defined by the X and Z directions, but instead the middle, fifth piece of pipe  43  lies in the positive Y direction, above the plane defined by the X and Z directions, while the fourth piece of pipe  42  and the sixth piece of pipe  44  lie underneath the plane defined by the X and Z directions. However, all three paint outlet openings still lie together in a plane parallel to a plane defined by the Y and Z directions. The three paint outlet openings therefore form as it were an imaginary triangle which is situated in a plane parallel to the plane defined by the Y and Z directions. This design has the advantage that the distance between the paint outlet openings can be further increased without leaving the chosen, one maximum of the sound particle velocity. In this way, the atomization can be further improved and at the same time the rate of paint can also be increased.  
       FIG. 4  shows a fourth ultrasonic standing-wave atomizer arrangement  60  with a second reflection body  46 , which is arranged lying opposite a second sonotrode  48 . Three first small paint pipes  52  are in turn arranged midway between the second reflection body  46  and the second sonotrode  48 . In a way similar to that already shown in  FIG. 1 , the paint outlet openings of the first small paint pipes are aligned along an imaginary line in the Z direction. A special feature of the arrangement shown is that a second sound body  54  on the second sonotrode  48  and also the second reflection body  46  have approximately a cuboidal form, the opposing sound faces of the second sound body  54  and of the second reflection body  46 , that is to say the third sound face  56  on the second sound body  54  and the fourth sound face  48  on the second reflection body  46 , having a form which corresponds to a portion of the generated surface of a cylindrical body.  
      In this case, it proves to be an advantage if the imaginary centre axis of the cylindrical body runs parallel to that line  62  which runs through the paint outlet openings of the first small paint pipes  52 . The projections  64  of the centre axis of the imaginary cylinder on the third sound face  56  and on the fourth sound face  58  are drawn as dotted lines. Such an arrangement achieves the effect that the maximum of the sound particle velocity in the stationary ultrasonic field is as wide as possible, that is to say it has an extent which is as great as possible in the direction of the line  62 , which coincides here with the Z direction.  
      A fifth ultrasonic standing-wave atomizer arrangement  70  is shown in  FIG. 5 . In this case, the arrangement shown is similar to that from  FIG. 4 , with the result that the second small paint pipes  52  are in turn arranged midway between a fifth sound face  66  and a sixth sound face  68 . As a difference from the sound faces shown in  FIG. 4 , the fifth sound face  66  and the sixth sound face  68  are made up of planar subfaces, the form of which however resembles a portion of the generated surface of a cylindrical body. In this way too, widening of the region of the maximum sound particle velocity in the standing ultrasonic field is likewise achieved.  
      Finally,  FIG. 6  shows a sixth ultrasonic standing-wave atomizer arrangement, which is based on the arrangement of the first sonotrode  12  with the first reflection body  14 , as shown in  FIG. 1 . The reference numerals have been correspondingly taken over from  FIG. 1 . In this case, three second small paint pipes  72  are arranged in a way corresponding to the pieces of pipe  30 ,  31 ,  32 , as shown in  FIG. 1 , and therefore have an equal distance from the sonotrode  12  and from the first reflection body  14 , which is shown here by indicating the second distance  36 . Also shown in this figure are three third small paint pipes  74 , which are shown in the position which corresponds to the position of the pieces of pipe  30 ,  31 ,  32  in  FIG. 2 . That is to say that the distance between the third small paint pipes  74  and the sound body  18  corresponds to the third distance  38  according to  FIG. 2 . This is correspondingly drawn in this figure. In this refinement of the subject-matter of the invention, it is therefore provided that a total of six small paint pipes  72 ,  74  are arranged between the first sonotrode  12  and the first reflection body  14 , to be precise respectively in two groups of in each case three small paint pipes  72 ,  74 , with the result that three small paint pipes  74  are respectively arranged at the second maximum of the sound particle velocity, proceeding from the sound body  18 , and three small paint pipes  72  are arranged at the third maximum, and consequently over the maximum of sound particle velocity. With such an arrangement, the rate of the paint atomization can be increased still further.  
      In none of the arrangements given above as examples was it shown in detail which further measures can act favourably on the atomization or on the painting process as such. For example, cleaning air can be used in the generally known way for substantially avoiding adherence of atomized paint to the sonotrode or to the reflection body. In addition, directing air can be used to make the atomized paint particles preferably fly in the desired direction of painting. The process of directed painting can also be assisted by the paint particles being electrostatically charged. This charging may be achieved internally, in the generally known way, that is to say with paint that is at a high-voltage potential being fed in, or by what is known as external charging, which usually charges the atomized paint through needles which carry a high voltage and are arranged in the vicinity of the atomizing location. The workpiece to be painted is then usually connected to earth potential, so that the electrically charged paint particles preferably fly towards the workpiece. A combination of internal and external charging is also quite possible.  
      Otherwise, it is quite conceivable that the reflection body is a further sonotrode, with the particular advantage that the standing ultrasonic field can be formed particularly strongly. Moreover, such a measure improves the controllability of the ultrasonic field.  
     LIST OF DESCRIPTION  
     
         
           10  first ultrasonic standing-wave atomizer arrangement  
           12  first sonotrode  
           14  first reflection body  
           16  basic body  
           18  first sound body  
           20  first sound face  
           22  second sound face  
           24  first line  
           26  second line  
           28  centre axis  
           30  first piece of pipe  
           31  second piece of pipe  
           32  third piece of pipe  
           34  first distance  
           36  second distance  
           38  third distance  
           39  fourth distance  
           40  second ultrasonic standing-wave atomizer arrangement  
           42  fourth piece of pipe  
           43  fifth piece of pipe  
           44  sixth piece of pipe  
           46  second reflection body  
           48  second sonotrode  
           50  third ultrasonic standing-wave atomizer arrangement  
           52  first small paint pipes  
           54  second sound body  
           56  third sound body  
           58  fourth sound body  
           60  fourth ultrasonic standing-wave atomizer arrangement.  
           62  line  
           64  projections  
           66  fifth sound face  
           68  sixth sound face  
           70  fifth ultrasonic standing-wave atomizer arrangement  
           72  second small paint pipes  
           74  third small paint pipes  
           80  sixth ultrasonic standing-wave atomizer arrangement