EP1641568B1 - Method for coating objects, electrode arrangement, and coating system - Google Patents

Description

The invention relates to a method for electrostatically assisted coating of articles with a coating material. Furthermore, the invention relates to an electrode assembly for generating an electric field in electrostatically assisted coating equipment and an electrostatic coating system.

The electrostatic influence of coating material in the coating of objects is well known and aims at the best possible deposition of the coating particles on the object. Application finds this type of coating in many areas, especially when coating the weld on the inside of can bodies with a powdery coating material, such as from DE-A-42 27455 known. It is also known to protect the field-generating electrode by a ring against the direct impingement of the powder / air mixture and also to flush the electrode with an air flow in order to minimize the contamination of the electrode by powder particles. Depending on the type of powder, however, a more or less adherent accumulation of powder on the electrode may still occur, with polyamide coating powder in particular being problematic in this respect and prone to sintering at the electrode. A soiled electrode changes the operating conditions during the coating and possibly causes a breakdown in the operation of the coating system. The invention has for its object to provide an improved electrostatically assisted coating process.

This object is achieved with the characterizing features of claim 1.

It has been found that a vibrated electrode contaminated much less. The adhesion of particles is greatly reduced or virtually completely prevented. The coating quality is not adversely affected by the vibrating electrode and it is usually not necessary to change the operating parameters of the coating process. The vibration takes place substantially transversely to the electric field line direction and / or in the field line direction of the field caused by the electrode.

Moving objects are preferably coated in this way, since as a rule the objects are moved past the coating site once at high speed, as a result of which a reduced coating due to contamination of the electrode can no longer be corrected. In particular, this is the case when coating the weld seam area inside can bodies, which is the preferred application of the method.

An oscillatable design of the electrode is preferred so that it can be set into oscillation or vibrated by an excitation means. The excitation means may e.g. act electromagnetically or piezoelectrically. However, an embodiment of the electrode is preferred such that it can be set in vibration by an air flow, similar to the blown tongue of a musical wind instrument. As mentioned, it is known to provide a scavenging air flow to the electrode, and it is preferable to use just scavenging air as the drive for generating vibration.

As an alternative or in addition to the vibration of the electrode, it is possible, with the stationary electrode arrangement and objects moved past it, to make the electrode driven, for example as a rotating electrode. Such a movement of the electrode in a basically stationary during the coating electrode assembly, the electrode contamination reduce or avoid by coating material. The electrode movement, eg rotation, can be effected by an electric motor but also pneumatically and, for example, again by the mentioned scavenging air.

It is a further object of the invention to provide an improved electrode assembly and an improved coating apparatus.

This object is achieved according to claims 10 and 17, respectively.

In this way, the advantages explained by the method are achieved.

The advantages of preferred embodiments explained with reference to the method also apply to the preferred embodiments of the electrode arrangement or the coating apparatus. In particular, the vibration excitation by an air flow results in a particularly simple construction in coating systems, and especially can coating systems, which are already designed to provide an air flow (scavenge air flow). This allows a simple retrofitting of such electrode arrangements in already existing coating devices.

Alternatively, even in a stationary electrode assembly or a stationary electrode assembly coating apparatus, a moving electrode, e.g. a rotating electrode, be provided; this at best in combination with a vibration of the electrode.

• Figur 1 schematisch in Schnittdarstellung die Beschichtung einer Dosenzarge;
• Figur 2 eine Schnittdarstellung einer Elektrodenanordnung gemäss der Erfindung;
• Figur 3 eine Draufsicht auf die Elektrodenanordnung von Figur 2;
• Figur 4 schematisch eine weitere Ausführungsform der Erfindung; und
• Figur 5 noch eine Ausführungsform der Erfindung.

In the following embodiments of the invention will be explained in more detail with reference to the drawings. It shows

• FIG. 1 schematically in sectional view the coating of a can body;
• FIG. 2 a sectional view of an electrode assembly according to the invention;
• FIG. 3 a plan view of the electrode assembly of FIG. 2 ;
• FIG. 4 schematically a further embodiment of the invention; and
• FIG. 5 another embodiment of the invention.

