JPH08108108A - Method for electrostatic coating of powder coating and electrostatic coating machine for powder coating - Google Patents

Abstract

PURPOSE: To perform electrostatic coating with high electrostatically charging rate, accordingly with high coating efficiency, and uniformly regardless of unevenness of the surface of an article to be coated by using a powder coating. CONSTITUTION: A main discharging electrode 2 on which a DC high electric voltage is applied and a pulse discharging electrode 6 with the same polarity as that of the main discharging electrode 2 and on which a DC pulse high electric voltage is applied are arranged to generate corona discharge between both electrodes and a powder coating is passed through between both electrodes to charge it electrostatically.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic coating method for powder coating and an electrostatic coating machine.

[0002]

2. Description of the Related Art Problems and Problems of the Prior Art For coating powder paint, an electrostatic coating method using electrostatic force is used because the paint itself has no adhesiveness. That is, a conductive object to be grounded is used as a counter electrode, and on the other hand, a negative DC high voltage is usually applied to the main discharge electrode of the electrostatic coating machine to generate corona discharge, and the generated negative ions are generated. The powder paint suspended in the pressurized air is adsorbed and charged, and the paint that collides with the object to be coated is applied by the Van der Waals force. Therefore, the higher the charge ratio of the powder coating, the higher the coating efficiency.

Conventionally, however, as a method for charging powder coating material, as shown in FIG. 5 (a), the discharge electrode 2 is provided at the outer center of the jet outlet 76 of the tubular conveying means 75 toward the article 121 to be coated. Since the lines of electric force 110 are formed in parallel with the traveling direction of the powder coating material 102, the coating material which is out of the passage position of the powder coating material 102 from the position of the corona discharge 119 is generated, and the charging efficiency is not charged. Was low. Therefore, the coating cost was significantly higher than that of the electrostatic coating of liquid-based paint. Further, when the discharge electrode is arranged in the middle of the conveying path of the tubular conveying means 75 as shown in FIG. 5B, most of the powder coating material 102 comes into contact with the corona discharge 119, but the charging rate is improved. The negative electrode-charged powder coating material 102 is adsorbed and deposited on the inner wall surface of the tubular conveying means 75 that is positively dielectrically polarized by the discharge electrode 2. The accumulated powder coating material is solidified and peeled off by the air flow velocity, scattered in the air flow, and adhered to the article 121 to be coated, thus increasing coating defects.

The object to be coated 1 at any electrode position
When the surface of No. 21 has irregularities, the lines of electric force are concentrated on the convex portions, so that there is a problem that the coating cannot be performed uniformly. In particular, in the case of a box-shaped product, electric lines of force are concentrated on the tip, which causes a problem that the inner surface of the corner cannot be coated. Therefore, due to the low pollution of the powder coating, it is rarely used for coating automobiles and the like, although it is expected to spread.

To solve these problems, Japanese Patent Publication No. 60-109 discloses that a plurality of pairs of electrodes are arranged as planar discharge electrodes of a ring shape or the like, and an AC voltage or an AC pulse is applied to the AC voltage between the electrodes. A method is disclosed in which a voltage is superposed and applied, and a powder current is charged by a minute electric current flowing therebetween, while a high direct current voltage for flying from a coating machine to an object is superposed and applied. . However, since this method uses an alternating voltage, the charging is alternately performed and the charging is almost erased, so that the coating efficiency cannot be improved.

Therefore, it is an object of the present invention to provide an electrostatic coating method for powder coating, which has a high charging rate and a high coating efficiency in electrostatic coating of powder coating. It is to provide an electrostatic coating machine. At the same time, another object of the present invention is to provide an electrostatic coating method and an electrostatic coating machine capable of preventing electric lines of force from concentrating and coating evenly on uneven surfaces and corners of box-shaped products.

