JP2024079323A - Coating method - Google Patents

Abstract

To implement electrostatic atomization coating without coating a primer paint.SOLUTION: A coating method for coating a surface 2 of a coated matter 1 made of an insulator includes: an arrangement step of arranging an electric conductor 4 that is grounded, and for arranging the coated matter 1 between a nozzle and the electric conductor 4; a grounding step of securing grounding connection of at least one place 21 of the surface 2 of the coated matter 1; and a coating step of applying coating to the surface 2 of the coated matter 1, after the arrangement step and the grounding step. In the coating step, the paint is atomized according to action of an electrical field formed between the nozzle and the electric conductor 4 to which voltage is applied.SELECTED DRAWING: Figure 1

Description

The present invention relates to a coating method.

Electrostatic atomization coating is a coating method in which paint is broken into fine droplets by the action of an electric field formed between an electrically charged nozzle and a conductor placed opposite the nozzle, and these are then sprayed onto the substrate. Since the paint flies in an electrically charged state and adheres to the surface of the substrate, if the substrate is made of an insulator, when the droplet particles adhere to the substrate's surface, the electric charge of the droplet particles has no choice but to remain in place and accumulate on the surface. As the coating progresses, the surface of the substrate and the nozzle become charged with the same polarity, and eventually the paint cannot be atomized.

To avoid this situation, a commonly used method has been to apply a conductive primer paint prior to electrostatic atomization painting. For example, Patent Documents 1 and 2 disclose primer paints for imparting electrical conductivity to the surface of insulating objects to be painted, and these primer paints can be used as undercoats for electrostatic atomization painting.

International Publication No. 2021/054352 International Publication No. 2021/132132

However, the need to apply a primer before applying decorative paint made the painting process more complicated.

Therefore, it is desirable to develop a painting method that allows electrostatic atomization painting without applying a primer paint.

The coating method according to the present invention is a coating method for coating the surface of an insulating substrate, and includes a positioning step of positioning a grounded conductor and positioning the substrate between a nozzle and the conductor, a grounding step of ensuring a ground connection at least at one point on the surface of the substrate, and a coating step of applying paint to the surface of the substrate after the positioning step and the grounding step, and is characterized in that in the coating step, the paint is atomized by the action of an electric field formed between the nozzle, to which a voltage is applied, and the conductor.

With this configuration, the surface of the object to be coated is connected to ground, and the charge held by the paint can be released through this ground connection, making it possible to carry out electrostatic atomization coating without applying a primer paint.

Further features and advantages of the present invention will become more apparent from the following description of exemplary and non-limiting embodiments, which are given with reference to the drawings.

1 is a diagram showing a state in which a coating method according to an embodiment of the present invention is carried out; FIG. 1 is a diagram showing a first example of the embodiment of the present invention in which an edge portion is painted. FIG. 1 is a diagram showing an example of painting an edge portion according to a conventional technique. FIG. 11 is a diagram showing a second example of the embodiment of the present invention in which an edge portion is painted. FIG. 1 is a diagram showing a first example of the embodiment of the present invention in which an opening is painted. FIG. 1 is a diagram showing an example of painting an opening using a conventional technique. FIG. 13 is a diagram showing a second example of the embodiment of the present invention in which an opening is painted. FIG. 13 is a diagram showing an example of painting a curved surface portion according to an embodiment of the present invention. FIG. 1 is a diagram showing an example of painting a curved surface part using a conventional technique. FIG. 13 is a diagram showing an example of an embodiment of the present invention in which a ground connection is secured via a coating film.

[Overview of the embodiment]
An embodiment of the coating method according to the present invention will be described with reference to the drawings. In the following, an example in which the coating method according to the present invention is applied to an automobile bumper 1 (an example of an object to be coated) will be described.

The bumper 1 is made of an insulating material such as polypropylene, fiber-reinforced plastic, or ABS resin, and has a front surface 2 that is positioned on the outside when the bumper is attached to the vehicle body, and a back surface 3 on the rear side of the front surface 2. In the painting method according to this embodiment, it is the front surface 2 of the bumper 1 that is to be painted.

The painting method according to this embodiment includes a placement process for placing a conductor 4 in contact with the back surface 3 of the bumper 1, a grounding process for grounding the front surface 2 of the bumper 1, and a painting process for painting the front surface 2 of the bumper 1.

[Placement process]
In the placement step, the grounded conductor 4 is placed, and the bumper 1 is placed between the nozzle head H (nozzle N) and the conductor 4 (FIG. 1). The coating method according to this embodiment atomizes the paint P uniformly by atomizing the paint P using the action of an electric field formed between the nozzle head H, which ejects the paint, and the conductor 4, which is placed on the back surface 3 side of the bumper 1, and belongs to the technical field of so-called electrostatic atomization coating. The position of the nozzle head H is controlled by an industrial robot or the like.