FIG. 1 shows schematically the conditions when coating can weld seams on the inside of cans as an example of objects to be coated. However, the invention explained below is also applicable to the electrostatically assisted coating of other articles, although the can weld seam coating is the preferred application. In this case, a powdery coating material is usually used, for example a polyamide powder. Such coating materials are known and are not explained here in detail. In the coating of articles, it is also known to use liquid coating materials in drop form, wherein the present invention may also find application. Also, such materials are known and will not be explained here. In FIG. 1 a can body 3 is indicated, which is located above the arm 2 of a coating device 1. The can is transported by a not shown, known transport device at high speed in the direction of arrow A. The can body 3 has previously been welded in a likewise known manner between the welding rollers 11 and 12 of a not-shown can welding machine, wherein usually a Drahtzwischenelektrode on the welding rollers 11 and 12 is used. The lower welding roller 11 is rotatably mounted on the lower arm 10 of the welding machine. Passing through this lower arm 10 and thereafter into the arm 2 of the coating device, at least one line 13 for conveying a coating powder-air mixture is provided. The coating powder is thereby passed through conveying air through the welding machine in a known manner conveyed along the line 13 to the coating device 1. In the figure, the powder-air mixture is shown in the line 13 with arrows 15 and at the exit point 4 of the coating device 1 as a cloud 15. The coating powder passes at the outlet at the exit point 4 on the inside of the can body 3 on the uncoated weld and forms there a layer which subsequently baked in a known manner and cooled above the weld forms a dense coating, which completes the inner coating of the can body in the weld seam area. This is known and need not be explained here. It is known, as in FIG. 1 shown to provide a tip electrode 7, which is supplied via a voltage source 8 with high voltage. The correspondingly forming electrostatic field leads in a known manner to an influence of the coating material and its better adhesion to the can body 3. It is known to protect the tip electrode by a guard ring 18 before the coating powder and also via a line 14 powder-free air 17 than Purge air around the electrode to lead and leave in the space 5 to protect the electrode also before coating by the coating material. It turns out, however, that it still leads to powder deposits on the known electrode 7, especially when using polyamide powder as a coating material. It has already been tried to counteract the powder contamination of the electrode 7 on the one hand by increasing the air velocity of the scavenging air, but this has not brought the desired success. The speed of the scavenging air is also limited to the top, since the exiting scavenging air, the cloud 15 may not be disturbed excessively. Further, it has been attempted to further increase the high voltage at the electrode 7 to thereby also achieve increased repulsion of the powder from the electrode. It appears but that the usual range of about 20 kV voltage and a maximum voltage of 25 kV should not be exceeded. If the voltage is too high, this results in the effect that the lacquer layer lying next to the uncoated weld seam region is also charged on the inside of the can, which in turn leads to a repulsion of the coating powder. It is therefore desirable to maintain as far as possible the parameters of purging air velocity customary in such an arrangement corresponding to a purging air volume of approximately 1 to 2 liters / minute and the high voltage in the range of approximately 20 kV. Of course, for other coating situations, such as other articles to be coated and other coating materials, each have different default values, there is a desire to maintain the default values used for a good coating as much as possible.

The FIGS. 2 and 3 Now show an embodiment of an electrode assembly 6 according to the invention and to explain the inventive method. This electrode arrangement 6 can be used directly instead of the electrode arrangement 6 'shown in FIG. The same reference numerals show the same elements, in particular again the scavenging air line is denoted by 14 and the scavenging air with the arrows 17. The arrow 15 above the electrode assembly 6 shows the course of the air-powder mixture 15 accordingly FIG. 1 , The electrode assembly 6 is usually in the device 1 of FIG. 1 arranged so that there is a similar, substantially constant from the object surface to be coated spaced position of the electrode 9 of the inventive electrode assembly 6, as corresponds to the electrode 7 according to the prior art. However, other installation options are also possible if this generates the desired electrostatic field through the electrode 9. This is in turn connected to a high voltage source 8, which preferably also generates DC voltage in the range of 20 kV.