[0007]

Means for Solving the Problem In order to solve the problem, a means provided by the present invention is to provide an electrode for applying a DC high voltage and a DC pulse high voltage of the same polarity as the electrode in electrostatic coating of powder paint. A corona discharge is periodically formed between both electrodes by using an applying electrode, and the powder coating material is conveyed in a direction intersecting with the corona discharge to charge the powder coating material. is there. Further, the electrostatic coating machine is disposed at the center of the ejection port of the tubular conveying means for ejecting the powder coating material, and the main discharge electrode to which a high DC voltage is applied, and the inner wall surface of the conveying means, or from the inner wall surface. It is provided with a pulse discharge electrode which is arranged outside and to which a DC pulse high voltage having the same polarity as that of the main discharge electrode is applied.

[0008]

When the electrostatic discharge is applied by the above means, the negative direct current high voltage is applied to the main discharge electrode and the pulse voltage of the pulse discharge electrode is not excited, that is, when it becomes the ground potential. As shown in FIG. 4 (a), an electric force line 110 extending from the main discharge electrode 2 to the pulse discharge electrode 6
a and a corona discharge 119A are formed. The powder coating material 102 that has approached the ejection port 76 of the tubular conveying means 75 is negatively charged by the corona discharge 119A, and most of the powder coating material 102 is ejected from the ejection port 76 by the air flow, and part of it is pulsed. Toward the discharge electrode 6, a tubular conveying means 75
Adheres to the inner wall surface of the spout. The powder coating material 102 ejected in the direction of the coating object by the jet flow of air is diffused by the repulsive force due to the electric charge between the powder coating materials and moves toward the coating object 121 by the attractive force from the coating object 121.

Next, when the pulse voltage is excited to have a negative polarity, the potential difference between the main discharge electrode 2 and the pulse discharge electrode 6 disappears as shown in FIG. 4 (b), and the lines of electric force are applied from both electrodes. It occurs toward the object 121. Therefore, the electric force lines 110 of the main discharge electrode 2 are affected by the pulse discharge electrode 6 and are compressed toward the object 121 to be coated. Further, the electric force lines 109 from the pulse discharge electrode 6 are affected by the main discharge electrode 2 and the object 12 to be coated is affected.
Head to 1. In the corona discharge, 119B and 119C are generated from both the main discharge electrode 2 and the pulse discharge electrode 6 toward the article 121 to be coated. At this time, the pulse discharge electrode 6 and the powder coating adhered to the inner wall surface of the tubular transfer means 75 lose their adhesive force due to the repulsive force from the pulse discharge electrode 6, are dispersed in the air flow, and head toward the article 121 to be coated. . The powder coating material 102 transported to the ejection port 76 by the tubular transport means 75 does not form dielectric polarization even when it approaches the pulse discharge electrode 6 or the main discharge electrode 2, and therefore the pulse discharge electrode 6 or the tubular transport means 75. It does not adhere to the side and keeps the dispersed state that has been conveyed.

The voltage is applied to the pulse discharge electrode 6 in a pulsed manner, so that the corona discharge 119A is intermittently formed, but an uncharged powder coating material is not generated. The pulse excitation time width is 119 A for corona discharge of the powder coating material 102.
This is because the powder coating material 102 will surely cross the corona discharge and be charged if the time is shorter than the time for passing through.
It is sufficient if the excitation time width of the normal pulse is 100 μsec or less. Further, if the number of times the pulse discharge electrode 6 is excited in one second, that is, the pulse frequency (pps) is large, the powder coating material having the powder coating material adhered to the inner wall of the conveying means or the electrode is periodically peeled off, and thus the deposition is performed. There is nothing to do. Therefore, the lumps of the deposited paint are not peeled off and are not adhered to cause coating failure. It is usually sufficient that the pulse frequency is 50 pps or more.

