Coating processes – Nonuniform coating – Applying superposed diverse coatings or coating a coated base
Reexamination Certificate
2002-02-26
2003-06-24
Parker, Fred J. (Department: 1762)
Coating processes
Nonuniform coating
Applying superposed diverse coatings or coating a coated base
C427S258000, C427S284000, C427S287000, C427S484000
Reexamination Certificate
active
06582766
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for coating a work object in two tones, particularly for coating surfaces of a work object such as a vehicle body or the like in two tones.
BACKGROUND ART
Generally speaking, coating machines which are currently in use in the art are largely constituted by a coating action mechanism like a coating robot which is located in a coating booth, and a paint sprayer unit which is adapted to spray atomized paint particles toward a work object. In the case of coating vehicle bodies, for instance, when a work object is delivered to a coating booth by means of a work object transfer system, paint is sprayed toward the work object from a sprayer unit which is mounted on an arm distal end portion of a coating robot to move the sprayer unit along coating surfaces of the work object, keeping a predetermined distance from the latter.
In this connection, as is well known in the art, in addition to coating in one and single color, the so-called two-tone coating is often resorted to in painting vehicle bodies, for example, coating an upper half of a vehicle body in one color and a lower half in a different color.
More specifically, according to a prior art two-tone coating method, as shown in the block diagram of
FIG. 35
, an upper half of a vehicle body is coated with a first color, for example, by the use of paint of color A (Step
1
), a clear paint is coated on the surface of the coating of color A (Step
2
), and the vehicle body is put in a baking furnace to cure the coated films (Step
3
). In the next place, masking tapes are put on the vehicle body along borders of the coated area, masking the coated area of color A to prevent deposition of a second color,
namely, to prevent deposition of paint of color B on the color A area (Step
4
). After masking the color A area, the color B is coated on the lower half of the vehicle body in such a way as that upper portions of the color B area partly overlaps the color A area (Step
5
). A clear paint is then coated on the coated color A (Step
6
), and the vehicle body is put in the baking furnace again to cure the coated films (Step
7
), followed by removal of the masking tapes (Step
8
). The removal of masking tapes reveals two coated areas of different colors (two tones) which are clearly separated from each other by a border line.
In the case of the above-described two-tone coating method, however, coated films of the second color B are forcibly stripped off together with the masking tapes at the time of removal of the latter, leaving a stepped surface along border lines between the color A and color B areas. In addition, upon peeling off masking tapes, fine sawtooth-like notches appear continuously along bordering edges of the color B area to degrade the quality of finish coating to a considerable degree.
The masking involves the jobs of sticking masking tapes and sheet on and over a masking area and peeling off the masking tapes and sheets afterwards. Because of the difficulty of performing these jobs by automation, it has been the usual practice to rely on manual labors in masking and unmasking particular coating areas despite a conspicuous drop in production efficiency.
Further, in the case of the two-tone coating method using masking tapes, it is a paramount requisite to bake and cure the coating films of the color A area into a dried state before adhering masking tapes on the surfaces of the color A or of the clear paint. For this purpose, the coating process should include a step of coating a color paint, a step for coating clear paint and steps of baking coating films separately for each one of the colors A and B, which is obviously disadvantageous in view of degradations in productivity and increases in production cost.
In an attempt to improve the productivity of the two-tone coating process by aborting the masking step or by aborting the use of masking tapes, there have been developed a number of coating methods as disclosed, for example, in Laid-Open Japanese Patent No. S58-58168 and Laid-Open Japanese Patent No. H11-57606.
Firstly, above-mentioned Laid-Open Japanese Patent No. S58-58168 discloses a method for coating heavy anti-rust or corrosion-proof paint, (hereinafter referred to simply as “the first prior art” for brevity), by the use of a coating robot having a sprayer unit and a masking device in the form of a masking plate on a fore distal end portion of a robot arm. According to this coating method, a fore end portion of the masking plate is abutted against a vehicle body at the time of a heavy anti-rust coating operation thereby covering the vehicle body on the upper side of the masking plate. In this state, lower portions of the vehicle body are coated with a heavy corrosion-proof paint which is sprayed from the sprayer unit.
On the other hand, disclosed in above-mentioned Laid-Open Japanese Patent No. H11-57606 (hereinafter referred to simply as “the second prior art” for brevity) is a two-tone coating method employing, for the purpose of demarcating a border line on a work object
101
, a sprayer unit which employs a binary fluid nozzle spray gun (or the so-called air brush gun)
102
with straight directionability, namely, with a narrow spray pattern which is smaller than 30 degrees in diverging angle &thgr; as shown in FIG.
36
. An air gun (not shown) is located over the spray gun
102
thereby to spurt air toward a border line of a coating area. In addition, according to the coating method of the second prior art, the spray gun
102
is tilted with respect to a work object
101
by an angle which is greater than half the diverging angle &thgr; of the spray pattern (>½&thgr;). Namely, a border line of a coating area is demarcated on the work object
101
by spraying paint from the spray gun
102
in the tilted position. During a coating operation, air is spurted toward the border line from the air gun to prevent deposition, for example, of a color B paint on an adjoining coating area which was coated with a color A paint in a preceding stage.
In the case of the coating method according to the first prior art, that is, in the case of the coating method according to Laid-Open Japanese Patent No. S58-58168, the masking plate is adapted to cover surface areas other than a target coating area. Therefore, a large mount of paint inevitably deposits on the masking plate during a coating operation, necessitating the provision of a paint scraper in association with the masking plate to scrape deposited paint off and as a consequence requiring larger and more complicated equipment. Besides, the fore end of the masking plate which is held in abutting engagement with a vehicle body during a coating operation always has the possibility of damaging or bruising coating surfaces. For this reason, the coating method according to the first prior art may be applicable to undercoatings like heavy anti-corrosive coatings which would not require a quality finish, but does not suit for application to finish coatings.
Further, in the case of the coating method according to the above-mentioned second prior art, disclosed in Laid-Open Japanese Patent No. H11-57606, a binary fluid nozzle spray gun
102
(an air brush gun) is employed as a sprayer unit for demarcating a border line. The spray gun
102
of this sort is adapted to spurt paint under the pressure of jet air. That is, paint is vigorously spurted out from the spray gun
102
together with jet air. Therefore, there is always a problem that, while demarcating a border line by the use of a color B paint, splashes of color B paint particles rebounding on the surfaces of the work object
101
tend to scatter around and deposit on surfaces of the coating film in the adjoining color A area.
Further, according to the coating method of the abovementioned second prior art, air is spurted toward the position of the border line from an air gun. A problem with an air gun of this sort is that, because of the difficulty of controlling the direction of an air jet precisely toward an aimed position, the spray of paint from the spray gu
Mitsui Michio
Takayama Shinichi
Yamada Shiro
ABB K.K.
Parker Fred J.
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