According to the invention, the electrode 9 is designed as a vibrating or oscillating electrode. In the embodiment according to the FIGS. 2 and 3 the electrode is designed as a spring tongue-like, one end fixed electrode and the other end exposed electrode, which is preferably arranged in front of the opening 25 'of a resonance chamber 25. FIG. 2 shows a vertical section through the electrode assembly 6 with the electrode 9 and FIG. 3 shows a frontal view in the direction of arrow B of FIG. 2 , It can be seen that the vibratable electrode 9 is arranged with a fastening means 20 in front of the opening 25 'of the resonance chamber 25 and in its outer shape substantially corresponds to this opening, so that only a narrow gap 19 between the opening 25' and the electrode. 9 formed. The width of this gap is for example only 1/10 mm. Smaller gap sizes are usually to be avoided for conventional coating powder for cans, since otherwise powder particles in the gap 19 can clamp. A slightly larger width of the gap 19 can be selected, but this must be such that a swinging of the tongue-shaped electrode 9 results due to the purge air 17. The purge air exits through the gap and thereby stimulates the electrode 9 to the vibration or vibration desired according to the invention. In this preferred embodiment, therefore, the oscillation of the electrode is generated by an air flow, which preferably consists of the purge air flow 17. Of course, in other coating applications where no scavenge air flow is provided, other means of vibrational excitation of the electrode may be used, such as piezoelectric or electromagnetic vibrational excitation. However, even in such applications, a special air flow can be generated, which then serves for vibration excitation. Naturally The oscillation with an air flow can also be achieved in other known ways, for example by a stream of air emerging from a nozzle flowing over the upper edge of an electrode and deflecting it vibratingly.

As an indication of a vibration that keeps the electrode 9 effectively free of powder contamination, an oscillation frequency of about 500 Hz at an amplitude at the free end of the electrode 9 of about 0.5 mm can be given. Of course, the oscillation frequency and the amplitude can be freely selected within a fairly wide range. The above-mentioned oscillation frequency is set at the conventional air amount of 1 to 2 liters / minute, when for the electrode assembly 6 of FIG. 2 about the following mass be chosen. As a length x of the supply chamber 24 for the supply of air flow 17 into the resonance chamber 25 about 60 mm. As height y of the room 24 about 6 mm. It has been found that the vibration excitation and the maintenance of the vibration is better when the resonance chamber 25 such a space 24 is connected upstream, instead of the direct insertion of the line 14 in the resonance chamber 25. For the resonance chamber 25 itself has a height v of about 10 mm and a width u of about 5 mm has proven suitable if the height h of the free tongue part of the electrode 9 is about 6-7 mm and the width b of the electrode is about 2 mm and the thickness is about 0 , 05 mm. The electrode is formed from a spring steel and preferably has the in FIG. 3 shown tongue-like shape. But it is also possible to provide an eg rectangular shaped electrode 9 ', as in FIG. 3 indicated by broken lines. The housing of the electrode assembly 6 is preferably made of plastic. The voltage source 8 applied to the electrode 9 is the same voltage source as used in the conventional tip electrode. Of course, other coating applications may take a different shaped and other dimensions design Electrode be used. Of course, the dimensions of the electrode assembly 6 are variable in a wide range and a resonance chamber is usually only required if the vibration excitation is carried out by an air flow. If another possibility of vibrational excitation is used, for example a piezoelectric or an electromagnetic vibrational excitation, by means of which the oscillatable electrode is forcibly vibrated directly, then of course the air supply and the spaces 24 and 25 are not needed. Also, vibration excitation by a beating element, which causes the electrode 9 only temporarily by a blow to the electrode or possibly an adjacent housing part or support member in vibration, can also be used to avoid the deposition of coating material and satisfy. The excitation for oscillation of the electrode can be done periodically or at random intervals.

FIG. 4 schematically shows a further embodiment in which a per se conventional tip electrode is provided within the electrode assembly 6. This tip electrode 29, which is again connected to the voltage source 8, is mounted on a base 30, which forms the actual vibrating element. The electrode 29 is rigid and follows only the movement of the vibrating base. This pedestal may in turn be driven piezoelectrically, electromagnetically or electromotively, or in some other known way to effect vibration. These vibrations can be done in the direction of arrow C and / or arrow D. In this way, a cylindrical and rigid tip electrode in its lower region can be vibrated, for example in a range from 100 Hz to also 500 Hz or higher. Also in this way, the deposition of coating material by vibration of the electrode can be prevented. In addition, scavenging air 17 can be used in a known manner, so that the scavenging air the rinse serves and is not used for vibration generation.