As described above, in the present invention, the corona discharge 119A is generated in the powder coating material conveying path and cross-contacts with the powder coating material 102 and the corona discharge 119A. Adsorbs and becomes charged. Therefore, the charging rate becomes extremely good and the coating efficiency is improved.
Moreover, since the powder coating material does not adhere to or deposit on the inner wall of the conveying means or the electrodes, stable powder coating can be continued.

Further, according to the means of the present invention, the electric field lines 110a of the main discharge electrode 2 are compressed inward by the excitation of the pulse discharge electrode 6 and diffused outward by the cancellation of the excitation of the pulse discharge electrode. . Therefore, the powder coating material 102 flying on the line of electric force 110 of the main discharge electrode and flying toward the object 121 to be coated.
Is excited and de-energized several times during the flight when the pulse frequency is high, typically above 50 pps, and therefore the powder paint 102 is redirected several times during the flight. That is, the powder coating material 102 advances toward the object to be coated while vibrating. Therefore, the powder coating material 102 as a whole is the object 12 to be coated.
Even if there is unevenness in the shape of the article to be coated, the coating is applied uniformly without concentrating on the projections. When applied to electrostatic coating of box-shaped products, the powder paint flies while vibrating, so that the inside of the box-shaped products can be painted without concentrating on the tip.

The effect of improving the coating rate using the pulse discharge electrode 6 of the present invention, the effect of changing the direction of the lines of electric force, and the effect of removing the paint adhering to the electrode are the voltage of the pulse discharge electrode 6 and the pulse. It varies depending on the frequency, the pulse width, the shape of the pulse discharge electrode 6, and the shape of the main discharge electrode. Further, if a shaping discharge electrode (hereinafter referred to as shaping electrode 5) is installed outside the pulse discharge electrode 6, the direction of the lines of electric force is limited, and the coating efficiency is further improved. First, the details of the electrostatic coating machine according to the present invention will be described.

[0014]

First Embodiment FIG. 1 shows a funnel-shaped main discharge electrode 2, a pulse discharge electrode 6 arranged on the inner wall of a transfer means 75 and facing the main discharge electrode 2, and a tubular transfer formed of a dielectric material. Means 75
A cross section of an electrostatic coating machine 61 for powder coating equipped with the ring-shaped shaping electrode 5 is shown on the outer side of FIG. FIG. 3 is an example of a circuit diagram applied to each electrode.

The main discharge electrode 2 is a funnel-shaped electrode that faces the pulse discharge electrode 6 and forms a sharp ring at the tip, and is formed of a copper wire and a copper plate, and the jet port 7 of the tubular carrier means 75.
It is arranged in the central part of 6. The distance from the tip of the ring-shaped main discharge electrode 2 to the pulse discharge electrode 6 is 3 cm. The main discharge electrode 2 is covered with an insulating material except the tip.

The pulse discharge electrode 2 is a funnel-shaped electrode made of stainless steel having a sharp tip, and is formed so that the position of the tip coincides with the tip of the main discharge electrode 2. In addition, the buffer material 9 made of vinyl chloride is used to prevent the current from the main discharge electrode from being concentrated.
Is provided on the entire circumference of the inner wall of the pulse discharge electrode 6 except the tip. The thickness of the vinyl chloride cushioning material 9 is such that no dielectric breakdown occurs in the shortest distance from the main discharge electrode 2 to the pulse discharge electrode. As a result, a large non-uniform electric field is formed at the tip of the main discharge electrode 2, and when the pulse electrode 6 is not excited, as shown in FIG.
A is generated.

Numeral 5 is a stainless steel ring-shaped shaping discharge electrode surrounding the ejection port 76 of the tubular conveying means, which is supported by the position adjusting means 72 and is shaped so that the lines of electric force from the main discharge electrode do not spread excessively. The tip position of the electrode 5 is adjusted. Normally, the tip is set to be slightly behind the tip of the pulse discharge electrode 6.