In electrostatic atomization coating, the uniformity of the electric field formed can be an important factor for uniformly atomizing the paint P. For example, when the nozzle head H and the conductor 4 are directly facing each other, the distance from the tip of each of the nozzles N provided in the nozzle head H to the conductor 4 is uniform, so the electric field formed between each nozzle N and the conductor 4 tends to be uniform, and the paint P tends to be uniformly atomized. Note that when the nozzle head H and the conductor 4 are directly facing each other, this means that the surface to which the base end of the nozzle N is connected and the conductor 4 are substantially parallel. On the other hand, if the conductor 4 has corners or protrusions, the electric field tends to become non-uniform because the electric field lines concentrate in these areas, which may result in non-uniform particle size of the paint.

Therefore, in the painting method according to this embodiment, the shape of the conductor 4 is devised to increase the uniformity of the electric field. In particular, since it is preferable to design the conductor 4 according to the partial shape of the opposing bumper 1, a dedicated mold prepared for each type of bumper 1 to be painted is suitable as the conductor 4. In addition, the conductor 4 needs to be connected to ground.

(When painting edges)
First, an example of painting the edge portion 11 of the bumper 1 will be described ( FIG. 2 ). Here, the edge portion 11 is a ridge-like portion formed at the joining portion of at least two adjacent faces 12. In this case, a curved portion 41 spaced apart from the back surface 31 is formed in the portion of the conductor 4 that faces the back surface 31 of the edge portion 11.

In conventional technology, it was common to provide a conductor that conforms to the shape of the back side of the object to be painted. When this method is applied to painting the edge portion 11 of a bumper 1, an edge portion 42 is provided on the conductor 4 that is located on the back side of the edge portion 11 (Figure 3). Such an edge portion 42 can generate a non-uniform electric field due to the concentration of electric field lines.

In this embodiment, the concentration of electric field lines is avoided by using a conductor 4 with a curved surface portion 41 instead of an edge portion 42. This makes it easier to form a uniform electric field around the edge portion 11 of the bumper 1, making it easier to uniformly atomize the paint P. The radius of curvature of the curved surface portion 41 is not particularly limited as long as it is possible to avoid the concentration of electric field lines, but can be, for example, 10 mm or more.

In addition, a conductor 4 having a chamfered portion 43 instead of the curved portion 41 may be used (Figure 4). In this case, the shape of the chamfer can be appropriately set to a shape that can avoid the concentration of electric field lines.

(When painting openings, etc.)
Secondly, an example of painting the opening 14 (one example of a specific portion) of the bumper 1 will be described ( FIG. 5 ). In this case, a curved portion 44 is formed at a portion of the conductor 4 that faces the back surface 34 of the opening 14 and is spaced apart from the back surface 34. In addition, a flat surface portion 45 connected to the curved portion 44 is formed as a portion of the conductor 4 that faces the flat surface portion 15 connected to the opening 14 of the bumper 1.

The nozzle head H may not be able to enter the opening 14 due to conditions such as the dimensions and position, and may not be able to face a part of the opening 14 (especially the back part). Therefore, if the shape of the conductor 4 is made to fit the back surface 34, the distance from the conductor 4 to the tip of each nozzle N may change abruptly around the opening 14 (Figure 6).

In this embodiment, the conductor 4 is provided with a curved portion 44, thereby suppressing the rate of change in the distance from the conductor 4 to the tip of each nozzle N around the opening 14. This reduces the change in the electric field around the opening 14, making it easier to apply paint uniformly. The radius of curvature of the curved portion 44 can be, for example, more than twice the radius of curvature of the surface of the workpiece.

In addition, instead of the curved portion 44, a conductor 4 having a planar inclined portion 46 may be used (Figure 7). This configuration also suppresses the rate of change in the distance from the conductor 4 to the tip of each nozzle N around the opening 14, making it easier to uniformly atomize the paint P, just as in the case of the curved portion 44.

If the connection portion 47 between the curved portion 44 or the inclined portion 46 and the flat portion 45 is sharp, the aforementioned concentration of electric field lines may occur, so it is preferable that the connection portion 47 is rounded or chamfered.

The same procedure is followed when painting the recessed or protruding parts of the bumper 1 as when painting the opening 14. In other words, the conductor used should have a curved or inclined part that faces the back surface of the recessed or protruding part.

(When painting curved surfaces)
Thirdly, an example of painting the curved portion 18 of the bumper 1 will be described ( FIG. 8 ). In this case, a flat portion 48 that is spaced apart from the back surface 38 and has a flat shape is formed on the portion of the conductor 4 that faces the back surface 38 of the curved portion 18.