According to a further aspect of the invention, which in FIG. 5 can be seen, the electrode is not vibrated, but moved in another way, preferably rotated about an axis E. In the example shown by FIG. 5 In the electrode arrangement 6, two electrodes 39 are arranged on a support 38, which together with the shaft 37 is rotatable about the said axis of rotation E. The electrodes are in turn connected to the voltage source U. The rotation can take place, for example, by an electric motor, or in turn an air flow, for example the scavenging air flow 17, could be used to drive the electrodes 39, which is possible by means of a corresponding wind-wheel-shaped design of the carrier 38. The electrode assembly 6 is stationary with respect to the again moving objects 3 and only the single electrode or the individual electrodes 39 are moved.

Claims (17)

1. A method of electrostatically supported coating of articles (1) with a coating material (15), characterised in that the at least one field-producing electrode (9; 29) is caused to vibrate at least at times and preferably continuously during the coating operation.
2. A method according to claim 1 characterised in that the articles (3) are moved past the stationary vibrating electrode (9; 29) and in particular are bodies of tins, the inside seam region of which is coated, in particular with a coating material in powder form.
3. A method according to claim 1 or claim 2 characterised in that the electrode is in the form of an oscillating element (9) and is excited to vibrate by an excitation means (17).
4. A method according to claim 3 characterised in that the electrode is excited to vibrate by an air flow (17), in particular a flushing air flow.
5. A method according to claim 4 characterised in that the electrode (9) is arranged in leaf form and in particular tongue form in front of the opening (25') of a resonance chamber (25), and the air (17) is passed through a gap (19) between the electrode (9) and the opening (25').
6. A method according to claim 1 or claim 2 characterised in that the electrode (29) is in the form of a substantially rigid element which is moved with an oscillating motion by a drive means (30).
7. A method according to claim 6 characterised in that flushing air flows around the electrode (29).
8. A method of electrostatically supported coating of moving articles (3) with a coating material, comprising an electrode arrangement (6) disposed stationarily at a spacing from the articles and including at least one electrode (39), characterised in that the electrode (39) in the coating operation is moved driven at least at times and in particular is moved in driven rotation about an axis of rotation (E).
9. A method according to claim 8 characterised in that the electrode (39) is moved by an air flow (17) or by electric motor means.
10. An electrode arrangement (6) for producing an electrical field in electrostatically supported coating apparatuses, characterised in that the arrangement (6) has at least one vibratingly movable electrode (9; 29).
11. An electrode arrangement according to claim 10 characterised in that the electrode is in the form of a flexible electrode (9) which can be excited to oscillate, in particular being in the form of an electrode (9) which can be excited to vibrate by an air flow.
12. An electrode arrangement according to claim 11 characterised in that the electrode (9) is of a substantially leaf-shaped, in particular tongue-shaped configuration, and is fixed at one end.
13. An electrode arrangement according to one of claims 10 to 12 characterised in that the electrode is fixed in front of the opening (25') of a chamber (25) of the arrangement, forming an air gap (19), which chamber is in communication with an air inlet of the arrangement (6).
14. An electrode arrangement according to claim 10 characterised in that the electrode (29) is in the form of a substantially rigid electrode, in particular a point electrode which is fixed on or to a vibrator element (30).
15. An electrode arrangement (36) for producing an electrical field in electrostatically supported coating apparatuses, characterised in that the arrangement (36) has at least one electrode (39) arranged driven rotatably about an axis of rotation.
16. An electrode arrangement according to claim 15 characterised in that it includes an electric-motor drive means for the rotary movement or a drive means (38) driven rotatably by an air flow.
17. A coating apparatus (1) for the coating of articles (3), in particular moving articles, comprising an electrode arrangement according to one of claims 10 to 16.

Images