A DEVICE FOR CONTROL OF A FLOW OF POWDER
Field of Invention
The present invention relates to a device for control of a flow of electrostatically charged particles issued from a sprayhead, and it is particularly intended for use in powder coating operations wherein spray guns are utilised.
Technical Background
A commonly practised method of applying paint particles on an object to be painted is the powder coating method using spray guns.
Certain objects to be coated are formed with areas that are not easily accessible, such as corners and cavities. In order to ensure that the powder material will stick also to these areas and form an even layer on the object, it is common practice to charge the powder material electrostatically and to earth the object.
Two prior-art methods exist of charging the powder. In accordance with a first technique, an electric field is created intermediate the spray gun and the object. According to a second technique, the charge is generated by the friction arising upon contact between the powder particles and the discharge equipment. This latter technique is disclosed in US-A- 4 597 534.
One problem encountered when using this latter technique is to achieve even distribution of the powder evenly over the entire surface of the object. The concentration of powder particles in the flow exiting from the spray gun varies, and despite being charged the particles may aggregate in too large amounts to ensure even coating results. Gradients in the flow pattern, such as eddies, may also cause uneven coating.
In order to solve this problem it is already known to equip the spray gun with a sprayhead comprising a plurality of nozzles located side by side in rows.
A sprayhead of this kind generates a flow in the form of a spread-out cloud, which impacts on a major part of the object. As a result, the powder particles impinge also on difficult-of-access parts. However, there are objects of such a complex and difficult-to-reach structure that not even this kind of sprayhead will provide a satisfactory result. As examples of structures of this kind may be mentioned profiled sections formed with sharp corners and angles, and finely-mesh grid structures.
The sprayhead referred to does not either fully remove the problem of uneven coating. The flow still exhibits a somewhat irregular flow pattern and particle concentration. Summary of the Invention
The present invention has for its object to solve the above problems and to make it possible to coat evenly also very complex structures formed with difficult-to- reach surfaces. This object is obtained in accordance with the invention by means of a device of the kind defined in the introduction hereto and which is characterised by the provision of a deflector means in the form of an impingement body having a surface extending crosswise relative to the direction of travel of the flow, said deflector means being positioned adjacent the mouth (s) of the sprayhead and extending at an acute angle with respect to the direction of travel of the flow being discharged from the sprayhead. In several various ways, the deflector means thus affects the flow of powder material exiting from the sprayhead nozzles. Firstly, the flows leaving the various nozzles are dispersed and interwoven upon their contact with the deflector means. The result is that any varia- tions of particle concentration in the flows are evened out. Secondly, the movements of the discrete powder particles become more uniform and less turbulent.
Formation of gradients in the flow pattern is counteracted, because the deflector means restricts the possibilities of movement of the powder particles. Thirdly, the flow is given a flatter and more compact spreading pattern and adopts the configuration of a curtain rather than of a cloud.
The flow may be changed without it being necessary to essentially alter the direction of travel of the flow. This means that the kinetic energy of the powder par- tides in the direction of travel of the flow is maintained and is not absorbed by the deflector means. Conservation of this kinetic energy ensures long powder- particle range and that the particles thus reach the intended object. Tests have shown that the more uniformly travelling flow in the form of a curtain is better suited to coat evenly also very difficult-to-reach parts. For instance, in painting automobile radiators extremely good paint coverage has been obtained. The deflector means may be designed in different ways, for instance as a flat plate, for even more uniform coating of the object in question.
The angle formed between the deflector means and the direction of travel of the flow may be in the range of 0- 45 degrees, preferably in the range of 0-20 degrees and most preferably in the range of 0-10 degrees.
In addition, the deflector means may be formed with areas that present mutually differing angles relative to the direction of travel of the flow, in order to ensure that flow guidance is effected gradually. For this purpose, the deflector means may be formed with one or several bends in the cross-flow direction.
Furthermore, the edge of the deflector means may be rounded, in order to further suppress the generation of turbulence as the flow passes over the edge.
In addition, the deflector means may consist of several separate parts, in case the guiding effect is not
to influence the entire flow, or the flows emanating from the various nozzles are to be guided in different ways.
Preferably, the deflector means is removably mounted on the sprayhead in order to facilitate handling and increase flexibility. This makes it easy for the user to choose whether the powder coating should be effected with or without the use of deflector means.
Finally, the deflector means may be mounted on a sprayhead comprising a plurality of nozzles located side by side in rows, preferably in one plane.
Furthermore, the device preferably is used in coating operations using friction-charged powders, such as paint powders
Brief Description of the Drawings The invention will be described in the following in more detail with reference to the enclosed schematical drawings, which for exemplifying purposes show one presently preferred embodiment of the invention. In the drawing: Fig 1 is a view from above of a deflector means mounted on a sprayhead having several nozzles located in one plane, and schematically a flow of powder being discharged from said nozzles.
Fig 2 is a lateral view of the deflector means of Fig 1.
Description of a Preferred Embodiment The deflector means in accordance with the preferred embodiment consists of an impact body in the form of a rectangular plane plate 1 of a non-pervious material, preferably plastics. By the expression "plane" should be understood in this context an essentially flat but not necessarily even surface.
The plate comprises two bent portions 2, 3 extending across the entire length of the plate in parallel with the long sides of the plate. The first bent portion 2 extends at an obtuse angle α of approximately 170-180 degrees relative to one side of the plate. The other bent
portion 3 extends at an obtuse angle β of approximately 140-160 degrees relative to the opposite side of the plate.
Several snap-fastening means 4 in the form of partly open cylinders, spaced evenly apart, are fixed to one face of the plate, along the sharply bent portion 3.
With the aid of the snap-fastening means the plate is attached to a sprayhead 5 comprising several nozzles 6 disposed in one plane, imparting to the flow 7 being discharged from the sprayhead a direction of travel that is essentially parallel with the short sides of the plate. The sprayhead nozzles project in over the plate approximately to the middle of the plate as seen in the direction of travel of the flow and they extend essen- tially over the full length of the plate in the cross- flow direction.
The snap-fastening means press the plate against the nozzle mouths 8 and also against a rail 9 which is a part of the sprayhead and which extends transversely across the nozzles in order to keep the nozzles together and stabilise them. The sharply bent portion 3 of the plate is arranged around the rail 9 in order to further anchor the plate to the sprayhead.
Because of the plate thus being pressed against the nozzle mouths 8 and against the sprayhead rail 9, the plate forms an angle φ of approximately 8 degrees relative to the direction of travel of the flow 7 leaving the sprayhead. The slightly bent portion 2 is deflected away from the flow and forms an angle of only approximately 0- 10 degrees relative to the direction of travel of the flow 7 leaving the sprayhead.
When the flow of powder particles leaves the sprayhead nozzles it impinges on the plate 1 at an angle φ, as a result of which the various powder streams will be distributed and interwoven. In addition, the deflector means guides the particles in a more uniform direction, essentially along the plate surface. The flow follows the
slightly bent portion 2 of the plate, which portion defines the final direction of travel of the flow as the latter leaves the plate.
When the flow has left the plate it spreads trans- versely across the plane of the plate and adopts the shape of a curtain. The direction of travel of the flow is essentially maintained because the slightly bent end portion of the plate is inclined at a small angle only relative to the direction of travel of the flow leaving the sprayhead.
As will be appreciated a number of modifications of the embodiment described above are possible within the scope of protection of the invention as defined in the appended claims. For instance, the impingement body may be cylindrical in shape in order to be applied on a sprayhead having cylindrically arranged nozzles. In addition, the impingement body may consist of several plates arranged at individually different angles to the flow being discharged from the sprayhead.