When the shape of the conductor 4 is made to fit the back surface 38 of the curved portion 18, the conductor 4 becomes curved (Figure 9). This makes it difficult to keep the distance from the tip of each nozzle N to the conductor 4 constant around the periphery of the curved portion 18, making it difficult to paint the curved portion 18 uniformly.

In this embodiment, the use of a conductor 4 with a flat portion 48 makes it easier to keep the distance between the tip of each nozzle N and the conductor 4 constant around the curved portion 18. This reduces the change in the electric field around the curved portion 18, making it easier to atomize the paint P uniformly.

[Grounding process]
The grounding step is a step of ensuring a ground connection at least at one point on the surface 2 of the bumper 1. The method is not limited, and examples thereof include a method in which a part of the conductor 4 is extended to reach a ground point 21 on the surface 2, thereby grounding one point on the surface 2 via the conductor 4 (FIG. 1), and a method in which a part of the conductor 4 is disposed in a manner that it protrudes from the bumper 1, and the protruding part and the surface 2 of the bumper 1 are painted together to ground the surface 2 via the coating film C (FIG. 10).

In principle, the painting method according to this embodiment converts paint into electrically charged droplets by the action of an electric field formed between the nozzle head H (nozzle N) and the conductor 4, and then paints the paint by attaching the droplets to the surface 2 of the bumper 1. The droplets of paint fly toward the bumper 1, attracted to the grounded (or oppositely charged) conductor 4 while being charged with the same polarity as the nozzle N. If the surface 2 of the bumper 1 is not grounded, the bumper 1 is made of an insulator, and when the droplets attach to the surface 2 of the bumper 1, the charge of the droplets remains there and accumulates on the surface 2. As the painting progresses, the nozzle N and the surface 2 become charged with the same polarity, and eventually the paint cannot be atomized. To avoid this situation, a method of applying a conductive primer paint prior to electrostatic atomization painting has been commonly used in the past.

In contrast, in this embodiment, by connecting the surface 2 of the bumper 1 to ground, the charge of the droplet particles can be released even on bumpers 1 that have not been primed. Therefore, even as painting progresses, charge does not accumulate on the surface 2, and electrostatic atomization painting can be continued.

〔Painting process〕
The painting step is a step of painting the surface 2 of the bumper 1 after the above-mentioned arrangement step and grounding step are performed.

In this embodiment, painting begins at the ground point 21 on the surface 2 of the bumper 1. Generally, the paint used in electrostatic atomization painting has a lower electrical resistivity (for example, 10 MΩcm or less) than the insulator that constitutes the bumper 1, so by bringing the ground point 21 into contact with the painted surface, a path for the charge is secured. This makes it easier to prevent the surface 2 from becoming charged during painting.

In addition, in this embodiment, the sweep speed of the nozzle head H is controlled based on the distance between the nozzle head H (nozzle N) and the conductor 4. Since the distance between the nozzle head H and the conductor 4 and the ability to atomize the paint are negatively correlated, in order to uniformly atomize the paint P, it is preferable to control the operation of the nozzle head H so that the distance between the nozzle head H and the conductor 4 is constant. However, when painting the above-mentioned edge portion 11, opening 14, curved portion 18, etc., it may be difficult to keep the distance between the nozzle head H and the conductor 4 constant for the purpose of avoiding interference between the bumper 1 and the nozzle head H. Therefore, when painting these places, if the distance between the nozzle head H and the conductor 4 is larger than when painting other places (for example, the flat portion 15), the paint discharge amount is reduced to prevent a decrease in atomization of the paint, or the sweep speed of the nozzle head H is reduced to extend the painting time for the place, so that the same film thickness can be ensured for other places. The position of the nozzle head H can be determined, for example, based on the control amount of an industrial robot to which the nozzle head H is attached, and the positions of the bumper 1 and the conductor 4 can be determined by correcting a previously prepared drawing of the bumper 1 based on an image captured by a camera installed in the paint booth, so the distance between the nozzle head H and the conductor 4 can be determined based on these.

In addition, it is advisable to control the nozzle head H so that it faces the conductor 4 as directly as possible, as far as the shapes of the bumper 1, the conductor 4, and the nozzle head H allow. For example, if the conductor 4 has an inclined portion 46, it is advisable to have the nozzle head H face the inclined portion 46 directly. Also, if the conductor 4 has a curved portion 44, it is advisable to change the angle of the nozzle head H to match the curvature direction of the curved portion 44. This makes it easier to make the distance from the tip of each nozzle N to the conductor 4 constant, making it easier to atomize the paint P uniformly.