A DC high voltage is applied to the main discharge electrode 2 and the shaping electrode 5 from the DC power supply 41 through the protective resistor 14, the cables 81 and 83, the connectors 90 and 92, and the current limiting resistors 15 and 20. Further, the pulse discharge electrode 6 has a protection resistor 16, a pulse circuit 52, a cable 8 and a DC power source 41.
2. A pulse high voltage is applied through the connector 91 and the current limiting resistor 18. The powder coating material is supplied by a coating material supply means (not shown) through the connector 93 for the coating material, and a tubular conveying means 75 is provided.
Is supplied to.

The current limiting resistor 18 for the pulse discharge electrode 6 is a resistor 5 to 10 times larger than the current limiting resistor 15 for the main discharge electrode, but when the pulse high voltage is not excited, Corona discharge 119A from the electrode 2 toward the pulse discharge electrode 6 can be prevented from developing into a spark discharge, which is preferable.

FIG. 3 shows an example of the power supply circuit. Main discharge electrode 2
The DC power supply 41 for use is a variable voltage type, and in the present invention, it is normally used with a negative polarity of 15 to 30 kv. Since the pulse circuit 52 for the pulse discharge electrode 6 is also supplied at 15 to 30 kv, an output of about 15 to 30 kv can be obtained. The pulse forming circuit 52 includes a pulse capacitor 33, a load resistor 12,
And a gap switch 27 with a trigger.
As the switch, a gap switch driven by compressed air, a thyristor, or a thyratron can be used instead of the gap switch with a trigger. Any of these may be commonly used.

In FIG. 3, when the switch 27 is short-circuited, a high voltage from the DC power supply 41 is applied to the pulse electrode 6 so that the potential difference from the main discharge electrode 2 disappears and the powder coating adhered to the pulse discharge electrode 6 is eliminated. Peels off, and as shown in FIG. 4B, rides on the lines of electric force 109 or 110 toward the article 121 to be coated. When the switch 27 is cut off, the corona discharge current from the main discharge electrode 2 is pulse discharge electrode 6, current limiting resistance 1
8. Because it flows to the earth circuit through the load resistor 12,
A corona discharge 119A is formed between the main discharge electrode 2 and the pulse discharge electrode 6. Therefore, the powder coating material passing between the main discharge electrode 2 and the pulse discharge electrode 6 is certainly charged.

The voltage applied to the main discharge electrode 2 can be set lower than in the conventional method. In the conventional method, a corona discharge is generated between the main discharge electrode 2 and the article 121 to be coated, so that a high voltage is required. Further, in the conventional method, the electric field lines of the main discharge electrode 2 were diffused in a direction of 180 degrees or more. However, in the present invention, the coating material charged by the main discharge electrode 2 is applied by the pulse discharge electrode 6 from another direction. This is because the lines of electric force in the 112 direction are superposed, so that the lines are concentrated on the object to be coated 112 as a whole even if the voltage is not excessively increased.

The higher the voltage applied to the pulse discharge electrode 6, the greater the effect of changing the direction of the lines of electric force. However, if the voltage is made too high, the atomized paint will concentrate too much in the center, so it is usually preferable to make it approximately equal to the voltage applied to the main discharge electrode 2.

The pulse waveform applied to the pulse discharge electrode 6 has a pulse width of 10 μsec and a pulse frequency of 150 pp.
s vibration waves are used. The pulse shape is a trapezoidal wave,
A rectangular wave, a sawtooth wave, or a direct current obtained by rectifying an alternating current with a half wave can also be used, and is not particularly limited. The shorter the pulse width is, the longer the duration of the corona discharge 119A is, and therefore the charging rate is improved. Therefore, the pulse width is usually 100 μsec or less. Preferably. Further, the pulse frequency needs to be at least 50 pps because if the frequency is too low, the adhesion of the powder coating material to the electrodes increases. However, it is not necessary to increase the pulse frequency excessively.