In the painting process, it is preferable to control the operation of the nozzle head H so that the distance between the nozzle N and the conductor 4 is 60 mm or more and 140 mm or less. If the distance between the nozzle N and the conductor 4 is 60 mm or more, sparks are less likely to occur between the nozzle N and the conductor 4. Also, if the distance between the nozzle N and the conductor 4 is 140 mm or less, it is easier to uniformly atomize the paint P.

During the painting process, it is preferable to control the operation of the nozzle head H so that the distance between the nozzle N and the surface 2 of the bumper 1 is 60 mm or more and 140 mm or less. When the distance between the nozzle N and the surface 2 is within the above range, it is easy to uniformly atomize the paint P.

Other embodiments
Finally, other embodiments of the coating method according to the present invention will be described. Note that the configurations disclosed in the following embodiments can be combined with the configurations disclosed in other embodiments as long as no contradiction occurs.

In the above embodiment, the conductor 4 is provided with a characteristic structure when painting the edge portion 11, the opening 14, and the curved portion 18 of the bumper 1. However, in the present invention, from the viewpoint of omitting the primer coating, the conductor may have any structure as long as it can ensure a ground connection at least in one place on the surface of the object to be coated.

In the above embodiment, an example has been described in which the ground point 21 on the surface 2 of the bumper 1 is connected to the conductor 4, and the ground point 21 is connected to ground via the conductor 4. However, in the present invention, the method of connecting at least one point on the surface of the object to ground is not limited to the above, and for example, the surface of the object to be coated may be connected to ground via a conductor different from the conductor grounded to the back surface of the object to be coated.

As for other configurations, it should be understood that the embodiments disclosed in this specification are illustrative in all respects and that the scope of the present invention is not limited thereto. Those skilled in the art will easily understand that appropriate modifications are possible without departing from the spirit of the present invention. Therefore, other embodiments that are modified without departing from the spirit of the present invention are naturally included in the scope of the present invention.

1: Bumper 2: Surface 21: Ground point 3: Back surface 4: Conductor

Claims (12)

A coating method for coating a surface of an insulating substrate, comprising the steps of:
a placement step of placing a grounded conductor and placing the workpiece between the nozzle and the conductor;
A grounding step of ensuring a ground connection at at least one point on the surface of the substrate;
A coating step of coating a surface of the object to be coated after the placement step and the grounding step,
In the coating process, the coating material is atomized by the action of an electric field formed between the nozzle and the conductor to which a voltage is applied. The substrate has at least two adjacent surfaces and an edge portion formed at the meeting point of the two surfaces;
2. The coating method according to claim 1, wherein at least one of a curved portion and a chamfered portion is formed in a portion of the conductor that faces the back surface of the edge portion and is spaced apart from the back surface. The coating method according to claim 2, wherein at least one of the curved surface portion and the chamfered portion has a shape that can avoid the concentration of electric field lines. The substrate has a specific portion including at least one of an opening, a recess, and a protrusion,
The conductor has a curved portion arranged to face a back surface of the periphery of the specific portion, and a flat portion connected to the curved portion,
2. The coating method according to claim 1, wherein in the coating step, the nozzle is controlled so that a discharge rate at a portion of the nozzle facing the curved portion is smaller than a discharge rate at a portion of the nozzle facing the flat portion. The coating method according to claim 4, wherein at least one of rounding and chamfering is performed on the connection between the curved portion and the flat portion. The substrate has a specific portion including at least one of an opening, a recess, and a protrusion,
The conductor has a planar inclined portion that faces the rear surface of the periphery of the specific portion and is spaced apart from the rear surface of the substrate,
The coating method according to claim 1 , wherein in the coating step, the nozzle is controlled so that the nozzle and the inclined portion are directly opposed to each other. the conductor has a flat portion connected to the inclined portion,
7. The coating method according to claim 6, wherein a connecting portion between said inclined portion and said flat portion is rounded or chamfered. The object to be coated has a curved surface portion,
The coating method according to claim 1 , wherein a portion of the conductor that faces the rear surface of the curved portion is formed in a flat shape. The coating method according to any one of claims 1 to 8, wherein the nozzle sweep speed is controlled based on the distance between the nozzle and the conductor during the coating process. The coating method according to any one of claims 1 to 8, wherein in the grounding step, at least one point on the surface of the workpiece is connected to the conductor, and the one point is grounded via the conductor. The coating method according to any one of claims 1 to 8, wherein the distance between the nozzle and the conductor in the coating process is 60 mm or more and 140 mm or less. The coating method according to any one of claims 1 to 8, wherein the distance between the nozzle and the surface of the workpiece during the coating process is 60 mm or more and 140 mm or less.

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