In this embodiment, since the applied voltage is low, the article to be coated is relatively small, and therefore the distance between the main discharge electrode 2 and the article to be coated can be set short, and the product having a complicated shape capable of reducing the applied voltage can be obtained. It is applied to improve the coating efficiency.

[0026]

[Embodiment 2] FIG. 2 shows a needle-shaped main discharge electrode 2 facing the object 121 to be coated and a ring-shaped pulse discharge electrode 6 arranged outside the tubular carrier means 75 for powder coating. The electrostatic coating machine 61 is shown.

The main discharge electrode 2 is a needle-shaped electrode 2 facing the grounded object 121 and is formed of a copper wire, and is arranged 2 cm inside at the center of the jet outlet 76 of the tubular conveying means 75. . The distance from the tip of the needle electrode 2 to the pulse electrode 6 is 6
It has become cm.

The pulse discharge electrode 6 is formed of a copper plate having a thickness of 0.5 mm and forms a ring having a diameter of 120 mm.
The position of the pulse discharge electrode 6 is arranged so as to coincide with the tip of the main discharge electrode 2. As a result, a large non-uniform electric field is formed at the tip of the main discharge electrode 2, and a corona discharge 119A is generated when the pulse discharge electrode 6 is not excited.

A DC high voltage of 40 kv is applied to the main discharge electrode 2 by a DC power supply 41 in the same manner as in the case of the first embodiment. In addition, the pulse discharge electrode 6 is similarly 40 k
A pulsed high voltage of v is applied.

In this embodiment, the distance between the main discharge electrode 2 and the pulse discharge electrode 6 can be set relatively long. Therefore, the applied voltage to the main discharge electrode 2 can be increased. Therefore the main discharge electrode 2
Since it is possible to set a long distance between the object 121 and the object to be coated 121, it can be applied to a relatively large product.

[0031]

According to the present invention, the corona discharge direction and the powder coating material conveying direction intersect each other, so that the ions and the powder coating material are surely brought into contact with each other. Therefore, the charging rate, and thus the coating efficiency, is greatly improved. Moreover, since the main discharge electrode and the pulse discharge electrode periodically have the same potential, the powder coating material does not adhere to and accumulate on the electrodes. Therefore, electrostatic coating with stable quality can be continued. Also, since the direction of the lines of electric force can be changed periodically, the lines of electric force do not concentrate on the convex portions, and the powder coating material flies toward the object to be coated while vibrating. Regardless of uniform electrostatic coating. Also, since it does not concentrate on the tip of the box-shaped product, the corners inside can be painted.

Needless to say, the electrostatic coating machine for powder coatings of the present invention can be used for powder products other than powder coatings to improve uniform coating and coating efficiency, and the present invention is not limited thereto.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment.

FIG. 2 is a diagram showing a second embodiment.

FIG. 3 is a diagram showing an example of a DC power supply circuit including a pulse circuit 52.

FIG. 4 is a schematic diagram illustrating the principle of the present invention.

FIG. 5 is a schematic diagram illustrating a conventional method.

[Explanation of symbols] 2, main discharge electrode 5, shaping electrode 6, pulse discharge electrode 61, electrostatic coating machine 75, tubular transfer means 102, powder coating material

Claims (2)

[Claims] 1. In electrostatic coating of powder coating, a corona discharge is periodically applied between both electrodes by using an electrode applying a high DC voltage and an electrode applying a high DC pulse voltage having the same polarity as the electrode. Electrostatically coating a powder coating material, wherein the powder coating material is formed, and the powder coating material is conveyed in a direction intersecting with the corona discharge to charge the powder coating material. 2. A main discharge electrode, which is disposed in the central portion of the ejection port of a tubular conveying means for ejecting powder coating material and to which a high DC voltage is applied, and an inner wall surface of the conveying means, or outside the inner wall surface. Electrostatic coating machine for powder coating, which is provided with a pulse discharge electrode to which a DC pulse high voltage having the same polarity as that of the main discharge electrode is applied.